Space-time separation during obstacle-avoidance learning in monkeys

Is the movement duration time known before we move? To answer this question, a new experimental paradigm is introduced that for the first time monitors the acquisition of a new motor skill in rhesus monkeys. Straight reaches were interleaved with reaches around physical obstacles that elicited a different path geometry. Curved and longer spatial paths were immediately resolved and consistent over months of training. A new temporal strategy separately evolved over repetitions from multiple to a single velocity peak. We propose that the obstacle-avoidance spatial paths were resolved before motion execution and used as reference in the computation of the new dynamics. Path conservation from the first trial occurred both at the hand and at the joint angle levels, whereas the speed profile dramatically changed over time. The spatial solution required no learning and was anticipated by the spontaneous repositioning of the initial arm posture. The learning was in the temporal domain, involving the adjustment of the speed during the motion's first impulse. Within the movement initiation, the partial distance traveled by the hand up to the first velocity peak was finely tuned under a constant time. For a given space location, the time of the first impulse remained robust to learning, but significantly shifted for different targets and obstacle configurations. Differences in the temporal-related parameters across time provided a clear distinction between learning and automatic behavior.