REACHING TO IPSILATERAL OR CONTRALATERAL TARGETS - WITHIN-HEMISPHERE VISUOMOTOR PROCESSING CANNOT EXPLAIN HEMISPATIAL DIFFERENCES IN MOTOR CONTROL

Aiming movements made to visual targets on the same side of the body a s the reaching hand typically show advantages as compared to aiming mo vements made to targets on the opposite side of the body midline in th e contralateral visual field. These advantages for ipsilateral reaches include shorter reaction time, higher peak velocity, shorter duration and greater endpoint accuracy. It is commonly hypothesized that such advantages are related to the efficiency of intrahemispheric processin g, since, for example, a left-sided target would be initially processe d in the visual cortex of the right hemisphere and that same hemispher e controls the motor output to the left hand. We tested this hypothesi s by examining the kinematics of aiming movements made by 26 right-han ded subjects to visual targets briefly presented in either the left or the right visual field. In one block of trials, the subjects aimed th eir finger directly towards the target; in the other block, subjects w ere required to aim their movement to the mirror symmetrical position on the opposite side of the fixation light from the target. For the th ree kinematic measures in which hemispatial differences were obtained (peak velocity, duration and percentage of movement time spent in dece leration), the advantages were related to the side to which the motor response was directed and not to the side where the target was present ed. In addition, these effects tended to be larger in the right hand t han in the left, particularly for the percentage of the movement time spent in deceleration. The results are interpreted in terms of models of biomechanical constraints on contralateral movements, which are ind ependent of the hemispace of target presentation.