On neural signals that mediate induced blackness

A small patch of achromatic light viewed within a large achromatic surround appears gray or black when the radiance of the surround is well above that of the patch. No single light can match the appearance of the patch because no light in isolation appears blackish; blackness is induced by a second stimulus. The present experiments examine the locus of the mechanisms mediating induction of blackness. They test whether induced blackness can be completely explained by interactions among signals from only one eye (retinal lateral inhibition, for example, though no explicit process is assumed here). If so, a surround affects the appearance of a patch only by modifying a signal that represents the patch at a monocular neural level. This signal may depend on retinal mechanisms and on purely monocular central processes. The alternative hypothesis is that induced blackness depends, at least in part, on a central binocular process driven by more information from each eye than can be carried by a neural signal representing only the patch. Measurements with fused binocular stimuli support the alternative hypothesis, implying retinal mechanisms alone are an incomplete explanation of induced blackness.