Forward-backward masking of contrast patterns: the role of transients

Graham and Wolfson have shown that, after adaptation to an array of Gabor patterns, observers discriminate poorly between spatially identical Gabor arrays in which the contrasts are symmetrically above and below the adapt contrast. Ten experiments are reported that show that the effect occurs in simple contrast discrimination tasks and examine its properties using a two-alternative spatial forced-choice paradigm. The experiments show that the effect is quite robust and manifests itself in several different experimental paradigms. It occurs with masks as short as 50 ms, but it disappears when an interval as short as 30 ms intervenes between the test patterns and the masks. It occurs at 4 and 8 c/deg. The decrease in discrimination for straddle contrasts is accompanied by an increase in discrimination for contrasts above the mask contrast. A model is presented that describes and predicts the results. The key idea of the model is that patterns which suddenly change contrast produce two responses in the visual system, a V-response that is a function of the absolute value of the difference between the test contrast and the mask contrast and an S-response that is an increasing function of contrast.