Abstract
The snake illusion is a lightness effect in which identical gray targets embedded in complex displays may look either strongly different (‘snake’) or very similar (‘anti-snake’), depending on the luminance of some non-adjacent patches. It has been suggested (Logvinenko et al., Perception & Psychophysics, 2005, 67, 120-128) that this effect is based on the classification of luminance edges as either reflectance or illumination edges (which also involve a sense of transparency or shadow), and the tendency of the visual system to interpret edges as the former rather than the latter if they are curved rather than straight. To examine these notions, we used five pairs of snake/anti-snake displays (created by switching the luminance levels of certain portions of displays), each containing six targets (a high luminance, a medium luminance, and a low luminance pair). The displays were presented on a calibrated monitor placed in a dark void. Each of our nine naïve observers participated in four individual sessions. They performed lightness matches by adjusting the luminance of comparison patches on the screen. We replicated the basic effect with a snake/anti-snake stimulus pair slightly modified from original, which contained straight luminance edges and a sense of shadow or transparency. However, we obtained an effect of the same strength with a variant display which retained much the same structure as the first, including straight edges, but involved inverted transparency conditions. For displays involving curved or jagged edges we found that the strength of the effect was clearly diminished. Our results confirm previous findings that the shape of luminance edges may affect the strength of this class of illusions, but argue against theories based on edge classification and transparency/shadow perception. We also found that the illusion was strongest for medium luminance targets, weaker for high luminance targets, and weakest for low luminance targets.
This research was supported by Grant #149039D from the Serbian Ministry of Science.