Abstract

Electrophysiological investigation has shown that illusory contours arise in V2 but not in V1.¹ This study investigates the fineness of localization in areas 18 and higher by measuring the ability of human observers to bisect a space defined by illusory lines. Subjects viewed a bisecting space formed by either an illusory square, an implied square, or two real lines. A test line was briefly presented within the bisecting space. The subject determined whether the test line appeared to the left of, to the right of, or at the point of bisection. Illusory contours produced thresholds 10-20% smaller than implied contours when the bisecting space was between 1 and 7°. Outside of this range, illusory and implied thresholds were similar, but illusory contours were difficult to perceive. Thus, when visible, illusory contours provide information improving localization ability. When the illusory contours were strong, thresholds were at most a factor of 2 worse than real-line thresholds. This surprising similarity suggests that the spatial grain of the cortex when area 17 is bypassed is only slightly coarser than that of area 17.

© 1988 Optical Society of America