Surround suppression in human V1 explains psychophysical lateral masking

Abstract

Purpose: We used fMRI and psychophysics to test whether surround suppression in primary visual cortex (V1) can explain lateral masking. Methods: Observers performed a contrast discrimination task on a contrast-reversing sinusoidal grating target (1 c/d, 4 Hz), restricted to an annulus (4–7 deg eccentricity) both in the presence and absence of a full contrast lateral mask. We measured fMRI responses and psychophysical thresholds for each of several target contrasts, both with and without the lateral mask. V1 was identified using standard mapping procedures, and the analysis was restricted to the subregion of V1 responding to the target. Results: Contrast discrimination thresholds were increased in the presence of the lateral mask, in particular for low target contrasts. We used the psychophysical data to infer a nonlinear contrast response function, assuming that a fixed response difference is required for correct contrast discrimination. FMRI responses increased with increasing target contrast. In the presence of the lateral mask, responses were reduced by about a factor of two. The amount of suppression we observed in the fMRI responses was very similar to that inferred from the psychophysical data. Measurements in a control experiment ensured that the reduction in the fMRI signal reflected a neural suppresion and not some confounding effect of blood flow. In the control experiment, the lateral mask was presented with a temporal delay (375 ms after the target offset). This eliminated the psychophysical masking effect and eliminated most of the reduction in fMRI signal caused by the lateral mask. Conclusion: V1 is a plausible candidate for mediating the lateral masking observed behaviorally.