Hemispheric asymmetry and interhemispheric transfer in reaching programming

The purpose of this study was to explore the intrahemispheric processes and the interhemispheric transfer that occur during the programming of a pointing movement. Twenty five subjects participated in this experiment: 12 were right-handed (Rhr), 12 left-handed (Lhr), and 1 was left-handed with a posterior callosal lesion. The task consisted in producing an open loop pointing response toward a visual target appearing briefly on the right or the left of a central fixation point. Reaction times (RTs) were shorter for the Rhrs when reaching with the left hand than with the right hand. No such hand-related difference was observed in the Lhrs. The left hand advantage indicates that one process was faster in the right hemisphere of Rhrs. This faster process appears not to be visual but motor or visuomotor. For either hand, responses were faster when the target appeared in the visual field homolateral to the pointing hand (uncrossed condition) than when it appeared contralaterally to the hand (crossed condition). The crossed vs uncrossed difference did not vary between Rhrs and Lhrs or between the hands. The transfer time between the hemispheres was symmetrical whatever its direction. The partially callosotomized left-handed subject was two-fold slower than the control Lhrs. His uncrossed responses were faster than the crossed ones, but his interhemispheric transfer time was very asymmetrical: it was normal from right to left hemisphere but was highly increased in the opposite direction. An attempt at modelling the RT data is proposed and the possibility of different callosal locations for the interhemispheric transfer is discussed.