Phantom contours: A new class of visual patterns that selectively activates the magnocellular pathway in man

Abstract

The visual system of primates has two anatomical pathways: magno (M) and parvo (P). Here we report a novel technique for selectively stimulating the magnocellular pathway in man. The stimulus was a texture border between black spots and white spots displayed on a uniform gray field. This stimulus (Frame 1) was followed by Frame 2, in which all the black spots were replaced with white spots and all the white spots with black. The procedure was repeated in a continuous cycle; that is, spots reversed polarity without changing positions. At high temporal frequencies (20 Hz), subjects could not see the difference between the flickering spots, but could see a phantom contour separating the two indiscriminable regions. The difference between the spots themselves could be discriminated only at about 17 Hz. Since the M pathway can follow high flicker rates but is insensitive to the sign of the border, we suggest that the phantom border is seen exclusively by the M system. The P system, on the other hand, can report the sign of the border, but only at low flicker rates. Phantom contours provide a psychophysical scalpel for producing a temporary “lesion” in the par-vocellular pathway of intact human subjects. One could therefore repeat all of classical psychophysics and physiology using phantom contours to determine whether or not a given visual process receives an M input. Also, these stimuli might provide a simple diagnostic test for revealing the loss of M-cell function that occurs in early glaucoma. © 1991, Psychonomic Society, Inc.. All rights reserved.