Nonlinear combination of luminance excursions during flicker, simultaneous contrast, afterimages and binocular fusion

Abstract

The changes in apparent brightness or color, induced into a test spot by a surround, can be greatly enhanced either by flickering the test spot between two luminances, or by binocularly fusing a pair of test spots of different luminances. Simultaneous contrast, in which a white surround makes a grey spot look darker, is greatly enhanced if the spot (not the surround) flickers between black and white. Colour contrast is likewise enhanced by chromatic flicker: on a blue surround, a grey spot looks slightly yellowish, but a yellow/blue flickering spot looks strongly yellow. Temporal successive contrasts, or negative afterimages, are also enhanced by flickering the test field. The negative afterimage of a half-white, half-black rectangle looked dark grey and light grey when projected on a grey test field, but it looked almost black and almost white when projected on a test field that flickered between black and white. Coloured negative afterimages were also enhanced by projecting them on a chromatic flickering test field. We examined the combination rules for pairs of luminances which were presented either successively as flicker or else dichoptically (and fused binocularly). The brightness averaging functions for spatial increments (light spots) on dark surrounds were quasi-linear for binocular fusion but quadratic for flicker. For spatial decrements (dark spots) on white surrounds, the brightness averaging functions were strongly nonlinear winner-take-alll for both binocular fusion and flicker. We also found temporal analogues of Fechner's [(1860). Elements of psychophysics. New York: Holt, Rinehart, Winston, 1966] paradox and Levelt's [(1965). British Journal of Psychology, 56, 1–13] dichoptic contour effect. We conclude that the visual rules for combining luminance excursions, whether in flicker or binocular fusion, favour disproportionately the spot with the higher contrast.