P1-5: Effect of Luminance Contrast on the Color Selective Responses in the Inferior Temporal Cortex Neurons of the Macaque Monkey

Although the relationship between color signal and luminance signal is an important problem in visual perception, relatively little is known about how the luminance contrast affects the responses of color selective neurons in the visual cortex. In this study, we examined this problem in the inferior temporal (IT) of the awake monkey performing a visual fixation task. Single neuron activities were recorded from the anterior and posterior color selective regions in IT cortex (AITC and PITC) identified in previous studies where color selective neurons are accumulated. Color stimuli consisted of 28 stimuli that evenly distribute across the gamut of the CRT display defined on the CIE- xy chromaticity diagram at two different luminance levels (5 cd/m 2 or 20 cd/m 2 ) and 2 stimuli at white points. The background was maintained at 10 cd/m 2 gray. We found that the effect of luminance contrast on the color selectivity was markedly different between AITC and PITC. When we examined the correlation between the responses to the bright stimuli and those to the dark stimuli with the same chromaticity coordinates, most AITC neurons exhibited high correlation whereas many PITC neurons showed no correlation or only weak correlation. In PITC, the effect was specifically large for neutral colors (white, gray, black) and for colors with low saturation. These results indicate that the effect of luminance contrast on the color selective responses differs across different areas and suggest that the separation between color signal and luminance signal involves a higher stage of the cortical color processing.