Does the Ebbinghaus Illusion Affect Optimal Pointing?

Abstract

We investigated speeded pointing movements (Trommershauser, Maloney, and Landy, 2003) in the presence of the Ebbinghaus illusion. Franz et al (2000) reported systematic effects of the Ebbinghaus illusion on grasping movements, contradicting earlier results (Aglioti, et al, 1995) and casting doubt on a strict dissociation between action and perception (Goodale and Milner, 1992). Here, we try to extend this finding by explicitly manipulating the consequence of motor actions in a speeded pointing task. We hypothesised that large, medium, and small circular surrounds induce the perception of smaller, unchanged, and larger target and penalty areas at the centre. If subjects take the visual illusion into account, then on average they should overshoot, hit, and undershoot the optimal point in the three illusion conditions, respectively. Subjects were asked to hit a circular target area on a touch screen with their right index finger. They were awarded +100 points for hitting the target, and in separate condition...