Chromatic sensitivity of surround suppression in macaque V1 and V2

Abstract

Stimulation of the silent surrounds of cortical neurons is known to affect the responsivity and tuning of the classical receptive field in several stimulus dimensions. We have examined the chromatic sensitivity of silent surrounds by measuring size-tuning curves for gratings of differing chromaticities. We recorded from 119 neurons in V1 and V2 of macaques prepared for acute electrophysiological experiments. Receptive fields were stimulated with patches of drifting grating of optimal spatial frequency and orientation. Gratings were modulated along either achromatic or isoluminant (L-M or S-cone isolating) color directions and the diameter of the grating patch was varied. The responses of most neurons declined as the patch was enlarged beyond the optimal size (surround suppression). In V1, the amount of suppression evoked by either L-M or S-cone isolating gratings was less than half that evoked by achromatic gratings. Neurons preferring isoluminant stimuli showed little surround suppression for any color direction. In V2, all neurons (including those preferring isoluminant stimuli) showed as much surround suppression from isoluminant gratings as from achromatic gratings. Thus, cortical mechanisms sensitive to isoluminance provide little input to the surrounds of V1 neurons but substantial input to the surrounds of neurons in V2.