Binocular deficits associated with early alternating monocular defocus. II. Neurophysiological observations

Experiencing binocularly conflicting signals early in life dramatically alters the binocular responses of cortical neurons. Because visual cortex is highly plastic during a critical period of development, cortical deficits resulting from early abnormal visual experience often mirror the nature of interocular decorrelation of neural signals from the two eyes. In the preceding paper, we demonstrated that monkeys that experienced early alternating monocular defocus (-1.5, -3.0, or -6.0 D) show deficits in stereopsis that generally reflected the magnitude of imposed monocular defocus. Because these results indicated that alternating monocular defocus affected the higher spatial frequency components of visual scenes more severely, we employed microelectrode recording methods to investigate whether V1 neurons in these lens-reared monkeys exhibited spatial-frequency-dependent alterations in their binocular response properties. We found that a neuron's sensitivity to interocular spatial phase disparity was reduced in the treated monkeys and that this reduction was generally more severe for units tuned to higher spatial frequencies. In the majority of the affected units, the disparity-sensitivity loss was associated with interocular differences in monocular receptive field properties. The present results suggest that the behavioral deficits in stereopsis produced by abnormal visual experience reflect at least in part the constraints imposed by alterations at the earliest stages of binocular cortical processing and support the hypothesis that the local disparity processing mechanisms in primates are spatially tuned and can be independently compromised by early abnormal visual experience.     

Cumulative effect of brief daily periods of monocular vision on kitten striate cortex

Summary Eight kittens were subjected to daily 4-h sessions with vision restricted to one eye between the 4th and 9th postnatal weeks. The total duration of monocular exposure ranged from 76 to 152 h. Between monocular exposure periods, four kittens were kept in total darkness, and four experienced binocular vision under normal colony conditions. At the end of the rearing period, all experimental animals and four normally reared controls were studied by means of single-unit recording in primary visual cortex. In kittens with inter-session dark rearing, very few neurons could be driven through the deprived eye. This effect was more extreme than that observed after continuous dark rearing. Visual responses mediated by the experienced eye appeared normal. In contrast, in kittens with inter-session binocular vision, the large majority of visual cortical neurons responded to visual stimulation of either eye. A few neurons appeared to have lost their responsiveness to the deprived eye, but this effect was small. We conclude that monocular vision delivered during brief daily sessions produces a cumulative competitive inactivation of transmission in the pathway from the deprived eye to striate cortex so long as no visual stimulation occurs outside the monocular periods. Binocular visual stimulation received between sessions rapidly and almost completely reverses the effects of monocular vision.     

Disparity tuning and binocularity of single neurons in cat visual cortex

Summary Activity from single neurons in the visual cortex of anaesthetized and paralyzed cats were recorded in response to monocular and binocular stimuli of different retinal disparities.Three different types of disparity sensitive neurons were found, characterized by (1) a response maximum near zero disparity, (2) a response minimum near zero disparity, and (3) an asymmetric disparity sensitivity.Usually, neurons of the first type receive about the same monocular input from the two eyes; the other disparity sensitive neurons respond better to monocular stimulation of one eye as compared to the other.The results closely resemble those obtained recently in the rhesus monkey.     

Visual cortical recovery from reverse occlusion depends on concordant binocular experience

The effects of early monocular deprivation on visual acuity and visual cortical responses can be reversed quickly if vision is restored to the deprived eye and the other eye is deprived instead, a procedure known as reverse occlusion. However, recovery of vision through the originally deprived eye (ODE) is not stable. Following re-opening of the recently deprived (originally nondeprived) eye (ONDE), vision in the ODE typically deteriorates rapidly, possibly because of competitive interactions, whereas vision in the ONDE also remains compromised, resulting in bilateral amblyopia, the reasons for which are unknown. Here we monitor the physiological changes in the visual cortex during recovery from reverse occlusion in a longitudinal study, using optical imaging of intrinsic signals and single-cell recording in anesthetized cats. We show that a brief period of just 4 days of concordant binocular vision intercalated between the two periods of monocular experience allows close to equal responses to develop through both eyes, both in terms of cortical territory and orientation selectivity. In contrast, with no binocular vision or discordant binocular experience, cortical territory dominated by the ONDE is significantly reduced, and orientation tuning of cells dominated by the ODE is wider than that of cells dominated by the ONDE. These results support the notion that a brief period of binocular vision is sufficient to prevent bilateral acuity loss caused by reverse occlusion.     

Postnatal development of disparity sensitivity in visual area 2 (v2) of macaque monkeys

Macaque monkeys do not reliably discriminate binocular depth cues until about 8 wk of age. The neural factors that limit the development of fine depth perception in primates are not known. In adults, binocular depth perception critically depends on detection of relative binocular disparities and the earliest site in the primate visual brain where a substantial proportion of neurons are capable of discriminating relative disparity is visual area 2 (V2). We examined the disparity sensitivity of V2 neurons during the first 8 wk of life in infant monkeys and compared the responses of V2 neurons to those of V1 neurons. We found that the magnitude of response modulation in V2 and V1 neurons as a function of interocular spatial phase disparity was adult-like as early as 2 wk of age. However, the optimal spatial frequency and binocular response rate of these disparity sensitive neurons were more than an octave lower in 2- and 4-wk-old infants than in adults. Consequently, despite the lower variability of neuronal firing in V2 and V1 neurons of infant monkeys, the ability of these neurons to discriminate fine disparity differences was significantly reduced compared with adults. This reduction in disparity sensitivity of V2 and V1 neurons is likely to limit binocular depth perception during the first several weeks of a monkey's life.     

The effects of reverse monocular deprivation in monkeys II. Electrophysiological and anatomical studies

Summary Monkeys had one eye closed at about 30 days of age for 14, 30, 60, or 90 days, then opened, and the fellow eye closed for another 120 days. The animals then had at least 10 months of binocular visual experience before extensive behavioral training and testing were carried out. In terminal experiments concluded more than 18 months later, microelectrode investigations of the striate cortex demonstrated that there was almost a complete absence of binocular neurons in all animals. The initially deprived eyes (IDEs) dominated the majority of cortical neurons, even when soma size measurements of lateral geniculate neurons indicated that the LGN cells driven by the IDE had not regained their normal size. The monkeys which had significant interocular differences in spatial vision also exhibited abnormalities in the distribution of the metabolic enzyme, cytochrome oxidase (CO), within the striate cortex. These results demonstrate that many of the severe alterations in cortical physiology and eye dominance produced by early monocular form deprivation can be reversed, with recovery of normal cortical function, via the reverse-deprivation procedure.Supported by National Eye Institute grants R01 EY01120, R01 EY03611, R01 EY01139, and EY02520     

BOLD fMRI response of early visual areas to perceived contrast in human amblyopia

In this study, we used a temporal two-alternative forced choice psychophysical procedure to measure the observer's perception of a 22% physical contrast grating for each eye as a function of spatial frequency in four subjects with unilateral amblyopia and in six subjects with normal vision. Contrast thresholds were also measured using a standard staircase method. Additionally, blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) was used to measure the neuronal response within early visual cortical areas to monocular presentations of the same 22% physical contrast gratings as a function of spatial frequency. For all six subjects with normal vision and for three subjects with amblyopia, the psychophysically measured perception of 22% contrast as a function of spatial frequency was the same for both eyes. Threshold contrast, however, was elevated for the amblyopic eye for all subjects, as expected. The magnitude of the fMRI response to 22% physical contrast within "activated" voxels was the same for each eye as a function of spatial frequency, regardless of the presence of amblyopia. However, there were always fewer "activated" fMRI voxels during amblyopic stimulation than during normal eye stimulation. These results are consistent with the hypotheses that contrast thresholds are elevated in amblyopia because fewer neurons are responsive during amblyopic stimulation, and that the average firing rate of the responsive neurons, which reflects the perception of contrast, is unaffected in amblyopia.     

Changes in pattern discrimination learning induced by visual deprivation in normal and commissurotomized pigeons

Summary The effect of monocular (MD), binocular (BD) and alternating monocular (AMD) deprivation on monocular pattern discrimination learning and interocular transfer was investigated in pigeons reared with intact and sectioned supraoptic decussation (DSO). In BD and AMD animals acquisition and interocular transfer of two different pattern discrimination problems remained as good as in the control animals (CO). Monocularly deprived animals (MDE) required significantly more trials to learn the discriminations through the deprived eye, when it was trained first, and interocular transfer from the deprived to the experienced eye was absent. However, if the experienced eye was trained first (MED), learning with the deprived eye was at least as rapid as with the experienced eye. This indicates positive interocular transfer from the experienced to the deprived eye.Interocular transfer of pattern discriminations was completely blocked by section of the DSO in both adults (DSOad) and newly hatched animals (DSOjuv). The elimination of binocular interaction by commissurotomy from the beginning of the postnatal life failed to produce an impaired pattern discrimination learning in monocularly deprived animals (MD+DSO).The results are discussed in terms of competition between the projections from both eyes in the visual Wulst.     

Laminar,columnar and topographic aspects of ocular dominance in the primary visual cortex ofCebus monkeys

Summary The representation of the two eyes in striate cortex (V1) ofCebus monkeys was studied by electrophysiological single-unit recordings in normal animals and by morphometric analysis of the pattern of ocular dominance (OD) stripes, as revealed by cytochrome oxidase histochemistry in V1 flat-mounts of enucleated animals. Single-unit recordings revealed that the large majority of V1 neurons respond to the stimulation of either eye but are more strongly activated by one of them. As in other species of monkey, neurons with preference for the stimulation of the same eye are grouped in columns 300–400 µm wide, spanning all cortical layers. Monocular neurons are clustered in layer IVc, specially in its deeper half (IVc-beta), and constitute less than 10% of the population of other layers. Neurons with equal responses to each eye are more commonly found in layer V than elsewhere in V1. In the supragranular layers and in granular layer IVc-alpha neurons strongly dominated by one of the eyes tend to be broadly tuned for orientation, while binocularly balanced neurons tend to be sharply tuned for this parameter. No such correlation was detected in the infragranular layers, and most neurons in layer IVc-beta responded regardless of stimulus orientation. Ocular dominance stripes are present throughout most of V1 as long, parallel or bifurcating bands alternately dominated by the ipsi- or the contralateral eye. They are absent from the cortical representations of the blind spot and the monocular crescent. The domains of each eye occupy nearly equal portions of the surface of binocular V1, except for the representation of the periphery, where the contralateral eye has a larger domain, and a narrow strip along the border of V1 with V2, where either eye may predominate. The orderliness of the pattern of stripes and the relationship between stripe arrangement and the representation of the visual meridians vary with eccentricity and polar angle but follow the same rules in different animals. These results demonstrate that the laminar, columnar and topographic distribution of neurons with different degrees of OD in V1 is qualitatively similar in New- and Old World monkeys of similar sizes and suggest that common ancestry, rather than parallel evolution, may account for the OD phenotypes of contemporaneous simians.     

Effect of interocular delay on disparity-selective v1 neurons: relationship to stereoacuity and the pulfrich effect

The temporal properties of disparity-sensitive neurons place important temporal constraints on stereo matching. We examined these constraints by measuring the responses of disparity-selective neurons in striate cortex of awake behaving monkeys to random-dot stereograms that contained interocular delays. Disparity selectivity was gradually abolished by increasing interocular delay (when the delay exceeds the integration time, the inputs from the 2 eyes become uncorrelated). The amplitude of the disparity-selective response was a Gaussian function of interocular delay, with a mean of 16 ms (+/-5 ms, SD). Psychophysical measures of stereoacuity, in both monkey and human observers, showed a closely similar dependency on time, suggesting that temporal integration in V1 neurons is what determines psychophysical matching constraints over time. There was a slight but consistent asymmetry in the neuronal responses, as if the optimum stimulus is one in which the right stimulus leads by about 4 ms. Because all recordings were made in the left hemisphere, this probably reflects nasotemporal differences in conduction times; psychophysical data are compatible with this interpretation. In only a few neurons (5/72), interocular delay caused a change in the preferred disparity. Such tilted disparity/delay profiles have been invoked previously to explain depth perception in the stroboscopic version of the Pulfrich effect (and other variants). However, the great majority of the neurons did not show tilted disparity/delay profiles. This suggests that either the activity of these neurons is ignored when viewing Pulfrich stimuli, or that current theories relating neuronal properties to perception in the Pulfrich effect need to be reevaluated.     

The effects of reverse monocular deprivation in monkeys I. Psychophysical experiments

Summary Monkeys had one eye closed at about 30 days of age for 14, 30, 60, or 90 days, then opened, and the fellow eye closed for another 120 days. The animals then had at least 10 months of binocular visual experience before behavioral training and testing were begun. All subjects were used in a series of psychophysical investigations during the next two years. The results of the behavioral studies indicated that the initially deprived eyes (IDE) of the two monkeys that were subjected to initial deprivation periods of 14 or 30 days recovered normal or nearnormal spatial contrast sensitivity. In contrast, the two animals which underwent longer periods of initial deprivation showed incomplete recovery, especially for high spatial frequency stimuli. All of the monkeys exhibited a reduction in spatial contrast sensitivity for their reverse deprived eyes (RDE); the earlier the onset of the reverse-deprivation procedures (i.e., the, shorter the initial period of deprivation), the greater the deficit in the RDE's spatial contrast sensitivity. Measurements of temporal contrast sensitivity showed that all of the subjects' IDEs had normal or near-normal sensitivity levels. However, the reverse-deprivation procedures initiated at 90 days of age or earlier produced a frequency dependent reduction in the RDE's temporal modulation sensitivity. The measures of increment-threshold spectral sensitivity revealed that only the RDE of the monkey that had the shortest initial deprivation period had an abnormal spectral sensitivity function. The results demonstrate that many of the severe behavioral deficits produced by early monocular form deprivation can be recovered via reverse deprivation procedures. However, depending upon the length of the initial deprivation period and the age at which the reversal procedure is initiated, the second deprivation period can also adversely affect the functional capacity of the RDE.Supported by National Eye Institute grants R01 EY01120, R01 EY03611, R01 EY01139, and EY02520     

Characteristics of postural instability induced by ischemic blocking of leg afferents

Summary Beginning near the peak of the sensitive period to monocular deprivation, kittens were reared in darkness except for daily sessions during which the left eye was exposed first followed immediately by an equal amount of right eye exposure. The notion was that the sequence of stimulation may be an important determinant in cortical representation of each eye. Although study of single neurons in area 17 showed that nearly all cells were monocular, no systematic imbalance was found in the numbers of units controlled by each eye.     

Gaze-stabilizing deficits and latent nystagmus in monkeys with early-onset visual deprivation: role of the pretectal not

We studied the role of the pretectal nucleus of the optic tract (NOT) in the development of monocular optokinetic nystagmus (OKN) asymmetries and latent nystagmus (LN) in two monkeys reared with binocular deprivation (BD) caused by binocular eyelid suture for either the first 25 or 55 days of life. Single-unit recordings were performed in the right and left NOT of both monkeys at 2-3 yr of age and compared with similar unit recordings in normally reared monkeys. We also examined ocular motor behavior during electrical stimulation of the NOT and during pharmacological inactivation and activation using GABA(A) agonists and antagonists. In BD animals a large proportion of NOT units was dominated by the contralateral eye, in striking contrast to normal animals where 100% of NOT units were sensitive to stimuli delivered to either eye. In the 55-day BD animal no binocularly sensitive neurons were found, while in the 25-day BD animal 60% of NOT units retained at least some binocular sensitivity. Differences in direction sensitivity were also observed in BD animals. We found that 56% of units in the 55-day BD monkey and 10% of units in the 25-day BD monkey responded preferentially to contraversive visual motion. In contrast, only 5% of the NOT units encountered in normally reared monkeys respond preferentially during contraversive visual motion, the rest were most sensitive to ipsiversive visual motion. NOT neurons of BD monkeys showed a wide range of speed sensitivities similar to that of normal monkeys. Unilateral electrical stimulation of the NOT in BD animals induced a conjugate nystagmus with slow phases directed toward the side of stimulation. When we blocked the activity of NOT units with muscimol, a potent GABA(A) agonist, LN was abolished. In contrast, LN was increased when spontaneous activity of the NOT was enhanced with bicuculline, a GABA(A) antagonist. Our results indicate that the NOT in BD monkeys plays an important role in the OKN deficits and LN generation during monocular viewing. We hypothesize that the large proportion of units dominated by the contralateral eye contribute to the development of monocular OKN asymmetries and LN.     

Binocular deficits associated with early alternating monocular defocus. I. Behavioral observations

To study the binocular vision deficits associated with anisometropia, monkeys were reared with alternating monocular defocus, which allowed monocular mechanisms to develop normally while binocular mechanisms were selectively compromised. A defocusing contact lens of -1.5 D, -3 D, or -6 D was worn on alternate eyes on successive days (n = 3 per lens power) from 3 wk to 9 mo of age. The control subjects were two normally reared monkeys and two human observers. Functional binocular vision was assessed through behavioral measurements of stereoscopic depth discrimination thresholds as a function of spatial frequency. To characterize the extent of the deficits in disparity processing at a given spatial frequency, the contrast required to support stereopsis was determined for a range of disparities that exceeded the subjects' measured stereoacuity. The lens-reared monkeys showed spatial-frequency-selective deficits in stereopsis that depended on the magnitude of the simulated anisometropia experienced during the rearing period. For a given spatial frequency, the treated monkeys generally required higher than normal contrasts to support stereopsis even for large disparities. Moreover, a given increase in contrast produced smaller than normal improvements in stereo discrimination in our treated subjects, which suggests that in addition to deficits in contrast sensitivity, disparity-sensitive mechanisms exhibited low contrast gains. The spatial-frequency selective nature of the binocular deficits produced by the imposed anisometropia indicate that disparity processing mechanisms are normally spatial-frequency selective and that mechanisms tuned to different spatial frequencies can be differentially affected by abnormal binocular visual experience.     

Cortical binocularity in convergent strabismus after section of the optic chiasm

In convergent strabismus (esotropia) the spatial asynchrony of the two eye inputs unbalances the interocular interactions, leading to the functional advantage of the nondeviated eye and the inhibition of the esotropic eye. It may be argued that the strabismic suppression, if it is the effect of inhibitory interactions between the eyes, could be removed by interrupting the interocular pathways at the optic chiasm. After chiasmatic section, each eye is connected only to the ipsilateral cortex through the uncrossed retinal projections and so the functionality of each eye's input is no longer interfered with by interocular mechanisms. In strabismic cats submitted, as adults, to section of the optic chiasm, we performed electrophysiological recordings from the striate cortex. Results show that in these animals: (1) the cortical responsiveness to the strabismic eye is strikingly higher than in esotropes with an intact optic chiasm; (2) the effectiveness of stimulation of the deviated eye is not different from that of the nondeviated eye in driving neurons of corresponding cortex; (3) surprisingly, a high degree of binocular activation is present in the cortex ipsilateral to the deviated eye, while in the cortex connected to the nondeviated eye the greatest majority of neurons are monocularly driven. Cortical binocularity depends on the corpus callosum, which conveys the input from the nondeviated eye to the opposite cortex (which receives the direct strabismic input), but not vice versa. The asymmetry of callosal transmission parallels the morphological asymmetry of callosal connections that occur in convergent strabismus. All together the findings indicate that the impaired effectiveness of esotropic input does not result from developmental deficit of the strabismic afferents but, rather, from inhibitory influences that are actively exerted through the interocular pathways. Strabismic suppression may be accomplished by the same interocular mechanisms underlying binocular rivalry.     

Short‐term monocular deprivation alters early components of visual evoked potentials

Key points

• Short‐term monocular deprivation in adult humans produces a perceptual boost of the deprived eye reflecting homeostatic plasticity.
• Visual evoked potentials (VEPs) to transient stimuli change after 150 min of monocular deprivation in adult humans.
• The amplitude of the C1 component of the VEP at a latency of about 100 ms increases for the deprived eye and decreases for the non‐deprived eye after deprivation, the two effects being highly negatively correlated.
• Similarly, the evoked alpha rhythm increases after deprivation for the deprived eye and decreases for the non‐deprived eye.
• The data demonstrate that primary visual cortex excitability is altered by a short period of monocular deprivation, reflecting homeostatic plasticity.

Abstract

Very little is known about plasticity in the adult visual cortex. In recent years psychophysical studies have shown that short‐term monocular deprivation alters visual perception in adult humans. Specifically, after 150 min of monocular deprivation the deprived eye strongly dominates the dynamics of binocular rivalry, reflecting homeostatic plasticity. Here we investigate the neural mechanisms underlying this form of short‐term visual cortical plasticity by measuring visual evoked potentials (VEPs) on the scalp of adult humans during monocular stimulation before and after 150 min of monocular deprivation. We found that monocular deprivation had opposite effects on the amplitude of the earliest component of the VEP (C1) for the deprived and non‐deprived eye stimulation. C1 amplitude increased (+66%) for the deprived eye, while it decreased (−29%) for the non‐deprived eye. Source localization analysis confirmed that the C1 originates in the primary visual cortex. We further report that following monocular deprivation, the amplitude of the peak of the evoked alpha spectrum increased on average by 23% for the deprived eye and decreased on average by 10% for the non‐deprived eye, indicating a change in cortical excitability. These results indicate that a brief period of monocular deprivation alters interocular balance in the primary visual cortex of adult humans by both boosting the activity of the deprived eye and reducing the activity of the non‐deprived eye. This indicates a high level of residual homeostatic plasticity in the adult human primary visual cortex, probably mediated by a change in cortical excitability.

Abbreviations

EEG

electroencephalography

ERP

event related potential

TMS

transcranial magnetic stimulation

VEP

visual evoked potential

     

Modification of parietal association cortex and functional blindness after binocular deprivation in young monkeys

Summary Clinical experience from humans deprived of vision at an early age indicates that recovery of vision is often rudimentary after late correction of the abnormalities in the eyes. Binocular deprivation in cats does not change the function of the visual cortex equally much as does monocular deprivation. Therefore, the behavioural blindness observed after binocular deprivation is probably caused by changes central to area 17. We studied four monkeys deprived of vision by binocular lid closure from shortly after birth until the ages of 7 to 11 months and three control animals of the same age. After opening, the eyes of the deprived animals were normal, optokinetic nystagmus could be elicited, and microelectrode recordings of multiple unit activity in areas 17 and 19 indicated brisk responses to visual stimulation. Behaviourally, all deprived animals were blind, however. They bumped into obstacles, fell from tables and used their somatic sense for exploration. Only minute recovery of visual orientation was observed during several months after the opening of the eyes. Approximately 400 multiple unit recording penetrations were made in Brodmann's area 7 in the deprived monkeys and a similar number in the control monkeys using the transdural recording technique in conscious, behaving animals. The results indicated a profound decrease in the representation of vision in this area: the representation of visual mechanisms was reduced by 92% and the combined visual and somatic representation was reduced by 97% in the deprived monkeys. On the other hand, the representation of active somatic movement had increased by 117% and that of passive somaesthesia by 53%. Also the proportion of cell groups that could not be activated (only spontaneously active) increased fourfold. These results show that early visual deprivation alters the associative systems of the brain by reducing the efficiency of transmission along pathways that mediate visual influences. Inputs from different sensory systems may compete for influence on the association cortex, disuse in one leading to its functional deterioration. Such changes may explain the lasting behavioural alterations that take place in man and monkeys after prolonged visual deprivation at an early age.This work has been supported by the Sigrid Juselius Foundation     

The regulation of vestibular afferent information during monocular vision while standing

The purpose of this study was to investigate the contribution of the vestibular system to postural control during monocular vision using binaural-bipolar galvanic vestibular stimulation (GVS). Four visual (both eyes, dominant eye, non-dominant eye, and no vision) conditions were tested during GVS in five healthy subjects while focusing on a target placed in front of them. GVS evoked similar upper body postural sway during both monocular and no vision conditions that were significantly greater to those during binocular vision. Changes in ground reaction forces to the anode side followed that same trend, although data for vision with the dominant eye were not significantly different from that for binocular vision. These data suggest an increase in the weighting of vestibular afferent information during monocular vision for standing postural control.     

Reversal of the physiological effects of monocular deprivation in kittens: further evidence for a sensitive period

1. It was confirmed that suturing the lids of one eye (monocular deprivation), until only 5 weeks of age, leaves virtually every neurone in the kitten's visual cortex entirely dominated by the other eye. On the other hand, deprivation of both eyes causes no change in the normal ocular dominance of cortical neurones, most cells being clearly binocularly driven.2. Kittens were monocularly deprived until various ages, from 5 to 14 weeks, at which time reverse suturing was performed: the initially deprived right eye was opened and the left eye closed for a further 9 weeks before recording from the visual cortex.3. Reverse suturing at 5 weeks caused a complete switch in ocular dominance: every cell was dominated by the initially deprived right eye. Reverse suturing at 14 weeks, however, had almost no further effect on ocular dominance: most cells were still driven solely by the left eye. Animals reverse sutured at intermediate ages had cortical neurones strongly dominated by one eye or the other, and they were organized into clear columnar groups according to ocular dominance.4. Thus, between 5 weeks and 4 months of age, there is a period of declining sensitivity to both the effects of an initial period of monocular deprivation and the reversal of those effects by reverse suturing.5. The small proportion of binocular cells in reverse sutured kittens (which have never had simultaneous binocular vision) often differed considerably in their receptive field properties in the two eyes. In particular, if the cells were orientation selective in both eyes the two preferred orientations could differ by up to 70 degrees .6. The relative importance of innate and environmental contributions to the properties of cortical cells is discussed.     

Representation of stereoscopic depth based on relative disparity in macaque area V4

Stereoscopic vision is characterized by greater visual acuity when a background feature serves as a reference. When a reference is present, the perceived depth of an object is predominantly dependent on this reference. Neural representations of stereoscopic depth are expected to have a relative frame of reference. The conversion of absolute disparity encoded in area V1 to relative disparity begins in area V2, although the information encoded in this area appears to be insufficient for stereopsis. This study examines whether relative disparity is encoded in a higher cortical area. We recorded the responses of V4 neurons from macaque monkeys to various combinations of the absolute disparities of two features: the center patch and surrounding annulus of a dynamic random-dot stereogram. We analyzed the effects of the disparity of the surrounding annulus on the tuning for the disparity of the center patch; the tuning curves of relative-disparity-selective neurons for disparities of the center patch should shift with changes in the disparity of the surrounding annulus. Most V4 tuning curves exhibited significant shifts. The magnitudes of the shifts were larger than those reported for V2 neurons and smaller than that expected for an ideal relative-disparity-selective cell. No correlation was found between the shift magnitude and the degree of size suppression, suggesting that the two phenomena are not the result of a common mechanism. Our results suggest that the coding of relative disparity advances as information flows along the cortical pathway that includes areas V2 and V4.