Spatial interactions in apparent contrast: individual differences in enhancement and suppression effects.

The apparent contrast of a small grating patch embedded in the center of an annular grating filled surround was studied as a function of the surround to central patch contrast ratio, the width of the annulus, and the contrast level of the central patch. Data from ten observers show large individual differences in apparent contrast, but these differences can be interpreted in terms of the relative strength of two interacting mechanisms. One mechanism mediates enhancement of the central patch. The other mediates suppression.