Manual reaction time asymmetries in human subjects: the role of movement planning and attention.

Hemispheric asymmetries in spatial processing are generally considered to be responsible for the shorter reaction time (RT) of the left hand classically observed for right-handers when pointing at targets. Surprisingly, despite the special role which the right cerebral hemisphere is known to play in visual attention, the attentional hypothesis for hand movement preparation asymmetries is currently rejected. This study aims to test the respective roles of visual attention and movement planning in the left hand RT advantage for goal-directed movements. Two experiments were conducted with the same subjects, a simple visual detection task and a classical pointing task, using the same lateralized stimuli. Subjects used the left hand and the right hand alternatively in order to react to the stimuli. In the detection task, the reaction consisted of simply releasing a switch as quickly as possible after the appearance of a target, whereas in the pointing task, it consisted of performing lateralized reaching movements towards the same target. The main results of this study revealed left hand shorter RTs for both tasks, emphasizing the role which right hemisphere dominance for visuospatial attention plays in manual aiming asymmetries. Moreover, a direct comparison of the RTs obtained in both experiments showed that the specific cost of movement planning was lower when using the left hand, therefore also revealing right hemisphere dominance for movement planning.