Exp. Brain Res. 28, 421--425 (1977)
Experimental Brain Research 0 Springer-Verlag1977
Extraocular Proprioceptive Projections to the Visual Cortex P. Buisseret 1 and L. Maffei Laboratorio di Neurofisiologia del C. N. R., Pisa, Italy
Summary. Electrical stimulation of the intraorbital part of the motor branches of extraocular muscles, where proprioceptive fibers run, evokes responses in 25 % of the units of the striate cortex of the cat. The latency ranges between 25 and 40 msec. Mechanical stretch of extraocular muscles evokes multiunit responses in the striate cortex. The response is abolished by injection of xylocaine into the stretched muscle. The suppression of the response is reversible. Key words: Visual system - Cat - Proprioceptive receptors - Extraocular muscles The extraocular muscles of the cat have neither typical muscle spindles nor tendon organs. They have, however, structures of spiral ending form, which are mostly in parallel with muscle fibers and respond to stretch (Bach-y-Rita and Ito, 1966). Afferent responses from extraocular muscles have been recorded in the cerebellum for mossy and climbing fibers (Baker et al., 1972; Batini et al., 1974; Fuchs and Kornhuber, 1969; Wolfe, 1971). Recently it has been shown (Maffei and Bisti, 1976; Mallei and Fiorentini, 1976) that surgical strabismus in kittens and paralytic strabismus in adult cats produces a decrease in the proportion of binocularly driven cells in the visual cortex (area 17) even if these animals are deprived of vision. The conclusion has been drawn that the motor asymmetry between the eyes per se is sufficient to decrease the proportion of binocularly driven neurons in the striate cortex. These effects might be mediated by proprioceptive signals from the extraocular muscles to the cortex. In this paper we report evidence for the existence of such projections. Experiments were performed on 28 adult cats. Thirteen of them were anesthetized with chloralose (70 mg/kg, i.v.) and 15 with pentobarbital (35 mg/kg as initial dose). In all experiments the distal tendons of all the muscles of 1 eye were tied with threads, cut distally to the ligature and the eye ball removed. The other eye was used for visual stimulation, using light and dark bars of various orientations. In the experiments where stretches were applied, muscles were isolated from 1 Partially supported by: European Training Program for Brain and Behaviour Research Present address: CollEge de France, Laboratoire de Neurophysiologie, 75231 Paris
422
P. Buisseret and L. Maffei
surrounding tissues as carefully as possible leaving their blood supply intact. The thread attached to one of the recti was connected to a galvanometer driven by a wave-form generator. Usually we applied sawtooth stimuli with a duration of 160 msec, and an amplitude of stretch of 2.5 mm. In the experiments where electric stimuli were applied, the branch of the III. nerve to the obliquus inferior, of the IV. nerve to the obliquus superior, and of the VI. nerve to the rectus lateralis were isolated. These nerves were severed and their central stump placed on a pair of silver hook electrodes. Single electric pulses were applied every 2 sec (duration of the pulse 0.1 msec, 2 - 6 volts). The threshold for muscle contraction was tested before nerve section and found to be of the order of 2 volts. After surgery the animal was immobilized with curarine and artifically ventilated. CO2 in the expired air was measured and maintained at 3.8%. Body temperature was kept at 38 ~. The animals in which stretches of extraocular muscles were applied were all anesthetized with pentobarbital. Recording was performed from area 17 (P1 to P3, L 0.5 to 1.5). We used either semimicroelectrodes for mulfiunit recordings or tungsten microelectrodes for single units. Recordings were performed in the hemisphere ipsilateral or contralateral to the removed eye with similar results. Cortical units were distinguished from geniculate ones on the basis of their orientation specificity. Cortical units were not distinguished, in the usual classification of simple, complex and hypercomplex since especially under chloralose anesthesia we found difficulties in doing so in a reasonable time.
The aim of this experiment was to demonstrate the existence of a pathway from proprioceptive receptors of the extraocular muscles to the striate visual cortex. Proprioceptive afferents are known to run intraorbitally with the motor branches of eye muscles. If information from eye muscle proprioceptive receptors reaches the visual cortex, the electrical stimulation of proprioceptive afferents at the intraorbital level should evoke cortical responses. A n example of the results of this experiment is reported in Figure 1. Figure 1A shows the poststimulus time histogram for a single visual cortical unit of area 17 in response to repeated electric pulses applied to the IV. nerve. The response of the unit develops very clearly at a latency of about 40 msec. The latencies of other cortical units ranged from 2 5 - 4 0 msec with an average of 31 msec. The histogram in Figure 1B is obtained in the same experimental conditions as that in Figure 1A, but following section of the nerve proximally to the stimulating electrodes. There is complete suppression of the response. The section of the nerve was done in such a way that the two stumps of the nerve remained in contact. Although we interpreted the cortical responses as being due to electrical stimulation of afferent fibers in the IV. nerve, antidromic stimulation of possible, but as yet unknown, recurrent collaterals of motor fibers to the visual cortex cannot be excluded.
B
]1o
Fig. 1. Responsiveness of a visual cortical unit of area 17 to electrical stimulation of the intraorbital part of the IV. nerve before A and after section B of the nerve. Single electric pulses (0.1 msec, 6 V) were applied by means of a pair of silver hook electrodes. The post stimulus time histogram is the average of 100 responses, sampled every 2 sec. (Each bin of the histogram is 10 msec. The vertical calibration gives the impulses in each bin). The arrow marks the time at which the electric pulse was applied. In B the response was taken after section of the IV. nerve proximally to the stimulating electrodes. The section of the nerve was done in such a way that the 2 stumps of the nerve remained in contact. The visual responsiveness of the neuron and its orientation specificity was tested using light or dark bars
Extraocular Proprioceptive Projections to the Visual Cortex
A V 160msec
B
423
Fig. 2. Responsiveness of a visual unit to stretch of the muscle rectus superior before A and after B injection of xylocaine into the stretched muscle (0.1 cc, 1%). The histogram shows the average response to 100 stretches, one every 2 sec. The record in B was taken 5 rain after the injection. The responsiveness of the visual unit to stretch was tested in full darkness, the visual responsiveness having been tested through the intact eye
In each penetration of the visual cortex we found, on average, only 1 or 2 units responding to electrical stimulation of a given nerve, while the other 5 or 6 units did not show any detectable response. Out of a total of 90 cortical units only 22 ( - 2 5 % ) showed clear responses to electric stimulation of motor nerves. Of the 30 geniculate fibers none showed any sign of response at all. Usually, for technical reasons, we restricted the electrical stimulation to the nerves of the muscles rectus late~:alis, obliquus inferior and obliquus superior. In several cases we noticed that a cortical unit responding to electrical stimulation of one muscle, did not respond to electrical stimulation of the other. Thus, we cannot exclude the possibility that cortical units unresponsive to electrical stimulation of the tested nerves were responsive to stimulation of other muscle nerves. Therefore the proportion of cortical units responding to stimulation of ocular nerves is probably greater than that reported above. This conclusion is also supported by multiunit recordings with the same or with a separate electrode. In animals anesthetized either with chloralose or in light pentobarbital anesthesia an evoked response was invariably present at each depth level of the cortex. The largest response was obtained at a depth of about 1 mm below the cortical surface. Also the multiunit activity was always abolished by section of the nerve stimulated. In a second series of experiments, muscle receptors were stimulated by stretching single extraocular muscles, thus avoiding the possibility of antidromic invasion of collaterals of motor fibers to the visual cortex. Chloralose anesthesia, which is known to increase nervous excitability, was not used in these experiments and animals were anesthetized with pentobarbital. An example of the results of this experiment is shown in Figure 2. In Figure 2A the histogram shows the average response of a visual cortical unit to stretch of the rectus superior muscle. This response was abolished by injecting a small dose of xylocaine into the stretched muscle (0.1 ml, 1%) (Fig. 2B). This experiment was also repeated by recording multiunit activity. The abolition of stretch responses by intramuscular injection of xylocaine into the stretched muscle was fully reversible within 6 0 - 9 0 rain from the injection (Fig. 3). Figure 3A, B and C shows the averaged cortical mass responses
424
P. Buisseret and L. Maffei
IR.S.
R ~ m~
E
. •
2,5 mm
I 0.1 mV
160msec
Fig. 3. Responsiveness of multiunit mass activity (area 17) to stretches of the muscle rectus lateral& (RL) before A and 3 min after B injection of xylocaine (0.1 cc, 1%). C Recovery of the responsiveness 90 min after injection. D-E Test responses to stretches of the muscles rectus superior (RS). The response in D was taken before xylocaine injection into the rectus lateralis, the response in E was taken 5 min after injection. The responsiveness of visual units was always tested in full darkness the visual responsiveness having been tested through the remaining intact eye; one was removed. Each record is the average of 100 responses. The electrical nervous activity was filtered (band pass 60-6000 Hz) and processed in an average computer
to stretch of the muscle rectus lateralis before (A), 3 min (B) and 90 min (C) after intramuscular injection of 0.1 ml of xylocaine. Figure 3D and E shows control responses from the same electrode to stretch of the rectus superior muscle. The response in Figure 3D was taken before the injection of xylocaine into the rectus lateralis, and that in Figure 3E, 5 rain after the injection. The similarity of these responses is a sign that cortical excitability did not vary in this lapse of time. In conclusion we think that the present findings are evidence in favor of the existence of a proprioceptive projection from extraocular muscles to the visual neurons. It remains t o be investigated whether all the classes of visual cortical neurons (simple, complex and hyper.complex) or only one class of them is responsive to stimulation of eye muscle proprioceptive afferents and if neurons of cortical areas other than area 17 show the same property. The functional role of extraocular muscle proprioceptive afferents of the visual cortex is not yet known. The experiments on strabismic animals deprived of vision indicate a possible role of proprioceptive afferents in maintaining binocular interaction in the visual cortex. Recent experiments of Brindley et al. (1976) and of Stevens et al. (1976) have revived Sherrington's view that extraocular proprioception could play a role in the stabilization of perceived objects during eye movements (Sherrington, 1918) by demonstrating the stationariness of the perceived world during attempted movements of a totally paralyzed human eye.
Extraocular Proprioceptive Projections to the Visual Cortex
425
References Bach-y-Rita, P., Ito, F.: Properties of stretch receptors in cat extraocular muscles. J. Physiol. (Lond.) 186, 663~588 (1966) Baker, R., Precht, W., Llinfis, R.: Mossy and climbing fiber projections of extraocular muscle afferents to the cerebellum. Brain Res. 38, 440-445 (1972) Batini, C., Buisseret, P., Kado, R.T.: Extraocular proprioceptive and trigeminal projections to the Purkinje cells of the cerebellar cortex. Arch. ital. Biol. 112, 1-17 (1974) Brindley, G.S., Goodwin, G.M. Kulikowsky, J.J., Leighton, D.: Stability of vision with a paralysed eye. J. Physiol. (Lond.) 258, 65-66P (1976) Fuchs, A.F., Kornhuber, H.H.: Extraocular muscle afferents to the cerebellum of the cat. J. Physiol. (Lond.) 200, 713-722 (1969) Maffei, L., Bisti, S.: Binocular interaction in strabismic kittens deprived of vision. Science 191, 579-580 (1976) Maffei, L., Fiorentini, A.: Asymmetry of motility of the eyes and change of binocular properties of cortical cells in adult cats. Brain Res. 105, 73-78 (1976) Sherrington, C.: Observations on the sensual role of the proprioceptive nerve supply of the extrinsic eye muscles. Brain 41, 332-342 (1918) Stevens, J.K., Emerson, R.C., Gerstein, R.L., Kallos, J., Neufeld, G.R., Nichols, L.W., Rosenquist, A.C.: Paralysis of the awake human: visual perceptions. Vision Res. 16, 93-98 (1976) Wolfe, J.W.: Relationship of cerebellar potentials to saccadic eye movements. Brain Res. 30, 204-206 (1971)
Received January 10, 1977
Experimental Brain Research 0 Springer-Verlag1977
Extraocular Proprioceptive Projections to the Visual Cortex P. Buisseret 1 and L. Maffei Laboratorio di Neurofisiologia del C. N. R., Pisa, Italy
Summary. Electrical stimulation of the intraorbital part of the motor branches of extraocular muscles, where proprioceptive fibers run, evokes responses in 25 % of the units of the striate cortex of the cat. The latency ranges between 25 and 40 msec. Mechanical stretch of extraocular muscles evokes multiunit responses in the striate cortex. The response is abolished by injection of xylocaine into the stretched muscle. The suppression of the response is reversible. Key words: Visual system - Cat - Proprioceptive receptors - Extraocular muscles The extraocular muscles of the cat have neither typical muscle spindles nor tendon organs. They have, however, structures of spiral ending form, which are mostly in parallel with muscle fibers and respond to stretch (Bach-y-Rita and Ito, 1966). Afferent responses from extraocular muscles have been recorded in the cerebellum for mossy and climbing fibers (Baker et al., 1972; Batini et al., 1974; Fuchs and Kornhuber, 1969; Wolfe, 1971). Recently it has been shown (Maffei and Bisti, 1976; Mallei and Fiorentini, 1976) that surgical strabismus in kittens and paralytic strabismus in adult cats produces a decrease in the proportion of binocularly driven cells in the visual cortex (area 17) even if these animals are deprived of vision. The conclusion has been drawn that the motor asymmetry between the eyes per se is sufficient to decrease the proportion of binocularly driven neurons in the striate cortex. These effects might be mediated by proprioceptive signals from the extraocular muscles to the cortex. In this paper we report evidence for the existence of such projections. Experiments were performed on 28 adult cats. Thirteen of them were anesthetized with chloralose (70 mg/kg, i.v.) and 15 with pentobarbital (35 mg/kg as initial dose). In all experiments the distal tendons of all the muscles of 1 eye were tied with threads, cut distally to the ligature and the eye ball removed. The other eye was used for visual stimulation, using light and dark bars of various orientations. In the experiments where stretches were applied, muscles were isolated from 1 Partially supported by: European Training Program for Brain and Behaviour Research Present address: CollEge de France, Laboratoire de Neurophysiologie, 75231 Paris
422
P. Buisseret and L. Maffei
surrounding tissues as carefully as possible leaving their blood supply intact. The thread attached to one of the recti was connected to a galvanometer driven by a wave-form generator. Usually we applied sawtooth stimuli with a duration of 160 msec, and an amplitude of stretch of 2.5 mm. In the experiments where electric stimuli were applied, the branch of the III. nerve to the obliquus inferior, of the IV. nerve to the obliquus superior, and of the VI. nerve to the rectus lateralis were isolated. These nerves were severed and their central stump placed on a pair of silver hook electrodes. Single electric pulses were applied every 2 sec (duration of the pulse 0.1 msec, 2 - 6 volts). The threshold for muscle contraction was tested before nerve section and found to be of the order of 2 volts. After surgery the animal was immobilized with curarine and artifically ventilated. CO2 in the expired air was measured and maintained at 3.8%. Body temperature was kept at 38 ~. The animals in which stretches of extraocular muscles were applied were all anesthetized with pentobarbital. Recording was performed from area 17 (P1 to P3, L 0.5 to 1.5). We used either semimicroelectrodes for mulfiunit recordings or tungsten microelectrodes for single units. Recordings were performed in the hemisphere ipsilateral or contralateral to the removed eye with similar results. Cortical units were distinguished from geniculate ones on the basis of their orientation specificity. Cortical units were not distinguished, in the usual classification of simple, complex and hypercomplex since especially under chloralose anesthesia we found difficulties in doing so in a reasonable time.
The aim of this experiment was to demonstrate the existence of a pathway from proprioceptive receptors of the extraocular muscles to the striate visual cortex. Proprioceptive afferents are known to run intraorbitally with the motor branches of eye muscles. If information from eye muscle proprioceptive receptors reaches the visual cortex, the electrical stimulation of proprioceptive afferents at the intraorbital level should evoke cortical responses. A n example of the results of this experiment is reported in Figure 1. Figure 1A shows the poststimulus time histogram for a single visual cortical unit of area 17 in response to repeated electric pulses applied to the IV. nerve. The response of the unit develops very clearly at a latency of about 40 msec. The latencies of other cortical units ranged from 2 5 - 4 0 msec with an average of 31 msec. The histogram in Figure 1B is obtained in the same experimental conditions as that in Figure 1A, but following section of the nerve proximally to the stimulating electrodes. There is complete suppression of the response. The section of the nerve was done in such a way that the two stumps of the nerve remained in contact. Although we interpreted the cortical responses as being due to electrical stimulation of afferent fibers in the IV. nerve, antidromic stimulation of possible, but as yet unknown, recurrent collaterals of motor fibers to the visual cortex cannot be excluded.
B
]1o
Fig. 1. Responsiveness of a visual cortical unit of area 17 to electrical stimulation of the intraorbital part of the IV. nerve before A and after section B of the nerve. Single electric pulses (0.1 msec, 6 V) were applied by means of a pair of silver hook electrodes. The post stimulus time histogram is the average of 100 responses, sampled every 2 sec. (Each bin of the histogram is 10 msec. The vertical calibration gives the impulses in each bin). The arrow marks the time at which the electric pulse was applied. In B the response was taken after section of the IV. nerve proximally to the stimulating electrodes. The section of the nerve was done in such a way that the 2 stumps of the nerve remained in contact. The visual responsiveness of the neuron and its orientation specificity was tested using light or dark bars
Extraocular Proprioceptive Projections to the Visual Cortex
A V 160msec
B
423
Fig. 2. Responsiveness of a visual unit to stretch of the muscle rectus superior before A and after B injection of xylocaine into the stretched muscle (0.1 cc, 1%). The histogram shows the average response to 100 stretches, one every 2 sec. The record in B was taken 5 rain after the injection. The responsiveness of the visual unit to stretch was tested in full darkness, the visual responsiveness having been tested through the intact eye
In each penetration of the visual cortex we found, on average, only 1 or 2 units responding to electrical stimulation of a given nerve, while the other 5 or 6 units did not show any detectable response. Out of a total of 90 cortical units only 22 ( - 2 5 % ) showed clear responses to electric stimulation of motor nerves. Of the 30 geniculate fibers none showed any sign of response at all. Usually, for technical reasons, we restricted the electrical stimulation to the nerves of the muscles rectus late~:alis, obliquus inferior and obliquus superior. In several cases we noticed that a cortical unit responding to electrical stimulation of one muscle, did not respond to electrical stimulation of the other. Thus, we cannot exclude the possibility that cortical units unresponsive to electrical stimulation of the tested nerves were responsive to stimulation of other muscle nerves. Therefore the proportion of cortical units responding to stimulation of ocular nerves is probably greater than that reported above. This conclusion is also supported by multiunit recordings with the same or with a separate electrode. In animals anesthetized either with chloralose or in light pentobarbital anesthesia an evoked response was invariably present at each depth level of the cortex. The largest response was obtained at a depth of about 1 mm below the cortical surface. Also the multiunit activity was always abolished by section of the nerve stimulated. In a second series of experiments, muscle receptors were stimulated by stretching single extraocular muscles, thus avoiding the possibility of antidromic invasion of collaterals of motor fibers to the visual cortex. Chloralose anesthesia, which is known to increase nervous excitability, was not used in these experiments and animals were anesthetized with pentobarbital. An example of the results of this experiment is shown in Figure 2. In Figure 2A the histogram shows the average response of a visual cortical unit to stretch of the rectus superior muscle. This response was abolished by injecting a small dose of xylocaine into the stretched muscle (0.1 ml, 1%) (Fig. 2B). This experiment was also repeated by recording multiunit activity. The abolition of stretch responses by intramuscular injection of xylocaine into the stretched muscle was fully reversible within 6 0 - 9 0 rain from the injection (Fig. 3). Figure 3A, B and C shows the averaged cortical mass responses
424
P. Buisseret and L. Maffei
IR.S.
R ~ m~
E
. •
2,5 mm
I 0.1 mV
160msec
Fig. 3. Responsiveness of multiunit mass activity (area 17) to stretches of the muscle rectus lateral& (RL) before A and 3 min after B injection of xylocaine (0.1 cc, 1%). C Recovery of the responsiveness 90 min after injection. D-E Test responses to stretches of the muscles rectus superior (RS). The response in D was taken before xylocaine injection into the rectus lateralis, the response in E was taken 5 min after injection. The responsiveness of visual units was always tested in full darkness the visual responsiveness having been tested through the remaining intact eye; one was removed. Each record is the average of 100 responses. The electrical nervous activity was filtered (band pass 60-6000 Hz) and processed in an average computer
to stretch of the muscle rectus lateralis before (A), 3 min (B) and 90 min (C) after intramuscular injection of 0.1 ml of xylocaine. Figure 3D and E shows control responses from the same electrode to stretch of the rectus superior muscle. The response in Figure 3D was taken before the injection of xylocaine into the rectus lateralis, and that in Figure 3E, 5 rain after the injection. The similarity of these responses is a sign that cortical excitability did not vary in this lapse of time. In conclusion we think that the present findings are evidence in favor of the existence of a proprioceptive projection from extraocular muscles to the visual neurons. It remains t o be investigated whether all the classes of visual cortical neurons (simple, complex and hyper.complex) or only one class of them is responsive to stimulation of eye muscle proprioceptive afferents and if neurons of cortical areas other than area 17 show the same property. The functional role of extraocular muscle proprioceptive afferents of the visual cortex is not yet known. The experiments on strabismic animals deprived of vision indicate a possible role of proprioceptive afferents in maintaining binocular interaction in the visual cortex. Recent experiments of Brindley et al. (1976) and of Stevens et al. (1976) have revived Sherrington's view that extraocular proprioception could play a role in the stabilization of perceived objects during eye movements (Sherrington, 1918) by demonstrating the stationariness of the perceived world during attempted movements of a totally paralyzed human eye.
Extraocular Proprioceptive Projections to the Visual Cortex
425
References Bach-y-Rita, P., Ito, F.: Properties of stretch receptors in cat extraocular muscles. J. Physiol. (Lond.) 186, 663~588 (1966) Baker, R., Precht, W., Llinfis, R.: Mossy and climbing fiber projections of extraocular muscle afferents to the cerebellum. Brain Res. 38, 440-445 (1972) Batini, C., Buisseret, P., Kado, R.T.: Extraocular proprioceptive and trigeminal projections to the Purkinje cells of the cerebellar cortex. Arch. ital. Biol. 112, 1-17 (1974) Brindley, G.S., Goodwin, G.M. Kulikowsky, J.J., Leighton, D.: Stability of vision with a paralysed eye. J. Physiol. (Lond.) 258, 65-66P (1976) Fuchs, A.F., Kornhuber, H.H.: Extraocular muscle afferents to the cerebellum of the cat. J. Physiol. (Lond.) 200, 713-722 (1969) Maffei, L., Bisti, S.: Binocular interaction in strabismic kittens deprived of vision. Science 191, 579-580 (1976) Maffei, L., Fiorentini, A.: Asymmetry of motility of the eyes and change of binocular properties of cortical cells in adult cats. Brain Res. 105, 73-78 (1976) Sherrington, C.: Observations on the sensual role of the proprioceptive nerve supply of the extrinsic eye muscles. Brain 41, 332-342 (1918) Stevens, J.K., Emerson, R.C., Gerstein, R.L., Kallos, J., Neufeld, G.R., Nichols, L.W., Rosenquist, A.C.: Paralysis of the awake human: visual perceptions. Vision Res. 16, 93-98 (1976) Wolfe, J.W.: Relationship of cerebellar potentials to saccadic eye movements. Brain Res. 30, 204-206 (1971)
Received January 10, 1977
