Rw. Bowen, ISOLATION AND INTERACTION OF ON AND OFF PATHWAYS IN HUMAN VISION - CONTRAST DISCRIMINATION AT PATTERN OFFSET, Vision research, 37(2), 1997, pp. 185-198
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.