Nature of perceptual encoding of hand positions according to subjects' perceptuomotor expertise.

We investigated whether the nature of the perceptual information encoded for the memorization of successive hand positions varied according to subjects' perceptuomotor expertise. The experimental task used perceptual conflicts, induced by prisms, between visual and proprioceptive afferences during the encoding of various static hand positions. Control subjects were compared with skilled fencers, whose expertise was derived from the optimization. required and developed with practice, of the perceptuomotor processes involved in movement control. The analysis of spatial errors observed in a remembered target location test indicated that control subjects were dependent on the use of static visual information for the encoding of hand positions, whereas skilled fencers were not. Results are discussed in the light of the role of expertise in sensory integration and perceptual dominance.

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.

The gap effect for eye and hand movements in double-step pointing.

The existence of a temporal gap between the offset of a fixation target and the onset of a peripheral target generally reduces the saccadic and manual reaction time in response to the peripheral target. Using a double-step paradigm, the present experiment investigated whether a temporal gap between the extinction of the first target and the presentation of the second target can help in reducing the time to trigger the corrective eye movements and to correct the arm trajectory towards the final target position. A gap was introduced between the presentation of the initial target and a new unexpected goal-target during the movement. The results replicated the gap effect for the corrective saccade to the second target, but revealed an opposite effect for the correction of the reaching movements as the arm correction occurred later in the Gap than in the No-Gap conditions. These results suggest that the information available for the arm motor system to correct the trajectory in relation to the second target was different in the Gap and No-Gap conditions. In the No-Gap condition, the correction of reaching movements would be based on retinal errors between the first and the second targets whereas, in the Gap condition, the correction would be based on information derived from the corrective saccade-related signals to the second target.

Manual asymmetries in reaching movement control. I: Study of right-handers.

Two experiments investigated manual asymmetries in the control of rapid reaching movements according to the movement parameters to be controlled. Single- and double-step reaching movements were performed by right-handed subjects with both hands. Pro and retroactive processes involved in rapid movement control were investigated. Manual performances and kinematic properties of hand movements showed that various forms of hemispheric specialization were involved in sensori-motor information processing. It was shown that the effects of hemispheric specialization were specific to the task constraints, that is, to the various operations involved in movement control.

Manual asymmetries in reaching movement control. II: Study of left-handers.

Two experiments performed with left-handed subjects investigated how the manual asymmetries and hemispheric specialization involved in visuo-manual coordination are associated with handedness. Pro and retroactive processes involved in rapid movement control were analyzed according to the different movement parameters to be controlled, similar to studies performed with right-handers (Boulinguez, Nougier and Velay, 2001). Manual performances and kinematic properties of reaching movements showed that the left and right hands of left-handers behaved in the same way as the left and right hands of right-handers. Results are discussed in the light of the independence of handedness and other forms of cerebral dominance in sensori-motor information processing involved in hand movements.

Influence of the movement parameter to be controlled on manual RT asymmetries in right-handers.

In this experiment we test whether the effects of manual asymmetries on movement preparation depend on the parameter (amplitude or direction) to be programmed. In two experiments, only the amplitude, or the direction, of aiming movements was constrained. Reaction and movement times were measured. Results show that RTs are always shorter for left-hand than for right-hand movements. There is an effect of target extent in the amplitude condition, but not in the direction one. RTs for ipsilateral movements are shorter than RTs for contralateral movements. These results are discussed in the light of the processes involved in setting the amplitude or direction of the movement and with regard to the competency of the two hemispheres regarding these processes.

Control of goal-directed movements: the contribution of orienting of visual attention and motor preparation.

Three experiments investigated the role of attention and motor preparation for the control of goal-directed movements. In Experiment 1 (double step paradigm), a movement correction was required on 25% of the trials towards the left or right of the initial target. Within these 25% of trials, the probability of location of the second target was manipulated. The efficiency of movement control increased when increasing the probability of correcting the movement in a given direction. In Experiment 2, attentional processes were isolated by asking the subjects to verbally detect the more or less probable target displacement, without correcting their movement. Subjects were able to orient visual attention during movement execution, thus improving the processing of visual feedbacks from target displacement. In Experiment 3, motor preparation processes were isolated by asking the subjects to correct their movement towards a fixed target in response to a more or less probable mechanical perturbation. It was shown that motor preparation not only specifies the initial movement parameters but may also include some parameters of the most probable movement modulations. Overall, these results highlight the role of both attentional and motor preparation processes in the control of goal-directed movements and suggest that the feedback-based corrections of the movement are modulated by a feedforward control.