title: Dissociable contribution of the parietal and frontal cortex to coding movement direction and amplitude.
abstract: To reach for an object, we must convert its spatial location into an appropriate motor command, merging movement direction and amplitude. In humans, it has been suggested that this visuo-motor transformation occurs in a dorsomedial parieto-frontal pathway, although the causal contribution of the areas constituting the "reaching circuit" remains unknown. Here we used transcranial magnetic stimulation (TMS) in healthy volunteers to disrupt the function of either the medial intraparietal area (mIPS) or dorsal premotor cortex (PMd), in each hemisphere. The task consisted in performing step-tracking movements with the right wrist towards targets located in different directions and eccentricities; targets were either visible for the whole trial (Target-ON) or flashed for 200 ms (Target-OFF). Left and right mIPS disruption led to errors in the initial direction of movements performed towards contralateral targets. These errors were corrected online in the Target-ON condition but when the target was flashed for 200 ms, mIPS TMS manifested as a larger endpoint spreading. In contrast, left PMd virtual lesions led to higher acceleration and velocity peaks-two parameters typically used to probe the planned movement amplitude-irrespective of the target position, hemifield and presentation condition; in the Target-OFF condition, left PMd TMS induced overshooting and increased the endpoint dispersion along the axis of the target direction. These results indicate that left PMd intervenes in coding amplitude during movement preparation. The critical TMS timings leading to errors in direction and amplitude were different, namely 160-100 ms before movement onset for mIPS and 100-40 ms for left PMd. TMS applied over right PMd had no significant effect. These results demonstrate that, during motor preparation, direction and amplitude of goal-directed movements are processed by different cortical areas, at distinct timings, and according to a specific hemispheric organization.
authors.affiliation: Institute of Neuroscience (IoNS), School of Medicine, University of Louvain Brussels, Belgium ; Sobell Department of Motor Neuroscience and Movement Disorders, UCL Institute of Neurology, University College London London, UK ; Department of Kinesiology, Movement Control and Neuroplasticity Research Group, Biomedical Sciences Group, KU Leuven Leuven, Belgium.
authors.forename: Marco
authors.initials: M
authors.lastname: Davare
authors.affiliation: Institute of Neuroscience (IoNS), School of Medicine, University of Louvain Brussels, Belgium.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Zénon
authors.affiliation: Centre de Neurosciences Cognitives, CNRS, UMR 5229 Bron, France.
authors.forename: Michel
authors.initials: M
authors.lastname: Desmurget
authors.affiliation: Institute of Neuroscience (IoNS), School of Medicine, University of Louvain Brussels, Belgium.
authors.forename: Etienne
authors.initials: E
authors.lastname: Olivier
keywords: action planning; goal-directed; posterior parietal cortex; prehension; reaching; sensorimotor transformation; step-tracking; transcranial magnetic stimulation