Biomedical subjects

Philippe Vindras

Publications and source records attributed to Philippe Vindras.

3 recordsLinked to original sources

PubMed2005

Error parsing in visuomotor pointing reveals independent processing of amplitude and direction.

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.

Adult↗

PubMed2004

Planning short pointing sequences.

An experiment tested the hypothesis that fast, short sequences of movements are planned as a whole, before movement inception. The experimental task consisted of pointing to either one (one-step condition), or two (two-step condition) visual targets aligned along the mid-sagittal axis in a horizontal plane. There were nine possible arrangements of the targets resulting from all combinations of three distances (5, 10, 15 cm), and two trial orders (blocked or random). Performances were characterised by reaction time (RT), movement kinematics, and spatial accuracy. Compared with one-step trials, the first movements of two-step trials had longer RTs (length effect), particularly in random sessions, and when the sequences included short-distance targets. There were also differences in duration (one-target advantage), velocity profile and spatial accuracy that did not depend on the characteristics of the second movement. The results are inconsistent with the assumption that two-step sequences are planned as a whole. Instead, they are in keeping with the alternative hypothesis that part of the preparation of the second step takes place during the execution of the first step.

Adult↗

PubMed2002

Altering the visuomotor gain. Evidence that motor plans deal with vector quantities.

Two experiments investigated the effects of providing nonveridical knowledge of the results (KR) in a visuomanual task in which participants pointed to briefly (200 ms) presented targets without seeing their hand. By showing after each trial the movement endpoint displaced radially with respect to its true position, we were able to alter progressively the gain of the visuomanual loop. In experiment 1, the KR was provided only for transversal movements and for one target distance, but the effect generalized to all directions and all distances. Moreover, it also generalized to the other hand that had never been biased. In experiment 2, nonveridical KR was supplied for movements along the two major diagonals which require sharply different muscle synergies. The transfer to other directions and to the other hand was equally substantial. It is argued that the results support the vector coding hypothesis, which holds that the input to the motor execution stage is supplied by specifying independently the amplitude and the direction of the vector from the initial to the final position in an extrinsic frame of reference. We also discuss the possible brain structures involved in the biasing action of the KR.

Adaptation, Psychological↗