This project is concerned with the rules by which signals from left and right eyes are combined in the visual cortex. Normal binocular vision will be examined as well as abnormal conditions in which convergence from left and right eyes has been interrupted and pathways from one eye are generally dominant. Electrophysiological approaches will be used to study visual responses in cats with normal or abnormal visual experience and psychophysical investigations of human subjects will also be made. These approaches are intended to yield complementary results. There are four principal goals. First, mechanisms of absolute retinal disparity processing will be studied to link the disparity selectivity of cortical cells with size or spatial frequency selectivity and receptive field detail. The main hypothesis to be tested is that disparity information is coded and represented in the striate cortex by phase-disparity selective neurons at different size or spatial frequency scales. Simultaneous recording of two cortical cells is also planned to obtain precise measures of spatial and temporal phase tuning of neighboring cells. Dynamic random-dot stereograms will be used as a visual evoked potential stimulus and to activate single cortical cells to study basic mechanisms of stereopsis. Second, the binocular organization of inhibitory processes will be investigated. Responses of cortical cells will be monitored while a suppressive stimulus is added to one which is maximally effective in activating the units. The inhibitory stimuli will consist of non-optimal gratings or texture (noise) patterns. The major hypothesis in the case of texture is that complex but not simple cells will respond to this stimulus. Texture will also be employed to mask gratings used in psychophysical tests of subjects with normal or abnormal binocular vision to compare relative effects in each group. A third major goal is concerned with the organization of the visual pathways when input from one eye is absent. Deafferentation by unilateral enucleation will be studied physiologically in cats by monitoring responses of individual cells in the lateral geniculate nucleus and visual cortex and by recording gross potentials. Human subjects who have undergone the same procedure will be studied also by recording of gross potentials and by psychophysical tests of visual sensitivity. In both cases, a search will be made for higher than normal visual sensitivity. The fourth goal is to investigate binocular processes that are independent of stereoscopic mechanisms. This involves a unique test which consists of gratings drifting in opposite directions before the two eyes. Each grating is an ineffective visual evoked potential stimulus but the neural combination of both causes a robust response. This test will be used to assess function objectively in cats and humans with abnormal binocular vision.
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