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Resting-state functional connectivity (RSFC) studies have provided strong evidences that visual deprivation influences the brain's functional architecture. In particular, reduced RSFC coupling between occipital (visual) and temporal (auditory) regions has been reliably observed in early blind individuals (EB) at rest. In contrast, task-dependent activation studies have repeatedly demonstrated enhanced co-activation and connectivity of occipital and temporal regions during auditory processing in EB. To investigate this apparent discrepancy, the functional coupling between temporal and occipital networks at rest was directly compared to that of an auditory task in both EB and sighted controls (SC). Functional brain clusters shared across groups and cognitive states (rest and auditory task) were defined. In EBs, we observed higher occipito-temporal correlations in activity during the task than at rest. The reverse pattern was observed in SC. We also observed higher temporal variability of occipito-temporal RSFC in EB suggesting that occipital regions in this population may play the role of a multiple demand system. Our study reveals how the connectivity profile of sighted and early blind people is differentially influenced by their cognitive state, bridging the gap between previous task-dependent and RSFC studies. Our results also highlight how inferring group-differences in functional brain architecture solely based on resting-state acquisition has to be considered with caution., (Copyright © 2016 Elsevier Inc. All rights reserved.)

How early blindness reorganizes the brain circuitry that supports auditory motion processing remains controversial. We used fMRI to characterize brain responses to in-depth, laterally moving, and static sounds in early blind and sighted individuals. Whole-brain univariate analyses revealed that the right posterior middle temporal gyrus and superior occipital gyrus selectively responded to both in-depth and laterally moving sounds only in the blind. These regions overlapped with regions selective for visual motion (hMT+/V5 and V3A) that were independently localized in the sighted. In the early blind, the right planum temporale showed enhanced functional connectivity with right occipito-temporal regions during auditory motion processing and a concomitant reduced functional connectivity with parietal and frontal regions. Whole-brain searchlight multivariate analyses demonstrated higher auditory motion decoding in the right posterior middle temporal gyrus in the blind compared to the sighted, while decoding accuracy was enhanced in the auditory cortex bilaterally in the sighted compared to the blind. Analyses targeting individually defined visual area hMT+/V5 however indicated that auditory motion information could be reliably decoded within this area even in the sighted group. Taken together, the present findings demonstrate that early visual deprivation triggers a large-scale imbalance between auditory and "visual" brain regions that typically support the processing of motion information., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Animal and human studies have demonstrated that transient visual deprivation early in life, even for a very short period, permanently alters the response properties of neurons in the visual cortex and leads to corresponding behavioral visual deficits. While it is acknowledged that early-onset and longstanding blindness leads the occipital cortex to respond to non-visual stimulation, it remains unknown whether a short and transient period of postnatal visual deprivation is sufficient to trigger crossmodal reorganization that persists after years of visual experience. In the present study, we characterized brain responses to auditory stimuli in 11 adults who had been deprived of all patterned vision at birth by congenital cataracts in both eyes until they were treated at 9 to 238 days of age. When compared to controls with typical visual experience, the cataract-reversal group showed enhanced auditory-driven activity in focal visual regions. A combination of dynamic causal modeling with Bayesian model selection indicated that this auditory-driven activity in the occipital cortex was better explained by direct cortico-cortical connections with the primary auditory cortex than by subcortical connections. Thus, a short and transient period of visual deprivation early in life leads to enduring large-scale crossmodal reorganization of the brain circuitry typically dedicated to vision., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Visual deprivation leads to massive reorganization in both the structure and function of the occipital cortex, raising crucial challenges for sight restoration. We tracked the behavioral, structural, and neurofunctional changes occurring in an early and severely visually impaired patient before and 1.5 and 7 mo after sight restoration with magnetic resonance imaging. Robust presurgical auditory responses were found in occipital cortex despite residual preoperative vision. In primary visual cortex, crossmodal auditory responses overlapped with visual responses and remained elevated even 7 mo after surgery. However, these crossmodal responses decreased in extrastriate occipital regions after surgery, together with improved behavioral vision and with increases in both gray matter density and neural activation in low-level visual regions. Selective responses in high-level visual regions involved in motion and face processing were observable even before surgery and did not evolve after surgery. Taken together, these findings demonstrate that structural and functional reorganization of occipital regions are present in an individual with a long-standing history of severe visual impairment and that such reorganizations can be partially reversed by visual restoration in adulthood., (Copyright © 2015 the American Physiological Society.)

Based on multiple invasive reports and neuroimaging studies, it is well established that the cytoarchitecture of the visual cortex is related to its functional organization, namely, its retinotopy. The present study aimed to further investigate retinotopic mapping as well as specific vertical and horizontal functional asymmetries within the human visual cortex using functional near-infrared spectroscopy (fNIRS). Black and white wedge checkerboard stimuli were randomly presented to the four visual field (VF) quadrants of eight healthy adults in order to quantify and compare the localization and the amplitude of hemodynamic cortical responses to each VF quadrant. Results showed the expected activation in the contralateral hemisphere, with respect to the side of the stimulated quadrant. We also measured significantly stronger activations in the upper visual cortex when low hemifield stimuli were presented compared to activations in the lower visual cortex when upper hemifield stimuli were shown, especially when the stimulation was presented in the right visual field. These findings confirm the vertical asymmetry of the visual cortex previously reported by neuroimaging and behavioral studies. More importantly, the present work confirms the reliability of the fNIRS technique for functional mapping of the human brain., (Copyright © 2011 Elsevier B.V. All rights reserved.)

How the brain processes visual stimuli has been extensively studied using scalp surface electrodes and magnetic resonance imaging. Using these and other methods, complex gratings have been shown to activate the ventral visual stream, whereas moving stimuli preferentially activate the dorsal stream. In the current study, a first experiment assessed brain activations evoked by complex gratings using intracranial electroencephalography in 10 epileptic patients implanted with subdural electrodes. These stimuli of intermediate levels of complexity were presented in such a way that transformational apparent motion (TAM) was perceived. Responses from both the ventral and the dorsal pathways were obtained. The response characteristics of visual area 4 and the fusiform cortex were of similar amplitudes, suggesting that both ventral areas are recruited for the processing of complex gratings. On the other hand, TAM-induced responses of dorsal pathway areas were relatively noisier and of lower amplitudes, suggesting that TAM does not activate motion-specific structures to the same extent as does real motion. To test this hypothesis, we examined the activity evoked by TAM in comparison to the one produced by real motion in a patient implanted with the same subdural electrodes. Findings demonstrated that neural response to real motion was much stronger than that evoked by TAM, in both the primary visual cortex (V1) and other motion-sensitive areas within the dorsal pathway. These results support the conclusion that apparent motion, even if perceptually similar to real motion, is not processed in a similar manner.

Febrile seizures occurring during childhood have been shown to interfere with the development of cognitive functions. However, an alteration of the developing sensory systems might also result from febrile seizures. In order to test this hypothesis, seizures were induced by hyperthermia in Long Evans rats on postnatal day 10. Extracellular single neuron recordings were carried out from postnatal days 15 to 30 and at adulthood. The response of neurons in the primary visual cortex to drifting sinusoidal gratings was recorded in anaesthetized rats. As soon as postnatal day 15, the neurons of rats having experienced a hyperthermic seizure showed significantly lower optimal spatial frequencies (SF), broader directional and temporal bandwidths, as well as higher contrast thresholds than did neurons recorded in normal rats. At adulthood, significantly broader spatial bandwidths and lower optimal temporal frequencies (TF) were obtained from neurons of rats subjected to hyperthermia. These results suggest that febrile seizures during infancy could affect the development of spatio-temporal receptive field properties of neurons in primary visual cortex. Such alterations of a sensory system might contribute to the cognitive deficits associated with early-onset febrile seizures., (Copyright © 2010 IBRO. Published by Elsevier Ltd. All rights reserved.)

Although rat is commonly used for studying the visual system, the development of spatio-temporal receptive field properties of neurons in its primary visual cortex remains relatively unknown. Extracellular single neuron recordings were thus carried out in anaesthetized newborn rats between postnatal (P) day 15 and 30 and in adult rats. The first neuronal responses evoked by drifting sinusoidal gratings were obtained at P16. From P27-P30 to adulthood, the neurons' optimal temporal frequency significantly increased, whereas the contrast threshold decreased. However, the optimal spatial frequency, spatial resolution and spatial bandwidth were, soon after eye opening, similar to those observed in the adult rat. The differential maturation of receptive field properties is discussed with regard to the functional pathways of the rat visual system.

We used functional Magnetic Resonance Imaging (fMRI) to explore the areas underlying the processing of two similar motion stimuli that evoke different types of processing. The results indicated that while form-from-motion (FFM) stimuli activated both lateral occipital complex (LOC) and MT complex (MT+), only the LOC remained significantly activated when contrasted with a global motion stimulus (GMS) with different coherence levels. Because of the large number of common characteristics shared between the stimuli, this contrast enabled us to isolate the regions implicated in form processing. The GMS on the other hand only activated MT+, reaching maximal intensity for low coherence. Overall, these data illustrate how two similar motion stimuli can elicit the participation of different cortical visual regions.

The present study investigated the spatial properties of cells in the postero-lateral lateral suprasylvian (PLLS) area of the cat and assessed their sensitivity to edges defined by motion. A total of one hundred and seventeen (117) single units were isolated. First, drifting sinusoidal gratings were used to assess the spatial properties of the cells' receptive fields and to determine their spatial frequency tuning functions. Second, random-dot kinematograms were used to create illusory edges by drifting textured stimuli (i.e. a horizontal bar) against a similarly textured but static background. Almost all the cells recorded in PLLS (96.0%) were binocular, and a substantial majority of receptive fields (79.2%) were end-stopped. Most units (81.0%) had band-pass spatial frequency tuning functions and responded optimally to low spatial frequencies (mean spatial frequency: 0.08 c./degree). The remaining units (19.0%) were low-pass. All the recorded cells responded vigorously to edges defined by motion. The vast majority (96.0%) of cells responded optimally to large texture elements; approximately half the cells (57.3%) also responded to finer texture elements. Moreover, 38.5% of the cells were selective to the width of the bar (i.e., the distance between the leading and the trailing edges). Finally, some (9.0%) cells responded in a transient fashion to leading and to trailing edges. In conclusion, cells in the PLLS area are low spatial frequency analyzers that are sensitive to texture and to the distance between edges defined by motion.

There are important developmental changes occurring during infancy in visual cortical structures that underlie higher-order perceptual abilities. Using high-density electrophysiological recording techniques, the present study aimed to examine the development of visual mechanisms, during the first year of life, associated with texture segregation. Forty-two normal full term infants were tested at 1, 3, 6 or 12 months of age. Visual-evoked potentials to low-level stimuli varying in orientation (oriVEP) and higher-level textured stimuli (texVEP) were recorded from 128 scalp electrodes. Difference potentials were obtained to extract the VEP component associated specifically with texture segregation (tsVEP). Results show a clear developmental pattern regarding amplitude, latency and scalp distribution of tsVEP, which appears at around 3 months but does not reach maturity by 12 months of age. A reduction in latency is particularly evident between 3 and 6 months, whereas amplitude shows a gradual increase with a marked increment between 3 and 6 months for low-level orientation stimuli and between 6 and 12 months for higher-level textured stimuli. These developmental patterns are attributed to neural maturational processes such as myelination and synaptogenesis. The differential developmental rates can be explained by delayed maturational processes of brain regions involved in more complex visual processing.

A number of studies on humans and animals have demonstrated better auditory abilities in blind with respect to sighted subjects and have tried to define the mechanisms through which this compensation occurs. The aim of the present study, therefore, was to examine the participation of primary visual cortex (V1) to auditory processing in early enucleated rats. Here we show, using gaussian noise bursts, that about a third of the cells in V1 responded to auditory stimulation in blind rats and most of these (78%) had ON-type responses and low spontaneous activity. Moreover, they were distributed throughout visual cortex without any apparent tonotopic organization. Optimal frequencies determined using pure tones were rather high but comparable to those found in auditory cortex of blind and sighted rats. On the other hand, sensory thresholds determined at these frequencies were higher and bandwidths were wider in V1 of the blind animals. Blind and sighted rats were also stimulated for 60 min with gaussian noise, their brains removed and processed for c-Fos immunohistochemistry. Results revealed that c-Fos positive cells were not only present in auditory cortex of both groups of rats but there was a 10-fold increase in labeled cells in V1 and a fivefold increase in secondary visual cortex (V2) of early enucleated rats in comparisons to sighted ones. Also, the pattern of distribution of these labeled cells across layers suggests that the recruitment of V1 could originate at least in part through inputs arising from the thalamus. The ensemble of results appears to indicate that cross-modal compensation leading to improved performance in the blind depends on cell recruitment in V1 but probably also plastic changes in lower- and higher-order visual structures and possibly in the auditory system.

Early blind (EB) individuals can recognize bidimensional shapes using a prosthesis substituting vision with audition (PSVA) and activate right dorsal extrastriate visual cortex during the execution of this task. The present study used repetitive transcranial magnetic stimulation (rTMS) to further examine the functional role of this structure in the successful use of the PSVA. Moreover, we investigated which auditory parameter used in the prosthesis (pitch, intensity, or spatial location) might contribute to this occipital activation. Results revealed that rTMS applied to right dorsal extrastriate cortex in EB subjects interferes with both the PSVA use and the auditory spatial location task but not with pitch and intensity discriminations. By contrast, rTMS targeting the same cortical areas in sighted subjects did not affect performance on any auditory tasks. Early visual deprivation thus leads to functional cerebral cross-modal reorganization in the processing of auditory information and auditory-to-visual sensory substitution. The findings also point to the specific involvement of the dorsal visual stream for auditory spatial processing in blind subjects. Moreover, this suggests that sensory substitution prostheses can be developed using these additional neural resources to perform tasks that partially compensate for the loss of vision.

Individuals deprived of vision early in life often demonstrate exceptional abilities in their remaining sensory modalities in order to compensate for their handicap. Recent studies have shown that some of these abilities also extend to those who have lost their sight later in life. It is not clear, however, what mechanisms underlie these abilities. Here, we examined cortical activation using positron emission tomography in late-onset blind participants during a free-field auditory localization task. Even though no behavioral enhancements were observed in this testing condition relative to sighted controls, the results revealed that the occipital cortex was nonetheless activated during task execution. We conclude that late-onset blind individuals do manifest cerebral reorganization, although its functional relevance to the task is less clear.

The current experiment examined the importance of visual input on the construction of inner spatial representations. Early and late-onset blind and paired control participants performed a tactile spatial orientation task. No significant group differences were observed, indicating that the blind can represent space. More errors, however, were committed by the early blind than by the late blind and sighted individuals in portions of the task that involved mental rotation skills, suggesting a potential facilitating role for vision in the proper development of spatial constructs.

The visual processing of radially modulated concentric patterns was studied in human participants, aged 3-22 years, by recording event-related potentials. These stimuli are known to activate the fusiform face area as well as area V4 in normal adults. The electrophysiological data showed a P1 latency that reached a maturation asymptote before 3 years of age, whereas that of N1 and P2 became adultlike by 13 years of age. In addition, the distribution of the P2 component over the scalp was focalized in the primary visual cortex before adolescence and became distributed over the entire brain after adolescence. Radially modulated concentric stimuli thus induce brain activation that is not mature until 13 years of age.

This study assessed the early mechanisms underlying perception of plaid motion. Thus, two superimposed gratings drifting in a rightward direction composed plaid stimuli whose global motion direction was perceived as the vector sum of the two components. The first experiment was aimed at comparing the perception of plaid motion when both components were presented to both eyes (dioptic) or separately to each eye (dichoptic). When components of the patterns had identical spatial frequencies, coherent motion was correctly perceived under dioptic and dichoptic viewing condition. However, the perceived direction deviated from the predicted direction when spatial frequency differences were introduced between components in both conditions. The results suggest that motion integration follows similar rules for dioptic and dichoptic plaids even though performance under dichoptic viewing did not reach dioptic levels. In the second experiment, the role of early cortical areas in the processing of both plaids was examined. As convergence of monocular inputs is needed for dichoptic perception, we tested the hypothesis that primary visual cortex (V1) is required for dichoptic plaid processing by delivering repetitive transcranial magnetic stimulation to this area. Ten minutes of magnetic stimulation disrupted subsequent dichoptic perception for approximately 15 min, whereas no significant changes were observed for dioptic plaid perception. Taken together, these findings suggest that V1 is not crucial for the processing of dioptic plaids but it is necessary for the binocular integration underlying dichoptic plaid motion perception.

Blind individuals often demonstrate enhanced nonvisual perceptual abilities. However, the neural substrate that underlies this improved performance remains to be fully understood. An earlier behavioral study demonstrated that some early-blind people localize sounds more accurately than sighted controls using monaural cues. In order to investigate the neural basis of these behavioral differences in humans, we carried out functional imaging studies using positron emission tomography and a speaker array that permitted pseudo-free-field presentations within the scanner. During binaural sound localization, a sighted control group showed decreased cerebral blood flow in the occipital lobe, which was not seen in early-blind individuals. During monaural sound localization (one ear plugged), the subgroup of early-blind subjects who were behaviorally superior at sound localization displayed two activation foci in the occipital cortex. This effect was not seen in blind persons who did not have superior monaural sound localization abilities, nor in sighted individuals. The degree of activation of one of these foci was strongly correlated with sound localization accuracy across the entire group of blind subjects. The results show that those blind persons who perform better than sighted persons recruit occipital areas to carry out auditory localization under monaural conditions. We therefore conclude that computations carried out in the occipital cortex specifically underlie the enhanced capacity to use monaural cues. Our findings shed light not only on intermodal compensatory mechanisms, but also on individual differences in these mechanisms and on inhibitory patterns that differ between sighted individuals and those deprived of vision early in life.

This investigation examines how neighboring neurons of area 18 react when area 17 inputs are excited or depressed. In anesthetized cats, area 18 responses to a sine-wave grating in the receptive field were analyzed, while a second grating was positioned in its periphery and responses were recorded in area 17. This latter site was also inactivated with GABA. A waveform template process sorted out at least two individual, neighboring cells with similar orientation preferences in area 18. These cells frequently displayed opposite reactions to stimulation and inactivation in area 17. Experiments suggest that nearby neurons belonging to the same functional domain in the visual cortex may simultaneously carry disparate information.

As shown by various human psychophysical studies, interocular spatial frequency disparities can yield a variety of percepts. In order to examine how binocular fusion is affected by spatial frequency differences, we have recorded cells in the border region of areas 17/18 of anesthetized cats. The optic axes of the eyes were deviated onto cathode-ray screens, and the optimal spatial frequency of each eye was assessed by monocular stimulations using drifting sinusoidal gratings. The optimal relative phase using identical spatial frequencies in both eyes was first determined. Spatial frequency differences were then introduced by keeping the optimal spatial frequency constant in one eye and varying the spatial frequency in the other. Results indicate that cells (39%) responded with an increased firing rate (facilitation) to similar spatial frequencies in each eye and with a gradual attenuation (occlusion or suppression) when spatial frequency differences were increased. However, binocular facilitation did not always occur to the presentation of identical stimuli. For 16% of the cells, maximal responses were observed when lower spatial frequencies than the optimal one were presented in one eye while higher spatial frequencies produced suppression. The opposite pattern was observed only for two cells. These findings are discussed in terms of binocular fusion and suppression.

Binocular interactions were investigated in area 19 of the anaesthetized cat using dichoptically presented phase-shifted static spatial frequency gratings that flickered at a fixed temporal rate. More than two-thirds of the binocular cells showed phase specificity to static phase disparities leading to either summation or facilitation interactions. This proportion of spatial disparity selectivity was higher than that shown for the same area (one-third of the units) when drifting light bars or drifting spatial frequencies were used to create disparities. The range of phase disparities encoded by binocular cells in area 19 is inversely related to the optimal spatial frequency of the dominant eye. Thus, cells in this area are tuned to coarse spatial disparities which, as supported by behavioural studies, could reflect its involvement in the analysis of stereoscopic pattern having gross disparities but devoid of motion cues. Because of the nature of its interconnections with numerous visual cortical areas, area 19 could serve as a way station where stereoscopic information could be first analysed and sent to other higher order areas for a complete representation of three-dimensional objects.

Binocular cells in area 19 are tuned to positional disparities. In effect, up to one-third of the cells respond preferentially to small incongruities between the optimal bar stimuli presented within the receptive fields of each eye. The aim of the present study was to determine whether cells in area 19 are also sensitive to phase disparities. Both types of disparities have been proposed as mechanisms through which stereoperception is achieved. Results indicate that phase disparities produced coherent interactions in 38% of the binocular cells, resulting in facilitation or summation. The remaining cells were phase insensitive. The overall results suggest that cells in area 19 code phase disparities in a proportion comparable to stimulus disparities, confirming that this area is implicated in binocular integration, albeit in a relatively smaller proportion than some of the other visual areas.

The posteromedial lateral suprasylvian area of the cat is known to be involved in the analysis of motion and motion in depth. However, it remains unclear whether binocular cells in the posteromedial lateral suprasylvian area rely upon phase or positional offsets between their receptive fields in order to code binocular disparity. The present study aims at clarifying more precisely the neural mechanisms underlying stereoperception with two objectives in mind. First, to determine whether cells in the posteromedial lateral suprasylvian area code phase disparities. Secondly, to examine whether the cells sensitive to phase disparity are the same as those which code for position disparities or whether each group represent a different sub-population of disparity-sensitive neurons. We investigated this by testing both types of disparities on single neurons in this area. The results show that the vast majority of cells (74%), in the posteromedial lateral suprasylvian area, are sensitive to relative interocular phase disparities. These cells showed mostly facilitation (95%) and a few (5%) summation interactions. Moreover, most cells (81%) were sensitive to both position and phase disparities. The results of this study show that most binocular cells in the posteromedial lateral suprasylvian area are sensitive to both positional and phase offsets which demonstrate the importance of this area in stereopsis.

Sixteen consecutive patients with a partially or completely preserved temporal crescent (PTC) in right or left eye were identified by Goldmann kinetic perimetry. PTC etiologies were stroke, birth injury, trauma, aneurysm, and migraine. PTC eludes detection by automated static perimetry of the central visual field, but its ascertainment with Goldmann perimetry usually implies contralateral occipital lobe ischemia sparing a small portion of anterior primary visual cortex.

Previous studies of the visual system of Siamese cats have shown that binocular cells are scarce in areas 17, 18 and 19, yet significantly more abundant in suprasylvian areas such as the postero-medial lateral suprasylvian area (PMLS). The present study aims at evaluating the sensitivity to spatial disparity of PMLS binocular cells in paralyzed and anesthetized Siamese cats. Centrally located receptive fields were mapped, separated using prisms and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory receptive fields. Delays were introduced in the arrival of the luminous bars in the receptive fields so as to create the desired spatial disparities. Results indicate that approximately a third of PMLS units are binocular and that these binocular cells can detect spatial disparity cues. Indeed, although the sample was relatively small, cells of the tuned excitatory (14/34), tuned inhibitory (2/34), near (6/34) and far (1/34) types were identified. The spatial selectivity, as measured by the width at half height of the tuning curves of the excitatory and inhibitory cells and the slopes of the near and far cells, was similar to that obtained in PMLS of normal cats but not as precise as that found for primary visual areas in these animals. This suggests that these cells might serve as a substrate for coarse stereopsis.

Single units in the posteromedial lateral suprasylvian area of the cat are known to be very sensitive to movement. A proportion of these cells can encode movement in depth, but it is unclear whether posteromedial lateral suprasylvian cells only rely upon motion cues to evaluate stimulus depth or whether they can also code for spatial cues. The present study aims at assessing the sensitivity to spatial disparity of binocular cells, in the postero-medial lateral suprasylvian area, in order to determine whether these units are tuned to positional depth cues. A total of 126 single cells located in the posteromedial lateral suprasylvian area of anesthetized, paralyzed cats were examined. As recordings were performed in the central visual field representation, receptive fields were small. A third of the receptive fields were surrounded by an inhibitory region and almost three-quarters of the cells were direction-selective. Most cells (110/114) were binocular, and a large proportion of single neurons responded to stimuli appearing on the fixation plane by increasing (tuned excitatory cells, 43%) or decreasing (tuned inhibitory cells, 14%) their response rate. A smaller proportion of cells increased their firing rate in response to crossed (near cells, 10%) or uncrossed (far cells, 6%) spatial disparities, hence demonstrating respective preference for stimuli presumably appearing in front of or behind the fixation plane. As compared to primary visual cortex, the proportion of disparity-sensitive cells in posteromedial lateral suprasylvian area is similar, but selectivity is significantly coarser. As the posteromedial lateral suprasylvian area can code for both spatial and temporal aspects of stimuli, this area might be involved in the spatiotemporal integration of depth cues, a process that may also participate in the control of accommodation and vergence.

The receptive field properties of single units were assessed in area 21b of the cat visual cortex. Visual cells in this area were binocular and showed relatively large receptive fields. Most cells were strongly sensitive to the direction of drifting gratings. The mean value of the half-widths of the direction tuning curves (32 degrees ) suggests broader direction tunings than are typically found in other visual areas. The spatial frequency tuning functions were either band-pass or low-pass. Cells responded optimally to low spatial frequencies (mean =0.08c/deg) and also showed low spatial resolution (mean =0.29c/deg.). The estimated values of spatial bandwidths (mean=2.2 octaves) suggest that area 21b cells act as relatively good spatial filters. Although some cells exhibited a low contrast threshold, most cells began to respond at intermediate or high contrast values (mean threshold =15.5%). Temporal frequency tuning functions were mostly band-pass and usually broad (mean temporal bandwidth=3.3 octaves). Cells were found that responded optimally to various temporal frequencies (mean optimal temporal frequency=3.2Hz), although the majority preferred a temporal frequency below 4Hz.These results suggest that visual properties (receptive fields sizes, spatial resolution and orientation/direction selectivity) of cells in area 21b differ from those of cells previously observed in the adjoining area 21a. These differences provide evidence in support of functional distinction between these two visual areas.

The spatial and temporal properties of single neurons were investigated in area 19 of the cat. We evaluated the matching of binocular receptive field properties with regard to the respective strength of the ipsilateral and contralateral inputs. Results indicate that most cells in area 19 are well tuned to spatial and temporal frequencies and exhibit relatively low contrast threshold (mean=6.8%) when assessed using optimal parameters and tested through the dominant eye. Spatial resolution (mean=0.75 c/degree), optimal spatial frequencies (mean=0.16 c/degree) were relatively low and spatial bandwidths (mean=2.1 octaves) were broader as compared to those of cells in area 17 but comparable to those of cells in other extrastriate areas. On the other hand temporal resolution (mean=10.7 Hz), optimal temporal frequency (mean=4.5 Hz) and temporal bandwidths (mean=2.9 octaves) were higher and broader than in primary visual cortex. A significant relationship exists between most of the cell's properties assessed through either eye. For some parameters, such as spatial and temporal resolution, ocular dominance was shown to be significantly related to the extent of matching between the two eyes. For these parameters, binocular cells that exhibited a balanced ocular dominance were generally well matched with regard to the receptive field properties of each eye whereas the largest mismatches were found in cells that were more strongly dominated by one eye. These results suggest that visual input contributes to the activation of cells in area 19 in a redundant manner, possibly attesting to the multiplicity of parallel pathways to this area in the cat.

The spatial disparity sensitivity of single units in the primary visual cortex (17-18 border), in extrastriate area 19 and in the superficial layers of the superior colliculus of the cat brain were compared in the present study. Unit recordings were performed in paralyzed and anesthetized animals. Centrally located receptive fields were mapped, separated using prisms and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory receptive fields. In the three regions studied, cells selective to spatial disparity were found and four classes of disparity sensitivity profiles emerged. Although the disparity sensitivity profiles of the cells in the three regions appeared to have the same general shape, selectivity was clearly different. Cells at the 17-18 border were sharply tuned, those of area 19 were not only less numerous but also less well tuned and collicular cells exhibited coarse selectivity. These differences in selectivity appear to be linked to the projection pattern of the X, Y and W systems to these regions and the roles that these cells might play in vision.

Electrophysiological recordings were carried out in the callosal recipient zone of area 19 in normal and split-chiasm cats and, for comparison purposes, at the border of areas 17 and 18 of split-chiasm cats. The influences of retinothalamic and callosal inputs on a single cortical neurons were thereby evaluated. Extracellular recordings of single cells were made in anaesthetized and paralysed cats in the zone representing the central visual field. Receptive field properties were assessed using sine wave gratings drifting in optimal directions. Results showed that in area 19 and areas 17/18 one-third of the cells were binocularly driven after section of the optic chiasm. In area 19, the spatial resolution and contrast sensitivity of cells driven via the dominant eye were similar in the normal and split-chiasm groups. In areas 17/18 and area 19 of split-chiasm cats, binocular cells showed significant interocular matching of their receptive field properties (spatial resolution and contrast threshold), although small differences were observed. These small interocular differences were related to the cell's ocular dominance rather than to the signal transmission route (thalamic or callosal).

The spatial and temporal selectivities of single neurons in area 21a of the adult cat were investigated using sinusoidal gratings. Optimal spatial frequencies and visual acuity (high cut-off frequency) were fairly low and spatial bandwidth was mainly narrow. Contrast threshold was generally low but a substantial number of cells were only excited by high contrast stimuli. The temporal selectivity suggests that cells responded to a wide range of temporal frequencies.

The present study investigated the postnatal establishment of the laminar pattern of acetylcholinesterase (AChE) activity in the visual cortex (Oc1) of normal and neonatally hemidecorticate rates. Rat pups received a hemidecortication on post-natal day (PND) 3 and sacrificed at three day intervals starting at PND-6 through PND-24. Laminar patterns of AChE activity in Oc1 are described qualitatively and quantitatively using optical densitometry. The postnatal development of the laminar distribution of AChE activity is similar in normal and hemidecorticate rats. In both cases, AChE activity is intense in layer I, in the deep layer III as well as in layer IV. This pattern is first detected at the end of the first postnatal week. AChE activity reaches a peak intensity during the third postnatal week and gradually declines to adult levels during the fourth postnatal week. Hemidecortication has no significant effect on the intensity of AChE activity measured in the visual cortex. Neonatal hemidecortication does not affect AChE activity levels, structure of AChE neurites or the laminar distribution pattern, nor does it affect the time course of the establishment of this pattern in Oc1 of the remaining cortex. These data do not support the hypothesis that massive cortical lesions in rats result in an increase in contralateral cholinesterase activity nor do they suggest terminal sprouting of basal forebrain projections to the visual cortex.

Alpha-1 and alpha-2 adrenergic receptors were localized in developing cat visual cortex by using [3H]prazosin and [3H]rauwolscine, respectively as selective ligands. The effects of neuronal input on the development of the two receptor subtypes were also studied in animals with lesions at various sites within the central visual pathways. Binding densities for both ligands increased during the first few postnatal weeks and declined thereafter. For both receptor subtypes, the highest concentration of binding sites was found in the subplate zone of the cortex in neonatal animals. Both ligands showed their highest concentrations in cortical layer IV beginning at postnatal day 30 and in the superficial cortical layers in adulthood. However, the developmental redistribution of alpha-1 receptors began at earlier ages than that of the alpha-2 sites. The alpha-1 sites were still concentrated in the subplate zone up to 60 days postnatal, while the alpha-2 sites in this region disappeared much earlier. Receptor binding densities were also examined in animals with quinolinic acid lesions within cortex, lesions of the lateral geniculate nucleus and lesions of the optic tract. The results indicate that both alpha-adrenoceptor subtypes were mainly located on cortical cells, and that the absence of neuronal activity during development resulted in a reduction of the binding density for both subtypes in the visual cortex. An additional major reduction in alpha-2 but not alpha-1 binding sites was observed following the lateral geniculate nucleus lesion, suggesting that the development of alpha-2 receptors is also dependent on input from the lateral geniculate nucleus. Removal of the lateral geniculate nucleus early in life resulted in a significant increase in alpha-1 receptors in the subplate region, indicating that receptor densities in this zone may be negatively regulated by the lateral geniculate nucleus afferents. These results show that adrenergic receptors reorganize during postnatal cortical development with a strong temporary concentration in the subplate zone. The reorganization process is heavily influenced by cortical inputs.

Binocular disparity, resulting from the projection of a three-dimensional object on the two spatially separated retinae, constitutes one of the principal cues for stereoscopic perception. The binocularity of cells in one hemisphere stems from two sources: (1) the ganglion cells in the homonymous temporal and nasal hemiretinae and (2) the contralateral hemisphere via the corpus callosum (CC). The objectives of this study were, on one hand, to determine whether disparity-sensitive cells are present in a "higher order" area, namely area 19 of the visual cortex, of the cat and, on the other hand, to ascertain whether the CC contributes to the formation of these cells. As in areas 17-18, two types of disparity-sensitive neurons were found: one type, showing maximal interactive effects around zero disparity, responded with strong excitation or inhibition when the stimuli presented independently to the two eyes were in register. These neurons are presumed to signal stimuli situated about the fixation plane. The other type, also made up of two subtypes of opposed valencies, gave maximum responses at one set of disparities and inhibitory responses to the other set. These are presumed to signal stimuli situated in front of or behind the fixation plane. Unlike areas 17-18, however, disparity-sensitive cells in area 19 of the normal cat were less finely tuned and their proportion was lower. In the split-chiasm animal, very few cells were sensitive to disparity. These results, when coupled with behavioral data obtained with destriate animals, indicate that (1) area 19 is probably less involved in the analysis of disparity information than area 17, (2) the disparity-sensitive neurons that are sensitive to disparity are not involved in the resolution of very fine three-dimensional spatial detail, and (3) the CC only determines a limited number of these cells in the absence of normal binocular input.

Binocular disparity, resulting from the projection of a three-dimensional object on the two spatially separated retinae, constitutes one of the fundamental cues for stereoscopic perception. The binocularity of cells in one hemisphere stems from two sources: i) from the ipsilateral ganglion cells in the temporal retina which converge with inputs coming from the contralateral nasal retina; the latter axons cross at the chiasma; ii) from inputs originating in the opposite hemisphere which cross in the corpus callosum. The objective of this study was to demonstrate that interactions from both types of inputs can result in the formation of disparity sensitive neurons and presumably that either type could mediate stereoperception based on disparity cues. Two types of disparity sensitive neurons were found in the normal cat: one type, showing maximal interactive effects around zero disparity responded with strong excitation or inhibition when the stimuli were in register. These neurons are presumed to signal stimuli situated about the fixation plane. The other type, also made up of two subtypes of opposed valencies, gave maximum responses at one set of disparities and inhibitory responses to the other set. These were presumed to signal stimuli situated in front of or behind the fixation plane. In the split-chiasm cat, whose cortical binocularity is presumably assured by converging ipsilateral and callosal inputs, three of the four subtypes of disparity sensitive neurons were found, the uncrossed disparity cells being absent in these animals. Moreover, stimulating each eye individually indicated that nearly 80% of the cells in normal and about 40% in split-chiasm cats were binocularly driven. However, both these figures underestimated the amount of binocular interaction in the callosal recipient zone, since stimulating both eyes simultaneously showed that a proportionately larger number of cells were binocularly driven. Disparity sensitive cells also varied as a function of ocular dominance, i.e., cells signaling the fixation plane tended to have balanced dominance whereas units preferring stimuli situated in front of or behind the fixation plane were dominated by the ipsilateral and contralateral eyes, respectively.

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.

The present experiment examined the receptive field (R.F.) properties of cortical cells which receive part of their input from the contralateral hemisphere via the corpus callosum. Two groups of cats were used to recording unit activity: a normal control group, and an experimental group consisting of cats which had their optic chiasmas split across the midline prior to the recording sessions. Acute recordings were carried out in the conventional manner using tungsten microelectrodes and N2O: O2 anaesthesia. The recording site was the 17-18 border. The stimulus consisted of a thin bar generated on an oscilloscope screen by a computer. The bar, whose orientation was varied automatically from 0 degrees to 345 degrees in 15 degrees steps, was swept across the screen at constant speed orthogonal to the orientation. Various R.F. properties were studied using both quantitative and qualitative criteria. Thus, in the normal cat, simple, complex and hypercomplex type R.F.'s were found, whereas no callosally activated cell was of the simple type. The ocular dominance distribution found in the split chiasma cat was skewed towards the ipsilateral eye, although a fairly large number of cells could be driven with the two eyes. The R.F.'s of the callosally activated neurons were all situated close to the vertical neurons were all situated close to the vertical meridian, which they sometimes straddled. Both in the normal and in the chiasma sectioned cats, the complex cells had larger R.F.'s than the other cell types. However, the R.F.'s determined through the ipsilateral eye was essentially of the same dimensions as those obtained through the indirect interhemispheric pathway, and this irrespective of cell type. Orientation specificity was similar for the two eyes in the split chiasma cats as it was for the normal cats although in the former the orientation tuning curve was narrower for the callosal pathway than for the more direct thalamo-cortical pathway. The results are interpreted within the context of the different functions ascribed to the corpus callosum in vision.

Unitary discharges of the lateral geniculate nucleus (LGN) were analyzed in anesthetized and paralyzed rats after inactivation of visual cortical areas (VC) by cryoblockade or by depositing a cotton wick soaked in KCl (3 M). The receptive fields were mapped prior to and following the interruption of the cortico-geniculate feedback. The responsiveness of the VC was controlled by monitoring evoked potentials and the EEG. In most off-center and about half on-center cells the surround excitatory responses were markedly reduced and even totally abolished. In contrast, the center excitation remained unchanged or increased suggesting a parallel decline of the inhibitory surround. This differential influence of cortical blockade on on- or off-responses failed to appear in on-off cells whose receptive field was nonconcentrically organized. It is proposed that the VC exerts a complex influence upon geniculate physiology while the spatial center-surround relationships are under the control of the VC. The results of this investigation are comparable to those obtained in rabbits and cats.

Split-chiasm cats with unilateral or bilateral lesions largely removing the commissurally connected portions of visual cortical areas 17, 18 and 19 showed good interocular transfer of monocularly learned pattern discriminations. The capacity for interocular transfer in these cats was in fact little or not different from that of split-chiasm cats with an intact cortex. Split-chiasm cats with an additional section of the forebrain commissures, as well as two split-chiasm cats with 17-18 lesions also submitted to forebrain commissurotomy after having shown good interocular transfer, were generally incapable of transferring pattern discriminations between the eyes. It is concluded that interocular transfer of pattern discriminations, in split-chiasm cats does not require areas 17, 18 and 19 and must therefore depend on other cortical areas.

Cells in area 17 of the cortex are generally activated either directly through a retino-thalamic pathway or indirectly via a contralateral hemisphere-callosal pathway. The aim of the present experiment was to evaluate the effects of eliminating this second pathway on the binocular activation of cells in the primary visual cortex. The optic tract was sectioned on one side in 18 cats and unit activity was recorded in the contralateral hemisphere. This hemisphere should receive normal thalamo-cortical inputs but no visual callosal input. These animals were compared to 21 normal cats. Extracellular electrophysiological recordings were carried out in the conventional way using tungsten microelectrodes and N2O anaesthesia. Results indicated that the proportion of binocular cells found in the cortex of tract sectioned animals was lower than that found in normal animals. However, this decrease in binocularity could be essentially attributed to cells having receptive fields situated to within 4 degrees of the vertical meridian of the visual field. These results are interpreted as being congruent with the demonstrated anatomo-physiological projections of the callosal system.

The spatial separation of the eyes in animals with overlapping visual fields means that parts of a three dimensional object project to slightly disparate retinal points in each eye. This disparity, once interpreted by the brain, is thought to be a sufficient condition for stereoperception. In the present experiment, stereopsis based on spatial disparity cues was evaluated in cats using Julesz random-dot stereograms before and after sections of the optic chiasm, the corpus callosum or both. Normal cats were able to solve the random-dot problem. Optic chiasm transection drastically diminished this ability, callosal section had little effect and combined lesions of these two structures abolished stereoperception. These results suggest that central stereopsis based on spatial disparity is mainly mediated by binocular cortical cells receiving their input via the ipsilateral and the through-the-chiasm contralateral thalamo-cortical pathways.

In this experiment we studied the distribution of callosal cells in various visual cortical areas of both common and siamese cats using the HRP method. The results showed that the most notable difference between the two strains concerned the primary visual areas. Indeed, labeling in areas 17 and 18 was significantly lower in siamese cats and only a few scattered cells were encountered at the 17/18 border. In general, the number of callosal cells forming interhemispheric connections within areas 17, 18 and the 17/18 border was dramatically reduced in the siamese. For all other visual areas (19, lateral suprasylvian subdivisions, 20 a, b and 21 a, b) the distribution was comparable for both species. A study of the laminar distribution indicated that, for common and siamese cats, medium and large pyramidal cells were mainly concentrated in layers III and IV whereas in deeper layers we found a mixture of small pyramidal and fusiform neurons.

Split-brain adult cats with various visual cortex lesions were tested for their ability to acquire several pattern discriminations using monocular vision. Different lesions were made in the two hemispheres to control for possible individual differences in learning speed due to factors other than the lesion. Results indicated that removal of either areas 17, 17 + 18, 18 + 19, or 19 did not abolish the ability to learn pattern discriminations. However, when compared to control animals, lesioned animals learned more slowly the various discriminations, especially subjects having lesions in 18 + 19. These results tend to confirm the hypothesis that the integrity of individual visual cortical areas for the perception and discrimination of simple patterns is important but not essential.

Plasticity in the human and animal brain is the rule, the base for development, and the way to deal effectively with the environment for making the most efficient use of all the senses. When the brain is deprived of one sensory modality, plasticity becomes compensatory: the exception that invalidates the general loss hypothesis giving the opportunity of effective change. Sensory deprivation comes with massive alterations in brain structure and function, behavioural outcomes, and neural interactions. Blind individuals do as good as the sighted and even more, show superior abilities in auditory, tactile and olfactory processing. This behavioural enhancement is accompanied with changes in occipital cortex function, where visual areas at different levels become responsive to non-visual information. The intact senses are in general used more efficiently in the blind but are also used more exclusively. New findings are disentangling these two aspects of compensatory plasticity. What is due to visual deprivation and what is dependent on the extended use of spared modalities? The latter seems to contribute highly to compensatory changes in the congenitally blind. Short term deprivation through the use of blindfolds shows that cortical excitability of the visual cortex is likely to show rapid modulatory changes after few minutes of light deprivation and therefore changes are possible in adulthood. However, reorganization remains more pronounced in the congenitally blind. Cortico-cortical pathways between visual areas and the areas of preserved sensory modalities are inhibited in the presence of vision, but are unmasked after loss of vision or blindfolding as a mechanism likely to drive cross24 modal information to the deafferented visual cortex. The development of specialized higher order visual pathways independently from early sensory experience is likely to preserve their function and switch to the intact modalities. Plasticity in the blind is also accompanied with neurochemical and morphological changes; both intrinsic connectivity and functional coupling at rest are altered but are likewise dependent on different sensory experience and training. [ABSTRACT FROM AUTHOR]