Psychophysical evidence for boundary and surface systems in human vision.

Psychophysical evidence is given for the existence of two distinct systems in human vision: a fast, sign-invariant system concerned with extracting contours and a slower, sign-sensitive system concerned with assigning surface color. A class of stimuli we developed seems to selectively activate the fast, contour system. This stimulus is formed by adjacent fields of black and white spots, which flicker in counterphase at 15 Hz, on a uniform gray field. Although subjects can not discriminate the temporal phase relationship between the fields of spots, they can, nevertheless see a "Phantom Contour" separating the two indiscriminable fields. The surface characteristics (temporal phase relationship of the spots) can only be seen when the stimulus is significantly slower (flicker < 7 Hz). In addition, phantom contours disappear with equiluminant spots but can be seen with very low contrast spots (< 10% contrast), and are enhanced with peripheral viewing. Taken together, the results suggest that the fast contour-extracting system may be the magnocellular system or a magno-recipient area. Implications for a stimulus which could isolate a contour extracting system, or a magno-recipient area are discussed.