Response modulation by texture surround in primate area V1: Correlates of 'popout' under anesthesia

We studied the effects of contextual modulation in area V1 of anesthetized macaque monkeys. In 146 cells, responses to a single line over the center of the receptive field were compared with those to full texture patterns in which the center line was surrounded by similar lines at either the same orientation (uniform texture) or the orthogonal orientation (orientation contrast). On average, the responses to single lines were reduced by 42% when texture was presented in the surround. Uniform textures often produced stronger suppression (7% more, on average) so that lines with orientation contrast on average evoked larger responses than lines in uniform texture fields. This difference is correlated with perceptual differences between such stimuli, suggesting that physiological mechanisms contributing to the saliency ('popout') of textural stimuli operate, at least to some degree, even under anesthesia. Significant response modulation by the texture surround was seen in 112 cells (77%). Fifty-three cells (36%) responded differently to the two texture patterns; response preferences for orientation contrast (35 cells; 24%) were seen more often than preferences for uniform textures (18 cells; 12%). The remaining 59 cells (40%) were similarly suppressed by both texture surrounds. Detailed analysis of texture modulation revealed two major components of surround effects: (l) fast nonspecific ('general') suppression that occurred at about the same latency as excitatory responses and was found in all layers of striate cortex; and (2) differential response modulation that began about 60-70 ms after stimulus onset (about 15-20 ms after the onset of the excitatory response) and was less homogeneously distributed over cortical layers.