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

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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 (ΔC).C was a large, long-duration cosine mask andΔC was a brief, localized, spatially narrow-band "D6" pattern. For same polarityC andΔC, contrast discrimination followed a "dipper" pattern threshold facilitation at lowC and a power relation (exponent < 1.0) at highC. The facilitation is predicted from the low-contrast response of cortical neurons and seems to represent isolation of an ON or OFF pathway. Opposite polarityC andΔC give a monotonic function.ΔC increases at low baseC and remaining higher than the same-polarity function at higherC 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 lowC values and a divisive interaction between pathways at highC values. Test contrast thresholds upon uniform fields of varying luminance show that both the dipper effect and most of the rise inΔC withC 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. © 1995 Elsevier Science Ltd.




Bowen, R. W. (1995). Isolation and interaction of ON and OFF pathways in human vision: Pattern-polarity effects on contrast discrimination. Vision Research, 35(17), 2479–2490.

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