Discontinuities along lines: psychophysics and neurophysiology

If a segment of a line differs in luminance or color from the rest of the line, three illusory phenomena may be perceived: a reduction in contrast of the line segment relative to the background, subjective contours running perpendicularly to the ends of the line segment, and spread of color or brightness surrounding the line segment. All three phenomena cause a perceptual enhancement of the line discontinuity. Their psychophysical properties and possible neurophysiological correlates in the visual cortex are discussed. Contrast reduction has characteristics similar to end-stopped neuronal responses. Subjective contours elicit orientation-specific neuronal responses in areas 17 and 18. Neon color and brightness spreading may be mediated by "dot-responsive" cells in area 19.