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Abstract

In recent studies we found effects of visual illusions on the maximum grip aperture in grasping. Here, we ask whether these effects decay (or build up) during the execution of a grasp movement. Some recent studies suggest a decay (Glover Dixon, Perception and Psychophysics, 64, 266–278, 2002), while the view of others is more consistent with a build up (Carey, Trends in Cognitive Sciences, 5, 109–113, 2001). We reanalyzed the data of different studies on the Ebbinghaus / Titchener illusion (Franz et al., Psychological Science, 11, 20–25, 2000; Franz et al., Experimental Brain Research, in press) which used very large sample sizes (26 and 52 participants). The hand aperture of each grasp movement was analysed at different, normalized time points. Special care was taken to avoid possible artefacts which might arise from the hand already touching the target object. Also, we corrected at each time point for the responsiveness of the hand aperture to a physical variation of size. Results show that the illusion effects are remarkably constant over time. This suggests that either the neuronal signals which cause the motor illusion are constant over time, or that the grasp trajectory is largely preprogrammed before the movement starts.