Brabt Research, 152 (1978) 81-95 :.~;)Elsevier/North-Holland Biomedical Press

LATERAL GENICULATE CELL STIMULATION OF T H E RETINA

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S. MOLOTCHNIKOFF and P. LACHAPELLE Ddpartement des Sciences biologiques, Universitd de Montrdal C.P. 6128, Succursale A, Montrdal, Qudbec H3C 3J7 (Canada)

(Accepted December 22rid, 1977)

SUMMARY Electrical stimulation of the retina evokes at the optic tract level rythmic bursts of activity whose temporal structure is predictable from the polarity of the stimulation and the receptive field type. The reaction of lateral geniculate units to this input was studied in fast and slow relay cells as well as in interneurons. The results revealed that fast relay cells presented a response whose temporal structure remained essentially unmodified in comparison to that observed at the optic tract level: that both anodal and cathodal polarities produced rythmic pattern of excitation the latency of which depended upon receptive field type and polarity applied. In slow relay cells and interneurons responses with equal latencies could be evoked for both polarities. Following cortical depression with 3 M KCI the latency of first bursts was unaffected in relay cells, while about one third of interneurons showed a temporal pattern which was similar to that recorded at the optic tract level after the treatment. This suggests that both ON and OFF retinal networks converge upon one geniculate slow P cell and interneuron, whereas fast relay cells are mostly driven by one of the two systems. Furthermore this convergence may be achieved through visual cortex in some units.

INTRODUCTION The visual system possesses two reciprocal neuronal networks which signal the brightening step (light ON, B system) and the dimming step (light OFF, D system17). Both systems are widely distributed throughout the vertebrates where they are characterized by receptive fields organized concentrically: center-ON or center-OFF. The activation of these two systems which produces the brightness and darkness sensations may be obtained by two modes of stimulation: one is light ON and light OFF stimuli; the other, anodal and cathodal stimulation of the retina 4,5,17,

82 The retinal ganglion cell, which constitutes the major input to the lateral geniculate nucleus, responds to brief electrical pulses applied across the retina in a very characteristic way depending on the polarity of the pulse. A current in one direction (cornea positive) excites the ganglion ON center cells and inhibits the ganglion O F F center cells while a current of opposite direction (cornea negative) excites the OFF center cells and inhibits the ON center units 15. Recent studies on rabbits ~,s~2t,303~ and cats 46 have confirmed and extended Granit's observations and demonstrated that electrical polarization of the retina evokes, in retinal ganglion cells, rythmic sequences of bursts whose latency depends upon the direction of the current. In ON center cells an anodal pulse evokes a short latency burst as compared to the cathodal response. The reverse sequence is observed in O F F center cells. Furthermore different electrophysiological and pharmacological studies in rabbits 29, catfishes 35, frogs ls.~;~ concur that electrical stimulation of the retina acts upon distal retinal elements whose nervous message reaches the ganglion cells. Thus transretinal stimulation activates the entire retinal network in a predictable manner with a temporal sequence depending upon the center type of the receptive field. This typical activity of optic nerve fibers has been considered the input message to cells in the lateral geniculate body and it is the aim of this study to investigate the response pattern of geniculate units to electrical stimulation of the retina. METHODS

Animal preparation Rabbits (New Zealand) of 2.5-3.5 Kg used in this study were anesthetized with an intravenous injection of pentobarbital (30 mg/Kg) and paralyzed with 5 mgtKg of gallamine thriethiodide at a rate of I ml/h. The rabbits were artificially ventilated and fixed to a modified stereotaxic apparatus which did not obstruct the visual field. Local anesthesia was obtained by subcutaneous infiltration of xylocaine or zyljectine. Rectal temperature was kept constant and heart rate was continuously monitored.

Stimulations (a) Electrical stimulation of the retina (TRS). A short (0.5-1 msec) electrical pulse across the retina (the transretinal stimulus, TRS) was applied between an active platinum needle electrode positioned in the anterior chamber of the eye and reference electrodes placed behind the globe. Special care was exercised to avoid obstructing the pupil opening. The direction of the current was defined with reference to the polarity of the corneal electrode. An anodal pulse (AS) means that the corneal electrode was linked to the positive pole of the stimulator, hence the current was in the vitreousscleral direction. Alternatively, a corneal electrode connected to the negative pole of the stimulator defined a cathodal pulse (CS) with current flowing in the scleral-vitreous direction. (b) Optic nerve stimulation (ONS). Axons of retinal ganglion cells were activated directly by applying electrical shock to the optic nerve head at its exit from

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Fig. 1. Mode of response of relay cells (A) and interneurons (B) to optic nerve stimulation (ONS). T: thresholds 4 V in A and 6 V in B. Duration 0.2 msec. (C) Simultaneous recordings from a relay cell (higher biphasic spike) and an interneuron (negative small spike). VCS: visual cortex stimulation. Due to high amplification the biphasic spike is truncated. Cut-off frequencies in B 3-3000 Hz. Dark arrow in all figures points to the moment of application of the stimulus.

the eye 17,18. This permitted classification of the units as relay P cells or interneurons and dissociated fast and slow groups according to conduction velocities. Fig. 1 shows the difference between relay cells and interneurons. The former always responded with a single spike to any supra-threshold optic nerve stimulus (Fig. I A) while interneurons discharged with a burst of up to 8 spikes as the stimulus intensity was increased (Fig. 1B). This distinction is adopted by many investigators6,a7, 41. One hundred and fortyfive cells were classified in this manner: 70)(, were relay cells and 30% were interneurons. The distribution of latencies to optic nerve stimulation revealed that the mean latency for interneurons was 5.81 3- 3.09 msec, with a modal value of 4.0 msec. Two peaks could be discerned in the histogram of the latency distribution of P cells :

84 the first at 3.0-4.0 msec and the second at 5.0-5.5 msec; these were separated by a sharp dip at 4.0-4.5 msec. A statistical analysis of the distribution of latencies of P cells permitted the dissociation of two major classes: those having a latency shorter than 4.3 msec, the fast P cells (N -- 39) and those which only responded with a latent period exceeding 4.3 msec, the slow P cells (N ..... 35) 2°. A fast P cell responding to ONS is shown in the left side of Fig. 1A. In addition cells were antidromicatly activated from the visual cortex. As this study was concluded, a report suggested that the so-called 1 cells are situated in the immediate vicinity of the dorsal nucleus of the lateral geniculate nucleus 4a, and perhaps, they are not interneurones at all. Several factors indicated that both types of cells were recorded within the LGN. In a single penetration it was consistently observed that P and I cells are met with no definite clustering for l cells. In no instances has it been possible to record exclusively from I cells. Occasionally simultaneous recordings from P and ! cells were obtained (Fig. 1C). Both units responded to a stimulation of the same site of the visual cortex (Area I, electrodes 1.5 mm apart) (Fig. 1C, tracing 3) and exhibited an alternating pattern, and thus revealing reciprocal connections between them. Consequently cells of either types should generate response patterns to the same retinal stimulus. (c) Light stimulation. Square pulses of light (duration l sec) were applied either diffusely or onto a localized area of the receptive field. The method for mapping receptive fields was similar to that described previously ~,3,')~. Briefly, the receptive fields were mapped on a translucent screen located at a distance of 1.14 m from the eye. Thus, 2 cm on the screen approximated an angle of 1° on the rabbit's retina. The unattenuated intensity of light projected on the screen was 10 Lm/mL The cornea was protected by a contact lens ( + 6D to + 7D) which also corrected the rabbit's usual hypermetropia z. Optical arrangements and mirrors fixed to a galvanometer permitted projection of small light spots at various sites of the field and displacements of targets (bars or spots) at desired speeds and directions.

Recordings Single cell activity was recorded (8-20 Mf~ impedance at 1 KHa) for the lateral geniculate nucleus, reached stereotaxicatly z9 through varnished tungsten microelectrodes. Amplification and display were effected with conventional equipment. Cut-off frequencies 30--3000 Hz unless indicated differently. In the last experiments histograms were obtained by averaging (Nicolet 1072) spike trains over 64 successive presentations. In the early experiments the final position of the tip of the electrode was verified histologically. RESULTS

Fast P cells In geniculate fast P cells the temporal pattern evoked by electrical stimulation of the retina was identical to the pattern observed at the optic tract. The unit depicted in Fig. 2 responded to the optic nerve shock with a single spike at 1.7 msec (tracing 1)

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Fig. 2. Responses of a fast P cell. Tracing I : response to optic nerve stimulus (ons). Tracings 2-4: responses to diffuse light ON. Tracings 5 7: responses to diffuse light OFF. Tracing 8: response pattern to anodal transretina! pulse (AS). Tracing 9: response pattern to cathodal transretinal pulse (CS) the current strength being the same for both polarities. In this and subsequent figures the voltage ranged between 10 and 30 V. Three superimposed sweeps in 8 and 9. Notice the alternating pattern o f activity between AS and CS responses. The last two tracings show the movement sensitivity of the cell to I ~ spot swept across the receptive field: 3 0 / s e c in tracing 10; 300'/sec in tracing 11; lower traces in 10 and 11 are the photocell response.

which confirms that the cell belongs to the fast P cell group. The small variability of its latency suggests a small number of optic tract axons converging upon this geniculate cell. Following this early spike, almost all P cells resumed their firing around 150-210 msec. The fast P cells possessed a concentrically organized receptive field as mapped with a stationary spot of light and were particularly sensitive to a wide spectrum of movements (30°/sec-300°/sec) (Fig. 2, tracings l0 and l 1). The response of this group of units to electrical polarization of the retina appeared in the form of bursts containing high frequency spikes (tracings 8-9). The anodal pulse (tracing 8) evoked a first burst lasting 15 msec with a latent period of 30 msec; a second burst occurred around 60 msec; it was then followed by scattered activity. Cathodal stimulation (tracing 9) (the current strength being constant) produced a burst of a considerably shorter latency: 15 msec. This initial activity ceased abruptly at the time when the anodal first burst appeared. The second cathodal burst coincided with the silent pause observed in the response pattern to the anodal stimulus. As in optic tract responses1,3°, 31, one can

86 find a complete absence of spontaneous activity during the inter-burst interval. The structure of this pattern is clearly out of phase and exactly reproduces the alternating pattern recorded from the optic tract. F r o m what has been said above, and from the response pattern to T R S one would expect that the O F F stimulus would evoke a response o f shorter latency than that to the O N stimulus. Tracings 2-4 and 5-7 illustrate this difference in latency, and it is of interest to observe that the O N response fell during the silent pause which superseded the initial O F F discharge. Since the temporal pattern of this geniculate unit is a duplicate of the optic tract activity, it is possible to suggest that the sequences of the spike train are transmitted t h r o u g h this relay P cell with high fidelity. The second example of a fast P cell (Fig. 3) presented the same general temporal response structure as in the previous unit. The opposite stimuli, AS and CS. evoked firing patterns which were opposite in phase. Two main periods o f increased excitation were evoked by the anodal pulse (tracings 6-8), one between 16 and 25 msec and AS

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