lack of control of finger movements. In other words, after commissurotomy, the "target seeing" hemisphere would be able to use positional cues to trigger the program for the ballistic part of an ipsilateral pointing movement, but not to insure the error correcting mechanisms, in the final part of the trajectory. Such a hypothesis appears to be contradictory with the fair accuracy of the right hand, observed in the present experiment. In fact, the apparent contradiction can be resolved by taking into account the possibility of speed-accuracy trade-off. The imprecise ipsilateral pointing, observed in previous studies, seems to be associated with very fast responses for both limbs ( Paillard & Beaubaton, 1974). Conversely, in the present experiment, the nature of the task could have emphasized spatial requirement at the expense of speed. The accuracy of the right hand would therefore be related to the supplementary time spent in the execution phase.
Moreover, the analysis of movement kinematics may help to qualify the ipsilateral motor impairment. The discontinuous nature of acceleration patterns and the overall slowing of the right limb is suggestive of attempts to correct the hand's trajectory. Particularly revealing, in this respect, is the long lasting approach phase. When the right limb was used, a considerable amount of the total time expenditure took place in the last five centimeters, near the target. The question therefore arises as to whether the difficulties in the final approach were strictly due to a defective visuo-motor coordination affecting the whole limb or rather by an impaired control of distal segments. The fact that the execution of pointing responses with a stylus held in hand (Experiment 5) brought about identical performances for the two limbs, in terms of duration and accuracy, suggests a specific impairment when distal adjustments are needed.
The overall data are indicative of a possibility by the "sighted" hemisphere to trigger at a proper time, muscular activities in the proximal parts of the ipsilateral limb and to insure a correct goal-oriented response. However, the ipsilateral control of finger movements is poorly executed. The initiation of these movements is longer than with a contralateral hemisphere-hand pairing, and the difficult selection of appropriate finger position impairs the harmonious time-course of the trajectories.
We wish to thank Jean-René Duhamel who reviewed the translation of the manuscript.
Beaubaton D., & Requin J. ( 1973). "The time course of preparatory processes in split-brain monkeys performing a variable foreperiod reaction-time task". Physiology and Behavior, 10, 725- 730.