Depth perception, eye alignment and cortical ocular dominance of dark-related cats

Silent Synapse-Based Mechanisms of Critical Period Plasticity

Critical periods are postnatal, restricted time windows of heightened plasticity in cortical neural networks, during which experience refines principal neuron wiring configurations. Here, we propose a model with two distinct types of synapses, innate synapses that establish rudimentary networks with innate function, and gestalt synapses that govern the experience-dependent refinement process. Nascent gestalt synapses are constantly formed as AMPA receptor-silent synapses which are the substrates for critical period plasticity. Experience drives the unsilencing and stabilization of gestalt synapses, as well as synapse pruning. This maturation process changes synapse patterning and consequently the functional architecture of cortical excitatory networks. Ocular dominance plasticity (ODP) in the primary visual cortex (V1) is an established experimental model for cortical plasticity. While converging evidence indicates that the start of the critical period for ODP is marked by the maturation of local inhibitory circuits, recent results support our model that critical periods end through the progressive maturation of gestalt synapses. The cooperative yet opposing function of two postsynaptic signaling scaffolds of excitatory synapses, PSD-93 and PSD-95, governs the maturation of gestalt synapses. Without those proteins, networks do not progress far beyond their innate functionality, resulting in rather impaired perception. While cortical networks remain malleable throughout life, the cellular mechanisms and the scope of critical period and adult plasticity differ. Critical period ODP is initiated with the depression of deprived eye responses in V1, whereas adult ODP is characterized by an initial increase in non-deprived eye responses. Our model proposes the gestalt synapse-based mechanism for critical period ODP, and also predicts a different mechanism for adult ODP based on the sparsity of nascent gestalt synapses at that age. Under our model, early life experience shapes the boundaries (the gestalt) for network function, both for its optimal performance as well as for its pathological state. Thus, reintroducing nascent gestalt synapses as plasticity substrates into adults may improve the network gestalt to facilitate functional recovery.

tDCS recovers depth perception in adult amblyopic rats and reorganizes visual cortex activity

Amblyopia or lazy eye is a neurodevelopmental disorder that arises during the infancy and is caused by the interruption of binocular sensory activity before maturation of the nervous system. This impairment causes long-term deterioration of visual skills, particularly visual acuity and depth perception. Although visual function recovery has been supposed to be decreased with age as consequence of reduced neuronal plasticity, recent studies have shown that it is possible to promote plasticity and neurorestoration in the adult brain. Thus, transcranial direct current stimulation (tDCS) has been shown effective to treat amblyopia in the adulthood. In the present work we used postnatal monocular deprivation in Long Evans rats as an experimental model of amblyopia and the cliff test task to assess depth perception. Functional brain imaging PET was used to assess the effect of tDCS on cortical and subcortical activity. Visually deprived animals ability to perceive depth in the cliff test was significantly reduced in comparison to their controls. However, after 8 sessions of tDCS applied through 8 consecutive days, depth perception of amblyopic treated animals improved reaching control level. PET data showed 18F-FDG uptake asymmetries in the visual cortex of amblyopic animals, which disappeared after tDCS treatment. The possibility of cortical reorganization and stereoscopy recovery following brain stimulation points at tDCS as a useful strategy for treating amblyopia in adulthood. Furthermore, monocular deprivation in Long Evans rats is a valuable research model to study visual cortex mechanisms involved in depth perception and neural restoration after brain stimulation.

An opposing function of paralogs in balancing developmental synapse maturation

The disc-large (DLG)-membrane-associated guanylate kinase (MAGUK) family of proteins forms a central signaling hub of the glutamate receptor complex. Among this family, some proteins regulate developmental maturation of glutamatergic synapses, a process vulnerable to aberrations, which may lead to neurodevelopmental disorders. As is typical for paralogs, the DLG-MAGUK proteins postsynaptic density (PSD)-95 and PSD-93 share similar functional domains and were previously thought to regulate glutamatergic synapses similarly. Here, we show that they play opposing roles in glutamatergic synapse maturation. Specifically, PSD-95 promoted, whereas PSD-93 inhibited maturation of immature α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type glutamate receptor (AMPAR)-silent synapses in mouse cortex during development. Furthermore, through experience-dependent regulation of its protein levels, PSD-93 directly inhibited PSD-95's promoting effect on silent synapse maturation in the visual cortex. The concerted function of these two paralogs governed the critical period of juvenile ocular dominance plasticity (jODP), and fine-tuned visual perception during development. In contrast to the silent synapse-based mechanism of adjusting visual perception, visual acuity improved by different mechanisms. Thus, by controlling the pace of silent synapse maturation, the opposing but properly balanced actions of PSD-93 and PSD-95 are essential for fine-tuning cortical networks for receptive field integration during developmental critical periods, and imply aberrations in either direction of this process as potential causes for neurodevelopmental disorders.

Recovery of visual functions in amblyopic animals following brief exposure to total darkness

KEY POINTS

Occlusion of one eye of kittens (monocular deprivation) results in a severe and permanent loss of visual acuity in that eye, which parallels closely the vision loss characteristic of human amblyopia. We extended earlier work to demonstrate that amblyopic vision loss can be either blocked or erased very fast by a 10 day period of total darkness following a period of monocular deprivation that begins near birth and extends to at least 8 weeks of age. The parameters of darkness were strict because no visual recovery was observed after 5 days of darkness. In addition, short periods of light introduced each day during an otherwise 10 day period of darkness obliterated the benefits. Despite recovery of normal visual acuity, only one-quarter of the animals showed evidence of having attained normal stereoscopic vision. A period of total darkness may catalyse and improve treatment outcomes in amblyopic children. A 10 day period of total darkness has been shown to either block or erase the severe effects on vision of a prior short period of monocular deprivation (MD) in kittens depending on whether darkness is contiguous or is delayed with respect to the period of MD. We have extended these earlier findings from kittens for which the period of MD began at 1 month and lasted for 1 week to more clinically relevant situations where MD began near birth and lasted for ≥ 6 weeks. Despite the far longer MD and the absence of prior binocular vision, all animals recovered normal visual acuity in the previously deprived eye. As before, when the period of darkness followed immediately after MD, the vision of both eyes was initially very poor but, subsequently, the acuity of each eye increased gradually and equally to attain normal levels in ∼ 7 weeks. By contrast, when darkness was introduced 8 weeks after MD, the visual acuity of the deprived eye recovered quickly to normal levels in just 1 week without any change in the vision of the fellow (non-deprived) eye. Short (15 or 30 min) periods of illumination each day during an otherwise 10 day period of darkness obliterated all the benefits for vision, and a 5 day period of darkness was also completely ineffective. Measurements of depth perception indicated that, despite possessing normal visual acuity in both eyes, only about one-quarter of the animals showed evidence of having attained normal stereoscopic vision.

Binocular eyelid closure promotes anatomical but not behavioral recovery from monocular deprivation

Deprivation of patterned vision of frontal eyed mammals early in postnatal life alters structural and functional attributes of neurones in the central visual pathways, and can produce severe impairments of the vision of the deprived eye that resemble the visual loss observed in human amblyopia. A traditional approach to treatment of amblyopia has been the occlusion of the stronger fellow eye in order to force use of the weaker eye and thereby strengthen its connections in the visual cortex. Although this monocular treatment strategy can be effective at promoting recovery of visual acuity of the amblyopic eye, such binocular visual functions as stereoscopic vision often remain impaired due in part to the lack of concordant vision during the period of unilateral occlusion. The recent development of binocular approaches for treatment of amblyopia that improve the possibility for binocular interaction have achieved success in promoting visual recovery. The full and rapid recovery of visual acuity observed in amblyopic kittens placed in complete darkness is an example of a binocular treatment whose success may in part derive from a restored balance of visually-driven neural activity. In the current study we examined as an alternative to dark rearing the efficacy of binocular lid suture (BLS) to stimulate anatomical and visual recovery from a preceding amblyogenic period of monocular deprivation. In the dorsal lateral geniculate nucleus (dLGN) of monocularly deprived kittens, darkness or BLS for 10days produced a complete recovery of neurone soma size within initially deprived layers. The growth of neurone somata within initially deprived dLGN layers after darkness or BLS was accompanied by an increase in neurotrophin-4/5 labeling within these layers. Although anatomical recovery was observed in both recovery conditions, BLS failed to promote any improvement of the visual acuity of the deprived eye no matter whether it followed immediately or was delayed with respect to the prior period of monocular deprivation. Notwithstanding the lack of visual recovery with BLS, all animals in the BLS condition that were subsequently placed in darkness exhibited a substantial recovery of visual acuity in the amblyopic eye. We conclude that the balanced binocular visual input provided by BLS does not stimulate the collection of neural events necessary to support recovery from amblyopia. The complete absence of visually-driven activity that occurs with dark rearing evidently plays an important role in the recovery process.

Sight restoration

Cases of sight onset after extended periods of congenital blindness provide windows into visual development and brain plasticity. Such cases are extremely rare in the developed world. Here, we make the argument that in meeting a public health challenge in the developing world, that of providing treatment to curably blind children, we have the opportunity to have a beneficial impact on science and society simultaneously. A recent initiative, Project Prakash, is motivated by these twin goals. We briefly describe this effort, some of its early results, and also the caveats that need to be kept in mind when interpreting the findings.

The development and activity-dependent expression of aggrecan in the cat visual cortex

The Cat-301 monoclonal antibody identifies aggrecan, a chondroitin sulfate proteoglycan in the cat visual cortex and dorsal lateral geniculate nucleus (dLGN). During development, aggrecan expression increases in the dLGN with a time course that matches the decline in plasticity. Moreover, examination of tissue from selectively visually deprived cats shows that expression is activity dependent, suggesting a role for aggrecan in the termination of the sensitive period. Here, we demonstrate for the first time that the onset of aggrecan expression in area 17 also correlates with the decline in experience-dependent plasticity in visual cortex and that this expression is experience dependent. Dark rearing until 15 weeks of age dramatically reduced the density of aggrecan-positive neurons in the extragranular layers, but not in layer IV. This effect was reversible as dark-reared animals that were subsequently exposed to light showed normal numbers of Cat-301-positive cells. The reduction in aggrecan following certain early deprivation regimens is the first biochemical correlate of the functional changes to the γ-aminobutyric acidergic system that have been reported following early deprivation in cats.

Depth perception in monocularly deprived cats following part-time reverse occlusion

The behavioural effects of an early period of monocular deprivation can be extremely profound. However, it is possible to achieve a high degree of recovery, even to normal levels of visual acuity, by prompt imposition of certain regimes of part-time reverse occlusion where the initially non-deprived eye is occluded for only part of each day in order to allow a daily period of binocular visual exposure. In this paper we report on the depth perception of five monocularly deprived cats that had recovered normal visual acuity in both eyes following imposition of certain of the above occlusion regimes. Although three of the animals exhibited five- to sevenfold superiority of binocular over monocular depth thresholds, subsequent tests made on two of the animals revealed that they were unable to make stereoscopic discriminations with random-dot stereograms. Despite the recovery of normal visual acuity in both eyes, we conclude that these animals recover at best only local stereopsis.

Human strabismus: evaluation of the interhemispheric transmission time and hemiretinal differences using a reaction time task

Experimentally induced strabismus in visually immature cats leads to abnormal development of the posterior corpus callosum. This, in turn, should lead to abnormal interhemispheric integration of unilaterally presented visual information. To test whether strabismus produces deficits in the human commissural visual system, the interhemispheric transmission time (ITT) was compared in strabismic and normal subjects. Simple unimanual reaction times (RT) were tested in 30 subjects in response to a lateralized target presented monocularly at 4 degrees and 35 degrees nasally and temporally from the fovea along the horizontal meridian. This method was also used to examine the effect of strabismus on the central and peripheral portions of each hemiretina. The results showed that in strabismic subjects with or without amblyopia, the ITT did not differ significantly from normals at both eccentricities. In non-amblyopic strabismic patients, RTs in the central and peripheral portions of hemiretina were comparable to normals. However, a reduced speed of response was found in the central visual field (4 degrees) in the amblyopic eye. Our results suggest that the ITT is normal in strabismic subjects and that the longer RTs in the central portion of the nasal and temporal hemiretina of the amblyopic eye may be associated with the severe amblyopic condition.

Neurological and behavioral effects of a unilateral frontal cortical lesion in fetal kittens. II. Visual system tests, and proposing an "optimal developmental period" for lesion effects

Nine fetal kittens sustained removal of the left frontal cortex during the last third of gestation (E 43-55) and were compared to animals sustaining a similar lesion postnatally (P 8-14) as well as to intact littermates. Beginning after 6 months of age, the animals' visual field and depth perception were assessed. In addition, pupil size as well as eye alignment were measured. On two visual field tests the fetal-lesioned cats showed test dependent decrements for some angles of vision. In terms of depth perception, only the prenatal-lesioned animals showed a higher binocular threshold; they also showed ocular misalignment which may have contributed to their depth perception impairment. Moreover, these animals had a larger ipsilateral pupil. The neonatal-lesioned animals were like normal cats for all tests and measurements. We conclude that, as for the tests reported in the preceding paper, the outcome for visual related behaviors of a prenatal frontal cortical lesion in the cat is also worse than that of a similar lesion sustained neonatally. Dysgenetic anatomical changes of the visual system induced indirectly by the frontal lesion are proposed as a possible explanation for these age-at-lesion differences. Based on the present work as well as on the literature, we propose the existence of an "optimal developmental period" for the best behavioral and anatomical outcome of perinatal brain lesions. We argue that this concept fits contemporary data and can better explain the different age-at-lesion effects of brain injury across animals species than the "Kennard Principle" (or "infant-lesion effect").

Binocular depth perception following early experience with interocular torsional disparity

The relationship between the behavioral and physiological consequences of rearing with optically induced cyclotropia was assessed. Beginning at the age of 4 weeks, kittens wore goggles that rotated the visual field in opposite directions in each eye for several hours each day over a period of several weeks. The amounts of interocular rotation were 0 deg (control), 16 deg, and 32 deg. Subsequently, they were tested to determine their monocular and binocular depth thresholds and, in some cases, visual acuity. In several kittens recordings were also made from the visual cortex. Binocular performance of all kittens in the 0-deg condition and three out of six kittens in the 16-deg condition was comparable to, although slightly lower than, that of normally reared kittens. In contrast, none of the 32-deg kittens showed any evidence of the binocular superiority that would suggest the presence of stereopsis. Extracellular unit recordings from the visual cortex confirmed our earlier results with goggle-reared kittens. In 16-deg kittens, the distribution of the cells' preferred interocular disparities (IOD) in receptive-field orientation showed a compensating shift so that the mean matched the experienced rotational disparity. In the 32-deg kittens, binocularity was greatly disrupted and there was no compensatory shift in the IOD distribution. Two 32-deg kittens were afforded 3 years of subsequent normal visual experience. Both the behavioral and the physiological findings were unaffected by normal visual exposure in adulthood. Control measurements of acuity indicated that any deficits in depth perception were not due to reduced spatial-resolution abilities. The data indicate that the kitten visual system is able to maintain functional binocularity sufficient to subserve a moderate level of stereoacuity with interocular rotations of up to at least 16 deg.

Stereopsis in the cat: behavioral demonstration and underlying mechanisms

The neural substrates subserving stereopsis were investigated behaviorally and electrophysiologically in the cat. In one set of studies, we examined behaviorally the ability of normal cats to perceive depth on the sole basis of spatial disparity using random-dot stereograms. Results showed that the animals were able to carry out this discrimination. We then evaluated the contribution of the optic chiasm, the corpus callosum and the primary visual cortex to this function. Results indicated that: (1) chiasma transection drastically reduced the ability of the animals to solve the random-dot problem; (2) a callosal split had little or no effect on their ability to relearn the same discrimination; (3) a section of both the corpus callosum and optic chiasm abolished this ability; and (4) bilateral lesions of areas 17-18 also abolished it. In another set of studies, we examined electrophysiologically the properties of neurons in the various visual cortical areas where disparity-based depth discrimination processes are presumed to take place. We recorded from areas 17, 18 and 19 of normal and split-chiasm cats. Results showed that: (1) the primary visual cortex of the normal cat contained cells sensitive to stimulus disparity; (2) these disparity sensitive neurons were also present in area 19 although in a much lower proportion and were more widely tuned than those in areas 17-18; and (3) following the section of the optic chiasm, there was a significant decrease in the number of disparity sensitive cells in areas 17-18, whereas in area 19 they were nearly completely absent. The results obtained from the lesion studies and from the single unit recording experiments indicate that stereoscopic depth perception is highly dependent in the cat upon the integrity of the through-the-chiasm geniculo-striate pathway and its target primary visual cortex.

Depth perception and cortical physiology in normal and innate microstrabismic cats

Evidence is presented that innate microstrabismus and abnormal cortical visual receptive-field properties can occur also in cats without any apparent involvement of the Siamese or albino genetic abnormalities in their visual system. A possible cause for microstrabismus in these cats may be sought in an abnormally large horizontal distance between blind spot and area centralis indicated by a temporal displacement of the most central receptive fields on both retinae. Depth perception was found to be impaired in cats with innate microstrabismus. Behavioral measurements using a Y-maze revealed in four such cats that the performance in recognizing the nearer of two random-dot patterns did not improve when they were allowed to use both eyes instead of only one. The ability of microstrabismic cats to perceive depth under binocular viewing conditions only corresponded to the monocular performance of five normal cats. Electrophysiological recordings were performed in the visual cortex (areas 17 and 18) of four awake cats, two normal, and two innate microstrabismic animals. Ocular dominance and orientation tuning of single neurons in area 17 and 18 were analyzed quantitatively. The percentage of neurons in area 17 and 18 which could be activated through either eye was significantly reduced to 49.7% in the microstrabismic animals when compared to the normal cats (74.8%). "True binocular cells," which can only be activated by simultaneous stimulation of both eyes, were significantly less frequent (1.6%) in microstrabismic cats than in normal animals (10.4%). However, subthreshold binocular interactions were identical in both groups of animals. In the strabismic animals, long-term binocular stimulation of monocular neurons did not give a clear indication of alternating use of one or the other eye. The range of stimulus orientations leading to discharge rates above 50% of the maximal response, i.e. the half-width of the orientation tuning curves, was the same in the two groups of cats. However, orientation sensitivity, i.e. the alternation in discharge rate per degree change in stimulus orientation, was higher in cortical cells of normal cats than in those of microstrabismic cats. In normal and microstrabismic cats, no clear sign of an "oblique effect," i.e. the preference of cortical neurons for vertical and horizontal orientations compared to oblique orientations, could be found neither in the incidence of cells with horizontal or vertical preferred orientation nor in the sharpness of orientation tuning and sensitivity of these neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Long-term effects of dark rearing on a visually guided reaching movement in cats

The existence of long-term effects of dark-rearing on visuo-motor coordination is still controversial. In this study 2 dark-reared (DR) cats were trained, after 5-6 years of recovery, to perform a reaching movement towards a stationary or a moving target. The accuracy, and the reaction and movement times were evaluated. The scores obtained by the DR cats were compared to those of normal subjects after a similar period of training. The results showed that while the accuracy of DR cats was not impaired, the performance of their reaching movements was slower than normal and its triggering was delayed. These data are discussed with regards to electrophysiological and behavioural data obtained on analogous DR cats after long-term recovery.

Effects of brief monocular deprivation on binocular depth perception in the cat: a sensitive period for the loss of stereopsis

The period of susceptibility for binocular depth vision was studied in kittens by subjecting them to periods of monocular deprivation beginning at different ages. In an initial study, we found that normally reared kittens can learn a depth-discrimination task much more rapidly when tested binocularly than monocularly, even when testing is begun as early at 30 d. In subsequent experiments, kittens were monocularly deprived by eyelid suture, following which their monocular and binocular depth thresholds were measured using the jumping-stand procedure. We obtained the following results: (1) When monocular deprivation is applied before the time of natural eye opening but is discontinued by no later than 30 d, there is very little effect on binocular depth thresholds. (2) When deprivation is begun at 90 d, binocular depth thresholds are unaffected. (3) When deprivation is begun between these two ages, the magnitude of the deficit varies with the period of deprivation and the age at which it begins. (4) By imposing brief (5 or 10 d) periods of deprivation, beginning at different ages, we were able to demonstrate that the peak of the sensitive period is between the ages of 35 and 45 d, with a fairly rapid decline in susceptibility outside those age limits. (5) Even with as little as 5 d of deprivation, substantial permanent deficits in binocular depth vision can be induced.

Properties of area 17/18 border neurons contributing to the visual transcallosal pathway in the cat

In a series of physiological experiments, a total of 203 neurons at the Area 17/18 border were recorded with a callosal link either demonstrated by antidromic or transsynaptic activation from stimulating electrodes located in the homotopic contralateral hemisphere (CH), or in the splenial segment of the corpus callosum (CC). Forty-four percent of the transcallosal cells could also be driven from stimulating electrodes in or just above the lateral geniculate nucleus (OR1). The majority (69%) of transcallosal neurons were classifiable as belonging to the complex family (B and C cells) and most of these were found in the supragranular laminae and in lamina 4A. The ocular dominance distribution of transcallosal cells was trimodal, consisting of roughly equal numbers of monocularly dominated and binocularly balanced neurons. Estimates of conduction time and synaptic delay were obtained for neurons driven from CH, CC, and from OR1, and in most instances the response latency was short enough to suggest a monosynaptic input from either the ipsi- or contra-lateral hemisphere. The distribution of transcallosal conduction times showed that S cells, as a class, had significantly faster conduction than cells of the complex family but otherwise there was no obvious signs of multimodality in the distribution curve. An analysis of the synaptic delays in transcallosal activation produced a mean of 0.6 to 0.7 ms but some were too short to be consistent with a transsynaptic drive, suggesting that some cells with an antidromic drive may have been included in the transsynaptic category. Results are interpreted in terms of the contribution made by the corpus callosum to stereoscopic vision.

Depth perception in cats after cerebral hemispherectomy: comparisons between neonatal- and adult-lesioned animals

Depth perception was studied in adult cats following removal of the left cerebral hemisphere as a neonate or as an adult. Both monocular and binocular thresholds were determined using a visual cliff. Although both age-at-lesion groups showed depth perception deficits, the neonatal-lesioned animals performed much worse under binocular conditions on the visual cliff than either adult-lesioned or intact animals. This was primarily due to the lack of a binocular advantage in the neonatal-lesioned cats since their monocular thresholds were similar to that of adult-lesioned animals. Both lesioned groups showed higher monocular thresholds compared to intact animals but this effect reached significance only for the right eye. In addition, the neonatal-lesioned cats showed ocular misalignment which may have contributed to their lack of binocular depth perception. Regardless of these deficits neonatal-lesioned cats were more like intact controls regarding the types of errors made on the visual cliff. Neonatal-lesioned animals and intact controls made random errors, whereas adult-lesioned animals made most of their errors when the shallow shelf was presented on the animals' right side. This may indicate that the adult-lesioned animals have greater motor and/or visual field biases than do neonatal-lesioned cats.

Visual field in dark-reared cats after an extended period of recovery

The visual field of dark-reared cats was behaviourally measured after several years of recovery in a normal environment. A reduction of the visual field was observed and affected the contralateral field as well as the ipsilateral field when tested in monocular viewing. The longer the deprivation period, the more reduced was the visual field. Our results suggest that binocular deprivation might have stabilized the visual system in an immature state.

Role of visual experience in promoting segregation of eye dominance patches in the visual cortex of the cat

Transneuronal autoradiography was used to study the role of visual experience in the development of ocular dominance patches in the cat. In order to assess quantitatively the effects of visual deprivation, image analysis was used to measure the profiles of grain density in layer IV. Fourier power spectra of these profiles were computed to give a measure of the amplitudes and frequencies of the fluctuations in grain density that were present. Deprivation of normal patterned vision by binocular lid suture or by rearing in total darkness from shortly after birth abolished the dominant periodicity (of about 1.1 mm) in the distribution of left and right eye afferents in layer IV of area 17. A dominant periodicity of about 2.2 mm was, however, present in area 18 of both normal and dark-reared animals. Visual deprivation was not able to reverse segregation. One animal reared normally for 6 weeks was placed in the dark for a further 28 weeks and showed normal periodicities in the distribution of geniculate inputs to area 17. Another animal given 128 hours of experience and kept in the dark for the rest of the time until it was 12 weeks old also showed normal segregation. To determine the minimum amount of visual experience necessary for segregation to occur, four animals were given 8-, 24-, 48-, and 128-hour periods of visual experience and were studied at 12 weeks of age. Eight hours of experience had no detectable effect on segregation; periodicities of intermediate amplitude were present in animals that received 24 and 48 hours of experience, while 128 hours of experience resulted in periodicities of normal amplitude. Recovery from visual deprivation was studied by rearing kittens from birth in the dark for varying periods and then returning them to the normally lit colony room for periods of 6 to 22 weeks. Recovery from 6 weeks of dark rearing was found to be complete; much less recovery occurred following periods of 8 to 25 weeks of initial deprivation, and no recovery at all occurred after 30 weeks of deprivation. It is concluded that the spontaneous activity present in the geniculocortical afferents of dark-reared and lid-sutured cats is not adequate to drive normal periodic segregation in area 17, though it can do so in area 18. Between 48 and 128 hours of visual experience, given before 8 weeks of age, appears to be necessary and sufficient for normal periodic segregation of geniculate afferents in area 17 of the cat.

The cat as a model for visual deprivation

Both behavioural and neurophysiological changes can be observed in cats that have experienced interference with their normal visual environment. This visual deprivation may result from alterations to the path of light forming the normal image on the retina, and includes changes that cause the image to fall on an inappropriate part of the retina so that normal binocular interactions are affected. While some neurophysiological changes can be observed at the level of the lateral geniculate nucleus they become more prominent as information reaches the visual cortex, where cells commonly receive neural excitation from both eyes and require the information to come from corresponding parts of the two retinas and that the stimulus should have appropriate orientation and direction of movement. Many of the observations of deprivation in animals have clear parallels in the human environment.

Dark rearing prolongs physiological but not anatomical plasticity of the cat visual cortex

Recent studies (Cynader and Mitchell, '80; Mower et al., '81) have shown that total dark rearing prolongs susceptibility to the physiological effects of monocular deprivation (MD) in visual cortex beyond the normal age limits. The present study addressed whether this delayed physiological plasticity is accompanied by delayed anatomical plasticity in the geniculocortical pathway. Ocular dominance (OD) columns as defined by transsynaptic autoradiography following injection of 3H proline into one eye were studied both qualitatively and quantitatively in 17 cats. Compared to normal rearing (N-3), both binocular eyelid suture (N-2) and total dark rearing (N-3) resulted in incomplete segregation of OD columns in area 17. This apparent immaturity after binocular deprivation, however, did not reflect a delayed capacity for development and plasticity. Visual experience after dark rearing produced no marked changes. In cats who experienced MD after dark rearing, injection of either the nondeprived (N-2) or deprived eye (N-3) resulted in a nearly uniform distribution of label throughout layer IV of area 17. The same result occurred with binocular vision after dark rearing (N-1). MD from birth, however, produced expansion of columns from the nondeprived eye (N-1) and contraction of columns from the deprived eye (N-1). MD imposed after 4 months of normal vision resulted in normal OD columns (N-1). Electrophysiological studies revealed a high proportion of binocular cells within layer IV in cats who experienced monocular or binocular vision after dark rearing. Outside of layer IV there were clear environmental effects on OD of single cells in these cats. Measurements of cell sizes in the clateral geniculate nucleus showed shrinkage of cells innervated by the deprived eye when MD was initiated at birth (N-3). MD after dark rearing (N-4) produced no differences in cell sizes. It is concluded that visual input is necessary for the formation of normal OD columns, the critical period for formation and environmental modification of OD columns is limited to early life, and the physiological effects of visual experience after dark rearing reflect changes occurring beyond the geniculocortical pathway.

Deafferentation of oculomotor proprioception affects depth discrimination in adult cats

Depth discrimination was tested behaviourally in adult cats prior to and after chronic section of the ophthalmic branch of the Vth cranial nerve, that contains the majority of oculomotor proprioceptive fibers. Binocular depth discrimination was considerably impaired following either unilateral or bilateral oculomotor proprioceptive deafferentation.

Survival of early monocular deprivation effects in cortical cells of kittens following prolonged dark rearing

To study whether early visual experience survives the absence of consequent visual stimulation during development, experimental kittens were reared in the dark for 5-13.5 months following monocular deprivation (MD) periods of 2-11 weeks which were initiated at the time of natural eye opening (MD-dark). For comparison, experimental kittens, normally reared after equivalent MD periods (MD-bino.), were also studied. Cats raised with permanent MD, dark-reared cats and normal cats, served as controls. The proportion of responsive cells was considerably reduced by the dark-rearing. It was especially reduced for the MD-dark kittens following monocular deprivation limited to the first postnatal month (58.3% responsive cells) in comparison to the equivalent group of MD-bino. kittens (80.5%). This is also in keeping with the diminution in cortical responsiveness obtained in the kittens which were dark-reared from birth (55.5%). The responsiveness level found in the normal control cats was 87.3%. It was found that the duration of the MD period prior to the dark-rearing period was directly related to the ocular dominance (OD) distribution of cortical cells. The susceptibility period to MD in both MD-dark and MD-bino. groups ends at approximately 3 months of age; the lower limit for the susceptibility period is at approximately 1-2 weeks after natural eye opening. The main period of sensitivity within this period of time is the first 4 postnatal weeks following natural eye opening. It is concluded that once the effect of monocular deprivation has been established, it will survive for the rest of the cat's life, even under conditions of complete absence of consequent visual stimulation. Furthermore, a certain degree of consolidation of the MD effect takes place in the light (i.e. in MD-bino. cats) despite their return to normal binocular vision. A somewhat opposite occurrence takes place in the dark (in MD-dark cats) with a tendency for masking of the MD effect previously induced in the light to be found.

The existence of a separate, brief critical period for the corpus callosum to affect visual development

The critical period for the role of the corpus callosum in visual development was explored in terms of the length of time during which the callosum has any influence on the development of visual acuity. Cats were given a surgical section of the posterior corpus callosum at 1, 2, 3, 4 and 29 weeks of age. These, as well as normal and operated control cats, were behaviorally tested for visual acuity thresholds from 5 through 29 weeks of age. Only the 1, 2 and 3 week callosum-sectioned cats showed deficits in visual acuity; the 4 and 29 week callosum-sectioned cats had acuity thresholds equivalent to those of the control cats. These results define a relatively brief critical period of time during which the corpus callosum effectively interacts with the developing visual system. This critical period ends during the fourth postnatal week; throughout the first postnatal month the corpus callosum's effectiveness in altering subsequent visual development gradually declines. Using the amount of acuity deficit as the measure of alteration of visual development, the input of the corpus callosum during the first postnatal month is as critical to visual development as is normal visual input during the first 4-6 postnatal months.

Development of vernier acuity in infants

The development of vernier acuity in human infants aged 2-9 months was assessed by a preferential looking procedure using a vernier-motion display. The displacement of vernier offsets gives the impression of motion only when the vernier offsets are detected. Grating acuity of the same group of infants was also measured by a preferential looking procedure. Vernier acuity was found to be superior to grating acuity only after 3 months of age. This superiority of vernier acuity was compared with the superiority of stereoscopic acuity to grating acuity. The two classes of hyperacuity proved to be almost equivalent in terms of their developmental time-courses. A common physiological basis for the development of hyperacuities is suggested.

Prolonged sensitivity to monocular deprivation in dark-reared cats: effects of age and visual exposure

Previous studies have shown that the response properties of cortical cells of cats reared in darkness throughout the naturally-occurring critical period can still be modified by environmental manipulation. Here the effects of extremely prolonged dark-rearing, and of light exposure before or after dark-rearing on subsequent susceptibility to monocular deprivation are examined. Monocular deprivation remains effective in altering cortical ocular dominance even in cats which have been dark-reared for up to 2 years, long after the end of the naturally occurring critical period. The effects of monocular deprivation are, however, less pronounced in these cats than in animals dark-reared for shorter periods of time. The effects of visual exposure before dark-rearing were examined by allowing kittens 2 months of normal vision followed by 3 months of dark-rearing. Then, the effects of monocular suture were compared among these kittens and: (1) 2-month-old normal kittens (which had the same amount of visual exposure as the experimental animals); and (2) 5-month-old normal kittens (of the same age as the experimental animals). The results show that monocular deprivation is more effective in the experimental animals than in normal kittens of the same age, but less effective than in normal 2-month-old animals. The effects of visual exposure after dark-rearing were examined by allowing animals dark-reared for 4 months varied durations of binocular visual exposure before monocular suture. Susceptibility to monocular suture disappears about 6 weeks after the dark-reared animals are brought into the light.

The effects of early and late monocular deprivation on binocular depth perception in cats

Binocular and monocular depth discrimination thresholds were obtained from cats which had been monocularly deprived either from the time of natural eye opening or else at the age of 4 months. Among normal cats, binocular depth thresholds typically are very much better than monocular thresholds, allowing the inference that normal cats have good stereopsis. For the early-deprived animals in the present study, only those whose deprived eyes were opened by 30 days of age showed any binocular advantage. Deprivation periods lasting to 35 days or older completely eliminated the binocular superiority, with no sign of any recovery. These results provide behavioral evidence that binocular visual mechanisms are extremely susceptible to disruption and, unlike those underlying visual acuity, do not have the potential for recovery. The effect of deprivation imposed later in life was quite different. Three cats, deprived for 1, 2 or 3 months respectively, beginning at the age of 4 months, showed no deficits in binocular depth perception. This latter finding implies the existence of a sensitive period for stereopsis which is over completely by the age of 4 months.