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 bri ef (30 msec) localized, spatially narrow-band D6 test stimulus was Del ta 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 pos itive or negative contrast, and was masked by either positive or negat ive 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 posit ive contrast followed a ''dipper'' function, as if the OFF pathway wer e isolated, A dipper function was also obtained for a positive Delta C at the offset of negative contrast (ON pathway isolation). But same-p olarity Delta C and C yield a monotonic discrimination function (''bum per'' 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, Manipul ations of temporal asynchrony between patterns and manipulations of pa ttern polarity are thus functionally equivalent in determining the for m of the contrast discrimination function, In a second experiment, I d etermined Delta C at times before and after the offset of a high-contr ast C and manipulated pattern polarity, The time course of threshold c hange is different for same vs opposite-polarity test and mask The res ults suggest that interaction between ON and OFF pathways is delayed r elative to the masking process within a pathway. Interaction between p athways may function to improve temporal resolution by suppressing per sistence of neural response in the complementary pathway. The present pattern polarity and temporal asynchrony effects on the contrast discr imination function also decisively falsify the ''uncertainty'' hypothe sis for low-contrast threshold facilitation (the dipper). Copyright (C ) 1996 Elsevier Science Ltd.