The continuous Wagon Wheel Illusion depends on, but is not identical to neuronal adaptation.
The occurrence of perceived reversed motion while observers view a continuous, periodically moving stimulus (a bistable phenomenon coined the "continuous Wagon Wheel Illusion" or "c-WWI") has been taken as evidence that some aspects of motion perception rely on discrete sampling of visual information. Alternative accounts rely on the possibility of a motion aftereffect that may become visible even while the adapting stimulus is present. Here I show that motion adaptation might be necessary, but is not sufficient to explain the illusion. When local adaptation is prevented by slowly drifting the moving wheel across the retina, the c-WWI illusion tends to decrease, as do other bistable percepts (e.g. binocular rivalry). However, the strength of the c-WWI and that of adaptation (as measured by either the static or flicker motion aftereffects) are not directly related: although the c-WWI decreases with increasing eccentricity, the aftereffects actually intensify concurrently. A similar dissociation can be induced by manipulating stimulus contrast. This indicates that the c-WWI may be enabled by, but is not equivalent to, local motion adaptation - and that other factors such as discrete sampling may be involved in its generation.
VanRullen
Rufin
R
CNRS, Centre de Recherche Cerveau et Cognition, Université Paul Sabatier Toulouse 3, 31062 Toulouse Cedex 9, France. rufin@klab.caltech.edu
D000222:Adaptation, Physiological; D000328:Adult; D015350:Contrast Sensitivity; D005260:Female; D005366:Figural Aftereffect; D005403:Fixation, Ocular; D006801:Humans; D008297:Male; D009039:Motion Perception / Q000502:physiology*; D009903:Optical Illusions; D011601:Psychophysics; D012160:Retina / Q000502:physiology*