Length illusion of Benham’s top－Percept: specific elongation of black arc in white sector at white-to-black transition－

Abstract

Benham’s top makes the observer perceive apparent color when it is spinning. Our new finding is that the perceived length of a line segment is clearly enlarged. We examined three factors of stimuli (line style, position of the line, and background color of the line), and the length illusion in two ways in order to understand the perceptual mechanism; line segment length (magnitude estimation task) and endpoint location (alignment task using other rotating marks outside the disk). Experiments show that the perceived “line segment length” is clearly elongated in the “rear / white-background” condition (max 11 deg. at 60 rpm) whereas the “endpoint location” is mostly correct (max 2.6 deg. at 40 rpm). Results suggest that the length illusion occurs (1) at a relatively high perceptual stage, and (2) only at the white-to-black transition on the white sector.