Manual asymmetries in the directional coding of reaching: further evidence for hemispatial effects and right hemisphere dominance for movement planning.

Directional coding of hand movements is of primary importance in the proactive control of goal-directed aiming. At the same time, manual reaction times are known to be asymmetric when reaching at lateralized targets. Generally, ipsilateral movements and left hand advantages are interpreted using the classical model of interhemispheric transmission for simple visuomotor integration, but the use of this model was recently challenged when applied to reaching movements, arguing that attentional and biomechanical effects could also account for such asymmetries. In this work, we aimed at controlling both visual attention orienting and movement mechanical constraints in order to clarify the origin of manual reaction time asymmetries and hemispatial effects in the directional coding of reaching. Choice reaction time pointing tasks were assessed in two experiments in which identical movements were compared in different conditions of target lateralization and different conditions of head, eye and hand position. Results suggested that biomechanical constraints could account for hemispatial effects for movement execution but not for movement direction coding. These results are discussed in the light of models of interhemispheric cooperation and the right hemisphere dominance for spatial processing.