Metacontrast masking and the cortical representation of surfacecolor: dynamical aspects of edge integration and contrast gaincontrol

摘要

This paper reviews recent theoretical and experimental work supporting the idea that brightness is computed in a series of neural stages involving edge integration and contrast gain control. It is proposed here that metacontrast and paracontrast masking occur as byproducts of the dynamical properties of these neural mechanisms. The brightness computation model assumes, more specifically, that early visual neurons in the retina, and cortical areas V1 and V2, encode local edge signals whose magnitudes are proportional to the logarithms of the luminance ratios at luminance edges within the retinal image. These local edge signals give rise to secondary neural lightness and darkness spatial induction signals, which are summed at a later stage of cortical processing to produce a neural representation of surface color, or achromatic color, in the case of the chromatically neutral stimuli considered here. Prior to the spatial summation of these edge-based induction signals, the weights assigned to local edge contrast are adjusted by cortical gain mechanisms involving both lateral interactions between neural edge detectors and top-down attentional control. We have previously constructed and computer-simulated a neural model of achromatic color perception based on these principles and have shown that our model gives a goodquantitative account of the results of several brightness matching experiments.Adding to this model the realistic dynamical assumptions that 1) the neuronsthat encode local contrast exhibit transient firing rate enhancement at theonset of an edge, and 2) that the effects of contrast gain control take time tospread between edges, results in a dynamic model of brightness computation thatpredicts the existence Broca-Sulzer transient brightness enhancement of thetarget, Type B metacontrast masking, and a form of paracontrast masking in whichthe target brightness is enhanced when the mask precedes the target in time.