Binocular interaction and disparity coding at the 17-18 border: contribution of the corpus callosum

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

Neuronal Representation of 3-D Space in the Primary Visual Cortex and Control of Eye Movements

The aim of this article is to consider the correlations between the structure of the primary visual cortical area V1 and control of coordinated movements of the two eyes. Using the anatomical data available, a schematic map of 3-D space representation in the layer IV of area V1 containing only monocular cells has been constructed. The analysis of this map revealed that binocular neurons of V1, which are formed by convergence of monocular cells, should encode the absolute disparity. Participation of monocular and binocular neurons of V1 in the control of convergence, divergence, and version eye movements is discussed. It is proposed that synchronous contraction of corresponding extraocular muscles of both eyes for vergence might be ensured by duplicated transmission of information from the central part of retina to visual cortex of both hemispheres.

Does dominance of crossing retinal ganglion cells make the eyes cross? The temporal retina in the origin of infantile esotropia – a neuroanatomical and evolutionary analysis

A closer look at the evolution of the eye and the brain provides a possible explanation for both the origin of infantile esotropia and its motor characteristics. In the course of evolution, the eyes have moved from a lateral to a frontal position. Consequently, the monocular visual fields started to overlap resulting in a binocular visual field. In lateral-eyed animals, the retinae project to the contralateral visual cortices only. These projections are also found in binocular mammals and birds with binocular visual fields but in addition there are uncrossed projections from the temporal retinae to the visual cortex. The partial chiasmal decussation and the corpus callosum provide the necessary structure that allows binocular vision to develop. Disruption of normal binocular development causes a loss of binocularity in the primary visual cortex and beyond. Beyond the primary visual cortex, the contralateral eye dominates while the temporal retinal signal appears to lose influence. Loss or absence of binocular vision in infantile esotropia may be caused by inadequate retinotopic matching between the nasal and temporal retinal signals like in albinism with an abnormal or asymmetric chiasmal decussation or agenesis of the corpus callosum. Dominance of the crossing retinal signal might also explain the motor characteristics of infantile esotropia (asymmetric OKN, latent nystagmus, DVD). A normal binocular cortical signal will predominate over the evolutionary older, originally non-binocular, retinal projections to the superior colliculi (CS) and the accessory optic system (AOS). A suppressed temporal retinal signal paves the way for the re-emergence of eye movements driven by one eye, as in lateral-eyed non-binocular animals.

The visual callosal connection: a connection like any other?

Recent work about the role of visual callosal connections in ferrets and cats is reviewed, and morphological and functional homologies between the lateral intrinsic and callosal network in early visual areas are discussed. Both networks selectively link distributed neuronal groups with similar response properties, and the actions exerted by callosal input reflect the functional topography of those networks. This supports the notion that callosal connections perpetuate the function of the lateral intrahemispheric circuit onto the other hemisphere. Reversible deactivation studies indicate that the main action of visual callosal input is a multiplicative shift of responses rather than a changing response selectivity. Both the gain of that action and its excitatory-inhibitory balance seem to be dynamically adapted to the feedforward drive by the visual stimulus onto primary visual cortex. Taken together anatomical and functional evidence from corticocortical and lateral circuits further leads to the conclusion that visual callosal connections share more features with lateral intrahemispheric connections on the same hierarchical level and less with feedback connections. I propose that experimental results about the callosal circuit in early visual areas can be interpreted with respect to lateral connectivity in general.

Interhemisphere connections of eye dominance columns in the cat visual cortex in conditions of impaired binocular vision

Data from studies of interhemisphere connections in fields 17 and 18 of cats reared in conditions of impaired binocular vision (monocular deprivation, uni- and bilateral strabismus) are presented. Monosynaptic connections between neurons were studied by microiontophoretic application of horseradish peroxidase into cortical eye dominance columns and the distributions of retrograde labeled callosal cells were analyzed. Spatial asymmetry and eye-specific interhemisphere neuron connections persisted in conditions of monocular deprivation and strabismus. Quantitative changes in connections were less marked in monocular deprivation than strabismus. In cats with impaired binocular vision, as in intact animals, the widths of callosal-receiving zones were greater than the widths of the callosal cell zones, which is evidence for the non-reciprocity of interhemisphere connections in cortical areas distant from the projection of the vertical meridian. Morphofunctional differences between cells mediating connections in the opposite directions are proposed.

Role of primary visual cortex in the binocular integration of plaid motion perception

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.

Disparity sensitivity in the superior colliculus of the cat

The present study aims at evaluating the spatial disparity response profiles of binocular cells in the superficial layers of the superior colliculus of the cat using drifting light bars and phase-shifted spatial frequency gratings. Results show that a total of 64% of the cells were sensitive to phase disparities and had large tuning profiles. Similarly, a large proportion (75%) of those tested with position offsets showed one of the four classic disparity profiles, those of the tuned cells being rather coarse. When tested with both position and phase disparities, 54% of the cells showed sensitivity profiles to the two types of stimuli. The overall results suggest that the superior colliculus is involved in the analysis of coarse stereopsis and/or the planning and initiation of saccades during vergence eye movements and/or the control of fine adjustments to maintain fixation as the stimulus moves in depth.

Binocular fusion/suppression to spatial frequency differences at the border of areas 17/18 of the cat

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.

Phase-disparity coding in extrastriate area 19 of the cat

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.

Phase- and position-disparity coding in the posteromedial lateral suprasylvian area of the cat

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.

Phase disparity in area 19 of the cat

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.

Callosal connections correlate preferentially with ipsilateral cortical domains in cat areas 17 and 18, and with contralateral domains in the 17/18 transition zone

Previous studies have shown that the distribution of callosal connections in the 17/18 callosal zone of the cat is patchy at a small scale, but the mechanisms that determine this periodic pattern remain unclear. The present study investigated this issue by correlating the distribution of retrogradely labeled callosal cells with the underlying patterns of ocular dominance columns (ODCs) revealed transneuronally after intraocular injections of wheat germ agglutinin-horseradish peroxidase. The density of labeled callosal cells was found to vary significantly between adjacent territories dominated by different eyes, indicating that the distribution of callosal cells is significantly biased toward domains that are eye specific. Moreover, callosal connections relate to the pattern of ODCs in a rather unique way: callosal cells correlate preferentially with contralateral ODCs within the 17/18 transition zone (TZ), and with ipsilateral ODCs in regions of areas 17 and 18 located outside the TZ. Similar results were obtained in cats raised with strabismus, indicating that the overlap between right and left ODCs present in normal cats does not influence the correlation between callosal neurons and ODCs. The results are consistent with the hypothesis that callosal linkages are stabilized during development by interhemispheric correlated activity driven by bilateral projections from temporal retina. It is proposed that developmental constraints imposed by both this retinally driven mechanism and the pattern of ODCs are likely to determine not only the association of callosal clusters with specific sets of ODCs, but also important aspects of the functional characteristics of the callosal pathway in cat striate cortex.

The subregion correspondence model of binocular simple cells

We explore the hypothesis that binocular simple cells in cat areas 17 and 18 show subregion correspondence, defined as follows: within the region of overlap of the two eye's receptive fields, their ON subregions lie in corresponding locations, as do their OFF subregions. This hypothesis is motivated by a developmental model (Erwin and Miller, 1998) that suggested that simple cells could develop binocularly matched preferred orientations and spatial frequencies by developing subregion correspondence. Binocular organization of simple cell receptive fields is commonly characterized by two quantities: interocular position shift, the distance in visual space between the center positions of the two eye's receptive fields; and interocular phase shift, the difference in the spatial phases of those receptive fields, each measured relative to its center position. The subregion correspondence hypothesis implies that interocular position and phase shifts are linearly related. We compare this hypothesis with the null hypothesis, assumed by most previous models of binocular organization, that the two types of shift are uncorrelated. We demonstrate that the subregion correspondence and null hypotheses are equally consistent with previous measurements of binocular response properties of individual simple cells in the cat and other species and with measurements of the distribution of interocular phase shifts versus preferred orientations or versus interocular position shifts. However, the observed tendency of binocular simple cells in the cat to have "tuned excitatory" disparity tuning curves with preferred disparities tightly clustered around zero (Fischer and Krüger, 1979; Ferster, 1981; LeVay and Voigt, 1988) follows naturally from the subregion correspondence hypothesis but is inconsistent with the null hypothesis. We describe tests that could more conclusively differentiate between the hypotheses. The most straightforward test requires simultaneous determination of the receptive fields of groups of three or more binocular simple cells.

Spatial and temporal matching of receptive field properties of binocular cells in area 19 of the cat

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.

Spatial resolution and contrast sensitivity of single neurons in area 19 of split-chiasm cats: a comparison with primary visual cortex

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).

Metabolic mapping of visual areas in the behaving cat: a [14C]2-deoxyglucose study

Visually responsive cortical areas and subcortical nuclei were studied in the awake cat using the 2-deoxyglucose technique. Visual input was confined to one hemisphere by unilaterally sectioning the optic tract, the corpus callosum and the commissura anterior. Within the intact hemisphere, numerous cortical regions were distinguishable in the autoradiographs due to differential labelling. Comparison of the intact with the visually deafferented hemisphere confirmed the visual character of eighteen cortical areas (areas 17, 18, 19, 20a, 20b, 21a, 21b, the posteromedial lateral, posterolateral lateral, anteromedial lateral, anterolateral lateral, dorsal lateral, ventral lateral, and posterior suprasylvian areas, the splenial and anterior ectosylvian sylvian areas, insular visual area and posterior area 7) and revealed the visual nature of an area in the posterior cingulate gyrus which had not been described previously. We refer to this area as cingulate visual area (CVA). This area exhibits a gradient in interhemispheric differences along a caudorostral axis similar to that observed in posterior area 7 which is in keeping with the strong and topographic connections between CVA and posterior area 7. These results support the validity of metabolic mapping for the characterisation of cortical areas.

Visual hemispheric dominance induced in split brain cats during development: a model of deficient interhemispheric transfer derived from physiological evidence in single visual cortex cells

The effects of cancellation of both interhemispheric callosal transfer and interocular interactions, were studied in early monocularly deprived cats. The main purpose of this study was therefore to prove whether unilateral hemispheric dominance would result under these conditions and to what extent each hemisphere will be functionally independent. Secondly, we have attempted to establish such an experimental model physiologically, on the single cell level. Interhemispheric transfer was surgically canceled by sagittal transection of the corpus callosum. In addition, the ocular projections were separated by sagittal transection of the optic chiasm in the transbuccal approach. This condition had practically induced visual split brain condition in these cats. These manipulations were carried out concurrently with monocular deprivation (SBDK group) which was surgically done by eye closure during the critical period of development of the visual system. Thus, the hemisphere ipsilaterally to the visually deprived eye had developed under conditions of deficient visual experience while the hemisphere ipsilaterally to the normal eye had developed under conditions of unaltered visual experience. A group of cats (SBK) similarly operated but equally binocularly exposed during development was served as controls. In addition, adult cats similarly operated during adulthood either chronically or acutely were studied to evaluate the effects of interhemispheric and interocular separation. Other groups of cats were also studied for comparison, and included sham operated and normal adult cats. At adulthood, electrophysiological studies were done on these cats, in which action potentials were extracellularly recorded from single cells in the visual cortex (area 17-18 boundary) following anesthesia and paralysis. Stimulation was carried out manually and by a computer driven optical system, presenting on a tangent screen light bars at various spatial positions, orientations and directions. Receptive fields were thus mapped for all neurons and their dimensions and eccentricities were measured. The responsiveness, ocular dominance and other parameters were also studied for these cells. The results in the early deprived cats and in their controls, had shown a full separation between the two hemispheres, as reflected in the almost absolute ipsilateral eye responsiveness (> 97.0% cells). In comparison, in the sham operated and in the normal control cats only minor proportions of cells (13.0-18.7%) have been found as ipsilaterally and monocularly driven, showing almost full interhemispheric and interocular interaction. The main difference, however, in the results between the early monocularly deprived cats and their controls is that in the first group the two hemispheres were asymmetric concerning the amount of visual activation and in the second one they were very symmetric.(ABSTRACT TRUNCATED AT 400 WORDS)

Pattern of development of the callosal transfer of visual information to cortical areas 17 and 18 in the cat

The aim of this study was to investigate the development of visual callosal transfer in the normally reared cat. Two- to nine-week-old kittens and adults (used as controls) underwent section of the optic chiasm. Three days later, the animals were placed under anesthesia and paralysed; unit activities were recorded from visual cortical areas 17 and 18 and from the white matter in one hemisphere. The units were tested for their responses to visual stimulation of each eye successively. Out of 1036 recorded neurons, 185 could be activated through the eye contralateral to the explored cortex via callosal transfer. Most of them could also be driven through the ipsilateral eye via the 'direct' geniculo-cortical pathway. For animals aged > or = 2 weeks, virtually all of these units were located at the 17/18 border zone, with a majority in the supragranular layers. When activated through the corpus callosum, they displayed receptive fields located either on the central vertical meridian of the visual field or in the hemifield ipsilateral to the explored cortex. Such extension into the ipsilateral hemifield as well as receptive field disparities of binocular units decreased with age, while spontaneous activity, strength of response, orientation selectivity and ability to respond to slits moving at middle-range velocity increased. The main conclusion is that the transient callosal projections described by anatomists, which are present until 3 months of age, do not achieve supraliminar synaptic contacts with parts of areas 17 and 18 other than the 17/18 border zone, at least from 12 days after birth. However the visual callosal transfer in young animals displays some characteristics which disappear with age.

Binocular interaction and disparity coding in area 19 of visual cortex in normal and split-chiasm cats

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.