Temporal structure of a bimanual goal-directed movement sequence in monkeys.

The aim of the present investigation was to assess a bimanual goal-oriented movement sequence with particular emphasis on its temporal structure. The three monkeys (Macaca fascicularis) used in this study chose the left arm as the leading and more postural arm to reach out and pull back a spring-loaded drawer containing a food morsel. The right arm followed the left and picked up the food with a precision grip. Video recordings, trajectory recordings of the two index fingers, drawer displacement and the measurements of discrete events of the left and right hand revealed a considerable trial-by-trial variability in the temporal and spatial domain. The variability of latencies progressively increased from the initiation of the bimanual sequence to the left-hand and right-hand events defining goal achievement. The main result was that, in spite of this variability in each of the two limbs, there was an invariant left-right goal-related synchronization. The timing of the goal-related event pairs covaried and showed high correlation coefficients. Covariation of the two hands resulting in an invariant synchronization was particularly striking when monkeys performed the task without vision, and timing of right and left movement components was delayed with further increase in variability. The results indicate that, in the present bimanual skill, kinaesthetic signals may be sufficient to coordinate the two limbs in a goal-oriented unitary action in accord with a memorized plan.