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Paolo Bernardis, Paul C. Knox, Nicola Bruno; Pointing and saccading toward the Müller-Lyer illusion: common or separate mechanisms?. Journal of Vision 2004;4(8):835. doi: 10.1167/4.8.835.
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It has often been reported that limb movements and eye movements are driven by common spatial representations. In three experiments using spatially identical displays (variants of the Müller-Lyer illusion), we compared the accuracy of spatial verbal judgments with that of saccadic (eye) and pointing (hand) movements. Verbal judgments showed a clear effect of the illusion. The amplitudes of primary saccades from one endpoint of the pattern (at fixation) to the other also showed an effect of the illusion, but the qualitative pattern of this effect was not identical to that observed for verbal judgments. Conversely, movement amplitudes when pointing from one endpoint (initial finger position) to the other were significantly more accurate than both saccades and verbal responses. In several studies, systematic biases in conscious spatial judgments have been contrasted to accurate open-loop pointing in peripersonal space. It has been proposed (Post and Welch, 1996 Perception 25 569–581) that such seeming dissociations between vision-for-action and vision-for-consciousness may be in fact due to a simple oculomotor strategy: saccade to the target before it disappears, then use the efference copy of the saccadic movement to drive pointing. The present data do not support such an oculomotor hypothesis, and provide constraints on the current debate on perception — action dissociations.
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