The Müller-Lyer illusion: investigation of a center of gravity effect on the amplitudes of saccades.

Previous research has compared the effects of visual illusions on perception with their effects on action to investigate if the action system and the perceptual system use different or common codes. Appropriate conclusions based on this comparison rely on effects that reflect the internal parameter estimates of the action and of the perceptual system. We investigated an additional factor that can possibly change the amplitudes of saccades along the Müller-Lyer illusion, the center of gravity effect. It refers to the finding that the endpoints of saccades can be diverted from the target point in the direction of the center of gravity of a stimulus configuration. We measured the perceptual (adjustment method) and the action effects (amplitudes of saccades) of the illusion. In addition, we let subjects carry out saccades along Müller-Lyer figures and a neutral figure that appeared to have the same size (but differed in actual sizes). The amplitudes of saccades differed for these figures. This was interpreted as evidence for a center of gravity effect. Its quantification allowed a correction of the action effect, which was then remarkably similar to the perceptual effect. Our results are in agreement with the notion of a common internal representation for perception and action.