Central core control of developmental plasticity in the kitten visual cortex: II. Electrical activation of mesencephalic and diencephalic projections

Exp Brain Res (1982) 47:223-233 E ,Xl Li_'mental Bran Research �9 Springer-Verlag 1982

Central Core Control of Developmental Plasticity in the Kitten Visual Cortex: II. Electrical Activation of Mesencephalic and Diencephalic Projections*

W. Singer and J.P. Rauschecker

Max-Planck Institute for Psychiatry, Kraepelinstr. 2, D-8000 Miinchen 40, Federal Republic of Germany

Summary. Fifteen dark-reared, 4- to 5-week-old kittens were stimulated monocularly with patterned light while they were anesthetized and paralyzed. Six of these kittens were exposed to the light stimuli only, in four kittens the light stimuli were paired with electric stimulation of the mesencephalic reticular formation and in five kittens with electric activation of the medial thalamic nuclei. Throughout the conditioning period, the ocular dominance of neurons in the visual cortex was determined from evoked potentials that were elicited either with electric stimulation of the optic nerves or with phase reversing gratings of variable spatial frequencies. In two kittens, ocular dominance changes were assessed after the end of the conditioning period by analyzing single unit receptive fields. Monocular stimulation with patterned light induced a marked shift of ocular dominance toward the stimulated eye, when the light stimulus was paired with electric activation of either the mesencephalic reticular formation or of the medial thalamus. Moreover, a substantial fraction of cells acquired mature receptive fields. No such changes occurred with light or electric stimulation alone. It is con- cluded that central core projections which modulate cortical excitability gate experience-dependent modifications of connections in the kitten visual cortex.

Key words: Visual cortex - Development - Plasticity - Central core - Cat

tive field properties. However, several recent studies indicate that retinal responses to contours are only a necessary but not a sufficient condition for inducing such modifications. Gating signals of non-retinal origin appear to be required in addition (Kasamatsu and Pettigrew 1976, 1979; Buisseret et al. 1978; Buisseret and Gary-Bobo 1979; Freeman and Bonds 1979; Singer et al. 1979a, b, 1982). The preceding study suggested that the medial thalamus serves as relay for such permissive gating signals (Singer 1982). Unilateral destruction of the medial nuclear complex led to a sensory neglect in the contralateral visual hemifield and impaired experience-dependent modifications in the visual cortex on the side of the lesion. In addition, it attenuated the effects which electric stimulation of the mesencephalic reticular formation has in the visual cortex. This suggested that the non- retinal gating signals required for the manifestation of cortical plasticity might be related to reticular activation. It appeared conceivable therefore that electric stimulation of the mesencephalic reticular formation and of the medial thalamic nuclei substitutes the required gating signals. To test this hypothesis, it was attempted to induce ocular dominance changes in anesthetized and paralyzed kittens by pairing monocular light stimulation with conditioning electric stimulation of the reticular core and of the medial thalamic nuclei. Brief accounts of some of the results have been published previously (Singer 1979, 1980).

Introduction

Early visual experience influences the development of the mammalian visual cortex and modifies recep-

* Part of this work was supported by a grant from the Deutsche Forschungsgemeinschaft SFB50, A14

Offprint requests to." Prof. W. Singer (address see above)

Material and Methods

Seventeen 5-week-old dark-reared kittens were prepared as usual for electrophysiologic recording and visual stimulation. Through- out the experiment, anesthesia was maintained with nitrous oxide (70% N20, 30% Oz), supplemented by Nembutal (2 mg/kgha i.v.), and paralysis was induced with continuous i.v. infusion of Imbretil (1 mg/kg/h). Careful control of (COz) in the expired air, of the

0014-4819/82/0047/0223/$ 2.20

224 W. Singer and J.P. Rauschecker: Central Gating of Plasticity

body temperature, of the heart rate, and of the liquid balance allowed to keep the kittens in good conditions for up to 4 days. The kittens were fed through an brally inserted gastric catheter with a mixture of Laevulose and Ringer's solution. Urine excretion was monitored continuously, using a catheter, and the amount of substituted liquid and electrolytes was adjusted accordingly. The eyes were protected with black contact lenses containing artificial pupils of 2 mm diameter and were moistened continuously with saline. Pupils were dilated with Atropine sulfate, and refraction was corrected with spectacle lenses. The retinal landmarks were determined with a fundus camera.

The Conditioning Stimuli

The light stimulus used for the induction of ocular dominance changes consisted of a rotating (5 deg/s) star pattern which was projected onto a translucent tangent screen placed 1.5 m in front of the kitten. The axis of this pattern moved in addition along a circle of 15 deg diameter around the area centralis projection of the stimulated eye. Thus, all orientations appeared successively at any point within this circle. In early experiments, this pattern moved continuously, in later experiments, for reasons described below, movement sequences of 3 s were separated by 20-s pauses (see Fig. 2A). The electric stimuli to the mesencephalic reticular formation (MRF) and to the thalamus were applied through stereotactically inserted concentric electrodes and consisted of 90 ms long pulse trains (pulse duration 50 ~s, pulse separation 15 ms). Two electrodes were lowered into the MRF, one on either side, at Horsley Clarke coordinates A+2, L+2, H-2. To avoid penetrating the visual cortex, these electrodes were advanced through the cerebellun~. For thalamic stimulation, two concentric electrodes were lowered into the thalamus of the right hemisphere at coordinates L2, H4, A6 and L2, H4, A8. The final adjustment of electrode positions was made according to functional criteria before the kittens were paralyzed. The MRF electrodes were lowered until the negative cortical field potential and the associated facilitation of responses to optic nerve stimulation could be elicited with minimal stimulus intensities (from 2 to 5V). At this location, MRF stimuli elicited eye movements with the contralateral eye deviating more than the ipsilateral. The facilitating effect on cortical evoked responses was, however, always equally pronounced in both hemispheres, irrespective of the side of MRF stimulation. The thalamic electrodes were adjusted according to the same functional criteria as the MRF electrodes. Here, however, eye movements were less frequent. If they occurred, they were conjugate and directed into the hemifield, contralateral to the stimulated thalamus.

For histological verification of electrode positions, small elec- trolytic lesions were made by passing DC current at the end of the experiments. After perfusion with 4% formaldehyde, the brains were cut along coronal planes and embedded in celloidin. Elec- trode positions were reconstructed from 30 ~m thick serial sections stained with cresyl violet. The locations of the MRF and the thalamic electrodes are indicated in Fig. 1.

Determination of Ocular Dominance

The following three methods were used to assess the relative efficiency of the two eyes in exciting cortical cells. Evoked potentials elicited either by electric stimulation of the optic nerves (Singer 1977; Mitzdorf and Singer 1980) or by phase-alternating square-wave gratings (Snyder and Shapley 1979) served to repeatedly determine the relative efficiency of the two eyes. This allowed our assessment of the time course of ocular dominance changes during conditioning. To confirm the changes in ocular-

dominance observed with the evoked potential methods, receptive fields of single units were analyzed at the end of the conditioning period in two kittens from the thalamic stimulation group. For evoked potential recording, a pair of epidural silver ball electrodes was implanted over the visual cortex of each hemisphere. The electrodes connected to the positive leads of the amplifiers were located over the striate cortex at coordinates P2, L2, the reference electrodes were located over the suprasylvian gyrus at coordinates P2, L7.

For electric stimulation of the optic nerves, bipolar, fork- shaped electrodes were placed on the nerves right behind the eye. The stimuli consisted of 50 us double shocks delivered at an interval of 20 ms. Threshold and saturation intensities for the cortical evoked potential were below 2 and 10 V, respectively. All measurements were made at saturation intensity, and for each trial 20 responses were averaged with a PDP 8 computer.

For the generation of pattern-evoked potentials, a square- wave grating with a contrast of 38% and an overall luminance of 11.5 cd/m 2 was generated on an oscilloscope screen (Hewlett Packard, P4) which subtended 30x40 deg of visual angle. The stimulus consisted of two phase reversals separated by 800 ms. These stimuli were repeated every 2 s and presented 50 times to each eye in alternation for the compilation of average responses. For most measurements, only four spatial frequencies were used (0.05, 0.6, 1.3, and 3 c/deg). As described previously (Singer et al. 1980) the peak-to-peak amplitude of the first positive potential in each response was chosen as the relevant parameter. For each spatial frequency, two independent measurements were made. Since two reversals occurred per sweep, four averaged responses were available for the calculation of mean response amplitude.

For receptive field analysis, single units were recorded with K+-citrate filled micropipettes from area 17. Ocular dominance was assessed with hand held light stimuli and when this did not allow a safe classification, response histograms to optimally aligned stimuli were compiled. To save time and to sample as many units as possible, orientation tuning was not assessed quantitatively. As described previously (Singer et al. 1980; Singer 1982), in addition to the usual receptive field parameters, the vigor of responses to the optimal stimulus was recorded and classified on a non-parametric scale of five classes.

Results

Monocular Light Stimulation Without Central Core Activation

In six k i t t ens o n l y t h e op t i c n e r v e and t h e E E G

e l e c t r o d e s w e r e i m p l a n t e d to i n v e s t i g a t e t h e e f f ec t o f

m o n o c u l a r l igh t s t i m u l a t i o n in t h e a b s e n c e o f c e n t r a l

c o r e ac t iva t ion . I n all t h e s e k i t t ens t h e r e l a t i v e

e f f i c i ency o f t h e two eyes was a s sessed f r o m co r t i c a l

e v o k e d p o t e n t i a l s e l i c i t ed w i t h e l ec t r i c s t i m u l a t i o n o f

t h e op t i c ne rve s . I n t w o of t h e m , p a t t e r n e v o k e d

p o t e n t i a l s w e r e a n a l y z e d in add i t i on . L i g h t s t imu la -

t i o n o f t h e l e f t e y e , t h e r igh t was o c c l u d e d , s t a r t e d

5 h a f t e r t h e e n d of su rge ry . S e v e r a l t i m e s b e f o r e t h e

o n s e t o f c o n d i t i o n i n g l igh t s t i m u l a t i o n , a n d subse -

q u e n t l y at i n t e r v a l s o f 5 - 8 h, e v o k e d p o t e n t i a l s w e r e

a v e r a g e d to assess t h e e f f i c i ency of t h e t w o eyes . T o tes t fo r t h e poss ib i l i ty t h a t t h e e x p r e s s i o n o f c h a n g e s

in o c u l a r d o m i n a n c e m i g h t r e q u i r e c o n s o l i d a t i o n

W. Singer and J. P. Rauschecker: Central Gating of Plasticity 225

N

Fig. 1. Histologically verified stimulation sites in the mesencephalic reticular formation (2-4) and the medial nuclei of the thalamus (6, 8). The locations of the electrode tips are indicated by squares. The numbers on the graphs correspond to the frontal plane of the sections. Only one side of the brainstem is represented because electrode positions on the other side were similar. CL n. centralis lateralis; CM n. centralis medialis; CR superior colliculus; GL lateral geniculate; LA n. lateralis anterior; LM1 lamina medullaris interna; LP n. lateralis posterior; M n. medialis dorsalis; R n. ruber; p pyramidal tract

The decrease of the test responses after continuous conditioning indicated that uninterrupted light stimulation impaired excitatory transmission and hence might be inappropriate for the induction of ocular dominance changes. Therefore , the schedule of the conditioning stimulus was changed in subsequent experiments, as indicated in Fig. 2A. The star pattern now rotated for 3 s in one direction, stopped for 2 s, rotated for another 3 s in the opposite direction, and then stopped for 20 s. This schedule no longer reduced the amplitudes of the test responses and was adopted in all subsequent experiments. However, in the remaining four kittens in which conditioning light stimulation was not paired with electric activation of central core structures, this stimulation sequence, too, failed to induce any significant interocular differences. The amplitudes of test responses evoked with electric stimulation of the optic nerves varied only little and in a non-systematic manner. Even after 3 days of monocular conditioning, amounting to a total of 50 h of monocular light stimulation, the evoked responses remained essentially the same as before conditioning. In two of the four kittens, ocular dominance was also assessed from responses to phase-alternating gratings, but also this method failed to show any consistent interocular differences throughout the course of conditioning.

To exclude the possibility that evoked potentials are not sensitive enough to disclose smaller changes in ocular dominance, control recordings were made from two kittens which were dark-raised until 4 weeks of age and then monocularly deprived in the usual way by lid suture for 2 days. In both kittens evoked potentials were recorded at the end of exposure and showed a clear bias in favor of the open eye.

periods devoid of light stimulation, pauses of 5-6 h were intercalated usually after 15 and 24 h of conditioning.

In the first two experiments, the conditioning star pattern rotated continuously throughout the exposure periods. This had the effect to further reduce the anyway small responses to optic nerve stimulation, the reduction being somewhat more pronounced for responses obtained from the conditioned eye. This decrease was consistently observed in the first post- exposure measurement 5 h after the beginning of conditioning. During the "consolidation" periods, which were devoid of conditioning light stimulation, the responses recovered to the initial level, but there was no evidence for any systematic interocular differences.

Monocu lar L igh t St imulat ion C o m b i n e d with M R F St imulat ion

In four kittens conditioning light stimulation was paired with electric activation of the MRF. As indicated in Fig. 2A, a brief pulse train was applied through the MRF electrodes each time the star pattern started to move. Precise temporal coinci- dence between the onset of the two stimuli was chosen, because this is the most favorable condition for maximally facilitating cortical responses to light (Singer et al. 1976). In three of the four kittens, this conditioning paradigm produced a clear shift of ocular dominance toward the stimulated eye. Stable interocular asymmetries of the test-evoked responses were established after approximately 15 h of exposure. In the fourth kitten conditioning light stimula-


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Fig. 2. A Schematic representation of the exposure schedule for monocular light stimulation. The conditioning light stimulus consisted of a star pattern that rotated around its center and in addition moved along a circle (dia= 15 deg) around the visual axis of the stimulated eye. Thus, all orientations appeared successively at any point within this circle. The donditioning electrical stimuli lasted 90 ms and were applied each time the star pattern started to rotate. B Time-dependent changes of averaged (20 stimulus presentations) evoked potentials which were recorded from visual cortex and elicited by electric double shocks (50 ~s duration, 10 V intensity, 20 ms interval) applied to the optic nerve of the left (conditioned) and right (deprived) eye. Conditioning monocular light stimulation was paired with electric activation of MRF and started at 3 pm, 5 h after the end of surgery. Traces (1) were obtained prior to conditioning stimulation and show similarly small responses from either eye. After the first night of conditioning stimulation (traces 3), the reponse from the left (conditioned) eye has increased but only to the second of the double shocks. With continuing monocular conditioning the response from the left eye occurs with large amplitude already after the first shock (traces 4), the response to the second stimulus is now suppressed. The response from the right, deprived eye increases slightly during the initial phase of conditioning (traces 2 and 3) and then decreases again to the control level

tion was started only 24 h after the beginning of the experiment and could be followed for only 8 h more because respiratory problems terminated the experiment prematurely (see below). The changes of evoked responses in the three successful experiments followed a characteristic sequence and are illustrated in Fig. 2B. Before conditioning the test-evoked responses had small amplitudes and were unstable. On occasions, they were barely distinguishable from the noise level even with averaging (Fig. 2B, traces 1). Typically, after 3-4 h of conditioning stimulation the test responses started to increase in

amplitude and to become more stable, but at this early stage did not yet show interocular differences. With further conditioning, the test responses from the open eye continued to increase while those from the closed eye stopped growing. In all three kittens, the test responses from the two eyes differed to the extent illustrated in Fig. 2B after one night of conditioning stimulation (between 15 and 18 h of stimulation). At this stage the ratios between the positive peaks of responses from the conditioned and the deprived eye had attained values of 2.8:1, 2.6:1, and 2.1:1 in the three kittens. Thereafter, the responses

W. Singer and J.P. Rauschecker: Central Gating of Plasticity 227

from the conditioned eye increased only little further. Interocular differences continued to increase, however, because the test responses elicited from the deprived eye decreased again. As illustrated in Fig. 2B, this decrease continued nearly to the pre- exposure level in this kitten. In the other two kittens the responses from the deprived eye decreased also during the 2nd and 3rd day but they remained above the pre-exposure level. No evidence was obtained that consolidation periods devoid of light stimulation are required for the expression of ocular dominance changes. Interrupting the conditioning procedure for intervals of 5-6 h did not noticeably alter the once attained interocular differences in the test responses.

As shown in Fig. 2B, not only the amplitudes but also the temporal structure of the test responses varied in the course of conditioning. Initially, the responses to the second of the two optic nerve stimuli were consistently more prominent than the responses to the first stimulus. Later on, the condition reversed, and the first response increased at the expense of the second; the latter could even drop below the initial control level.

To test whether MRF stimulation alone might have been responsible for the induction of interocular differences, light stimulation was resumed in one of the four kittens only 24 h after the beginning of the experiment. Before that, the MRF stimuli were delivered alone for 15 h following exactly the same sequence as that used for light stimulation. This caused a slight enhancement of the test responses from both eyes but did not induce any noticeable interocular differences. The extent of the enhancement of responses was in the range of that seen a few hours after the beginning of light conditioning (traces 2 in Fig. 2B). Measurements taken 6 h after conditioning with light had been resumed showed the beginning of interocular differences. Unfortunately, these changes could not be further substantiated because this kitten died 2 h later.

Monocular Light Stimulation Paired with Thalamic Stimulation

The conditioning schedule in these experiments was the same as in the experiments with MRF stimulation, except that now the medial thalamus was stimulated instead of the MRF. Five kittens were examined and all provided evidence of a marked shift of ocular dominance toward the conditioned eye. Ocular dominance changes were assessed from potentials evoked by electric stimulation of the optic nerves in one kitten, from responses elicited with phase-alternating gratings in two kittens and from

single unit receptive fields in the remaining two kittens. Interocular differences in the test responses elicited by nerve stimulation developed in the same way and with the same time course as with combined MRF and light stimulation. The only difference was that the change in ocular dominance was now considerably more Pronounced in the hemisphere contralateral to the open eye; this is the hemisphere in which the thalamus was stimulated.

The responses to phase-alternating gratings, too, reflected the development of interocular differences in the course of conditioning. Interestingly, however, the extent of these differences was dependent on the spatial frequency of the grating pattern that was used to elicit the test responses. As is typical for pattern evoked responses, their shapes and amplitudes differed in the two kittens, but the changes in amplitude followed the same trend. Before conditioning, the grating with the lowest spatial frequency (0.05 c/deg) elicited the largest and ,the grating with the highest spatial frequency (3 c/deg) the smallest responses (Fig. 3). No significant interocular differences were apparent at any spatial frequency tested. With conditioning, the responses from the two eyes became different, and this dissociation followed the same time course as the development of interocular differences in the responses evoked with electrical stimulation of the optic nerves. As illustrated in Fig. 3, the responses to the grating with higher spatial frequencies increased for the conditioned eye and decreased for the closed eye, leading to amplitude ratios between the two eyes of up to 2.5. By contrast, the amplitudes of responses to the grating with the lowest spatial frequency (0.05 c/deg) decreased in the course of conditioning for both eyes, and interocular differences in response amplitude remained marginal.

As in the experiments with MRF stimulation, "consolidation" periods during which conditioning stimulation was arrested for 5 h did not further accentuate interocular differences. In both kittens the amplitudes of responses to gratings with high spatial frequencies increased during these pauses, but this augmentation was similar for both eyes. The responses to the Ganzfeld stimulus remained unchanged (Fig. 3).

Single Unit Analysis

In two further kittens, that were conditioned with light and thalamic stimulation, changes of ocular dominance were assessed by analyzing single unit receptive fields. The exposure schedule was the same as in the previous experiments. During receptive field analysis, which started 18 h after the beginning

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228 W. Singer and J. P. Rauschecker: Central Gating of Plasticity

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Fig. 3A-D. Time-dependent changes in the amplitudes of averaged (50 stimuli) evoked potentials that were recorded from striate cortex and elicited by the phase reversal of square wave gratings. Recordings are from a kitten in which conditioning monocular light stimulation was paired with electrical stimulation of medial thalamus. (A-C) show the absolute peak to peak amplitudes (in arbitrary units on the ordinate) of responses elicited from the two eyes with gratings of different spatial frequencies (0.05-3.0 c/deg). D shows the respective ratios between the amplitudes of responses from the conditioned and the deprived eye. Each value corresponds to the mean of four independent measurements, each of which represents in turn the average of 50 responses. The responses to gratings of 0.6 and 1.3 c/deg had very similar amplitudes and are therefore presented together. The vertical bars indicate the standard deviation of the respective means. The frst measurement at 3 pm is before the beginning of conditioning stimulation. During conditioning (time scale on abscissa) responses to the gratings with medium and high spatial frequencies (0.6, 1.3, 3.0 c/deg) increase from the conditioned eye and decrease from the deprived eye (B, C) leading to a marked interocular dissociation of response amplitudes (D). The responses to the test grating with very low spatial frequency (0.05 c/deg) show little if any interocular dissociation and the responses flom both eyes decrease with prolonged exposure to the conditioning light stimulus. After a pause of 5 h, during which conditioning monocular stimulation was arrested, responses to the finer gratings increased in both eyes (B, D), but this did not substantially alter the interocular differences attained at the end of conditioning (D)

of the conditioning period and lasted for another 16-18 h, the kittens received no further electric stimulation. All recordings were restricted to the striate cortex of the hemisphere in which the thalamus electrodes were located. A total of 90 units were analyzed in the two kittens, 40 in JN4 and 50 in JN5. Of these cells, 86 responded to light stimulation.

As indicated in Fig. 4A, in both kittens the ocular dominance distributions showed a clear and quantitatively similar bias in favor of the eye that had been

open during conditioning. However, many of the cells (67%) were still binocular, suggesting that changes of ocular dominance are gradual. Interest- ingly, in these binocular cells, the preferred stimulus orientation was always virtually identical in the two eyes, even though only one eye had experienced contours. On our scale of response vigor, responses to the Conditioned eye reached averages close to normal (for reference see Singer et al. 1980), while those of responses to the closed eye were significantly below normal (Fig. 4B). In the X2-test the distribu-

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W. Singer and J. P. Rauschecker: Central Gating of Plasticity 229

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Fig. 4A-D. Ocular dominance distributions (A, C, D) and average indices of response vigor (B) of single cells in area 17. The cells are from two kittens which had 18 h of conditioning monocular light stimulation paired with electric stimulation of medial thalamus. Cells in ocular dominance classes 1 and 5 responded exclusively either to the ipsilateral closed eye or to the contralateral conditioned eye. Cells in class 3 reacted equally well to stimulation of either eye and cells in classes 2 and 4 responded more vigorously to one of the two eyes. The distribution in A summarizes the ocular dominance distribution of all responsive cells in the two kittens In4 and In5. The graph in B compares the average vigour of responses from the two eyes with corresponding averages from kittens raised with conventional monocular deprivation (Singer et al. 1982). The average vigor of responses from the conditioned eye has attained nearly the same level as responses from the normal eye in cats raised with conventional monocular deprivation, while the vigor of responses from the deprived eye is abnormally low but not yet as poor as that of responses from the deprived eye in conventional MDs. The distribution in C shows the ocular dominance of cells whose response vigor was -> 3 and whose orientation selectivity was in the normal range while the distribution in D summarizes the remaining cells in which either property was rated abnormal. This comparison reveals that the bias in the total sample of cells (A) is essentially caused by cells which have developed normal response properties

tions of vigor indices d i f fered significantly (p <

0.0001) for responses f rom the dep r ived and f rom the condi t ioned eye. The p e r c e n t a g e of n o n - o r i e n t e d

cells was slightly e l eva t ed (14%) , and as far as it can

be in fer red f rom hand mapp ing , the o r i en ta t ion

tuning of the r ema in ing cells a p p e a r e d on the average

b roader than in ki t tens raised with no rma l visual

exper ience . H o w e v e r , in a substant ia l f rac t ion (56%)

of cells both the v igor of responses and the or ienta-

"tion tuning were in the n o r m a l range. In the fol low-

ing, these cells are addressed as " m a t u r e " cells; the

remain ing cells, in which e i the r response vigor or


orientation tuning were abnormal, are referred to as "immature" cells. Comparison of the ocular dominance distributions of mature and immature cells revealed that conditioning stimulation had modified ocular dominance preferentially in mature cells (Fig 4C, D). Of the 48 mature cells, 90% were dominated or driven exclusively by the conditioned eye. By contrast, of the 38 immature cells, 29% still responded equally well to both eyes and only 43% were dominated or driven exclusively by the conditioned eye.

Discussion

Justification of Methods

The principal reason for using evoked potentials rather than single unit responses for measuring changes of ocular dominance was that the former allow a more rapid, less invasive and hence repeat- able assessment of ocular dominance. Because of the limited duration of the experiments, only this method made it possible to establish baseline data before conditioning and to follow the time course of changes throughout conditioning. Previous studies have indicated that evoked potentials elicited with electric stimulation of the optic nerves (Singer 1977, 1982; Mitzdorf and Singer 1980) and with phase-alternating gratings (Snyder and Shapley 1979) do reflect with sufficient precision the ocular dominance of cortical cells. The general agreement between the single unit and evoked potential data in the kittens conditioned with thalamic stimulation further confirm the validity of evoked potential measurements.

Central Gating of Developmental Plasticity

The finding that retinal stimulation does not alter the functioning of striate cortex in anesthetized and paralyzed kittens confirms previous reports (Buis- seret et al. 1978; Freeman and Bonds 1979; Singer 1979). Since paralysis abolishes eye movements and since anesthesia might have modified the responses of retinal ganglion cells, this failure could have been due to the inappropriate nature of retinal responses. The present results exclude this possibility in demon- strating that electrical stimulation of the central core enables the very same retinal signals to induce cortical modifications which failed to do so when occurring alone. This complements the evidence that experience-dependent modifications of striate cortex functions require permissive gating signals of non- retinal origin (Kasamatsu and Pettigrew 1976; Buis-

seret et al. 1978; Buisseret and Gary-Bobo 1979; Freeman and Bonds 1979; Singer et al. 1979a, b, 1982). The evidence that electric activation of the medial thalamic nuclei substitutes these gating signals corroborates the results of the preceding study (Singer 1982) and provides complementary evidence for a gating function of these nuclei. The finding that MRF stimulation also facilitates cortical modifications suggests a functional linkage between MRF and medial thalamic nuclei, the latter probably serving as relay station for ascending gating signals. This is in agreement with anatomic (Scheibel and Scheibel 1958; Singer 1982) and physiologic evidence (Ropert and Steriade 1981) and with the results of the lesion experiments: destruction of the medial thalamic nuclei reduced the cortical effects of MRF stimulation. Finally, also the conclusion reached in the preceding study (Singer 1982) that the gating signals are related to attentional mechanisms is compatible with the presumed origin of these signals. Central core activation is a well established correlate of alerting, of orienting responses, and of attentional states (for reviews see Sokolov 1963; Hobson and Scheibel 1980).

As discussed in the preceding lesion study (Singer 1982) we cannot answer the question whether the effects of reticular and thalamic activation are ulti- mately mediated by norepinephrinergic (NE) pathways (Kasamatsu and Pettigrew 1976, 1979; Kasamatsu et al. 1979). While it is very likely that the reticular electrodes activated ascending NE pathways directly, only indirect synaptic activation of NE neurons is conceivable in the case of thalamic stimulation. If our effects are solely due to the activation of the NE projection from locus coeruleus this implies that the facilitatory effects of reticular activdtion are not responsible for the enhancement of cortical plasticity. The facilitatory effects are antagonized by Atropine and thus probably not dependent on the catecholamine projection (for review see Singer 1979).

The Nature of Experience-dependent Modifications

Monocular exposure to patterned light had three effects: First, it increased the safety factor of excitatory transmission in pathways of the stimulated eye. Second, it improved the selectivity of cortical receptive fields. Third, it transiently enhanced, but in the long run decreased the efficiency of the deprived eye. Direct evidence for improving transmission in pathways of the stimulated eye is the steady increase of the test-evoked potentials during conditioning. The test responses elicited with electric stimulation of the

w. Singer and J.P. Rauschecker: Central Gating of Plasticity 231

optic nerves indicated a change of the response pattern in addition. Initially, the second of the two stimuli elicited the larger response. The normal pattern with the first stimulus being more effective than the second developed only during later phases of conditioning. These changes, too, suggest that the efficiency of excitatory transmission improved during conditioning. The fact that the response was initially larger to the second of the two shocks implies (1) that the first stimulus failed to induce IPSPs and (2) that there was temporal summation between the two stimuli. Since the cortical IPSPs which follow optic nerve stimulation are mediated by interneurones (Toyama and Matsunami 1968; Tretter et al. 1975), the lack of IPSPs is as indicative of poor excitatory transmission as the low amplitude of the response itself. The fact that temporal summation occurred over as much as 20 ms suggests further that excitatory synaptic potentials have a remarkably long time- course in the immature visual cortex. As synaptic efficiency increased, already the first stimulus evoked cortical responses and now obviously drove inhibitory interneurons, too, which in turn supp~ressed responses to the second stimulus. Further but indirect evidence for an improvement of excitatory transmission is the finding that the response vigor of a substantial fraction of single units was close to normal after conditioning, while it is certainly sub- normal in kittens that lack visual experience (Hubel and Wiesel 1963; Buisseret and Imbert 1976).

Another observation worth mentioning with respect to changes of synaptic efficiency is the depression of test responses that occurred after continuous retinal stimulation in the first two experiments. This seems to indicate a particularly great fatigability of immature synaptic connections and probably needs to be taken into account in conditioning experiments. It may, however, reflect a more general phenomenon, since Creutzfeldt and Heg- gelund (1975) reported that habituation to prolonged stimulation occurred also in the visual cortex of adult cats.

An increase of receptive field selectivity is suggested by the differential changes of evoked potentials elicited with gratings of low and high spatial frequency. Responses to the fine gratings (above 0.6 c/deg) increased during conditioning while those to the coarse grating (0.05 c/deg) decreased. This suggests that an increasing number of cells stopped responding to changes in Ganzfeld luminance and became selective for spatial luminance gradients within the receptive field. The single unit data further support this suggestion. In 4- to 5-week-old dark- reared kittens only few cortical cells possess normal selectivity for stimulus orientation (Buisseret and

Imbert 1976; Fregnac and Imbert (1978). In the two kittens examined after 18 h of conditioning , more than half of the cells had receptive fields which appeared normal within the precision range of hand mapping. Evidence that this increase in receptive field specificity actually resulted from conditioning light stimulation is provided by the covariance of receptive field selectivity and ocular dominance: The cells which had aquired mature receptive fields also showed a bias in their ocular dominance toward the open eye, while the cells with immature receptive fields showed no such bias. Fregnac and Imbert (1978) have demonstrated that the few orientation selective cells in dark-reared kittens of this age tend to be monocular and in their majority are driven by the contralateral eye. Since we had conditioned the eye contralateral to the investigated hemisphere these cells contribute to the bias in the ocular dominance of mature cells. The contribution of these prespecifled cells is, however, only marginal since our sample of mature cells comprises more than half of the cortical neurons (56%). Moreover, the bias of ocular dominance of the mature cells is not solely due to monocular cells but is caused to a substantial extent by binocular neurons.

In agreement with the preceding study (Singer 1982) the binocular cells preferred the same orientations in the two eyes even though only one eye had experienced contours. This suggests that the prefer- ence of a cell for a particular orientation is determined by intracortical interactions rather than by specific arrangements of thalamo-cortical input connections.

The test responses evoked from the deprived eye increased during early phases of conditioning but subsequently decreased again. This suggests that activity dependent increases of synaptic gain occur faster than the competitive inactivation of the deprived afferents. The initial enhancement of responses from the deprived eye is explained by the fact that in inexperienced kittens most cortical cells are binocular. Thus, any improvement of transmission occurring beyond the level of binocular conver- gence also enhances the responses from the deprived eye. As the data suggest, this initial enhancement was antagonized only with some delay by subsequent inactivation of the afferents from the deprived eye. The single unit data are in agreement with this interpretation. They indicate that the shift of ocular dominance was gradual and not yet associated with a substantial reduction of binocularity. Such gradual changes in ocular dominance distributions were obtained also in kittens that were monocularly deprived for brief periods while alert (Peck and Blakemore 1975). The evidence that ocular domi-


nance changes are gradual is, however, at odds with the claim of Freeman and Bonds (1979) that monocular light stimulation first disrupts cortical binocularity, leading to U-shaped ocular dominance distributions and only later causes disconnection of the deprived eye. Since in Freeman and Bonds' study light stimulation was associated with passive eye movements, the discrepancy between the two studies can probably be attributed to methodological differences.

Both evoked potential and single unit analysis revealed clear modifications of cortical responses after at most 15 h of conditioning. Similar time courses have been described in alert kittens for ocular dominance changes after brief periods of monocular deprivation (Peck and Blakemore 1975; Schechter and Murphy 1975; Freeman and Olson 1979) and for the acquisition of mature receptive fields after brief periods of contour vision (Buisseret et al. 1978). In agreement with previous evidence from alert kittens (Freeman and Olson 1979) is further the observation that consolidation periods devoid of conditioning stimulation are not required for the manifestation of ocular dominance changes. In conclusion, then, the experience-dependent modifications in anesthetized and paralyzed kittens resemble in many respects those occurring in alert kittens.

Gawd Hebb Synapses as Common Mechanism in Developmental Plasticity

Previous experiments suggested that experience- dependent changes of striate cortex functioning follow modification rules which closely resemble those proposed by Hebb (1949) for adaptive synaptic connections (Rauschecker and Singer 1979, 1981). The crucial point of Hebb's rules is that modifications depend on the temporal contingency of pre- and postsynaptic activation. In the kittens exposed to the conditioning light stimulus only, cortical cells certainly responded to retinal signals, but Hebbian modifications did not occur. Thus, the classical Hebbian modification rules need to be extended by adding as further requirement permissive gating signals.

Since the gating required for Hebbian modifications are obviously provided by electric stimulation of the MRF and medial thalamus, and since the effects of such stimulation on visual processes have been investigated (for reviews see Burke and Cole 1978; Singer 1977, 1979), a brief consideration of possible gating mechanisms is warranted. In anesthetized preparations MRF stimulation facilitates transmission in the lateral geniculate by blocking

local inhibitory mechanisms (Singer 1973). It further enhances transmission of retinal signals along intracortical pathways, probably through heterosy- naptic facilitation of excitatory events (Singer et al. 1976; for review see Singer 1979). Activation of the medial thalamus with high frequency stimulation produces very similar effects in thalamic relay nuclei (for review see Purpura 1970) and in the visual cortex (unpublished observations). It is predictable from this evidence that conditioning MRF and thalamic stimulation considerably enhances dendritic depolarization of cortical neurones. The possibility may be considered, therefore, that Hebbian modifications have a threshold and occur only when dendritic depolarization of the postsynaptic target cells trespasses a critical level, the occurrence of soma spikes alone not being a sufficient condition. If such were the case, numerous non-retinal excitatory pathways would actually contribute in a cooperative and highly selective way to the gating of Hebbian modifications. This could explain why so many different manipulations abolish and why such coarse proce- dures as electrical activation of modulatory projections can facilitate experience-dependent modifications.

After submission of this manuscript a research note appeared by Tsumoto and Freeman (1981) in which they report results very similar to ours. Tsumoto and Freeman (1981) succeeded to induce rapid ocular dominance changes in anesthetized kittens by pairing monocular light stimulation with electric stimulation of the internal medullary lamina of the thalamus. They further succeeded to observe these changes while recording from single units. Their criterion for optimal stimulation sites were eye movements rather than facilitation of cortical responses but the locations of stimulation electrodes appear to be similar in the two studies. Even though the methods of conditioning differ in some aspects in the two studies the general conclusions agree. Thus, there is independent and converging evidence that structures of the intralaminar nuclear complex of the thalamus are involved in the gating of neuronal plasticity.

Acknowledgements. Margot Steinleitner was in charge of raising the kittens, Susanne Zieglgfinsberger provided technical assistance during the experiments and prepared the histological sections, and Mariele Kremling edited the manuscript. We gratefully acknow- ledge their competent help which made this study possible.

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Received October 12, 1981

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Central core control of developmental plasticity in the kitten visual cortex: II. Electrical activation of mesencephalic and diencephalic projections