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The photopic spectral sensitivity of one female Cebus Griseus was determined to complement published results showing that this monkey did not demonstrate the red deficiency typical of most New World monkeys. A modified method of limits was used to determine S's perceived brightness of different monochromatic stimuli. The results suggest that this monkey might be a normal trichromat.

The aim of this series of experiments was to evaluate the receptive field properties of visual cells receiving part of their input the corpus callosum. Normal (control) and chiasma sectioned cats were recorded using conventional methods. The recording sites were the 17-18 border and the lateral suprasylvian (LS) cortex. The results indicated (a) the ocular dominance distribution was shifted towards the ipsilateral eye in the split chiasma cats; (b) orientation tunning and/or directional specificity were identical for the two eyes; (c) R.F. positions of binocular cells were also similar for each eye and were clustered near the vertical meridian, which they sometimes straddled; (d) R.F. sizes were larger in L.S. than in primary visual cortex but generally of equal dimensions for each eye. The results are interpreted with respect to the various functions which have been postulated for the corpus callosum.

The visual system is capable of reorganization following experimental interventions, provided that these are carried out before the 'critical period' early in life. Direct evidence suggests that this plasticity may also apply to the commissural system. In the present experiment, we sectioned the posterior two-thirds of the corpus callosum in kittens either before this structure attained maturity, as defined by various anatomical and physiological parameters, or after callosal maturation but before the end of the critical period for most other visual functions. As adult, further transection of the optic chiasma was carried out and these animals were then compared with adult split-brain cats for their ability to transfer monocularly learned pattern discriminations from one hemisphere to the other. Our results indicate that only the first group of animals with the earlier callosal transection demonstrated significant interhemispheric transfer. This suggests that the secondary commissural system is subject to at least some degree of functional plasticity. However, its critical period is very brief and possibly pre-dates the optimal activation of the callosal pathway.

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.

The properties of the receptive fields of visual cortex neurons of cats were studied manually and by a computer controlled system using single lines, double lines and multiple lines (gratings). The multiple selectivities of each of the receptive fields studied make it necessary to abandon the concept that each cell functions as a feature detector. Instead, an attempt was made to classify the receptive field properties with the aim to delineate the transfer functions (of the total networks) served for each property. When tested with one-line stimulus, cells with simple receptive field properties differed from cells with complex receptive field properties as to their velocity selectivity (simple: 1 degree to 3 degrees/s; complex: 4 degrees to 10 degrees/s), spontaneous activity (lower for cells with simple properties) optimal firing rate (lower for cells with simple properties) and receptive field size (smaller for cells with simple properties) but not for orientation and direction selectivity. When tested with a 2-lines moving stimulus, the responses of cells with simple properties were facilitated by the progressive separation of the lines whereas the responses of cells with complex receptive field properties were inhibited. When multiple line, i.e., gratings, were used, an equivalence between simple and X properties and complex and Y properties was shown, while the sustained/transient classification proved to be independent of the simple/complex (X/Y) classification. Thus, receptive field properties can be classified into three categories: one reflects the input to the receptive fields; a second deals with the interactive properties of the fields; while a third appears more related to the overall properties of the network.

We report electrophysiological data regarding the contribution of the corpus callosum to visual responses in the cortex around the anterior ectosylvian sulcus (AES). The experiments were performed in cats in which the optic input from each eye was surgically restricted to the ipsilateral hemisphere (split-chiasm cats), and where neuronal responses to stimulation of the contralateral eye were mediated by interhemispheric connections. A very high proportion of cells were driven by stimuli presented to either eye indicating that they were activated not only through an intrahemispheric pathway from the ipsilateral eye, but also through an interhemispheric pathway from the contralateral eye. With few exceptions, both receptive fields (RFs) of each binocular neuron abutted or were in the vicinity of the vertical meridian. All neurons responded well to moving stimuli and most of them showed directional selectivity. A few cells were activated by stimuli moving in depth. Following an additional section of the posterior half of the corpus callosum, cells in AES responded only to stimulation of the ipsilateral eye, demonstrating thus that the input from the contralateral eye was conveyed by this part of the corpus callosum. By contrast following a section of the anterior half of the corpus callosum, all visually responsive AES neurons were binocularly activated. These results suggest that the interhemispheric visual input to this ectosylvian region is conveyed via a polysynaptic loop involving visual cortical areas that are connected through the posterior portion of the corpus callosum.

The aim of the present experiment was to evaluate the hypothesis that the lateral suprasylvian area is involved in the interhemispheric transfer of visual information. This area was surgically removed in 10 cats which had previously undergone a midsagittal transection of their optic chiasmas. The animals then learned a pattern discrimination using either one or the other hemisphere and were tested for transfer using the other, untrained hemisphere. The lateral suprasylvian area in the intact hemisphere was next ablated in 6 of these cats. Each hemisphere was trained on a new pattern discrimination and tested for transfer using the other. The results obtained with the unilaterally lesioned animals indicated that: (a) learning with the lesioned hemisphere was as rapid as with the intact hemisphere; and (b) that transfer in either direction was normal although slightly retarded, but not significantly so, when the information proceeded from the intact to the lesioned hemisphere. Learning with either hemisphere of the bilaterally lesioned animals also appeared to be normal. Learning with the hemisphere which was lesioned second and transferring to the one which was ablated first was within normal range whereas transfer was generally not as immediate when the procedure was reversed. As a whole, the results, when coupled with those of others, would tend to indicate that the lateral suprasylvian area is involved in interhemispheric transfer but shares this function with other callosally connected areas of the primary visual cortex.

A battery of neurological tests was administered to normal and colliculectomized monkeys to determine the effect of the lesion on visually guided behavior, acoustically guided behavior, general locomotor performance and affective behavior. The tests were given at one week and three months post-operatively. The behaviors of the animals were filmed and the results were derived from a careful analysis of the films. The operated monkeys showed important deficits on most tests. After three months a considerable improvement was noted in many functions. However, some behaviors were still totally or partially abnormal. An attempt is made to relate the completeness of the collicular resection and the duration of some of these symptoms.

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.