How attention and contrast gain control interact to regulate lightness contrast and assimilation: a computational neural model.

Recent theories of lightness perception assume that lightness (perceived reflectance) is computed by a process that contrasts the target's luminance with that of one or more regions in its spatial surround. A challenge for any such theory is the phenomenon of lightness assimilation, which occurs when increasing the luminance of a surround region increases the target lightness: the opposite of contrast. Here contrast and assimilation are studied quantitatively in lightness matching experiments utilizing concentric disk-and-ring displays. Whether contrast or assimilation is seen depends on a number of factors including: the luminance relations of the target, surround, and background; surround size; and matching instructions. When assimilation occurs, it is always part of a larger pattern in which assimilation and contrast both occur over different ranges of surround luminance. These findings are quantitatively modeled by a theory that assumes lightness is computed from a weighted sum of responses of edge detector neurons in visual cortex. The magnitude of the neural response to an edge is regulated by a combination of contrast gain control acting between neighboring edge detectors and a top-down attentional gain control that selectively weights the response to stimulus edges according to their task relevance.