Target recovery in metacontrast: the effect of contrast.

The visibility of a target stimulus (T) can be reduced by an aftercoming and spatially non-overlapping mask stimulus (M1), a phenomenon known as metacontrast masking. Interestingly, the visibility of the masked target can be recovered when a secondary mask (M2) is added to the T-M1 sequence. We analyzed a computational model of retino-cortical dynamics (RECOD) and derived the prediction that contrast dependence of metacontrast and target recovery should parallel the contrast dependence of afferent magnocellular and parvocellular pathways, respectively. In a psychophysical experiment, we tested this prediction by systematically varying (a) M2's contrast and (b) the M1-M2 onset asynchrony (SOA). At the optimal M1-M2 SOA, target recovery effect increased with M2's contrast without saturating, but at the optimal M1-M2 metacontrast SOA, reduction of M1's visibility saturated very rapidly as M2's contrast increased. Quantitative comparisons of psychophysical results with model simulations provide support for our prediction. We conclude that metacontrast masking is driven by signals originating from the magnocellular pathway and target recovery in metacontrast is driven by signals originating from the parvocellular pathway.