Rw. Bowen, ISOLATION AND INTERACTION OF ON AND OFF PATHWAYS IN HUMAN VISION - PATTERN-POLARITY EFFECTS ON CONTRAST DISCRIMINATION, Vision research, 35(17), 1995, pp. 2479-2490
To activate selectively cortical ON and OFF pathways, I measured patte
rn contrast discrimination functions and manipulated contrast polarity
(positive and negative) of base contrast (C) and added contrast (Delt
a C). C was a large, long-duration cosine mask and Delta C was a brief
, localized, spatially narrow-band ''D6'' pattern, For same polarity C
and Delta C, contrast discrimination followed a ''dipper'' pattern: t
hreshold facilitation at low C and a power relation (exponent < 1.0) a
t high C, The facilitation is predicted from the low-contrast response
of cortical neurons and seems to represent isolation of an ON or OFF
pathway. Opposite polarity C and Delta C give a monotonic function, De
lta C increases at low base C and remaining higher than the same-polar
ity function at higher C values, This represents interaction between O
N and OFF pathways, Pathway isolation also occurs: a positive test is
detected as a contrast increment if masked by negative contrast and a
negative test is detected as a contrast decrement if masked by positiv
e contrast, Quantitative aspects of the data suggest a subtractive int
eraction at low C values and a divisive interaction between pathways a
t high C values. Test contrast thresholds upon uniform fields of varyi
ng luminance show that both the dipper effect and most of the rise in
Delta C with C are mediated in pattern-selective pathways rather than
at a site of luminance adaptation, The pattern-polarity effects on con
trast discrimination rule out the ''channel uncertainty'' explanation
for the facilitation dipper. My results suggest that parallel ON and O
FF pathways evolved because stimulus-produced decreases in the respons
e of a single pathway are potentially confounded with the effects of c
ontrast adaptation, Thus transient decreases in response in either pat
hway are not processed and both decreases and increases in contrast ar
e expressed as response increases in separate pathways.