BINOCULAR COMBINATION OF CONTRAST SIGNALS BY STRIATE CORTICAL-NEURONSIN THE MONKEY

With the use of microelectrode recording techniques, rye investigated how the contrast signals from the two eyes are combined in individual cortical neurons in the striate cortex of anesthetized and paralyzed m acaque monkeys. For a given neuron, the optimal spatial frequency, ori entation, and direction of drift for sine wave grating stimuli were de termined for each eye. The cell's disparity tuning characteristics wer e determined by measuring responses as a function of the relative inte rocular spatial phase of dichoptic stimuli that consisted of the optim al monocular gratings, Binocular contrast summation was then investiga ted by measuring contrast response functions for optimal dichoptic gra ting pairs that had left to right-eye interocular contrast ratios that varied from 0.1 to 10. The goal was to determine the left- and right- eye contrast components required to produce a criterion threshold resp onse, For all functional classes of cortical neurons and for both coop erative and antagonistic binocular interactions, there was a linear re lationship between the left- and right-eye contrast components require d to produce a threshold response. Thus, for example for cooperative b inocular interactions, a reduction in contrast to one eye was counterb alanced by an equivalent increase in contrast to the other eye. These results showed that in simple cells and phase-specific complex cells, the contrast signals from the two eyes were linearly combined at the s ubunit level before nonlinear rectification. in nonphase-specific comp lex cells, the linear binocular convergence of contrast signals could have taken place either before or after the rectification process, but before spike generation. in addition, for simple cells, vector analys is of spatial summation showed that the inputs from the two eyes were also combined in a linear manner before nonlinear spike-generating mec hanisms. Thus simple cells showed linear spatial summation not only wi thin and between sub-regions in a given receptive field, but also betw een the left- and right-eye receptive fields. Overall. the results sho w that the effectiveness of a stimulus in producing a response reflect s interocular differences in the relative balance of inputs to a given cell, however, the eye of origin of a light-evoked signal has no spec ific consequence.