TMS reveals inhibitory extrastriate cortico-cortical feedback modulation of V1 activity in humans.

The interaction between the primary visual cortex (V1) and extrastriate visual areas provides the first building blocks in our perception of the world. V2, in particular, seems to play a crucial role in shaping contextual modulation information through feedback projections to V1. However, whether this feedback is inhibitory or excitatory is still unclear. In order to test the nature of V2 feedback to V1, we used neuronavigation-guided offline inhibitory transcranial magnetic stimulation (TMS) on V2 before testing participants on collinear facilitation, a contrast detection task with lateral masking. This contextual modulation task is thought to rely on horizontal connections in V1 and possibly extrastriate feedback. Results showed that when inhibitory TMS was delivered over V2, contrast thresholds decreased for targets presented in the contralateral hemifield, consistent with the retinotopic mapping of this area, while having no effect for targets presented in the ipsilateral hemifield or after control (CZ) stimulation. These results suggest that feedback from V2 to V1 during contextual modulation is mostly inhibitory, corroborating recent observations in monkey electrophysiology and extending this mechanism to human visual system. Moreover, we provide for the first time direct evidence of the involvement of extrastriate visual areas in collinear facilitation.

A PET study of photophobia during spontaneous migraine attacks.

BACKGROUND: Photophobia is an abnormal sensitivity to light experienced by migraineurs during attacks. The pathophysiology of photophobia is poorly understood. Nevertheless, 2 facts appear to have a link with photophobia: visual cortex hyperexcitability on the one hand and interactions between visual pathway and trigeminal nociception on the other. METHODS: We used H(2)(15)O PET to study photophobia induced by continuous luminous stimulation covering the whole visual field in 8 migraineurs during spontaneous migraine attacks, after headache relief by sumatriptan and during attack-free interval. The intensity of the luminous stimulation provoking photophobia with subsequent headache enhancement was specifically determined for each patient. RESULTS: We found that low luminous stimulation (median of 240 Cd/m(2)) activated the visual cortex during migraine attacks and after headache relief but not during the attack-free interval. The visual cortex activation was statistically stronger during migraine headache than after pain relief. CONCLUSION: These findings suggest that ictal photophobia is linked with a visual cortex hyperexcitability. The mechanism of this cortical hyperexcitability could not be explained only by trigeminal nociception because it persisted after headache relief. We hypothesize that modulation of cortical excitability during migraine attack could be under brainstem nuclei control.

Distance perception within near visual space.

We investigated the perception of distance of visual targets with constant size and luminance presented between 20 and 120 cm from subjects' eyes. When retinal disparity cues were present, the subjects could reproduce very accurately the distance of a seen reference in this area. When only extraretinal information was available, distance perception was still correct for distances of 40 cm or less. However, distances beyond 60 cm were underestimated. When forced to evaluate the distance between a reference and themselves, e.g. when evaluating the absolute distance or half the distance or twice the distance of a reference, subjects used an egocentric plane of reference located on average 10.4 cm in front of their eyes. Measurements of binocular eye movements indicated a clear relationship between vergence angle and target distance. The egocentric plane of reference at 10.4 cm also corresponds to the maximum achievable vergence. These results suggest that ocular convergence can be used as a reliable cue for distance within the arm's reaching space.

Gaze direction controls response gain in primary visual-cortex neurons.

To localize objects in space, the brain needs to combine information about the position of the stimulus on the retinae with information about the location of the eyes in their orbits. Interaction between these two types of information occurs in several cortical areas, but the role of the primary visual cortex (area V1) in this process has remained unclear. Here we show that, for half the cells recorded in area V1 of behaving monkeys, the classically described visual responses are strongly modulated by gaze direction. Specifically, we find that selectivity for horizontal retinal disparity-the difference in the position of a stimulus on each retina which relates to relative object distance-and for stimulus orientation may be present at a given gaze direction, but be absent or poorly expressed at another direction. Shifts in preferred disparity also occurred in several neurons. These neural changes were most often present at the beginning of the visual response, suggesting a feedforward gain control by eye position signals. Cortical neural processes for encoding information about the three-dimensional position of a stimulus in space therefore start as early as area V1.

Neural processing of stereopsis as a function of viewing distance in primate visual cortical area V1.

1. The influence of viewing distance on disparity selectivity was investigated in area V1 of behaving monkeys. While the animals performed a fixation task, cortical cells were recorded extracellularly in the foveal representation of the visual field. Disparity selectivity was assessed by using static random dot stereograms (RDSs) through red/green filters flashed over the central fixation target. To determine the influence of the viewing distance, a color video monitor was positioned at fixed distances of 20, 40, or 80 cm. The same RDSs with the same angular size of dots were used at the three distances. 2. Disparity sensitivity was tested on 139 cells, of which 78 were analyzed at two or more distances and the rest (61) at a single distance. When disparity selectivity was analyzed at a given distance, about half the cells were found to be selective at 40 or 80 cm, but only a third at 20 cm. Near cells were > or = 1.5 times more common than far cells at all three distances. The latency distribution of the responses of disparity-selective (DS) cells was similar at all three distances, with a mean distribution centered around 60 ms. 3. Changing the viewing distance drastically affected the neural activity of the V1 neurons. The visual responsiveness of 60 of 78 cells (77%) was significantly changed. Disparity selectivity could be present at a given distance and absent at other(s), with often a loss of visual response. This emergence of disparity coding was the strongest effect (28 of 78 or 36%) and occurred more frequently from short to long distances. Among the cells that remained disparity insensitive at all recorded distances (31 of 78 or 40%), about half also showed modulations of the amplitude of the visual response. For cells that remained DS at all recorded distances (13 of 78 or 17%), changing the viewing distance also affected the sharpness (or magnitude) of disparity coding in terms of level of visual responsiveness and those changes were often combined with variations in tuning width. In only two cells did the peak of selectivity type change. Finally, the activity of four DS cells was not affected at all by the viewing distance. 4. Another effect concerned the level of ongoing activity (OA), defined as being the neural activity in darkness preceding the flash of the visual stimulus while the monkey was fixating the small bright target. Changing the viewing distance resulted in significant changes in OA level for more than half of the cells (41 of 78 or 53%). The most common effect was an increase in OA level at the shorter distance. The modulations of both visual responsiveness and OA could occur simultaneously, although they often had opposite signs. Indeed, the two effects were statistically independent of each other, i.e., modulations of visual responses were not related to the level of excitability of the neurons. 5. Control experiments were performed that showed that the effects of changing the viewing distance were not due to the retinal patterns in that the modulations of visual responsiveness were independent of the dot density. Seventeen cells were also tested for a possible effect of vergence by the use of prisms. When there was an effect of distance, it could be closely or partially reproduced by using prisms. These controls, together with the effects observed on OA, strongly suggest that the modulations of neural activity of the V1 neurons by the viewing distance are extraretinal in origin, probably proprioceptive. 6. The modulation of visual responsiveness by the viewing distance in the primary visual cortex indicates that integration of information from both retinal and extraretinal sources can occur early in the visual processing pathway for cortical representation of three-dimensional space. A functional scheme of three-dimensional cortical circuitry is discussed that shows cortical areas where disparity selectivity and modulations of visual activity by the angle of gaze have been described so far.

Cortical representation of visual three-dimensional space.

Perception of real depth includes information on stereopsis and distance. How both interact in the visual pathway was the subject of a study performed on the behaving monkey. Neurons in the primary visual cortex (area V1) have their activity, visual and/or spontaneous, modulated by the viewing distance. Disparity selectivity may be present or better expressed at a given viewing distance. This modulation is independent of the visual pattern. The use of prisms shows that vergence is implicated in this phenomenon. Consequently, extraretinal signals related to ocular motility have access to area V1. Among them, proprioceptive signals from the eye muscles have been shown to be involved in visual cortical function and in the development of depth perception. It is possible that the same signals may also be involved in the distance modulation shown in V1 neurons, but this remains to be examined. A possible specialisation of disparity-selective cells in different cortical areas is discussed.

Dynamics of orientation coding in area V1 of the awake primate.

To investigate the importance of feedback loops in visual information processing, we have analyzed the dynamic aspects of neuronal responses to oriented gratings in cortical area V1 of the awake primate. If recurrent feedback is important in generating orientation selectivity, the initial part of the neuronal response should be relatively poorly selective, and full orientation selectivity should only appear after a delay. Thus, by examining the dynamics of the neuronal responses it should be possible to assess the importance of feedback processes in the development of orientation selectivity. The results were base on a sample of 259 cells recorded in two monkeys, of which 89% were visually responsive. Of these, approximately two-thirds were orientation selective. Response latency varied considerably between neurons, ranging from a minimum of 41 ms to over 150 ms, although most had latencies of 50-70 ms. Orientation tuning (defined as the bandwidth at half-height) ranged from 16 deg to over 90 deg, with a mean value of around 55 deg. By examining the selectivity of these different neurons by 10-ms time slices, starting at the onset of the neuronal response, we found that the orientation selectivity of virtually every neuron was fully developed at the very start of the neuronal response. Indeed, many neurons showed a marked tendency to respond at somewhat longer latencies to stimuli that were nonoptimally oriented, with the result that orientation selectivity was highest at the very start of the neuronal response. Furthermore, there was no evidence that the neurons with the shortest onset latencies were less selective. Such evidence is inconsistent with the hypothesis that recurrent intracortical feedback plays an important role in the generation of orientation selectivity. Instead, we suggest that orientation selectivity is primarily generated using feedforward mechanisms, including feedforward inhibition. Such a strategy has the advantage of allowing orientation to be computed rapidly, and avoids the initially poorly selective neuronal responses that characterize processing involving recurrent loops.

Long-term dysfunctions of neural stereoscopic mechanisms after unilateral extraocular muscle proprioceptive deafferentation.

1. Neural correlates of the permanent deficits in depth perception that occur when extraocular muscle proprioceptive (EMP) afferents are interrupted unilaterally in kittens were investigated by performing extracellular recordings in the primary visual cortex (area 17) in adulthood. Unilateral section of the ophthalmic branch of the trigeminal nerve (V1 nerve) were performed in 11 cats when they were between 5 and 12 weeks of age (uni-V1 group). Electrophysiological results were compared with those obtained in 17 normal adult cats (control group). 2. Binocular interactions were assessed by testing the sensitivity of cortical neurons to dichoptic presentations of moving sine-wave gratings whose interocular positional phase relationship was randomly varied. The amplitude modulation between the minimum and the maximum binocular responses defined the dynamic range. The degree of binocular suppression or facilitation was assessed by comparing these binocular response limits with the optimal monocular responses evoked through either eye at the best spatial frequency. The variability of both monocular and binocular responses was estimated by using the variation coefficient. 3. In uni-V1 cats, both the dynamic range and the degree of binocular suppression were significantly less pronounced than in controls, whereas binocular facilitation was not affected. The variability of the binocular responses was significantly increased, unlike monocular responses, whose variability was similar to control values. 4. From Fourier analysis of the poststimulus time histograms, two clear-cut categories of cells emerged that were differentially affected in the uni-V1 group. The "modulated" cells showed significantly less binocular suppression than in controls, and the "unmodulated" cells had binocular responses that were significantly more variable than in controls. Results from "simple" cells were similar to those of modulated cells, and results from "complex" cells were similar to those of unmodulated cells. However, in the unmodulated population, which was composed of both simple and complex cells, it was shown that the increase of variability was due to that of complex cells. 5. A nonparametric statistical test was applied on the interocular phase shift tuning curves to determine the minimum stimulus change necessary to elicit a significant change in the neural response. Two categories of cells were determined: the "discriminative" cells (80% in controls but 45% in uni-V1 cats) combined pronounced binocular suppression and dynamic range with relatively low variability. The reverse was true in the case of "nondiscriminative" cells (20% in controls and 55% in uni-V1 cats). 6. In uni-V1 cats, about half of the cells were monocularly activated.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of neural stereoscopic processing in primate area V1 by the viewing distance.

Accurate binocular depth perception requires information about both stereopsis (relative depth) and distance (absolute depth). It is unclear how these two types of information are integrated in the visual system. In alert, behaving monkeys the responsiveness of a large majority of neurons in the primary visual cortex (area V1) was modulated by the viewing distance. This phenomenon affected particularly disparity-related activity and background activity and was not dependent on the pattern of retinal stimulation. Therefore, extraretinal factors, probably related to ocular vergence or accommodation, or both, can affect processing early in the visual pathway. Such modulations could be useful for (i) judging true distance and (ii) scaling retinal disparity to give information about three-dimensional shape.

Temporal limits of the susceptibility of depth perception to proprioceptive deafferentations of extraocular muscles.

In a previous study, extraocular muscle proprioception (E.O.M.P.) was shown to play an important role in the postnatal development of depth perception: following unilateral or bilateral sections of the ophthalmic branch of the trigeminal nerve (V1th nerve) performed at 6-8 weeks of age, the binocular thresholds were 2 to 3 times higher than in control animals. Since the V1-sections produced no deficits when performed in adults, the temporal limits of a period of susceptibility remained to be determined. In order to assess the lower and upper limits of the period during which these perceptual deficits could be induced, unilateral or bilateral V1-sections were performed in kittens at different ages. Depth perception thresholds were measured by using the jumping stand technique. Sections of the V1 nerve only produced significant impairments of the binocular depth thresholds when performed after 3 weeks of age. They could be observed when unilateral sections were performed at up to 13 weeks of age and with bilateral sections at up to 10 weeks of age. These functional impairments appeared to remain permanently through adult life.

Influences of uniform and textured backgrounds on the impulse activity of neurons in area V1 of the alert macaque.

The influences of the visual background on the spontaneous and evoked activity of neurons in the striate cortex (V1) of the awake and behaving macaque were investigated using uniform (dark and bright) and textured (dynamic random-dot) large fields (10 degrees) centered on the receptive field of the cell under study. Rhesus monkeys were trained to fixate a small target while visual patterns were presented on monitor displays and the impulse activity of single cortical neurons recorded extracellularly with metal microelectrodes. The discharge rates of the ongoing, spontaneous activity of the vast majority of V1 neurons, as well as their responses to optimally adjusted bar stimuli, were not significantly influenced by the luminance of a uniform background. On the other hand, the activity of more than 50% of V1 neurons was clearly affected by a textured background. Comparison of the effects of a uniformly dark background and a background of dynamic random dots showed that the neuron's spontaneous discharge rate was typically higher in the presence of the textured background, while the evoked response was often reduced in amplitude or even suppressed. The opposite effects were observed in only a few neurons. These findings indicate that neurons in area V1 are highly sensitive to a textured background of dynamic random dots which exert on them an activating effect, chiefly by stimulation of the neuron's receptive field, with consequent increase in the ongoing discharge and a reduction of the dynamic range of impulse activity, leading to a reduction in the amplitude of the response evoked by a contrast stimulus.

Neuronal stereoscopic processing following extraocular proprioception deafferentation.

In adult cats, after section of extraocular muscle proprioceptive (EOMP) afferents during the 'critical period', most cortical area 17 cells loose their ability to discriminate changes in binocular spatial disparity. After unilateral section this loss depends on whether or not cortical cells modulate their responses to the presentation of sinusoidal gratings linearly. For 'modulated cells', this loss is due to a reduction of binocular suppression while for 'unmodulated cells', it is due to a selective increase in the variability of the binocular response. These permanent neural dysfunctions show that balance in EOMP inflow plays a crucial role in cortical processing of binocular depth discrimination.

The period of susceptibility of visual cortical binocularity to unilateral proprioceptive deafferentation of extraocular muscles.

1. The electrophysiological effects of section of extraocular muscle proprioceptive afferents have been investigated in kitten area 17. Extraocular proprioceptive afferents were interrupted by cutting the ophthalmic branch of the fifth trigeminal nerve (V1 nerve) unilaterally in 15 normally reared kittens (NR) between 3 and 12 wk postnatal, in 3 NR adult cats, and in 7 dark-reared (DR) kittens at 6 wk postnatal. Bilateral sections of the V1 nerve were performed in two kittens at 6 wk postnatal. NR kittens were maintained in a normal environment after the section. DR kittens were returned to the darkroom until the recording session. Receptive-field properties of area 17 neurons were studied after a postsurgical delay of 4-7 wk in most NR kittens and of 4 days to 5 wk in DR kittens. In one NR kitten and in the operated adult cats, the delay was 1-2.5 yr. This study is based on a total sample of 1,190 visual cortical units. 2. When performed in NR kittens between 4 and 8 wk of age, the unilateral section of extraocular proprioceptive afferents significantly reduced the proportion of binocular cells: 1 mo after the section of the V1 nerve, half of the visual cortical cells were monocularly activated. A similar reduction in the proportion of binocular cells was also observed in DR kittens operated at 6 wk of age and then maintained in the dark for 5-7 wk. In contrast to the unilateral section, the bilateral section of the V1 nerve performed in 6-wk-old NR kittens did not disrupt cortical binocularity. 3. In 10 of the 22 kittens that had undergone unilateral sections, there was a strong asymmetry in the ocular dominance distribution in favor of one eye. This asymmetry was not related to the side of the section and was the same in both hemispheres for a given kitten. 4. The postsurgical delay played an important role in the appearance of the cortical deficit: binocularity loss was not found within the week following the section but was present 1 mo after the section. This functional impairment appeared to be permanent, since it was still observed 2.5 yr after the section. 5. Cortical cells were classified in two ways on the basis of their receptive-field organization: 1) into S- or C-types (38, 73), and 2) into small area slow (SAS), large area slow (LAS), or Fast (F)-types (42, 57).(ABSTRACT TRUNCATED AT 400 WORDS)

Role of extraocular muscle proprioception in the development of depth perception in cats.

1. The ophthalmic branch of the trigeminal nerve (V1), which carries extraocular proprioceptive afferents, was sectioned unilaterally or bilaterally in kittens and adult cats. Depth perception was measured behaviorally in these sectioned cats, as well as in control cats. 2. For kittens that underwent unilateral V1 sections at 6-11 wk of age, postsurgical values of binocular depth perception--measured 1.5-3 mo later--were 2-3 times worse than in normal control animals. Cats that underwent unilateral V1 sections as adults, however, showed no postsurgical deficits in binocular depth perception. 3. For kittens that underwent bilateral V1 sections at 6.5-7.5 wk of age, similar longterm impairments were found in binocular depth perception. No impairment was found in two kittens bilaterally sectioned at 11.5 wk of age. A cat that underwent bilateral sections as an adult also showed no binocular depth perception deficits. 4. Although these behavioral effects were observed only when unilateral and bilateral V1 sections were performed up to a certain age in development, they differed in two ways. 1) Imbalance of extraocular proprioceptive inflow produced by unilateral section had a deleterious effect at an age when the final adult level had been reached. At that stage, complete suppression of inflow produced by the bilateral section failed to impair the final level of binocular performance. 2) Short-term effects observed during the week following the section appeared in bilaterally operated animals as a transient freezing of the presurgical binocular performance whatever the age of the section during the sensitive period. In contrast, short-term effects produced by unilateral section were found to be age dependent: a progressive slowing down in the normal rate of improvement of binocular thresholds was observed following a section performed at 5 wk of age; an arrest in development was found when surgery was done at 6-7 wk of age. A significant impairment appeared within 2 days when the section was performed at 11 wk of age. 5. In all experimental kittens, monocular depth perception thresholds were unaffected or impaired only to a minor extent (less than 15% change) following the unilateral or bilateral section. In unilaterally operated kittens, there were no consistent differences associated with the side of the section. 6. A sham-operated kitten, in which the V1 was visualized but not cut, showed no impairments in binocular or monocular depth perception.(ABSTRACT TRUNCATED AT 400 WORDS)

Noradrenaline and functional plasticity in kitten visual cortex: a re-examination.

A quantitative re-examination was made of the influence of noradrenergic depletion on the epigenesis of kitten visual cortex. Two methods were used to deplete noradrenaline at the cortical level: stereotaxically controlled injection of 6-hydroxydopamine (6-OHDA) in the coeruleus complex, from which the noradrenergic input to visual cortex arises; intraventricular injection of 6-OHDA. The latter chemical lesion also depleted dopamine levels in the brain. Lesion of the noradrenergic or catecholaminergic systems was performed neonatally or at an age of 3-4 weeks in kittens submitted to five different rearing procedures: normal rearing, dark rearing, monocular rearing, monocular exposure following dark rearing and monocular deprivation following normal rearing. Forty-two kittens between 3 and 12 weeks of age were used for this biochemical and electrophysiological study. Noradrenaline and dopamine levels were measured by a radioenzymatic method in the primary visual cortex of twenty-six kittens. A total of 1263 cells were recorded in area 17 of twenty-six kittens. Combined biochemical and electrophysiological data were obtained in ten 6-OHDA-lesioned kittens. Whatever the mode of chemical lesion used, cortical noradrenergic depletion failed to block either maturation or vision-dependent processes which are known to affect orientation selectivity and/or ocular dominance during the critical period. However, in some cases, the amplitude of the epigenetic functional modifications was slightly reduced in 6-OHDA-treated kittens. The cortical effects of monocular deprivation starting from the age of 5 weeks were studied quantitatively both in lesioned and intact kittens. Disappearance of noradrenaline in area 17 did not prevent the loss of binocularity in cortical cells. However, even when monocular occlusion had been maintained for 2 or 3 weeks in 6-OHDA-treated kittens, ocular dominance shifts were limited to a stage equivalent to that observed in the intact kitten after 5-8 days of monocular occlusion. The amplitude of this partial protective effect was found to be unrelated either to the delay following the chemical lesion, or to the level of noradrenaline remaining in lesioned kitten cortex. Although a putative gating role of noradrenaline cannot be excluded in the development of the intact animal, this report shows that its presence is not required for functional plasticity to occur in kitten area 17.

Responses of neurons in visual cortex (V1 and V2) of the alert macaque to dynamic random-dot stereograms.

A substantial proportion of both simple and complex neurons in the cortex subserving central vision are differentially sensitive to binocular disparity of isolated line patterns (local stereopsis), a sensitivity based on a positional disparity between the neuron's receptive fields in the two eyes. In addition, a subset of cortical neurons, nearly all complex neurons, responds to dynamic random-dot stereograms containing no depth cues other than disparity. These neurons are capable of signaling the correct binocular matches among a multitude of false matches in the stereograms (global stereopsis). The discovery of cyclopean neurons in striate cortex, at early stages of the processing neural network for stereoscopic vision provides a new insight of the basic neural mechanisms underlying binocular depth perception.

[Synergy of vision and extraocular proprioception in the mechanisms of functional plasticity of the primary visual cortex in the kitten]. / Synergie de la vision et de la proprioception extraoculaire dans les mécanismes de plasticité fonctionnelle du cortex visuel primaire du chaton.

In 6 week old dark reared kittens, a visual monocular exposure for 6 hrs. elicits major functional modifications in the primary visual cortex. Most cells become orientation selective and are activated through the open eye only. However, if a unilateral section of the ophthalmic branch of the fifth nerve is performed 1 week before the monocular exposure, these modifications are less pronounced, particularly when carried out on the same side as the open eye. Moreover, if the ophthalmic branch has been cut bilaterally, visual monocular exposure has almost no effect: cortical cells remain non selective to orientation and binocularly activated.

[Noradrenaline and plasticity of the visual cortex of the kitten: a reexamination]. / Noradrénaline et plasticité du cortex visuel du Chaton: un réexamen.

We have undertaken a study of the role of the noradrenergic system in the functional modifications, observed in the primary visual cortex of the Kitten, following monocular deprivation. The lids of one eye were sutured in 5 week old Kittens for a period of 1 or 2 weeks. Noradrenergic depletion was obtained by 6-OHDA injection, either intraventricular or localized in the coeruleus complex. Our results indicate that disappearance of noradrenaline in area 17 does not prevent the loss of binocularity of cortical cells, but appears to limit ocular dominance shifts at a stage equivalent to that observed in the intact Kitten after 6 days of monocular deprivation.

[Period of sensitivity of the primary visual cortex of the cat to the unilateral suppression of extraocular proprioceptive afferents]. / Période de sensibilité du cortex visual primaire du chat à la suppression unilatérale des afférences proprioceptives extraoculaires.

Unilateral suppression of extraocular proprioceptive afferents, which run through the ophthalmic branch of the Vth nerve, disrupted binocular integration in the primary visual cortex of normally reared kittens, when it was performed between 4 and 8 weeks of postnatal age. One month later, half the visual cortical cells were exclusively activated through one eye. A similar disorganization of binocularity was observed in visually deprived Kittens when unilateral lesion was performed at 6 weeks of age. We conclude that the loss of binocular integration is caused by non visual factors, probably related to asymmetry between proprioceptive informations from each eye.

Recovery of orientation selectivity in kitten primary visual cortex is slowed down by bilateral section of ophthalmic trigeminal afferents.

The recovery of orientation selectivity in the primary visual cortex has been studied in 6-week-old dark-reared (DR) kittens after visual exposure of various durations following bilateral section of either the ophthalmic (V1) or the maxillary (V2) branches of the Vth nerve. After 6 h of vision, visual cortical neurones become orientation selective in V2-operated kittens as well as in intact animals, while they remain non-specific in V1-operated kittens. However, in this latter case, if the duration of visual exposure is extended to 4 weeks, a slow and incomplete recovery of orientation selectivity takes place.