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1Faculty of Psychology and Educational Sciences, University of Geneva, Geneva, Switzerland; 2Department of Neurology, Neurological Hospital Pierre Wertheimer, Bron, France; 3Space and Action, Institut National de la Santé et de la Recherche Médicale, Bron, France; and 4Faculty of Psychology, Vita-Salute San Raffaele University, Milan, Italy
Submitted 17 December 2004; accepted in final form 14 April 2005
An experiment investigated systematic pointing errors in horizontal movements performed without visual feedback toward 48 targets placed symmetrically around two initial hand positions. Our main goal was to provide evidence in favor of the hypothesis that amplitude and direction of the movements are planned independently on the basis of the hand-target vector (vectorial parametric hypothesis, VP). The analysis was carried out mainly at the individual level. By screening a number of formal models of the potential error components, we found that only models compatible with the VP hypothesis provide an accurate description of the error pattern. A quantitative analysis showed that errors are explained mostly by a bias in the represented initial hand position (46% of the sum of squared errors) and a visuomotor gain bias (26%). Range effect (3%), directional biases (3%), and inertia-dependent amplitude modulations (1%) also provided significant contributions. The error pattern was incompatible with the view that movements are planned by specifying either a final posture or a final position. Instead, the results fully supported the view that, at least in the horizontal plane, amplitude, and direction of pointing movements are planned independently in a hand- or target-centered frame of reference.
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