Isolation and interaction of ON and OFF pathways in human vision: contrast discrimination at pattern offset.

Pattern contrast discrimination is typically studied with simultaneous onset of the base contrast (C) and added contrast (delta C) patterns. I measured contrast discrimination functions at pattern offset. A brief (30 msec) localized, spatially narrow-band D6 test stimulus was delta C. The onset of delta C was simultaneous with the offset of a large, 500 msec cosine pattern (the base contrast C). The D6 was either positive or negative contrast, and was masked by either positive or negative contrast, i.e., a light or dark bar of the cosine pattern. Stimuli were 3 cpd. Discrimination of negative delta C at the offset of positive contrast followed a "dipper" function, as if the OFF pathway were isolated. A dipper function was also obtained for a positive delta C at the offset of negative contrast (ON pathway isolation). But same-polarity delta C and C yield a monotonic discrimination function ("bumper" function) at the offset of C, suggesting inhibitory interaction. These discrimination functions for same-and opposite-polarity delta C and C are the reverse of functions obtained at pattern onset. Manipulations of temporal asynchrony between patterns and manipulations of pattern polarity are thus functionally equivalent in determining the form of the contrast discrimination function. In a second experiment, I determined delta C at times before and after the offset of a high-contrast C and manipulated pattern polarity. The time course of threshold change is different for same vs opposite-polarity test and mask. The results suggest that interaction between ON and OFF pathways is delayed relative to the masking process within a pathway. Interaction between pathways may function to improve temporal resolution by suppressing persistence of neural response in the complementary pathway. The present pattern polarity and temporal asynchrony effects on the contrast discrimination function also decisively falsify the "uncertainty" hypothesis for low-contrast threshold facilitation (the dipper).