Physiological and behavioral identification of vestibular nucleus neurons mediating the horizontal vestibuloocular reflex in trained rhesus monkeys.

JOURNALOFNEUROPHYSIOLOGY Vol. 68, No. 1, July 1992. Printed

in U.S.A.

Physiological and Behavioral Identification of Vestibular Nucleus Neurons Mediating the Horizontal Vestibuloocular Reflex in Trained Rhesus Monkeys C. A. SCUDDER AND A. F. FUCHS Department of Physiology and Biophysics and Regional University of Washington, Seattle, Washington 9819.5

SUMMARY

AND

Primate

CONCLUSIONS

1. To describein detail the secondaryneuronsof the horizontal vestibuloocularreflex (VOR) , we recordedthe extracellular activity of neuronsin the rostra1medial vestibular nucleus of alert, trained rhesusmonkeys.On the basisof their activity during horizontal headand eyemovements,neuronswere divided into several different types. Position-vestibular-pause(PVP) units dischargedin relation to headvelocity, eyevelocity, eyeposition, and ceasedfiring during somesaccades. Eye and headvelocity (EHV) units dischargedin relation to eyevelocity and headvelocity in the samedirection sothat the two signalspartially canceledduring the VOR. Two cell types dischargedin relation to eye position and velocity but not headvelocity; other typesdischargedin relation to headvelocity only. 2. The position in the neural path from the primary vestibular afferentsto abducensmotoneuronswasexamined for eachtype. Direct input from the vestibular nerve was indicated if the cell could be activated by shocksto the nerve at latencies5 1.4 ms.A projection to abducensmotoneuronswasindicated if spike-triggeredaveragingof lateral rectuselectromyographic(EMG) activity yielded responses with a sharponsetat monosynapticlatencies. 3. PVP neuronswere the principal interneuron in the VOR “three-neuron arc.” Eighty percent receivedprimary afferent input, and 66%madeexcitatory connectionswith contralateralabducensmotoneurons. Surprisingly few, - 1l%, made inhibitory connectionswith ipsilateralabducensmotoneurons.This imbalance in the ipsi- and contralateral projections wasconfirmed by measuringthe EMG activity evoked by electrical microstimulation in regionswherePVP neuronswerelocated. 4. EHV neurons whose activity increasedduring contralaterally directed heador eye movementswerealsointerneuronsin the ipsilateral inhibitory pathway. Eighty-nine percent received ipsilateralprimary afferent input, and 25%projectedto ipsilateral abducensmotoneurons.EHV neuronsexcited during ipsilateral movements received neither direct primary afferent input nor projected to either abducensnucleus.A smallproportion of each of two other cell types having sensitivity to contralateraleyeposition madeexcitatory connectionswith contralateralabducensmotoneurons.Other types rarely wereactivated from the eighth nerve or projectedto the abducensnucleus. 5. The significanceof the connectionsof VOR interneurons and the signalsthey convey is discussedfor three situations: smoothpursuit of a moving target, suppressionof the VOR, and the VOR itself. PVP neuronsconvey a signalwith a ratio of eye position and velocity componentsthat is inappropriate to drive motoneuronsduring pursuit or the VOR. Moreover, they convey a headvelocity signalto motoneuronsduring suppressionwhen the eyeisnot moving. EHV neuronsconvey signalsappropriateto 244

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remedymotoneuron input in eachsituation, but our data indicate that they do not project in sufficientnumbersto help significantly. INTRODUCTION

The vestibuloocular reflex (VOR) acts to stabilize the direction of gaze in space during a head movement by generating a movement of the eyes in the head that is opposite in direction but comparable in amplitude with the head movement. The semicircular canals of the inner ear sense the head movement and modulate the discharge of vestibular nerve afferents approximately in proportion to head velocity (Blanks et al. 1975; Fernandez and Goldberg 197 1; Keller 1976; Louie and Kimm 1976). The primary afferents synapse in the vestibular nuclei, from which head movement signals are relayed to the appropriate extraocular motoneuron pools and ultimately to the muscles themselves. Early work in anesthetized animals revealed the details of the neural pathways that mediate the VOR. It was found that primary afferents make exclusively excitatory connections on cells in the ipsilateral vestibular nucleus (cf. Kawai et al. 1969; Precht and Shimazu 1965 ) . To investigate subsequent pathways, Baker et al. ( 1969) electrically stimulated the medial vestibular nucleus (MVN) and recorded intracellularly from motoneurons in the abducens nucleus. They observed monosynaptic excitatory postsynaptic potentials ( EPSPs) in contralateral abducens motoneurons and monosynaptic inhibitory PSPs (IPSPs) in ipsilateral abducens motoneurons. Disynaptic PSPs with the same sign were obtained when the vestibular nerve was electrically stimulated. It was evident, therefore, that a single interneuron, located in the MVN, lay between the primary afferents and the motoneurons. A similar three-neuron arc was found to mediate the vertical VOR, but the interneurons lay in the superior vestibular nucleus (SVN) as well as the MVN (Baker and Berthoz 1974; Highstein 1973; Highstein et al. 197 1). Experiments on encephale isole cats revealed that the discharge of the secondary interneurons mediating the VOR was related not only to head velocity, but eye movements as well, because activity varied in conjunction with the fast and slow phases of vestibular nystagmus (Baker and Berthoz 1974; Hikosaka et al. 1977). The signals conveyed

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HORIZONTAL

VOR INTERNEURONS

by VOR secondary neurons were determined more precisely with the use of alert, trained monkeys In the medial longitudinal faSClC ulus (MLF) between the vestibu lar and oculomoto r nuclei, axons of presumed secondary neurons discharged in relation to vertical head velocity, vertical eye position, and firing ceased (paused) during saccades (King et al. 1976; Pola and Robinson 1978). They were named tonic-vestibular-pause (TVP), or alternatively positionvestibular-pause (PVP), cells for the signals that they carried. That these cells were secondary neurons was confirmed more recently with the use of the method of intraaxonal recording and staining in alert monkeys and cats. Vertical and horizontal PVP neurons were shown to project directly to the extraocular motor nuclei and to be activated at monosynaptic latencies by stimulation of the vestibular nerve (Ishizuka et al. 1980; McCrea et al. 1980, 1987a,b; Ohgaki et al. 1988). In addition to PVP neurons, the MVN and SVN contain a variety of other cell types, each with its own unique combination of vestibular and oculomotor signals (Chubb et al. 1984; Fuchs and Kimm 1975; Keller and Daniels 1975; Keller and Kamath 1975; Lisberger and Miles 1980; Miles 1974; Tomlinson and Robinson 198 1). These have been named vestibular, vestibular-plus-pause, eye-position, burst-position, gaze-velocity, and position-vestibular, according to the signals that they convey. Horizontal neurons with vestibular sensitivity are also subdivided into type I, excited during rotation toward the side of the cell body, and type II, excited duri ng ro tation away from the cell body. The contributio n of these various cell .s to the V-OR is unknown. It is evident, however, that a single pathway with PVP neurons as the only interneurons is inadequate to fully explain the VOR, and that some of these other types might also contact motoneurons or receive primary afferent input. Pola and Robinson ( 1978) showed that the signal carried by vertical PVP neurons in the MLF was not appropriate to produce the vertical VOR. Motoneurons require coordinated position velocity, and higher order dynamic commands to move the eyes w rith the correct time course and to achieve the correct final eye position (Fuchs et al. 1988; Optican and Miles 1985; Robinson 1970). Vertical PVP neurons conveyed too little position and too much velocity input, so that motoneurons needed additional position input ( Pola and Robinson 1978 ) . Possible sources of this input were suggested by Tomlinson and Robinson ( 1984), who were able to antidromically activate SVN and rostra1 MVN cells of several different types by stimulating the MLF. In addition, other inputs to motoneurons are needed to suppress the VOR when a subject must track a target that moves with the head. Because PVP cells still discharge in relation to head velocity in this condition, another neuron must provide an equal and opposite input to the motoneurons to eliminate any modulation. For the vertical VOR, this input might be provided by cells in the ygroup of the vestibular complex, which projects to the IIIrd nucleus and has cells that fire in relation to head velocity during suppression of the VOR (Chubb and Fuchs 1982; Graybiel and Hartwieg, 1974; Highstein 1973; Steiger and Biittner-Ennever 1979; Tomlinson and Robinson 1984). A similar input to the abducens nucleus has yet to be demonstrated. ‘.

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245

In summary, most of the data regarding connectivity between nuclei or subnuclei that are involved in the VOR have been gathered in the anesthetized animal with the use of intracellular recording. On the other hand, most of the data pertaining to the various types of discharge patterns has been obtained in alert animals where connectivity data is difficult to get. Only intra-axonal recording and staining has, to date, been able to obtain behavioral and connectivity data at the same time. These studies, however, suffer from being mainly anatomic, from small sample sizes, and possibly strong sampling biases. Studies using CNS stimulation and extracellular recording in alert animals are few, limited in the connections they can examine, and often cannot establish connectivity conclusively. Therefore we sought to confirm physiologically that PVP neurons contact motoneurons and have the anticipated postsynaptic effect. In addition, we wished to examine the connectivity of other possible VOR interneurons in a preparation where they could be reliably identified by a quantitative evaluation of their discharge patterns. To do so, we used extracellular unit recording in trained rhesus monkeys and employed spike-triggered averaging to examine connectivity with abducens motoneurons. We also examined whether any of these units received input from the vestibular nerve. Because several studies have quantified the discharges of vestibular neurons (Fuchs and Kimm 1975; King et al. 1976; Lisberger and Miles 1980; Pola and Robinson 1978), we concentrate here only on those neurons with demonstrated connections to the abducens nucleus and eighth nerve. Partial results have previously appeared as an abstract (Scudder and Fuchs 198 1). METHODS

Surgery Four rhesusmonkeyswerepreparedfor single-unitrecordingin two asepticsurgeries.The monkeysweredeeply anesthetizedwith the useof halothaneafter a presurgicaldoseof ketamine( 10mg/ kg). In the first procedurea searchcoil for the measurementof eye movementswasimplanted by the useof the methodof Fuchsand Robinson ( 1966): three turns of stranded stainlesssteel wire (Cooner AS630) werewrappedabout the eyeby successively passing it under the insertionsof the four rectusmuscles.At the same time, three lugsfor the stabilization of the head were built from dentalacrylic layeredabout stainlessscrewsattachedto the skullat the top front center and at the back behind both ears.Theselugs matedwith stout barsattachedto the primate chair in which monkeys satduring the recordingsessions. After a monkey wastrained, two recordingchamberswereimplanted on top of its headby meansof stainlesssteelscrewsand dental acrylic. One chamber,locatedon the left, wasinclined 15” to the parasagittalplaneand directedtoward the abducensnuclei. The other, located on the right, was directed toward the eighth nerve asit passedunder the flocculus.In the sameprocedure,two electromyographic(EMG) electrodeswere implanted in the lateral rectusmuscleof the left eyein a mannerdescribedpreviously (Scudder et al. 1988; Fuchset al. 1988). Theseelectrodeshad a longuninsulated region to record from asmany motor units as practical. They wereplacedasnearaspossibleto the entry point of the sixth nerve in an effort to ensurethat at leastone of the electrodes would yield averagedaction potentials that were entirely negative.


C. A. SCUDDER AND A. F. FUCHS

246

for activation from the eighth nerve. Most cellswere not tested using pitch oscillationsexcept for those cells exhibiting type-II Experiments were conducted in a dimly illuminated sound- activity (i.e., those potentially excited by the vertical canals)or proof booth with the monkeysseatedin a primate chair with their those having evident vertical eye-movement sensitivity during headsrestrained.Surrounding their headswere two pairs of per- pursuit and/ or spontaneousvertical eye movements. Because pendicular Helmholtz coils driven at high frequency (35 kHz) in cellswere sometimeslost when we switchedour apparatusfrom temporal quadrature. The voltage induced in the implanted yaw to pitch oscillations,not all type-II or vertical eyemovement searchcoil was demodulatedto produce signalsproportional to cellswere successfullytested. the horizontal and vertical positionsof the eye in the head. The systemwas calibrated by having the monkey alternately fixate stationary targets at & 10” horizontal and vertical. The coils and Vestibular nerve stimulation primate chair weremountedwithin a vertically orientedyoke that With the use of the right-hand chamber, tungsten electrodes could be driven by a DC motor to produce sinusoidaloscillations weredriven through the flocculusand into the eighthnerve. Entry of the monkeys’ head and body about an earth vertical axis (cf. into the eighth nerve wasmarkedby the absenceof all eyemoveFuchsand Kimm 1975) . Theseyaw oscillationstypically had 10’ ment-related activity, the appearanceof mainly regularunits repeakangulardisplacements at frequenciesfrom 0.3 to 1.4Hz. The sponsiveto headvelocity or static orientation, and a shift in acmonkeys’ heads were oriented in the stereotaxic plane (22’ tion-potential waveshapefrom initially negative (typical of sopitched down from the horizontal canal plane) (Blanks et al. matic recordings)to initially and mainly positive (typical of fiber 1985) , soall semicircularcanalswereactivated by horizontal rota- recordings). Tracks were run until a location was found where tions. Monkeys could alsobe oscillatedin the pitch plane(noseup horizontal canal afferents were recorded and microstimulation to nosedown) but only with ~fr10” displacementsat 0.5 Hz. ( 30 PA, 200/s, 0.1-ms biphasic pulses)elicited predominantly The monkeyswere trained to follow an illuminated red light- leftward eye movements of adequatevelocity (e.g., 4O/s). The emitting diode(LED) that moved back and forth alonga horizon- electrodewasthen fixed in placewith a setscrew,and the microtal rail 25 cm in front of their eyes.Vergenceunder thesecondi- drive wasremoved. The electrodewasleft in this location until its tions was5.8Oand wasalmostconstantover the pertinent rangeof efficacy deteriorated significantly (~50 PA to elicit an EMG reeye movements.In the “pursuit” condition, the LED target was sponse;typically 4- 14 days) after which the electrode was removed sinusoidallyat 0.3- 1.4Hz with the headstationary. In the moved and replaced.Stimuli usedto test for orthodromic activa“suppression”condition, the monkey wasoscillated,but the LED tion consistedof 0.1-msbiphasicpulsesdeliveredat a rate of 1/s. was held stationary relative to the head, thereby requiring the Current wasincreaseduntil the unit wasactivated or 500PA was monkey to suppressthe eyemovementsnormally inducedby the reached.Becausethe evoked EMG responsewasnearly maximal VOR. In the “stable-gaze”condition, the monkey wasoscillated, at 500PA, 90-95% of the horizontal primary afferentswereprobaand the LED wasmoved relative to the headsothat it wasstation- bly activated at this value. ary in space.The monkey’s eyestherefore moved asthey would Cellstransynaptically activated by vestibularnerve stimulation during the VOR. When the monkeys were pitched, the rail that were recordedin the right vestibular nucleus.The spikelatency guided the LED target was oriented vertically so that the same wasmeasuredfrom the onsetof the shockand not the onsetof the three paradigmscould be usedto control vertical head and eye artifact in the extracellular recording. Whenever possible,it was motion. measuredat 2 times threshold. For a variety of reasons,many units were only testedat the lowestcurrent that reliably activated the unit, - 1.3- 1.5times threshold.Becausespikesevokedat the Stimulation and recording lower currents occurred ~0.1 ms later than those evoked at 2 Electrodesfor extracellular recordingwereetchedfrom 0.005-in timesthresholdin the sameunits, latenciesobtainedonly at lower tungstenwire and insulatedwith Epoxylite except for the tip. A currentswere adjustedby subtracting0.1 ms. guidetube protected eachelectrodeasit wasloweredthrough the silasticsealingthe recording chamber,through dura and through Spike-triggered averaging cerebralcortex toward the brain stem.The electrodewasadvanced Data were collectedfor spike-triggeredaveragingwith the stabeyond the tip of the guide tube for the last 8-15 mm of the penetration with the use of a Trent Wells hydraulic microdrive. tionary monkey fixating a stationary target. Epochscontaining Signalsfrom the electrodewereconventionally amplified, filtered, saccades werediscarded.EMG activity from the lateralrectuswas and monitored by ear with the use of an audio amplifier and amplified,band-passfiltered (20 Hz to 6 kHz), and continuously sampledby a PDP- 11/ 73 computer at 50-psintervals. Unit activspeaker. The left-hand chamberwas usedto record from the abducens ity from the vestibular nuclei was amplified, passedthrough a nuclei and subsequentlythe vestibular nuclei (VN) bilaterally. thresholddetector, and the resultingpulsesusedto signalthe comWith the useof the stable-gazeparadigm,we searchedfor cellsthat puter to retain EMG data from 1 ms before the spike to 9 ms dischargedfor eyeand/or headmovementsand later testedthem ( sometimes14ms)after the spike.Spikesseparatedfrom succeedfor sensitivity to each independently with the useof the pursuit ing or precedingspikesby < 10 ms were discardedand did not and suppressionparadigms,respectively. Having thus classified triggerthe computer. EMG activity from all suchtriggeredsweeps the cell, we subjectedit to someor all of the following paradigms, was addedtogether to produce an averagedresponse.As in an describedin detail below: 1) spike-triggeredaveragingto test for earlier study (Fuchs et al. 1988), responses had to have a distinct connectionsto the left abducensnucleus;2) electricalstimulation onset at a physiologically reasonablelatency to be regardedas of the right eighth nerve to test for peripheralinputs; 3) recording evidence of the cell’s projection. Details appear in RESULTS. If of spike-discharge and eye-movementdata for quantitative analy- there wasambiguity about whether the unit yielded an averaged sis;and 4) pitch rotations to determine the sensitivity to vertical response,the averagewasaccumulateda secondtime. Large-amwere usually similar both times, but small-amrotations. It wasusually not possibleto completethis whole bat- plitude responses tery of tests,sowe tendedto concentrateon eachin turn. The early plitude responses ( l-2 pV) could differ. This wasresolvedby acmonkeysweremainly usedto collect quantitative data and spike- cumulating the averagea third time whenever practical. Units triggered averages;later monkeys were mainly used to test for yielding no averagedresponsewere allowed to accrue at least eighth nerve activation. Also, cellsin the left VN were not tested 6,000sweeps,and usually 10,000,beforewejudged that therewas

Behavioral

tasks


HORIZONTAL

VOR

INTERNEURONS

247

in fact no response.Units lostbefore a clearresponsewasobtained Monkeys werekilled under deeppentobarbitalsodiumanestheand before 6,000 sweepshad accruedwere discarded. siaby perfusionthrough the heart with salineand 10%Formalin. Fifty-micrometer frozen sectionswere collected, mounted, and counterstainedwith cresyl violet. With the use of the marking Collection of data and quantitative analysis lesionsasa guide, the coordinate systemof the recordingtracks Single-unit activity, eye position, and head position data were wasmappedonto the brain sections.The locationsof interesting recordedon FM magnetictape for subsequentquantitative analy- cellswere plotted at their recordedcoordinateson the brain secsis.The relation of firing rate to steady eye position wasdeter- tions. mined by having the monkey fixate for -5 s at eachof 11 evenly spacedpositionsfrom -25 to +25 O.The firing rate associated with RESULTS eachposition wasplotted, and the slopeof the straightline passing through the points wastaken asthe static rate position constant, In the rostra1 third of 7 vestibular nuclei from four monKS. For thoseunits having nonlinear rate position curves, KS was keys, we recorded and classified the activity of 948 neurons taken asthe averageslopefrom - 10to + lo”, the samerangeover that discharged in relation to rotation of the head and/or which most of the sinusoidaldata were collected. movement of the eyes. Because we were interested in findUnit firing was recordedduring the pursuit, suppression,and ing cells that mediated the horizontal VOR, the majority of stable-gazeconditions at 0.5 Hz. Someunits were studiedover a wider frequency range(0.3- 1.4 Hz). Storedsinusoidaldata were electrode penetrations were directed at the medial vestibulater digitized off-line by samplingat 1-ms intervals (spikeswere lar nucleus, and only a few traversed the other three princialsobinned with 1-msresolution). Digitized headposition, target pal nuclei. Penetrations were run from the caudal pole of position, eyeposition, and unit dischargeweredisplayedone cycle the abducens nucleus caudally for 2 mm, and from -0.75 at a time, and a usermarked the durations of saccadiceye move- to - 3.5 mm lateral of the midline. The nucleus prepositus ments.Eye position data during saccades werediscarded,and the hypoglossi was intentionally avoided because it was the positionsbefore and after each saccadewere joined by using a subject of a separate study (McFarland and Fuchs 1992). second-orderTaylor seriesto fill the gap.The period of eachcycle was divided into 64 equal bins, and data from typically 10, but ClassiJication of units and selecteddischarge characteristics never fewer than 5, cycleswereaveragedbin by bin. The averaged data for each signalwaspassedthrough a fast Fourier transform Units were designated as having eye position, eye veloc(FFT) to determineits amplitude and phase.For units whosefir- ity, or vestibular sensitivity according to their discharge ing rate droppedto zero over a portion of the cycle, a sinusoidwas during pursuit and suppression, as described in METHODS. fit to the nonzero portion by the methodof leastsquares.In either Units with vestibular sensitivity were further classified as case,firing rate wasfactoredinto componentsin phasewith stimutype I or type II, depending on whether they were excited lus position and velocity. Dependingon the behavioralcondition, heador eyevelocity sensitivity wascomputedby dividing the am- during horizontal head rotation toward or away from the plitude of the velocity componentby the amplitude of the heador cell body, respectively. Because, with the head upright, the eye velocity. Eye position sensitivity was computed by dividing anterior and posterior canal afferents are excited during conthe amplitude of the firing rate component in phasewith position tralateral rotations, many units having type-II responses by the eyeposition amplitude. As in our previous study, we have were also tested with pitch rotations. Units that were much calledthis the “dynamic” rate-positioncoefficient ( Kd) in recogni- more deeply modulated (e.g., >2 times) during pitch were tion of the fact that it is not purely position sensitivity that is being classified as having vertical (and not horizontal) vestibular measured(Fuchs et al. 1988). For a small minority of the cells, input. Those type-II neurons that were unmodulated were cardiac pulsationsinduced variations in spike amplitude large classified as horizontal units. A very few neurons were clasenoughto causebrief periods(e.g., 50 ms) of unreliabletriggering. In thesecaseswe displayedinstantaneousfrequency (reciprocal sified as mixed. interspike interval), which still had a discernableenvelope becauseof the periodsof reliabletriggering,and fit the pattern by eye Horizontal PVP with a curve generatedby the equation PVP neurons have head velocity sensitivity, oppositely FR = K:,E + R’l?

for pursuit or FR = G sin (2-lrft + ‘P)

for suppression and stable-gaze,whereFR is firing rate, E is angular eyeposition,& is angulareyevelocity, fis frequency, t is time, and K>, R’, G, and Cpare constantsmanipulatedby the user.As discussedpreviously (Fuchs et al. 1988), this analysisfits two independentvariablesto the two degreesof freedomof sinusoidal dataand doesnot presuppose that K> and R’ arethe actualweights of position and velocity inputs to vestibular neurons.

Anatomy and histology Near the end of recording in each monkey, marking lesions were made by the passageof 30.PA anodal current through the recordingelectrodefor 30 s.Typically, two lesionsweremadein or beneath the vestibular nuclei and another one or two near the abducensnucleus.

directed eye position sensitivity, and pause for some saccades. The discharge of a typical PVP is illustrated in Fig. 1. During smooth pursuit when the eye is moving but the head is not (Fig. 1, top), the peak firing rate (FR) occurs between the time of peak rightward eye velocity and peak rightward eye position. The unit therefore has eye position sensitivity and eye velocity sensitivity. They are 1.9 spikes/s/deg and 0.45 spikes/s/deg/s as determined by quantitative analysis. In the suppression condition, Fig. 1, middle, the head is moving but the eye is essentially still. The peak firing rate is almost precisely in phase with maximum leftward head velocity, leading it by only 1O. The unit, therefore, also has head velocity sensitivity ( 1.4 spikes/s/deg/s from quantitative analysis). Because this unit was recorded in the left vestibular nucleus and has leftward head velocity sensitivity, it is a type-1 unit, or I-PVP for short. The majority ( 74%) of our horizontal PVP neurons were type I. In the stable-gaze condition the eye and head are both moving,



248

Smooth Pursuit H

E\

VOR Suppression

I*

//

Stable Gaze

tivity and paused for saccades. If we could discern by ear any difference in firing rate as the monkey tracked a target stepping between -20 and +20”, the unit was classified as having position sensitivity. This was usually confirmed by subsequent quantitative analysis. Plotting firing rate against horizontal eye position for I-PVP neurons usually revealed linear relationships having slopes (static rate-position sensitivities, KS) ranging from 0.24 to 4.7 spikes/s/deg . The average KS was 1.73 t 0.93 (SD) spikes/s/deg (n = 69). Plots were nonlinear for about one-third ofthe units, and most units were active throughout the range of eye movements we examined ( t25” ) . From the 0.5-Hz pursuit data, we also measured the amplitude of the component of PVP discharge that was in phase with eye position (i.e., &, the dynamic position sensitivity ). Although some investigators have considered Kd an alternative measure of the static eye position sensitivity (KS), for motoneurons they are not equal (Fuchs et al. 1988). This was also true for PVP neurons where Kd was 8.2% greater than KS when both were obtained from the same units (P = 0.05, n = 65), although Kd on average (1.75 t 1.2; n = 91) was similar to& Figure 2 (top) shows that Kd was usually not constant but increased as frequency increased. Eye velocity sensitivity (R) was computed from 0.5-Hz smooth-pursuit data. The average sensitivity for 91 PVP units was 0.53 t 0.38 spikes/s/deg/s. Like Kd, R was not constant, but it declined with frequency (Fig. 2, bottom). The fact that neither Kd nor R were constant indicates that they are not measures of purely eye position and velocity 5

FIG. 1. Discharge of a position-vestibular-pause neuron for a single cycle during pursuit ( top), suppression ( middle), and stable gaze ( bottom). In each panel, the top trace is horizontal head position (H) and the middle is horizontal eye position (E). Upward deflection is rightward position, and downward is leftward. The bottom trace is instantaneous firing rate (FR): a vertical line is drawn at the position in time of each spike, and the height is proportional to the reciprocal of the interval between it and the succeeding spike. Calibration bars equal 10’ for head and eye position, 100 spikes/s for firing rate, 0.5 s for time, and apply to all panels.

but in opposite directions as in the VOR (Fig. 1, bottowl). The modulation due to eye movement and that due to head movement are therefore synergistic, resulting in a modulation deeper than that due to either alone. In all three panels, leftward saccades are accompanied by a pause, but rightward saccades are not. To distinguish PVP neurons from vestibular-plus-pause neurons, which have no eye position sensitivity (Fuchs and Kimm 1975; Keller and Kamath 1975; Tomlinson and Robinson 1984), we were scrupulous about examining the eye position sensitivity of all units that had vestibular sensi-

0

0.5

1.0

1.5

Frequency (Hz) 2. Plots of dynamic K and R computed from smooth-pursuit data at frequencies of OS- 1.5 Hz for 11 position-vestibular-pause ( PVP) neurons. Top: Kd is computed as the amplitude of PVP discharge modulation that is in phase with eye position divided by the amplitude of eye displacement. Bottom: R is the amplitude of the PVP modulation in phase with velocity divided by velocity. FIG.


HORIZONTAL

VOR INTERNEURONS

sensitivity, respectively, but also reflect higher order components of the movement. In previous studies the relative contributions of velocity and position sensitivity to total firing rate have been measured by the ratio, R/ Kd, which is the time constant (7) of a first-order system having the same phase lag as our averaged data. Figure 3 shows the distribution of 7 computed at 0.5 Hz. For units having 7 > 0.32 ( -l/2 the units), the component of firing rate due to eye velocity exceeded that due to eye position. Consequently, the total firing rate was relatively more in phase with eye velocity. A disproportionate number of these high 7 neurons came from two monkeys. The variability of 7 could be traced to the variability of Kd, which differed considerably betweenmonkeys(l.lO;n= 12to2.37;n=40),whereasR did not. Head velocity sensitivity was measured during the suppression condition. Firing rate was slightly corrected for the residual eye movements (usually < 1 O) by the use of the Kd and R measured previously. Head velocity sensitivity averaged 1.04 t 0.85 spikes/s/deg/s. For most units the phase of the modulation led pure head velocity, and the average was 10.7 t 10.4’ (n = 85). The distribution of the phase leads is very similar to that of squirrel monkey primary vestibular afferents (Fernandez and Goldberg 197 1) . The modulation of PVP neurons during the stable-gaze condition (average peak modulation, 49.7 t 27.0; n = 60) is larger than during either the pursuit ( 19.1 t 13.5 ) or suppression (35.7 t 29.6) condition alone. An analysis of similar data from monkey vertical PVP cells suggested to Tomlinson and Robinson ( 1984) that there was no contribution of eye velocity sensitivity to the total modulation during stable gaze. We examined this possibility quantitatively in 60 horizontal PVP neurons by comparing actual firing during stable gaze with that predicted by either 1) a linear vector addition of eye position, eye velocity, and head velocity sensitivities (additive model), or 2) a linear vector addition of only eye position and head velocity sensitivities (Tomlinson model). Figure 4 plots, for each unit and each model, the ratio of the predicted to actual amplitude of the firing rate modulation (G) against the predicted minus the actual phase (a). The point G = 1, + = 0 represents perfect prediction. Although neither model makes perfect predictions and there is much scatter, the predic30-

25P ,z20ii5 d5z ZlO2 50

0.4

TIME

0.6

CONSTANT

0.8

>0.8

(set)

FIG. 3. Dynamic behavior of 92 position-vestibular-pause neurons during OS-Hz pursuit summarized by a histogram of the equivalent time constant (7 = R/ Kd). The histogram shows the existence of a large number of units with very high time constants. The average time constant is 0.32 s.

X X

X

10.5 X

X

FIG. 4. Polar plot comparing position-vestibular-pause firing rate obtained during the stable-gaze condition at 0.5 Hz with the predictions of 2 models. Amplitude (G) is expressed as a ratio of the predicted to the actual amplitude, and phase as the predicted minus the actual phase. A net phase lag means the model had more phase lag than the data, a lead means the opposite. Each circle represents the prediction of 1 unit based on an additive model: firing rate = &H + &E + RE, where + is head velocity sensitivity, H is head velocity, E is eye displacement, E is eye velocity, Kd and R as in text: Each X represents that based on a nonadditive model: firing rate = &H + K,E. Fifty-five units are plotted; 5 having phase errors >30” (nonadditive model) were omitted from the plot.

tions of the additive model (circles) are better than those of the Tomlinson model (X’s). The average firing rate predicted by the additive model was 9.9% (t 11%) too high with 1.4O (k5.9) too much lag, whereas that predicted by the Tomlinson model was 22% (t 16%) too low with 8.9’ ( t 13) too much lag. For each unit, the predicted firing rate from the additive model was closer to perfect prediction (i.e., the actual distance to G = 1, + = 0 on the plot) for 54 of 60 units ( 90%). The predictions of the Tomlinson model were worst for units with high R values, exactly the situation where it should provide the best predictions. The final prominent feature of PVP discharge patterns was the pause for saccades. With few exceptions (see below) horizontal PVP neurons paused for all oFl+direction saccades (the direction opposite to their eye position sensitivity). We did not routinely quantify pause onset, but for 13 units, the last spike preceded the saccade by 13.3 t 3.8 ms. The duration of the pause, usually exceeded the duration of the saccade and could vary depending on the unit and final eye position. For a final position well into the cell’s ON-direction, steady firing was achieved soon after termination of the saccade, but for a final position far in the cell’s OFFdirection, the cell might not resume firing until 100-200 ms after the end of the saccade. When firing did resume, it achieved its final level gradually with a time constant of 30-200 ms (time to 63% of final rate). Presumably, both



250

phenomena reflect the underlying excitability of the neuron, which seemed to recover exponentially. A small minority of PVP units had such low or irregular discharge rates that it was impossible to determine whether they ever paused. Another small group definitely did not pause for any of the saccades we examined. Because discharges of both groups did not otherwise differ from the remaining PVP neurons and because they had the same afferent and efferent connections (see below), they were lumped in the PVP category. For ON-direction saccades, the majority of units (40/77, 52%) simply increased firing during the saccade to achieve their postsaccadic steady rate. Some burst (23/ 77, 30%), and a few paused ( 10/77, 13%). The burst was always smaller than that of a motoneuron (Fuchs and Luschei 1970) and usually consisted of 2-6 spikes with a peak firing typically 20-50% higher than the postsaccadic rate. There was rarely a “slide” in firing rate after the burst like that shown for motoneurons (Fuchs et al. 1988; Goldstein 1983).

from PVP neurons, whose eye and head sensitivities are in opposite directions. EHV neurons also have a much higher eye velocity sensitivity and proportionately less eye position sensitivity than PVP neurons. The discharge of a typical EHV is shown in Fig. 5. During smooth pursuit alone (top), the neuronal discharge increases during rightward movements. The unit has little eye position sensitivity and, in fact ,, the peak firing rate leads peak eye velocity by - 11 O. When thi: head alone is moving and the VOR is suppressed (middle), the neuron is excited during rightward head velocity. Finally, when the head is moving in one direction and the eye in the opposite direction (stable gaze, bottom), the effect of head velocity sensitivity on firing rate is opposite to that of eye velocity sensitivity, and the two signals

Smooth Pursuit

Vertical PVP neurons Vertical PVP neurons have vertical eye position sensitivity (up or down), oppositely directed head velocity sensitivity, and a pause for saccades. Because our search stimulus included horizontal but not vertical head rotation, units were considered to be candidate vertical PVP neurons if they had type-II discharge patterns as well as vertical eye position sensitivity and saccadic pauses. Candidate vertical PVP units were then tested during pitch rotations, and all tested did indeed have vertical head velocity sensitivity opposite to the eye position sensitivity. On the strength of this, the l/6 of the candidate units that could not be tested during pitch rotations were still classified as vertical PVP units. Because vertical PVP neurons have been reported to pause during saccadic eye movements in all directions (Pola and Robinson 1978; Tomlinson and Robinson 1984), whereas horizontal PVP neurons do not, we wondered whether this difference was real or just the result of methodological differences. The 27 confirmed vertical PVP units that we examined did pause more frequently than horizontal PVP units, but most did not pause for all saccades. Only 5 /27 units always exhibited a robust pause. For the majority ( 16/27) the pause was strongly size and/or direction dependent, varying from robust to brief (e.g., one doubled interspike interval), or even absent for small saccades ( 5 O) with frequencies approaching those reached by saccadic burst neurons ( 800/s). The remainder had bursts with frequencies between 300 and 800/s. The burst was usually followed by a gradual reduction or slide in discharge rate. The decay appeared to be either exponential or a fast exponential followed by a slower one. The time to 63% of the final rate ranged from 30 to 500 ms depending on the cell. For OFFdirection saccades, nearly all EHV units paused. The firing rate recovery from the pause was usually the mirror image of the recovery from a burst; i.e., units with a fast exponential decay after a burst had a fast exponential increase after a pause. A small number of units had more complicated saccadic responses. Three ( of 33 ) cells burst for small saccades (~5’) in the or;r;-direction and paused for saccades in the oN-direction. Another paused before each ON-direction burst, whereas two more, including the cell of Fig. 5, had a rebound burst after each pause.

Burst-position (BP) units BP units (also called burst tonics elsewhere) have eye position and velocity sensitivity, a burst in the ON-direction, a pause in the om-direction, and no head velocity sensitivity. Most cells had an ipsilateral eye position sensitivity, although some preferred contralateral eye movements. Vertical BP cells were very rare in the locations that we searched. BP units located just caudal to the abducens nucleus appeared to be just like BP cells within that nucleus. Although we could verify by spike-triggered averaging that these cells were not motoneurons, the accuracy of our reconstructions was not adequate to definitively place them outside the abducens nucleus. Data from these cells have been excluded from further analysis. BP units located more caudally within the vestibular nucleus had more irregular tonic firing with more pronounced bursts than abducens motoneurons. We undertook a quantitative analysis of only a small number of the caudal BP units. Because there was no evident difference between the discharge during pursuit and that during stable gaze, we report the data from the latter condition that was easier to analyze. For 11 BP units, KS = 3.6, Kd = 4.1, R = 1.1, and 7 = 0.32. These data are similar to those obtained by McFarland and Fuchs ( 1992) for BP units recorded in the prepositus nucleus and the adjacent medial vestibular nucleus.

Eye position units (EP) EP units (also called tonic units elsewhere) have eye position sensitivity, usually eye velocity sensitivity, but no head velocity sensitivity. Unlike BP units, they do not burst for ON-direction saccades. About one-half pause for or;r;-direction saccades. They were classified as EP rather than PVP units because we could not detect any modulation over the audio monitor during VOR suppression. Nonetheless, we collected and analyzed suppression data for some units. After subtracting the modulation due to the residual eye movements, a modest modulation was sometimes detect-



252

able (0.02-O. 1 spikes/s/deg/s). However, the modulation was not statistically significant and constituted ~3% of the tonic rate of the cell. Vestibular units This category includes all cells that were modulated during VOR suppression but were unmodulated during pursuit. It includes cells that pause during saccadic eye movements (vestibular-plus-pause units) (Fuchs and Kimm 1975; Keller and Kamath 1975) and those that do not (vestibular-only ) (Fuchs and Kimm 1975). We have lumped these together because they do not differ in their connections (see below), but we do not know if they have similar functional roles. We did subdivide vestibular units into type I, type II, and vertical, as described earlier. Because we were unable to test all units having type-II responses for modulation during pitch rotations, our type-II category undoubtedly includes some vertical units. Of those type-II units that were tested with the use of pitch rotations, 70% responded better during pitch, and 27% exhibited negligible modulation. Two units (3%) had about equal horizontal and vertical modulations. This percentage of true type-II units (27%) is considerably higher than the 10% found by Lisberger and Miles ( 1980).

Miscellaneous units In addition to the five major categories above, some other types occurred in insubstantial numbers. A “mixed” category of cells includes all cells with mixed horizontal and vertical sensitivities whether they had vestibular-vestibular or vestibular-eye mixtures. A “miscellaneous” category includes cells with vestibular and eye position sensitivity in the same direction but with very little eye velocity sensitivity (othenvise they would be EHV units), This category also includes cells that show increased activity at both left and right eccentric eye positions. In subsequent tables and figures, the designation “eye” will include EP and BP units when differentiation is not important. The prefix i- or c- will be used to designate ipsilateral and contralateral directions of eye position / velocity sensitivity, respectively. I-, II-, and V-, will designate type-I, type-II, or vertical (up or down) vestibular sensitivity, respectively. Location and numerical distribution by type Figure 6 shows the anatomic distribution of some of the cell types recorded in this study. Locations of individual cells (dots and X’s) are collapsed rostrocaudally onto a stereotaxic section of the brain stem located - 1 mm caudal to the caudal pole of the abducens nucleus. Type-I PVP units (top right) were found principally in the medial vestibular nucleus along its border with the lateral vestibular (Deiters’) nucleus. The most ventral and lateral area containing PVP units is coextensive with the “ventral lateral vestibular nucleus” (VLVN) (Langer et al. 1986; McCrea et al. 1980, 1987a,b). PVP neurons were the predominant neuron in the lateral MVN and VLVN, where some penetrations encountered as many as nine PVP units over 1 mm with no other cell types in between. More medially, PVP

FIG. 6. Locations of the major cell types plotted on a stereotaxic cross section of the brain stem 1 mm caudal to the abducens nucleus. Top right: dots mark locations of type-1 position-vestibular-pauses. Bottomleft: dots mark ipsilateral burst-position units (i-BPS), X’s mark contralateral BP units ( C-BPS). Bottomright : dots mark ipsilateral eye and head velocities (i-EHVs), X’s mark c-EHVs, See text for neuron abbreviations; MVN, medial vestibular nucleus; SVN, superior vestibular nucleus; LVN, lateral vestibular nucleus; DVN, descending vestibular nucleus; BC, brachium conjunctivum; MLF, medial longitudinal fasciculus; IV-V, fourth ventricle.

units were mixed with other cell types. Some subtle features, not revealed in the figure, are that I-PVP neurons were recorded in clusters, and that near the abducens nucleus, PVP neurons were pushed laterally and dorsally. The distribution of cells retrogradely labeled by horseradish peroxidase (HRP) injections in the contralateral abducens nucleus have these same features (C. A. Scudder, A. F. Fuchs, T. P. Langer, unpublished observations). BP units with ipsilateral eye movement sensitivity (dots, Fig. 6, bottom right) were found medially within the MVN. EHV units with ipsilateral velocity sensitivity (dots, Fig. 6, bottom left) were distributed widely, but the majority were also located medially within the MVN. The two types were frequently intermingled within a penetration. Both types may be distributed more medially than is depicted in Fig. 6, because we did not run tracks within 0.7 mm of the midline. EHV units with contralateral velocity sensitivity (X’s, bottom right) were located more dorsally and laterally, between the fourth ventricle and the SVN or LVN. They were frequently encountered just dorsal to PVP units in the same electrode penetrations. BP units with contralateral sensitivity, one of the more uncommon types in the MVN, were distributed like EHV units with contralateral sensitivity.


HORIZONTAL

VOR INTERNEURONS

Other unit types were distributed more uniformly. Vertical PVP units were located slightly more caudal than horizontal PVP units. There was a slight tendency for type-1 vestibular units (with or without saccadic sensitivity) to be located laterally. Type-II vestibular and PVP units tended slightly to be located medially. However, when the modulations due to eye movement and vestibular rotation are combined, as during the VOR, there was a striking preponderance of “type-II” responses medially, and of “type-I” responses laterally. The medially located i-BP, i-EHV, and II-vestibular units are all excited during contralateral head rotation when the eye is moving in a compensatory manner. The laterally located I-PVP, c-EHV, and I-vestibular units are all excited during ipsilateral rotations of the head during the VOR. Table 1 estimates the proportion of each cell type within the MVN. The first column tabulates the actual number of cells recorded, whereas the second column tabulates the corresponding percentage of the total. Because we made more penetrations laterally where PVP neurons were located, we attempted to correct for this bias. The counts of each type along a given track were weighted by the inverse of the number of tracks within a OS-mm circle around the track. The corrected percentages are shown in the third column. The major corrections were a decrease in the percentage of I-PVP neurons and an increase in the percentage of i-BP neurons. The latter are probably still underestimated because we did not probe the full medial extent of the MVN ( see McFarland and Fuchs 1992). Additionally, medially located cells tend to be smaller than laterally located ones and might be isolated at disproportionally low rates. Column 3 also estimated the number of type-II vestibular units that were actually vertical vestibular units by assuming that the proportion of the latter was the same in the sample of cells that were not tested with the use of pitch rotations as in the tested sample. TABLE

1.

Encounter rates of cells in the medial vestibular

nucleus

?I I-PVP II-PVP c-EP c-BP c-EHV i-EP i-BP i-EHV I-Vest II-Vest V-Vest v-PVP V-Eye Mixed Miscellaneous Total

246 77 50 15 41 45 91 54 97 62 35 77 13 22 23 948

Corresponding Percentages of Total 25.9 8.1 5.3 1.6 4.3 4.7 9.6 5.7 10.2 6.2 3.7 8.1 1.4 2.3 2.4 1oo:o

Corrected Percentage* 23.5 8.4 4.9 1.5 4.5 5.2 10.8 6.0 10.3 3.0 7.5 8.1 1.4 2.4 2.4 100.0

n is number of neurons encountered and classified. PVP, position-vestibular-pause; EP, eye position; BP, burst position; EHV, eye and head velocity; Vest, vestibular; I-, type I; II-, type II; c-, contralateral; i-, ipsilateral; V, vertical. *See text.

253

Although most unit types were encountered at approximately the same rates across the four monkeys, two types were not. Type-II PVP units were encountered over a range of 2.1-9.1%, and i-EHV units were encountered over a larger range, 0.6- 16.0%.

Connections to abducens nucleus revealed by spike-triggered averaging Spike-triggered averaging of left lateral rectus (LR) muscle EMG activity with the use of spikes of vestibular neurons as the trigger was carried out with the monkey fixating a stationary target. Artifacts were more likely to arise if the eye was moving. The size of the response grew as the eyes deviated leftward. Most averages were performed with the eye looking 15 O left, because beyond that point, the noise grew more rapidly than the response. Sometimes we moved the fixation point more in the cell’s om-direction to reduce the firing rate to < lOO/ s. Responses were classified as null, synchronous, ambiguous, or clear, depending on their shape, latency, and duration. Null responses were usually just flat lines perturbed only by noise, which tended to be ~0.2 pv RMS (root mean squared) for 8,000 sweeps and the monkey looking 15 O left. Most neurons recorded in this study yielded null responses. Because the purpose of the spike-triggered averaging was to determine which cells project to abducens (ABD) motoneurons, we defined “clear” responses as those that might reasonably arise from a PSP in the motoneurons. The simplest criteria are that the averages should have some qualitative similarity to spike-triggered averages from motoneurons, be relatively delayed, and be somewhat more temporally dispersed. Figure 7A shows a typical motoneuron spike-triggered average, and Fig. 7, B and C, shows examples of clear responses obtained from two I-PVP cells located in the right vestibular nucleus (contralateral to the EMG electrode) that meet the above criteria. They have a well-defined negative deflection beginning 1.4 (B) and 1.3 ( C) ms after the time of the trigger spike (dotted line). The negative deflection lasted 2.6 (B) and 2.2 ms (C). The distribution of response latencies for averages produced by triggering from contralateral I-PVP neurons is shown in Fig. 8. The mean was 1.4 ms, 0.5 ms later than the average motoneuron response (Fuchs et al. 1988). The range of latencies is wide, but it represents the combined spread of latencies for two synapses (PVP to ABD, ABD to LR). The minimum latency we were willing to accept as plausible for a transynaptic event was 0.8 ms: 0.5 ms for the earliest response from a motoneuron (Fuchs et al. 1988), plus 0.4 ms for synaptic delay, minus 0.1 ms delay from spike onset to triggering the computer and other sources of temporal uncertainty. As stated earlier, we attempted to place the EMG electrodes so that at least one would record entirely negative potentials from the muscle. This was largely achieved for the monkey used to generate Fig. 7. Because spike-triggered averages from motoneurons were negative and the averages from contralateral I-PVP neurons had the same sign, the latter responses must also represent excitation of the lateral rectus, in this case mediated by transynaptic excitation of the motoneurons.



254

D

f

Trigger Spike

2.0 ms

7. Spike-triggered averages of lateral rectus electromyographic activity obtained from 4 different units. A: abducens motoneuron, 2,000 sweeps. B and C: type-1 position-vestibular-pause units (PVPs) yielding positive-type averages, 4,900 and 1,900 sweeps. D: type-1 PVP yielding a synchronous-type average, 5,300 sweeps. Vertical dotted line indicates time of the trigger spike for each of the above cells. PVP averages have the same sign as abducens motoneuron averages and hence are excitatory. FIG.

Figure 70 shows another common, but less frequent, EMG average seen when triggering from I-PVP neurons. It has a longer duration than that of Fig. 7, B or C, and begins even before the occurrence of the trigger spike. Obviously, it cannot be caused by a projection from the triggering neuron to motoneurons. Averages of this type most likely arise when the spikes of the recorded neuron are loosely synchronized with those of another cell that does project to the motoneurons. This second cell fires both before and after the recorded cell fires, thus giving rise to activity in the averaged EMG both before and well after the time of the trigger spike. We have called these synchronization responses. Less frequently, these slow responses had a sharp

negative deflection superimposed at - 1.4 ms (not illustrated). We regarded those as the addition of synchronization responses with clear responses like those of Fig. 7, B or C, and treated them in subsequent analyses as clear responses. For the purpose of computing the proportion of cells making synaptic connections with motoneurons, pure synchronization responses 3 PV were treated as ambiguous because they could obscure smaller superimposed clear responses. Responses having some, but not all, of the features of clear responses were classified as ambiguous. Examples include very noisy responses with otherwise reasonable shapes, responses with sharp onsets but at marginally short latencies (e.g., 0.7 ms) or durations ( 1.O ms), or responses with reasonable shapes but poorly defined onsets. Usually such responses could be replicated, demonstrating that the activity of the trigger cell was genuinely correlated with the activity of one or more motoneurons. However, we were not convinced that a direct projection from the former to the latter could produce such correlations, so ambiguous responses were not included in the computations below. With the use of such criteria, we computed the proportion of each type of neuron recorded from the right vestibular nucleus that made excitatory connections with cells in the left abducens nucleus. The results are shown in Table 2 and Fig. 9 (top). Note that we have not plotted the actual number of cells because we did not compute averages for, nor did we probably encounter, cells in their natural proportions. The group making by far the most projections were the I-PVP cells. Two of every three cells (excluding ambiguous responses) were connected to motoneurons. IPVP cells also had the strongest projections based on the size of the averaged response: 3.5 PV t 1S, n = 70, compared with all others: 2.5 PV t 1.O, n = 10. About one in four non-PVP cells with contralateral eye position sensitivity projected to the contralateral abducens nucleus. More BP cells projected than did eye position cells. Negligible numbers of other cell types projected. Of interest, however, is the single contralateral projecting down eye vertical PVP, because a similar connection was found by McCrea et al. ( 1987b) using intra-axonal dye injection. Of the 296 cells TABLE 2. Excitatory EMG averagesfrom dlflerent cell types in contralateral vestibularnucleus

15

.6

.7

.8

.9

1.0

1.1

1.2

LATENCY

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

>2.1

(ms)

8. Histogram illustrating the distribution of latencies of spike-triggered-averaged responses from type-1 position-vestibular-pause units contralateral to the electromyographic electrodes. FIG.

I-PVP c-EP c-BP c-EHV i-Eye i-EHV I-Vest II-Vest v-PVP II-PVP Other

n

Clear

Synchronous

Ambiguous

122

70 (57) 3 (11)

19 (16) 4 (15)

10 (8) 3 (11)

23 (19) 17 (63) 13 (81)a

27

12

Null

4 (33)

2 (17)

0

0

21 10

1 (6)

0 1 (10)

0 0

2 (13)

0 0

21 (100) 9 (90)

22

0

19 (86)

0 1 (9) 0 0

3 (14)

0

19 11 23 13

1 (5) 0 0 2 (15)

1 (5) 0 2 (9) 1 (8)

17 (90) 10 (91) 21 (91) 10 (77)

16

6 (50)

Tabulated are the number of cells of each type giving rise to Clear, Synchronous, Ambiguous, and Null responses. Corresponding percentages of the total number of cells of each cell type are in parentheses. Abbreviations, see Table 1.


HORIZONTAL

VOR INTERNEURONS

255

100

Contralatera .I Projections 80

60

Trigger Spike I-PVP

80

1

c-Eye

c-EHV

Vest

i-Eye

Other

FIG. lo. Spike-triggered averages of lateral rectus electromyographic (EMG) activity obtained from 2 type-1 position-vestibular-pause cells located ipsilateral to the EMG electrodes. Top: 22,100 sweeps. Bottom: 28,400 sweeps.

lpsilateral Projections

60

I-PVP

1 .O ms

c-Eye

c-EHV

Vest

i -Eye

Other

FIG. 9. Top: histogram illustrating the proportion of each cell type that projects to the abducens nucleus contralateral to the soma as determined by spike-triggered averaging. Values are reduced from Table 2 by excluding ambiguous averages and - l/3 of synchronous averages (see text). The c-Eye and i-Eye categories include burst-position and eye-position units. Bottom: histogram illustrating the proportion of each cell type that projects to the ipsilateral abducens nucleus as determined by spike-triggered averaging. Values are reduced from Table 3 by excluding ambiguous averages and ‘13of synchronous averages.

averaged on this side of the brain stem, only three yielded clear averages of the opposite sign (inhibitory). Few clear averages were obtained when triggering from vestibular nucleus neurons ipsilateral to the EMG electrodes. In all cases, like those illustrated in Fig. 10, the responses were positive going, indicating a decrease of lateral rectus activity or an inhibition of abducens motoneurons. Compared with clear responses obtained from the contralatera1 side, they were smaller (2.0 t 1.4 pV, n = 11)) began later ( 1.6 t 0.5 ms, n = 1 1 ), and often were not mirror images of the contralateral averages. Figure 9 (bottom), which shows the proportion of the various neuron types that yielded clear averages, reveals other differences. Not only did many fewer vestibular nucleus neurons produce clear averages, but remarkably few PVP neurons did. In fact, the cell type that produced the most averages was the EHV units having contralateral eye and head sensitivity, although the proportion doing so (25%) was still quite low. None of the other types produced significant numbers of averages ( Table 3 ) . Although data from the cat (Ishizuka et al. 1980; McCrea et al. 1980; Ohgaki et al. 1988) indicate that vestibular nu-

cleus neurons have somewhat weaker projections to the ipsilateral abducens nucleus, we were surprised at the paucity of projections suggested by our monkey data. To corroborate the findings obtained from spike-triggered averaging, we measured and compared the EMG responses obtained by microstimulating the ipsilateral and contralateral vestibular nuclei. Biphasic 1~-PA, 100~ps shocks repeated at 40/s were passed through the recording electrode at several regions where PVP neurons were prevalent. Figure 11 compares the results from stimulation at the best site on the contralateral side (A ) with that on the ipsilateral side (B) . A and B are at the same calibration, whereas C is amplified to better reveal the waveshape. Microstimulation in the contralateral side was seven times more effective than that in the ipsilateral side.

Connections with the vestibular nerve Three hundred fifty-six cells located in the right vestibular nucleus were tested for transynaptic activation by microstimulation of the eighth nerve. The threshold current for evoking spikes ranged from 20 to 500 PA (the highest tested) with a mean of 118 t 92 PA. Thresholds were not significantly different for the different cell types, and both I-PVP as well as other types could have high thresholds. TABLE 3.

Inhibitory EMG averagesfrom diJ2rent celZtypesin ipsilateral vestibularnucleus

I-PVP c-Eye cEHV i-Eye i-EHV I-Vest II-Vest v-PVP II-PVP Other

n

Clear

Synchronous

54 50

5 (8) 2 (4)

7 (14 5 (10)

12

5 16 17 9 7 21 14

3 (25)

0 0 0 1W) 1 (14) 1 (5) 0

0 0 3 (1% 0 0 0 1 (5) 1 (7)

Ambiguous

6 (12) 5 (10) 0 0 1 (6) 0 0 0 2 (lo) 0

Null 36 (66) 38 (76) 9 (75) 5 (100) 12 (75) 17 (100)

8 (8% 6 (86) 17 (81) 13 (93)

Numbers in parentheses are percentages. Headings and abbreviations same as Table 2.



256

A

contra

:

PVP

:

ipsi PVP

: i

B

PVP

1 .O ms 15yA 11. Stimulus-triggered averages of lateral rectus electromyographic (EMG) activity obtained by stimulating the position-vestibular-pause region contralateral to the EMG electrodes (top), or the PVP region ipsilateral to the EMG electrodes (middle and bottom). Top and middle traces are plotted at the same gain, the bottom trace is an amplified version of the middle trace. Stimuli were 0.1 -ms biphasic, 1~-PA shocks repeated at 40/s. Each average represents 200 sweeps. FIG.

This result may reflect the small sample size of some nonPVP types and the large variability produced by the different distances of vestibular afferents from the stimulating electrode. The lack of significant differences need not imply that all types would be equally excitable when optimally stimulated. At higher currents, spike doublets were evoked in a few neurons, again irrespective of neuron type. For the sake of the stimulating electrode and the eighth nerve, we did not exceed 2 times threshold, so it is possible that doublets could have been evoked in more neurons at higher currents. If spikes evoked by stimulation matched, in size and shape, the spikes of the neuron previously classified at that site, the evoked spikes were attributed to the classified neuron. Usually, there was no difficulty because we also knew the sizes and shapes of spikes belonging to neighboring cells, if any, that were responsive to head and eye movements. When a discrimination could not be made, or when stimulation revealed a completely different spike (presumably from a behaviorally unresponsive cell), the results were classified as ambiguous and were not considered further. Stimulation of the vestibular nerve generated a negative field potential that peaked at -0.7-0.9 ms and that was often preceded by a positive potential. As shown in Fig. 12A, the orthodromic spike for units having the earliest latencies arose soon after the peak of the negative field but, more commonly, arose near the end of the field. For a small number of units, the field was so large that it could obscure a smaller evoked spike. These results were classified as ambiguous and were not considered further. Often, especially at more medial locations, there was no field, and the spikes were readily observed (Fig. 12 B) . A wide variety of types were activated by eighth nerve shocks (Table 4), and, along the border of the MVN and

LVN, often every cell was activated regardless of type. This area receives the densest projection of horizontal canal afferents according to some authors (Gacek 1969; Sato et al. 1989; Stein and Carpenter 1967). The two cell types most prevalent in this area, I-PVP neurons and EHV neurons with contralateral eye and head movement sensitivity, were

0.5 msec

12. Five superimposed traces of extracellular recordings from vestibular nucleus neurons activated by stimulation of the ipsilateral vestibular nerve. The stimulus artifact occupies the initial 0.3 ms after shock onset (vertical dotted line). A : the artifact is immediately followed by a positivegoing and then a negative-going field that have been interpreted as the afferent volley and the extracellular recording of excitatory postsynaptic potentials, respectively (Precht and Shimazu 1965 ). The spike arises from the rising phase of the negative field at 0.8 ms (4 ). B: there is little or no field, and the spike follows at a latency of - 1.3 ms (4 ) . Spike latencies in alert monkeys typically display more variability than illustrated. Traces were selected to include spikes with the earliest and with modal latencies, but the full range of latencies is not illustrated to reduce confusion. FIG.


HORIZONTAL

also the most frequently activated at 8 1 and 89%, respectively (excluding ambiguous responses). The former were expected on the basis of other work (Ishizuka et al. 1980; McCrea et al. 1980, 1987a; Nakao et al. 1982), but the latter have not been previously reported. Surprisingly, the activated EHV units have type-II vestibular sensitivity and would not be expected to be activated by horizontal canal afferents. Activation via vertical canal afferents is possible. Although a few cells did have measurable modulation during pitch rotation, the majority did not. The category of cells with contralateral eye position sensitivity (c-EYE, Table 4) includes left EP and left BP units. About one-half of these units were activated although they had no demonstrable vestibular sensitivity. Possible reasons for this, as well as the paradoxical activation of c-EHV units, will be addressed in the DISCUSSION. Finally, Table 4 shows that 9 of 34 type-II units were activated from the vestibular nerve. On the basis of the proportion of activated real type-II cells and the proportion of activated vertical vestibular units, we expect that seven of these nine are vertical units, and only two ( 12% ) are real type-II units. Therefore type-II vestibular units usually do not receive ipsilateral eighth nerve input. The latency from shock onset to evoked spike onset was measured in 149 units. For any given unit, the latency was not constant but varied by -0.3-0.4 ms from trial to trial. In part, this variability depended on the animal’s behavior. For example, evoked spikes in PVP neurons were delayed or even abolished during the pause for om-direction saccades. For each unit, we report modal values, which were typically 0.1 ms later than the earliest evoked spikes. Figure 13 shows the distributions of modal latencies for type-1 PVP cells (A), vertical PVP cells (B), and for the remaining types (C). The former two are unimodal and almost certainly reflect monosynaptic activation. Although it is technically possible that the cells with 1.3- and 1.4-ms latencies were disynaptically activated, it seems improbable that the spread of disynaptic latencies would be only 0.2 ms. The third distribution (Fig. 13C) is also unimodal but includes latencies up to 2.5 ms. It was unclear \whether units with long spike latencies were monosynaptically activated. On TABLE 4.

I-PVP c-Eye i-Eye c-EHV i-EHV I-Vest II-Vest V-Vest v-PVP II-PVP Miscellaneous Total

Cell types receiving vestibularnerve input n

Yes

69 32 69 9 8 41 34 32 43 17 15 356

50 (72) 12 (38) 3 (4) 8 (89) 1(14) 9 (22) 9 (26) 14 (44) 29 (67) 5 (29) 5 (33)

Late 0 3 2 0 0 2 2 3 0 0 0

0 (3) (5) (6) (9)

257

VOR INTERNEURONS

Ambiguous

No

7 (10) 3 (9) 2 (3) 0 0 2 (5) 0 0 4 (9 1 (6) 0

12 (17) 14 (44) 48 (70) 1 (11) 7 (86) 28 (68) 23 (68) 15 (47) 10 (23) 11 (65) 10 (67)

Numbers in parentheses are percentages. Yes column indicates number of cells definitely activated by vestibular nerve stimulation. Late responses are those with latencies > 1.4 ms. Ambiguous responses are those where a spike may have been evoked but was swamped by the field, or where a spike was evoked but did not sufficiently match the spike of the unit under examination to be sure they were produced by the same cell.

I-PVP N=53 x=1.03

.a

B

.9

1.0

1.1 1.2 1.3

1.4 1.5

1.6 1.7 v-PVP

5 .a

.9

1.0 1.1

1.2 1.3 1.4

1.5 1.6 1.7

cl :::::

Other

El

c-EYE, c-EHV

;;g;; I-V, v-v cl. . . .

.a

.9

1.0 1.1

1.2 1.3

LATENCY

1.4 1.5 1.6

1.7

21.8

(ms)

FIG. 13. Histograms illustrating the distribution of spike latencies for neurons activated by vestibular nerve stimulation. A : type-1 position-vestibular-pause ( PVPs) . B: vertical PVPs. C: all other activated types. The “Other” category includes II-PVPs, II-Vestibular (most probably are VVestibular units, see text), i-Eye, and miscellaneous cells.

the one hand, they were activated at currents similar to neurons on the same track having shorter latencies, whereas on the other hand, one-half had latencies that fluctuated more than usual. We tested a sufficient number of I-PVP units for both input from the eighth nerve and projections to the abdutens nucleus to determine whether the two connections occurred independently. They did not; there was a tendency for cells having one connection to also have the other (25 / 36), and for cells not having one connection to not have the other (6 / 36). This compares with expected values of 22 / 36 and 1.8 / 36, respectively, and the differences were significantly different from chance ( x2, P = 0.005 ) . There was a weak tendency for I-PVP neurons lacking one or both connections to be located dorsally in the MVN. DISCUSSION

Figure 14 summarizes the major findings of this study. Schematized for the right vestibular nucleus are the major cell types, their eighth nerve inputs, and their projections to the ipsilateral and contralateral abducens nuclei. Thicker lines indicate stronger pathways. According to our data, type-1 PVP neurons are the major conduit for head velocity information from the horizontal canal to the abducens nuclei. Not only are they the most abundant cell type in the rostra1 MVN (Table 1 ), but 80% of I-PVP neurons are activated from the eighth nerve, at least 77% contact abducens



258

weak, so most of their activated neurons were probably PVP cells whose position sensitivity was missed. In our sample, however, the activated vestibular-plus-pause units are not likely to be closet PVP neurons because none projected to the abducens nuclei. Although our data show that the contralateral projecting R-VN PVP pathway was quite strong, the ipsilateral pathway was quite weak. Only one-sixth as many ipsilateral cells proL-Eye I-PVP L-EHV I-vest duced averaged responses as did contralateral cells, and their responses were but 60% as large. In the cat, PVP neurons projecting ipsilaterally are fewer in number and have more restricted terminations in the abducens nuclei than their contralateral projecting counterparts (Ishizuka et al. 1980; McCrea et al. 1980; Nakao et al. 1982), but the monEighth Nerve Input key ipsilateral projection seems even weaker. Although FIG. 14. Diagram illustrating the cell types and their connections as revealed by this study. The right 8th nerve comes in from the bottom right there may be problems with the technique of spike-triggered averaging for inhibitory connections (seeAPPENDIX), and makes excitatory synapses with several cell types in the right vestibular nucleus. These in turn, project to either the left or right abducens nuclei, there are several reasons for believing that the number of which contain left and right burst-position cells, respectively. The diagram ipsilaterally projecting PVP cells in the monkey is close to is not intended to indicate that the same individual cells that receive 8th the 1 l- 18% we report. First, if at least 66% of the PVP cells nerve input necessarily project to motoneurons, although some do (see make contralateral projections and perhaps 10% form the text). Line thickness indicates the relative strength of each connection, data are available), that leaves at including the proportion of cells making/receiving the connection and an ATD (no quantitative estimate of the number of cells of each type. most 24% to project to the ipsilateral abducens. Second, stimulation of the PVP regions on both sides of the brain motoneurons ( 66% contralateral and 11% ipsilateral), and (Fig. 11) shows the ipsilateral pathway to be about one-sevthe strength of each cell’s projection is on average greater enth as strong as the contralateral. These results are supthan that of the other types as assessed by the amplitude of ported by the study of McCrea et al. ( 1987a) who were the spike-triggered average. The actual proportion of I-PVP unable to intra-axonally stain a single monkey PVP neuron neurons that contact abducens motoneurons might be projecting to the ipsilateral abducens. Finally, fewer ipsilatgreater than the above 77% because, in the monkey with the era1 than contralateral cells were also retrogradely labeled best placed EMG electrodes, this proportion was 9% higher by HRP injections in the rhesus monkey abducens nucleus than the average. This leaves somewhere between 14 and (Langer et al. 1986). 23% that could project elsewhere, such as to the medial Although PVP neurons have been regarded as the interrectus subdivision of the IIIrd nucleus via the ascending neurons of the “three-neuron arc” of the VOR, our study tract of Deiters’ ( ATD) (Highstein and Reisine 198 1; found other types that also receive eighth nerve input and/ McCrea et al. 1987a; Reisine et al. 198 1) or to the spinal or project to abducens motoneurons. This result agrees cord. It is currently uncertain whether monkey PVP neu- with the study of Tomlinson and Robinson ( 1984) in rons do project to the spinal cord (Nudo and Masterton which several types of vestibular nucleus neurons were anti1989; McCrea et al. 1987a,b). dromically activated by stimulation of the MLF near the The participation of PVP units in the VOR had been oculomotor nucleus. While studying the ipsilateral horizonanticipated from earlier experiments. In the cat, type-1 tal VOR pathway in ketamine-anesthetized cats, Ohgaki et PVPs that received direct input from the vestibular nerve al. ( 1988) found five different morphological types that were injected with HRP and found to project to the extraoc- may correspond to different behaviorally defined types. ular motor nuclei (Ishizuka et al. 1980; McCrea et al. 1980; The fact that other studies using intra-axonal staining often Ohgaki et al. 1988). PVP connections to motoneurons missed these cells may reflect the small sample sizes and were also demonstrated with spike-triggered averaging of potential biases due to axonal size and trajectory. killed-end abducens nerve potentials (Nakao et al. 1982). In our study 15% of c-EP and 40% of c-BP neurons (TaIn most studies the cats were not alert, however, in two, ble 2, excluding ambiguous and most synchronous reabducens-projecting neurons were demonstrated to have sponses) made excitatory projections to contralateral abdueye position and saccadic sensitivity (Iwamoto et al. tens motoneurons. Some of these neurons were also acti1990a,b; McCrea et al. 1980). In the squirrel monkey, cells vated by shocks to the vestibular nerve. There are several monosynaptically activated from the vestibular nerve and possibilities why the afferent vestibular input was not evihaving PVP-like position and saccadic sensitivity project to dent in the discharge of the neurons. First, they may have the oculomotor nucleus or the contralateral abducens nu- received canal afferent input, but the vestibular sensitivity cleus (McCrea et al. 1987a,b). Activated cells were pre- was subliminal. For instance, the c-EP cells could just lie at sumed to have type-1 vestibular sensitivity, however, as we one end of the spectrum of PVP cells. Second, cells could have discovered, the correlation between monosynaptic ac- receive utricular input and could be interneurons in the tivation and type-1 activity is not perfect. An earlier study linear VOR. Third, the modulation due to vestibular affer( Keller and Kamath 197 5 ) identified vestibular-plus-pause ent input could be canceled by another signal. This possibilcells as the only recipients of direct primary afferent input. ity will be discussed for the BP neurons in detail below. But the eye position sensitivity of PVP neurons can be Finally, activation could be an artifact of current spread to

c I

contra ABD

ipsi ABD

(L-BP)

(R-BP)


HORIZONTAL

VOR INTERNEURONS

the flocculus, which would cause antidromic activation of some cells and inhibition of others. We do not regard such current spread as likely. Although the currents we used were occasionally high, the electrode was imbedded in multiple layers of insulating neural sheaths (dura, endoneurium, perineurium), making current spread outside the nerve difficult. This is supported by the fact that all types of neurons were activated throughout the range of stimulus currents. There were no facial twitches that would indicate current spread to the adjacent facial nerve. Finally, inhibition of vestibular neurons was seen only twice, and that could arise from other routes besides the flocculus. Our data suggest that EHV neurons should join PVP neurons as interneurons in the ipsilateral three-neuron arc of the horizontal VOR. Of EHV cells having contralateral eye and head sensitivity, 25% projected to the ipsilateral abdutens nucleus, and 80% received vestibular afferent input. McCrea et al. ( 1987a) stained only two ipsilaterally projecting cells in the squirrel monkey that they classified as BP. However, those cells could also have been EHV neurons because the possibility of vestibular input was not examined. The finding that c-EHV cells were activated by vestibular nerve shocks was unexpected because they exhibited type-II activity when the monkeys suppressed the VOR. Input from vertical canal afferents is possible but unlikely because most neurons were not detectably modulated during pitch rotations. Presumably, EHV neurons received a type-II input that was stronger than the type-1 input from the ipsilatera1 eighth nerve. The cerebellar flocculus is an excellent candidate for providing this input. First, because most floccular Purkinje cells are type I (Lisberger and Fuchs 1978; Miles et al. 1980) and their output is inhibitory, they would produce type-II activity in their target neurons during VOR suppression. Second, the gaze-velocity signal conveyed by floccular Purkinje cells is similar to the discharge of EHV neurons except for the ratio of eye to head velocity sensitivity. The type-II signal from the flocculus might be reduced by the addition of a weaker type-1 vestibular signal from the ipsilateral eighth nerve to create the ratio of eye to head velocity sensitivity that is observed in EHV units. Finally, EHV units lie within the area of termination of floccular efferents (Langer et al. 1985 ). If this scenario is correct, then EHV units constitute some of the “floccular target neurons” (FTNs) studied by Lisberger and colleagues (Lisberger 1988; Lisberger and Pavelko 1988 ). Lisberger has hypothesized that FTNs mediate the adaptive capabilities of the VOR. In this context, the hypothetical addition of opposing vestibular inputs on EHV neurons makes sense because it expands their range of responses and amplifies the effect of small changes in one input upon the output. In the unadapted monkey, FTNs often behave like BP cells (Lisberger and Pavelko 1988) implying that their type-II floccular input and the (sometimes indirect) type-1 input from the ipsilateral eighth nerve virtually cancel. Because our monkeys were unadapted, presumably some of the BP cells recorded in the present study were in fact FTNs, i.e., EHV neurons with equal type-1 and -11 vestibular inputs. The case is more compelling for the c-BP units that are intermingled. with c-EHV units (Fig. 6) and receive primary afferent input as do EHV neurons.

259

The BP and EHV neurons with ipsilateral movement sensitivity are likewise intermingled, but in medial portions of the MVN. Together, they constitute a very large proportion of vestibular nucleus neurons with horizontal sensitivity. Unfortunately, because we found no connections with either abducens nucleus or the vestibular nerve, we have no clues regarding the role of these cells. However, only five i-BP neurons were examined for ipsilateral projections to abducens motoneurons (Table 3 ), so this negative finding is not conclusive. McCrea et al. ( 1987a) did stain two i-BP (maybe EHV) neurons that projected to the ipsilateral abducens. The i-EHV to ipsilateral abducens projection was tested in adequate numbers, but the ability of spike-triggered averaging to reliably reveal inhibitory connections is in question (see APPENDIX). In any case, an ipsilateral projection is not likely to be strong in the rhesus monkey because the density of ipsilateral cells retrogradely labeled by HRP injections in the abducens nucleus is low compared with the density of unlabeled cells in the area and the density of labeled cells contralaterally (Langer et al. 1986). Note that our neurons were recorded rostrally to the “marginal” cell group at the border of the prepositus and medial vestibular nuclei that projects heavily to the abducens nucleus (Langer et al. 1986). The possibility that i-BP or iEHV neurons make inhibitory connections with the contra-: lateral abducens nucleus seems unlikely because IPSPs have never been reported in contralateral motoneurons resulting from vestibular nucleus stimulation (Baker et al. 1969).

Addition of signals on motoneurons during smooth pursuit and the VOR As noted above, I-PVP cells constitute the principal interneuron in the three-neuron arc of the VOR. When excited by primary vestibular afferents during ipsilateral head movements, they in turn excite contralateral, or inhibit ipsilateral abducens motoneurons to drive the eyes contralaterally to produce compensatory eye movements. The eye position and velocity components of PVP firing act in synergy with the vestibular component to drive the eyes contralaterally. In addition to PVP neurons, we have found that three other cell types contribute inputs to motoneurons: c-EP, c-BP, and c-EHV. Because of their ON-directions, the sign of their synaptic effects, and the laterality of their projections (see Fig. 14)) each type augments the eye position and eye velocity input from PVP neurons during both pursuit and stable gaze. Pola and Robinson ( 1978) asked whether the combination of eye and head signals conveyed by PVP neurons was appropriate to drive motoneurons during the VOR. They concluded that the ratio of eye velocity to eye position sensitivity was too large so that motoneurons required at least one more input that conveyed a predominantly eye position signal. We reexamined this issue because our PVP neurons had relatively more velocity and less position sensitivity than theirs (R/K = 0.32 vs. 0.19 ) , and because we found that three additional cell types projected to motoneurons. As a first approximation of the influence of each type, we assumed that each neuron projected to the abducens nucleus with the same weight so that the relative influence of


C. A. SCUDDER

AND

each type depended on the ratio of their relative numbers. The ratios are PVP:c-EP:c-BP:c-EHV

= 1:0.034:0.027:0.062

Multiplying these coefficients times the eye position, velocity, and head velocity sensitivities (at 0.5 Hz) for each type and then adding them together yields the following estimate of the aggregate input to motoneurons Input

= 2.03

E + 0.72k - 0.97fi

(1)

where E and i are eye position and velocity, and fi is head velocity. For our current purposes, we have neglected the small eye acceleration term contributed by EHV cells and have omitted tonic firing rates at primary gaze. Note that the eye velocity/position ratio for this input (0.36) is slightly increased by the addition of the new cells when compared with the ratio for PVP neurons alone (0.32). The firing rate (FR) of motoneurons obtained under the same conditions as that of vestibular neurons is given by (Fuchs et al. 1988)

A.F.

FUCHS

cancel the VOR. No indication of an excitatory ipsilateral connection exists in our data or that of others (Baker et al. 1969 ) . EHV units also exist more caudally within the vestibular nucleus (McCrea 1992; McFarland and Fuchs 1992), and it has been suggested that they might provide input to abducens motoneurons (McCrea and Cullen 1992). The strongest projection from this caudal region is, however, from the marginal zone between the vestibular and prepositus nuclei (Langer et al. 1986), which probably contains only BP units and not EHV units (McFarland and Fuchs 1992). In summary, these calculations reveal that there are as yet unknown inputs to the abducens nuclei that need to be identified. Moreover, these other inputs may be quantitatively more important than the PVP neurons for driving abducens motoneurons during the VOR.

Cancellation of the VOR

When humans or animals visually track slowly moving objects with the use of combined head and eye movements, they must suppress their VOR to keep their eyes on target. FR = 5.9E + 0.94E (2) One possible mechanism of VOR suppression is that the After scaling Eqs. 1 and 2 to the same eye velocity sensitiv- vestibular input is nullified by an equal and opposite signal ity, the estimated input (Eq. 1) during pursuit (fi = 0) that is generated only during a head movement. It could be has only 45% of the required eye position signal. The situa- either a copy of the head velocity motor command (Robintion is considerably worse for the stable-gaze condition. Set- son 1982) or a gated copy of the vestibular input arising via ting fi = -E and scaling the velocity term as before, the a different pathway (Cullen et al. 199 1) . Alternatively, subestimated input provides only 19% of the required position jects could use the smooth-pursuit system to generate the signal that nullifies the vestibular input (Benson and signal. Barnes 1978; Lisberger et al. 198 1) . Tomlinson and RobinPola and Robinson ( 1978) assumed that the additional position input could be provided by “tonic neurons,” son ( 198 1, 1984) reasoned that, if the latter were true, a full which convey a pure, or nearly pure, eye position signal. or partial cancellation of the vestibular input might be eviThe difficulty is that most neurons having eye position sen- dent in the discharge of vestibular nucleus neurons. When sitivity also have eye velocity sensitivity (Chubb et al. 1984; they compared PVP neuron firing during suppression with Fuchs and Kimm 1975; McFarland and Fuchs 1992; Pola that during the VOR (after subtracting the component of and Robinson 1978; Tomlinson and Robinson 1984) so the modulation due to eye displacement), they found little that pure tonic neurons are very rare. This makes it difficult difference in most neurons. They concluded that there was to produce the missing position input noted above. For ex- no pursuit command available to decrease PVP modulaample, the BP cells reported here have an R/K ratio greater tion during suppression. than that of motoneurons (0.32 vs. 0.19), so no amount of For two types of neurons, we have made essentially the excitatory input from them onto motoneurons could ever same comparison and obtained a different answer. We bring about the appropriate ratio of position to velocity compared PVP modulation during stable gaze with a preinputs. diction obtained by the vector addition of the modulation obtained during suppression of the VOR with either 1) that The above calculations revealed that the ratio of velocity equivalent to position input on motoneurons differed for pursuit and due to eye displacement alone (mathematically stable gaze. This is due to the addition of a head velocity to the comparison of Tomlinson and Robinson), or 2) that signal from PVP units during stable gaze. So besides need- due to eye displacement and velocity (Additive model). ing more position input, motoneurons require an input that Our experiment was not identical to theirs because their adds eye velocity input during pursuit but adds less or none monkeys were in the dark during the VOR, and ours were during stable gaze. Because EHV cells carry such a signal, viewing a spot stationary in space. However, Tomlinson we computed the amount of pure position and EHV input and Robinson ( 198 1) maintain that this makes no differthat would be needed to drive motoneurons properly dur- ence. With the use of three different measures (see REing both pursuit and stable gaze. The proper ratio would be SULTS), use of both position and velocity sensitivities rather achieved if the c-EHV input had the same overall strength than position alone provided a better prediction of horizonas the PVP input. Unfortunately, this is 16 times more EHV tal PVP firing. The same was true for EHV neurons, alinput than indicated by our data. If every c-EHV projected though it might be argued that these neurons, like gaze-veto the ipsilateral abducens nucleus, their synaptic efficacy locity neurons, carry the hypothesized head velocity cancelwould still have to be four times that of PVP neurons. A lation command (Robinson 1982) and should be exempt larger population of EHV neurons exist medially, namely from the test. Finally, our BP cells always had an eye velocthe i-EHV neurons, but they would need to make excit- ity component to their discharge during stable gaze, conatory connections with the ipsilateral abducens nucleus to trary to the findings of Tomlinson and Robinson ( 1984). Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (130.130.211.199) on August 3, 2018. Copyright © 1992 American Physiological Society. All rights reserved.

HORIZONTAL

VOR INTERNEURONS

We did not examine whether the sensitivity was decreased when compared with the pursuit condition. There are other difficulties with Tomlinson and Robinsons’ assertion that a pursuit command is missing from some vestibular nucleus neurons. First, it presupposes that eye position and eye velocity inputs arrive from separate sources, and that the latter is really the pursuit command. There is no evidence to support either assertion. In fact, we have found very few vestibular nucleus neurons that convey only a position or a velocity signal without the other. Second, their hypothesis does not address the fact that PVP neurons continue to transmit a head velocity signal to the motoneurons during suppression of the VOR, whether or not it is decremented by an eye velocity signal. It is this signal that must be canceled, and to do so requires an as yet undiscovered input to abducens motoneurons. The ultimate source of the cancellation command is probably the flocculus (Lisberger and Fuchs 1978; Miles et al. 1980; Robinson 1982). Because the flocculus projects to the vestibular nucleus and not to the abducens nucleus (Langer et al. 1985 ) , we must look for vestibular nucleus neurons that relay the command. From our data, the only realistic candidates are the EHV cells. However, the calculations and arguments raised in the previous section also apply to suppression of the VOR, namely that c-EHV neurons probably do not have the numbers or synaptic weight to cancel the PVP head velocity signal and that other populations of EHV neurons may not project to the abducens nuclei. Hence we cannot yet identify the source of the cancellation signal.

Gaze saccades Combined head and eye movements are also used to produce rapid shifts in the direction of gaze. These have been called gaze saccades. For gaze shifts less than a certain size that depends on the animal, the VOR is operational and decreases the velocity of the eye movement (Bizzi et al. 197 1; Tomlinson 1990). For larger gaze saccades, the VOR is inoperative through most of the duration of the movement (Guitton et al. 1984; Laurutis and Robinson 1986; Tomlinson and Bahra 1986a,b). Because the VOR appears to be shut off during large gaze saccades and PVP neurons pause for some saccades, it has been asserted that the pause of PVP neurons underlies the disconnection of the VOR (Guitton 1988; Laurutis and Robinson 1986; Tomlinson 1990). This assertion is premature because the only neurophysiological experiments done to date have used low-amplitude saccades ( ~25” ) in headfixed animals. Moreover, existing data show that horizontal PVP neurons in rhesus monkey (our data), squirrel monkey.( McCrea et al. 1987a), and cat (McCrea et al. 1980) do not pause for low-amplitude ON-direction saccades and thus may well transmit the vestibular signal when their target motoneurons are active. They fail to transmit the vestibular signal (i:e., they pause) only during OR-direction saccades when their target motoneurons are also silenced (Fuchs and Luschei 1970). These neurophysiological data are in fact consistent with the behavior of the VOR during small saccades, i.e., the VOR is functioning and PVP neurons probably convey a vestibular signal to abducens moto-

261

neurons. PVP neurons need to be studied during large saccades when the VOR is thought to be inoperative. Vertical PVP neurons did not pause for every saccade, as has sometimes been claimed. However, more vertical PVP neurons did pause for ON-direction saccades than did horizontal PVP neurons. Therefore a useful experiment would be to compare the degree of VOR decoupling for vertical versus horizontal gaze saccades. It should be remembered that PVP neurons are not the only pathway by which vestibular input could affect motoneurons. An integrated version of the vestibular signal must be present on any vestibular nucleus neuron conveying an eye position signal ( e.g., a BP neuron) because their modulation during the VOR accurately reflects eye position. The effect of such a pathway could be quite sizable, because our earlier calculations show that motoneurons require a position input that is larger than that provided by PVP neurons. Another possible source of vestibular input is the saccadic burst neurons that have been shown to be modulated during rotation under the appropriate circumstances (Henn et al. 1984). Tomlinson ( 1990) has proposed that all head velocity information is conveyed to motoneurons by such a pathway during gaze saccades.

Classijkation of cells and plasticity of the VOR Although our criteria for classifying cells are well defined, small changes in the discharge properties of some neurons could cause them to be placed in a different category. For example, an EP unit with subliminal vestibular input would be a PVP if that input were slightly stronger. We suggest that this potential arbitrariness in categorizing cells simply reflects that real brain cells do not have fixed inputs but, within limits, can and do change their properties. A cell’s actual discharge properties would be determined by the inputs available at the specific location of the cell and by adaptive mechanisms operating to produce a properly functioning VOR. Such a hypothesis could explain some of the differences between the monkeys used in this study. Because of phenotypic and genotypic differences, different histories, surgeries to both eyes, and repeated electrode penetrations into the parts of the brain that control eye movements, adaptive mechanisms could be required to modify the characteristics and numbers of some cell types. In our animals, I-PVP time constants differed by a factor of 2, and encounter rates for II-PVP and for EHV neurons varied widely. According to Lisberger and Pavelko ( 1988 ) , flocculus target neurons in “unadapted” monkeys are unmodulated when monkeys suppress the VOR but carry an eye velocity signal and to some extent an eye position signal under other conditions. We would categorize them as BP cells despite the dominance of eye velocity input. As discussed earlier, small changes in their type-1 or type-II inputs would change their classification. With the use of i-BPS as an example, an increase of type-1 vestibular input due to VOR adaptation would give the cells the characteristics of i-EHV neurons. An increase of type-II vestibular input would give them the characteristics of II-PVP neurons with high eye velocity sensitivity. c-BP neurons would become either c-EHV or IPVP neurons with high eye velocity sensitivity. Because I-



262

PVP neurons are already numerous,

crease in their number change in the population

the proportionate inwould not be obvious, but the of the others would.

APPENDIX

Validity of the method of spike-triggered averaging This section contains a brief review of the literature on spiketriggered averaging (spike cross-correlations)and the resultsof simulationsbasedon the specificsituation reported in this paper. Spike-triggeredaveraging is usedto infer somethingabout the postsynapticeventsin an output neuron, commonly a motoneuron, generatedby the dischargeof an input (trigger) neuron. Empirical and computer studiesreveal three types of errorsthat can occur: falsepositivesdue to synchronization amongtrigger neurons, falsepositivesdue to periodicitiesin the firing of either the trigger or output neurons,and falsenegativesdue to lack of sensitivity (Kirkwood 1979;Knox and Poppele1977). The first two we do not believe are a problem in our casebecausewe have I) adjusted recordingconditionsto reducetheir occurrence,2) learned to recognizethem when they did occur, and 3) adopted criteria that eliminatetheir acceptanceasveridical evidenceof a synaptic connection.The third is a potentially seriousproblem that we will discussbelow. Most studiesthat have attempted to elucidatethe transformation from a PSPto a spike-triggeredaveragehave usedthe motoneuron asthe model output neuron. EPSPsare integratedby the membrane,causingthe membranepotential to rise at a nearly constantrate (u) but for superimposedsynaptic noise.When the neuron reachesthreshold,a spikeisgenerated,an afterhyperpolarization ensues,and the processof depolarization beginsagain.In this context, it hasbeenproposedthat cross-correlations (CC) are relatedto PSPsaccordingto the formula CC = a*PSP + b*d/dt(PSP)

wherea and b are “constants” that vary dependingon the cell and severalother factors(Kirkwood 1979). Theoretically and empirically, the derivative term is found to dominate for large EPSPs (0.2-3 mV) againsta backgroundof low synaptic noise(Fetz and Gustafsson1983)) whereasthe first term becomessignificant for smallEPSPs(x0.2 mV) againsta background of heavy synaptic noise(Kirkwood and Sears1982). Therefore the amplitude of a cross-correlationdependsnot just on PSPsize,but alsoon the PSP risetime. The risetime, in turn, dependson physiologicalfactors suchaslocation of the synapses on the cell, type of transmitter, etc. More accurately, the coefficient, b, dependson the differencebetweenthe membranepotential rate of rise(u) and the PSPrate of rise(Fetz and Gustafsson1983) . So for the sameEPSP,the derivative term will be largerin a neuronthat is firing rapidly than in one that is firing slowly. All these factors may be important for the presentstudy becausewe have been examining severaldifferent types of cellsthat might project to abducensmotoneurons.Each type could synapseon its own part of the motoneuronwith its own neurotransmitterand/ or could synapsedifferentially on motoneurons with different properties. We have no way of evaluating whether thesepossibilitiesoccur, but it doessuggestthat negative resultsshouldbe interpreted with caution. Most studieshave beenconcernedwith the effectsof EPSPson cross-correlations,and few have consideredIPSPs.Thosethat do considerIPSPshavedealt with largepotentialsthat drive motoneuron firing to zero for the duration of the IPSP declining phase (Fetz and Gustafsson1983;Knox and Poppele1977). This situation certainly doesnot apply to the presentstudy. Given the low proportion of cells making inhibitory connectionsin the present study and the nonlinear transformations inherent in spike-trig-

geredaveraging,we were concernedthat the technique wasless sensitivefor IPSPsthan EPSPs.Consequently,we createda computer model of our situation to answerthis question.The model was similar to that above: after a spike, the motoneuron membranepotential washyperpolarizedto 10mV belowthresholdand recoveredlinearly toward threshold.Superimposedwasnoisegeneratedby a pseudorandomnumber generatorand filtered at 800 Hz, and a PSP with a 1-mssigmoidalrise time and 7-mstime constantexponentialdecay. RMS noiseamplitudewas300- 1,200 pV, and PSPsrangedfrom 80 to 640 pV. One hundred suchneuronswerecontinuously firing at ratesdistributed from 20 to 200/ s.When eachfired, it contributed an EMG potential consistingof a cosinewave from - 180to + 180”, 1mshalfwidth, and upwardly displacedso that it beganand ended at baselinepotential. Each lo-ms sweepwascoincident with a PSPin each of the 100neurons. The results showedthat I) the cross-correlationwaveshape looked like a widenedderivative of the PSP;2) at high noiselevels (0.6- 1.2 mV), largeEPSPsand IPSPswereequally efficacious;3) at low noiselevels ( ~0.3 mV) EPSPsyielded responsestwo to three timeslargerthan IPSPsof the sameamplitude; 4) responses to smallEPSPsand IPSPs( 10 ms whenever possibleand looked at featuresof the responses at times < 10ms.That is sufficient to control problemsdue to input firing rate, but the firing rate of the output (motor) neuronsis impossibleto control becausethe EMG electrodessamplefrom so many (Fuchs et al. 1988). If the fastestfire at 300/s, then a responsecausedby this would begin at 3.3 ms. That is later than the latest responsewe have classifiedasa clear type. We thank M. Moran for assistance in preparing this manuscript. Research was supported in part by Grant EY-00745 from the National Eye Institute and Grant RR-00 166 to the Regional Primate Research Center, Seattle, WA. Current address and address for reprint requests: C. A. Scudder, Dept. of Otolaryngology, Eye and Ear Institute of Pittsburgh, 203 Lothrop St., Pittsburgh, PA 152 13. Received 19 September 199 1; accepted in final form 27 February 1992. REFERENCES R. AND BERTHOZ, A. Organization of vestibular nystagmus in oblique oculomotor system. J. ikurophysiol. 37: 195-2 17, 1974.

BAKER,


HORIZONTAL

VOR INTERNEURONS

R. G., MANO, N., AND SHIMAZU, H. Postsynaptic potentials in abducens motoneurons induced by vestibular stimulation. Brain Res. 15: 577-580, 1969. BENSON, A. J. AND BARNES, G. R. Vision during angular oscillation: the dynamic interaction of visual and vestibular mechanisms. Aviat. Space Environ. Med. 32: 340-345, 1978. BIZZI, E., KALIL, R. E., AND TAGLIASCO, V. Eye-head coordination in monkeys: evidence for centrally patterned organization. Science Wash. DC 173: 452-454, 1971. BLANKS, R. H., CURTHOYS, I. S., BENNET, M., AND MARKHAM, C. H. Planar relationships of the semicircular canals in rhesus and squirrel monkeys. Brain Res. 340: 3 15-324, 1985. BLANKS, R. H., ESTES, M. S., MARKHAM, C. H. Physiologic characteristics of vestibular first order canal neurons in the cat. II. Response to constant angular acceleration. J. Neurophysiol. 38: 1250- 1268, 1975. CHUBB, M. C. AND FUCHS, A. F. Contribution of y group of vestibular nuclei and dentate nucleus of cerebellum to generation of vertical smooth eye movements. J. Neurophysiol. 48: 75-99, 1982. CHUBB, M. C., FUCHS, A. F., AND SCUDDER, C. A. Neuron activity in monkey vestibular nuclei during vertical vestibular stimulation and eye movements. J. Neurophysiol. 52: 724-742, 1984. CULLEN, K. E., BELTON, T., AND MCCREA, R. A. A non-visual mechanism for voluntary cancelation of the vestibulo-ocular reflex. Exp. Brain Res. 83: 237-252, 1991. FERNANDEZ, C. AND GOLDBERG, J. M. Physiology of peripheral neurons innervating semicircular canals of the squirrel monkey. II. Response to sinusoidal stimulation and dynamics of peripheral vestibular system. J. Neurophysiol. 34: 66 l-675, 197 1. FETZ, E. E. AND GUSTAFSSON, B. Relation between shapes of post-synaptic potentials and changes in firing probability of cat motoneurons. J. Physiol. Land. 341: 387-410, 1983. FUCHS, A. F. AND KIMM, J. Unit activity in vestibular nucleus of the alert monkey during horizontal angular acceleration and eye movement. J. Neurophysiol. 38: 1140-l 161, 1975. FUCHS, A. F. AND LUSCHEI, E. S. Firing patterns of abducens neurons of alert monkeys in relationship to horizontal eye movement. J. Neurophysiol. 33: 382-392, 1970. FUCHS, A. F. AND ROBINSON, D. A. A method for measuring horizontal and vertical eye movement chronically in the monkey. J. Appl. Physiol. 21: 1068-1070, 1966. FUCHS, A. F., SCUDDER, C. A., AND KANEKO, C. R. S. Discharge patterns and recruitment order of identified motoneurons and internuclear neurons in the monkey abducens nucleus. J. Neurophysiol. 60: 1874- 1895, 1988. GACEK, R. R. The course and central termination of first order neurons supplying vestibular end organs in the cat. J. Comp. Neurol. 290: 423439, 1969. GOLDSTEIN, H. P. The Neural Encoding ofsaccades in the Rhesus Monkey (PhD thesis). Baltimore, MD: Johns Hopkins Univ., 1983. GRAYBIEL, A. M. AND HARTWIEG, E. A. Some afferent connections of the oculomotor complex in the cat: an experimental study with tracer techniques. Brain Res. 8 1: 543-55 1, 1974. GUITTON, D. Eye-head coordination in gaze control. In: Control of Head Movement, edited by B. W. Peterson and F. J. Richmond. New York: Oxford Univ. Press, 1988, p. 196-206. GUITTON, D., DOUGLAS, R. M., AND VOLLE, M. Eye head coordination in cats. J. Neurophysiol. 52: 1030- 1050, 1984. HENN, V., BALOH, R. W., AND HEPP, K. The sleep-wake transition in the oculomotor system. Exp. Brain Res. 54: 166-176, 1984. HIGHSTEIN, S. M. The organization of the vestibulo-oculomotor and trochlear reflex pathways in the rabbit. Exp. Brain Res. 17: 285-300, 1973. HIGHSTEIN, S. M., ITO, M., AND TSUCHIYA, T. Synaptic linkage in the vestibulo-ocular reflex pathways of the rabbit. Exp. Brain Res. 13: 306326, 1971. HIGHSTEIN, S. M. AND REISINE, H. The ascending tract of Deiters and horizontal gaze. Ann. NY Acad. Sci. 374: 102- 111, 198 1. HIKOSAKA, O., MAEDA, M., NAKAO, S., SHIMAZU, H., AND SHINODA, Y. Presynaptic impulses in the abducens nucleus and their relation to postsynaptic potentials in motorneurons during vestibular nystagmus. Exp. Brain Res. 37: 355-376, 1977. ISHIZUKA, N., MANNEN, H., SASAKI, S., AND SHIMAZU, H. Axonal branches and terminations in the cat abducens nucleus of secondary vestibular neurons in the horizontal canal system. Neurosci. Lett. 16: 143-148, 1980.

BAKER,

263

Y., KITAMA, T., AND YOSHIDA, K. Vertical eye movement-related secondary vestibular neurons ascending in the medial longitudinal fasciculus of the cat. I. Firing properties and projection pathways. J. Neurophysiol. 63: 902-9 17, 1990a. IWAMOTO, Y., KITAMA, T., AND YOSHIDA, K. Vertical eye movement-related secondary vestibular neurons ascending in the medial longitudinal fasciculus of the cat. II. Direct connections with extraocular motoneurons. J. Neurophysiol. 63: 9 18-937, 1990b. KAWAI, N., ITO, M., AND NOZUE, M. Postsynaptic influences on the vestibular non-Deiters’ nuclei from primary vestibular nerve. Exp. Brain Res. 8: 190-200, 1969. KELLER, E. L. Behavior of horizontal semicircular canal afferents in alert monkey during vestibular and optokinetic stimulation. Exp. Brain Res. 24: 459-47 1, 1976. KELLER, E. L. AND DANIELS, P. D. Oculomotor related interaction of vestibular and visual stimulation in vestibular nucleus cells in alert monkey. Exp. Neurol. 46: 187- 198, 1975. KELLER, E. L. AND KAMATH, B. Y. Characteristics of head rotation and eye movement-related neurons in alert monkey vestibular nucleus. Brain Res. 100: 182-187, 1975. KING, W. M., LISBERGER, S. G., AND FUCHS, A. F. Responses of fibers in medial longitudinal fasciculus (MLF) of alert monkeys during horizontal and vertical conjugate eye movements evoked by vestibular or visual stimuli. J. Neurophysiol. 39: 1135- 1149, 1976. KIRKWOOD, P. A. On the use and interpretation of cross-correlation measurements in the mammalian central nervous system. J. Neurosci. Methods 1: 107-132, 1979. KIRKWOOD, P. A. AND SEARS, T. A. The effectsof single afferent impulses on the probability of firing of external intercoastal motoneurones in the cat. J. Physiol. Lond. 322: 3 15-336, 1982. KNOX, C. K. AND POPPELE, R. E. Correlation analysis of stimulus-evoked changes in excitability of spontaneously firing neurons. J. Neurophysiol. 40: 616-625, 1977. LANGER, T., FUCHS, A. F., CHUBB, M. C., SCUDDER, C. A., AND LISBERGER, S. G. Floccular efferents in the rhesus macaque as revealed by autoradiography and horseradish peroxidase. J. Comp. Neurol. 235: 2637, 1985. LANGER, T., KANEKO, C. R. S., SCUDDER, C. A., AND FUCHS, A. F. Afferents to the abducens nucleus in the monkey and cat. J. Comp. Neurol. 245: 379-400, 1986. LAURUTIS, V. P. AND ROBINSON, D. A. The vestibulo-ocular reflex during human saccadic eye movements. J. Physiol. Land. 373: 209-234, 1986. LISBERGER, S. G. The neural basis for motor learning in the vestibulo-ocular reflex in monkeys. Trends Neurosci. 11: 147-152, 1988. LISBERGER, S. G., EVINGER, L. C., JOHANSON, G. W., AND FUCHS, A. F. Relationship between eye acceleration and retinal image velocity during fovea1smooth pursuit in man and monkey. J. Neurophysiol. 46: 229249, 1981. LISBERGER, S. G. AND FUCHS, A. F. Role of primate flocculus during rapid behavioral modification of vestibulo-ocular reflex. I. Purkinje cell activity during visually guided horizontal smooth pursuit eye movements and passive head rotation. J. Neurophysiol. 4 1: 733-763, 1978. LISBERGER, S. G. AND MILES, F. A. Role of the primate medial vestibular nucleus in long-term adaptive plasticity of the vestibulo-ocular reflex. J. Neurophysiol. 43: 1725- 1745, 1980. LISBERGER, S. G. AND PAVELKO, T. A. Brainstem neurons that are postsynaptic to the site of motor learning in the vestibulo-ocular reflex in monkeys. Science Wash. DC 242: 771-773, 1988. LOUIE, A. W. AND KIMM, J. The response of eighth nerve fibers to horizontal sinusoidal oscillation in the alert monkey. Exp. Brain Res. 24: 447457, 1976. MCCREA, R. A. AND CULLEN, K. E. Responses of vestibular and prepositus neurons to head movements during voluntary suppression of the vestibuloocular reflex. In: Sensing and Controlling Motion: Vestibular and Sensorimotor Function, edited by B. Cohen and D. Tomko. New York: NY Acad. Sci., 1992, p. 379-395. MCCREA, R. A., STRASSMAN, A., MAY, E., ANDHIGHSTEIN, S. M. Anatomical and physiological characteristics of vestibular neurons mediating the horizontal vestibulo-ocular reflex in the squirrel monkey. J. Comp. Neurol. 264: 547-570, 1987a. MCCREA, R. A., STRASSMAN, A., AND HIGHSTEIN, S. M. Anatomical and physiological characteristics of vestibular neurons mediating the vertical vestibulo-ocular reflex of the squirrel monkey. J. Comp. Neurol. 264: 571-594, 1987b. IWAMOTO,


264


R. A., YOSHIDA, K., BERTHOZ, A., AND BAKER, R. Eye movement related activity and morphology of second order vestibular neurons terminating in the cat abducens nucleus. Exp. Brain Res. 40: 46% 473, 1980. MCFARLAND, J. L. AND FUCHS, A. F. Discharge patterns of the nucleus prepositus hypoglossi and adjacent medial vestibular nucleus during horizontal eye movement in behaving macaques. J. Neurophysiol. 68: 3 19332, 1992. MILES, F. A. Single unit firing patterns in the vestibular nucleus related to voluntary eye movements and passive body rotations in conscious monkeys. Brain Res. 7 1: 2 15-224, 1974. MILES, F. A., FULLER, J. H., BRAITMAN, D. J., AND Dow, B. M. Long-term adaptive changes in primate vestibuloocular reflex. III. Electrophysiological observations in flocculus of normal monkeys. J. Neurophysiol. 43: 1437-1476, 1980. NAKAO, S., SASAIU, S., SCHOR, R. H., AND SHIMAZU, H. Functional organization of premotor neurons in the cat’s medial vestibular nucleus related to slow and fast phases of nystagmus. Exp. Brain Res. 45: 371-385, 1982. NUDO, R. D. AND MASTERTON, R. B. Descending pathways to the spinal cord: a comparative study of 22 mammals. J. Comp. Neural. 277: 5379, 1989. OHGAKI, T., CURTHOYS, I. S., AND MARKHAM, C. H. Morphology ofphysiologically identified second-order vestibular neurons in cat, with intracellularly injected HRP. J. Camp. Neural. 276: 387-411, 1988. OPTICAN, L. M. AND MILES, F. A. Visually induced adaptive changes in primate saccadic oculomotor control signals. J. Neurophysiol. 54: 940958, 1985. PRECHT, W. AND SHIMAZU, H. Functional connections of tonic and kinetic vestibular neurons with primary vestibular afferents. J. Neurophysiol. 28: 1014-1028, 1965. POLA, J. AND ROBINSON, D. A. Oculomotor signals in medial longitudinal fasciculus of the monkey. J. Neurophysiol. 4 1: 245-259, 1978. REISINE, H., STRASSMAN, A., AND HIGHSTEIN, S. M. Eye position and head velocity signals are conveyed to medial rectus motor neurons in the alert cat by the ascending tract of Deiters. Brain Res. 2 11: 153- 157, 1981. MCCREA,

D. A. Oculomotor unit behavior in the monkey. J. Neurophysiol. 33: 393-404, 1970. ROBINSON, D. A. A model of cancelation of the vestibulo-ocular reflex. In: Functional Basis of Ocular Motility Disorders, edited by G. Lennerstrand, D. S. Zee, and E. L. Keller. New York: Pergamon, 1982, p. 5- 16. SATO, Y., SASAIU, H., ISHIZUKA, N., AND SASAKI, S. Morphology of single primary vestibular afferents originating from the horizontal semicircular canal in the cat. J. Cornp. Neural. 290: 423-439, 1989. SCUDDER, C. A. AND FUCHS, A. F. Projections from the vestibular to abdutens nuclei as revealed by spike triggered averaging. Sot. Neurosci. Abstr. 7: 777, 198 1. SCUDDER, C. A., FUCHS, A. F., AND LANGER, T. P. Characteristics and functional identification of saccadic inhibitory burst neurons in the alert monkey. J. Neurophysiol. 59: 1430-1454, 1988. STEIGER, H.-J. AND B~~TTNER-ENNEVER, J. A. Oculomotor nucleus afferents in the monkey demonstrated with horseradish peroxidase. Brain Res. 160: 1-15, 1979. STEIN, B. M. AND CARPENTER, M. B. Central projections of the vestibular ganglia innervating the specific parts of the labyrinth in the rhesus monkey. Am. J. Anat. 120: 28 l-3 18, 1967. TOMLINSON, R. D. Combined eye-head gaze shifts in the primate. III. Contributions to the accuracy of gaze saccades. J. Neurophysiol. 64: 1873-1891, 1990. TOMLINSON, R. D. AND BAHRA, P. S. Combined eye head gaze shifts in the primate. I. Metrics. J. Neurophysiol. 56: 1542- 1557, 1986a. TOMLINSON, R. D. AND BAHRA, P. S. Combined eye head gaze shifts in the primate. II. Interactions between saccades and the vestibulo-ocular reflex. J. Neurophysiol. 56: 1558-1570, 1986b. TOMLINSON, R. D. AND ROBINSON, D. A. Is the vestibulo-ocular reflex canceled by smooth pursuit? In: Progress in Oculomotor Research, edited by A. F. Fuchs and W. Becker. New York: Elsevier, 198 1, p. 533-540. TOMLINSON, R. D. AND ROBINSON, D. A. Signals in the vestibular nucleus mediating vertical eye movements in the monkey. J. Neurophysiol. 5 1: 1121-l 136, 1984. ROBINSON,


Vestibuloocular reflex of rhesus monkeys after spaceflight.

Vestibular inputs to brain stem neurons that participate in motor learning in the primate vestibuloocular reflex.

Visual suppression of torsional vestibular nystagmus in rhesus monkeys.

GABA and glycine as inhibitory neurotransmitters in the vestibuloocular reflex.

Unit activity in vestibular nucleus of the alert monkey during horizontal angular acceleration and eye movement.

Blood levels do not predict behavioral or physiological effects of Δ⁹-tetrahydrocannabinol in rhesus monkeys with different patterns of exposure.

Physiological identification of interneurons and motoneurons in the abducens nucleus.

Neurotransmitter and peptide receptors on medial vestibular nucleus neurons.

Head-shaking nystagmus during vestibular compensation in humans and rhesus monkeys.

Inhibition of lateral vestibular nucleus neurons by 5-hydroxytryptamine derived from the dorsal raphe nucleus.

Vestibular nucleus neurons relaying excitation from the anterior canal to the oculomotor nucleus.

The mammalian efferent vestibular system plays a crucial role in the high-frequency response and short-term adaptation of the vestibuloocular reflex.

Discharge patterns in nucleus prepositus hypoglossi and adjacent medial vestibular nucleus during horizontal eye movement in behaving macaques.

Organization of local horizontal functional interactions between neurons in the inferior temporal cortex of macaque monkeys.

Behavioral effects of stimulating positive reward sites in the central tegmentum of rhesus monkeys.

Physiological, Behavioral, and Scientific Impact of Different Fluid Control Protocols in the Rhesus Macaque (Macaca mulatta).

Projections of vertical eye movement-related neurons in the interstitial nucleus of Cajal to the vestibular nucleus in the cat.

Inputs from regularly and irregularly discharging vestibular nerve afferents to secondary neurons in squirrel monkey vestibular nuclei. III. Correlation with vestibulospinal and vestibuloocular output pathways.

A behavioral taxonomy of loneliness in humans and rhesus monkeys (Macaca mulatta).

Experimental tests of a superposition hypothesis to explain the relationship between the vestibuloocular reflex and smooth pursuit during horizontal combined eye-head tracking in humans.

Effects of beta-carboline on fear-related behavioral and neurohormonal responses in infant rhesus monkeys.

Effects of alprazolam on fear-related behavioral, hormonal, and catecholamine responses in infant rhesus monkeys.

Spinal commissural neurons mediating vestibular input to neck motoneurons in the cat upper cervical spinal cord.

Behavioral and physiological consequences of unilateral ablation of the nucleus isthmi in the leopard frog.