Mechanism of positive color afterimage caused by dichoptical presented contours

The negative afterimage following an adaption to a specific color can be enhanced by a contour surrounding the adapting area in the test phase. However, if the contour is presented in a different eye from the adaptor, the observer would see a positive afterimage. To investigate the mechanism responsible for this positive aftereffect, we measured the target threshold versus pedestal contrast (TvC) functions under various adaptor and contour conditions. Each trial began with an adaptation phase in which adaptors (two red squares positioned above and below the fixation) were presented to either the left or right eye for 1s, followed by a test phase in which contours (two white square outlines matching the shape of the adaptors) were presented on either the same or the different eye from the adaptor, or not presented. Simultaneously, two pedestals (green squares) were displayed in the same position as the adaptors with a target square on top of one of the pedestals. Participants were to indicate the location of the target. Without the adaptor and the contour, the target threshold was flat at low and increased at high pedestal contrasts. The threshold increment caused by adaption was large at low pedestal contrast but weakened as the pedestal contrast increased. The same-eye contour decreased threshold at low pedestal contrasts and a masking effect at high pedestal contrasts, whereas the cross-eye contour increased threshold at low pedestal contrasts. These contour effects were weakened with adaptation. These data were fitted with a version of the divisive inhibition model with lateral sensitivity modulation, which showed that the same- and different-eye contours have an opposite effect on the visual mechanism’s sensitivities to the target and that adaption changes the dynamic range of the response function. These effects combine to produce a positive color aftereffect.