The time course of contrast masking reveals two distinct mechanisms of human surround suppression

Abstract

When a target in the periphery is surrounded by a matching mask, contrast sensitivity is suppressed, producing elevated thresholds. We explored the time course of this suppression and found clear evidence for two distinct mechanisms: one strong, transient, and monocular, the other weaker, sustained, and dichoptic. We measured thresholds for a 1.3 cpd Gabor target at 8 deg eccentricity surrounded by a large annulus of matching spatial frequency and orientation. We found that surround suppression is very strong (threshold elevated by up to 600%), but that it decreases by about three-fold when stimuli last longer than ∼100 msec. The strong transient component occurs almost instantaneously (< 1 frame delay, 12 msec) irrespective of the separation between target and surround. The transient suppression differs from the sustained suppression in two remarkable ways. First, it does not transfer dichoptically. Second, it occurs even when the surround is of much lower contrast than the target. Both suppression components are tightly tuned to orientation, peaking at target orientation, but not tuned to spatial phase. These results indicate that there are two distinct kinds of surround suppression in human vision. The properties of the weaker sustained mechanism suggest a cortical origin. The neural substrate of the strong transient suppression is much less evident: the absence of dichoptic transfer suggests a subcortical origin, yet the strong orientation selectivity indicates cortex.