Surround modulation in human vision unmasked by masking experiments.

The responses of neurons in cat and monkey primary visual cortex are modulated by stimuli outside the classical receptive field. Here we report psychophysical evidence from masking experiments for two distinct types of surround modulation, one narrowly tuned to iso-orientation (stimuli with center and surround at the same orientation) and the other broadly tuned to cross-orientation (center and surround at perpendicular orientations). Surround modulation at iso- and cross-orientations showed distinct contrast dependencies, and high-contrast cross-oriented surrounds were able to completely eliminate masking. Surround modulation was modeled by subtracting divisive inhibition that raised the gain of spatial filters.