Lateral interactions among contrast sensitive mechanisms

We have previously demonstrated suppression of the apparent contrast of a small centrally viewed, 0.25 contrast, sinewave grating patch by a peripheral grating, of 0.5 contrast, presented in an annular surround of variable width.1 We conducted two new experiments with eight subjects to extend our understanding of these lateral interactions by using several central patch contrasts and several surround to center contrast ratios. Experiment 1 determined how center apparent contrast changed as surround size, central patch contrast, and surround to center contrast ratio were changed systematically. This experiment revealed that central patch suppression is not universal for all subjects in all conditions studied. While two subjects did show universal suppression, two other subjects showed almost universal enhancement, and the other four showed a systematic change from suppression to enhancement as stimulus conditions changed. Experiment 2 investigated lateral interactions over a much larger surround to center contrast ratio at a fixed surround width. These data indicate that all subjects show suppression for surround to center contrast ratios of 4.0 and higher. Enhancement, when it exists, occurs only for surround to center ratios between 0.5 and 4.0. The data indicate that contrast perception for complex stimuli may be mediated by an interaction of both excitatory and inhibitory interconnections among contrast sensitive mechanisms. Possible reasons for the different types of behavior across subjects with essentially normal vision are discussed.