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 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.