Isolation and interaction of ON and OFF pathways in human vision: pattern--polarity effects on contrast discrimination.

To activate selectively cortical ON and OFF pathways, I measured pattern contrast discrimination functions and manipulated contrast polarity (positive and negative) of base contrast (C) and added contrast (delta 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: threshold facilitation at low C and a power relation (exponent < 1.0) at 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. delta C increases at low base C and remaining higher than the same-polarity function at higher C values. This represents interaction between ON 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 positive contrast. Quantitative aspects of the data suggest a subtractive interaction at low C values and a divisive interaction between pathways at high C values. Test contrast thresholds upon uniform fields of varying 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 contrast discrimination rule out the "channel uncertainty" explanation for the facilitation dipper. My results suggest that parallel ON and OFF pathways evolved because stimulus-produced decreases in the response of a single pathway are potentially confounded with the effects of contrast adaptation. Thus transient decreases in response in either pathway are not processed and both decreases and increases in contrast are expressed as response increases in separate pathways.