Effect of trial order and error magnitude on motor learning by observing

Abstract

Watching an actor make reaching movements in a perturbing force field provides the observer with information about how to compensate for that force field. Here we asked two questions about the nature of information provided to the observer. Is it important that the observer learn the difference between errant (curved) movements and goal (straight) movements by watching the actor progress in a relatively orderly fashion from highly curved to straight movements over a series of trials? Or is it sufficient that the observer sees only reaching errors in the force field (FF)? In the first experiment, we found that observers performed better if they observed reaches in a FF that was congruent, rather than incongruent, with the FF used in a later test. Observation-trial order had no effect on performance, suggesting that observers understood the goal in advance and perhaps learned about the force-field by observing movement curvature. Next we asked whether observers learn optimally by observing the actor's mistakes (high-error trials), if they learn by watching the actor perform with expertise in the FF (low-error trials), or if they need to see contrast between errant and goal behavior (a mixture of both high- and low-error trials). We found that observers who watched high-error trials were most affected by observation but that significant learning also occurred if observers watched only some high-error trials. This result suggests that observers learn to adapt their reaching to an unpredictable FF best when they see the actor making mistakes.

Fig. 1.

Experimental set-up. A: 1 frame from a video displayed during observation. The video showed the actor moving the manipulandum handle from the start position to the target. A synchronized, transparent view of the start position, target, and cursor that was viewed by the actor was layered over the video of arm movements. B: the entire set of targets and the central start position used in both experiments.

Fig. 2.

Trajectories from experiment 1. A: a representative subject from each experimental condition. B: the mean of trials 1–8 (all directions, no catch trials) and for all subjects in each experimental condition.

Fig. 3.

Results from experiment 1. A: learning over bins of 16 trials. Although there were differences in performance initially, these differences were no longer significant by the end of training. Error bars represent SE. B: participants' initial response to the clockwise velocity-dependent perturbing force field (CWFF) was significantly influenced by whether they previously observed a counterclockwise velocity-dependent perturbing force field (CCWFF) or a CWFF movie, but it was not significantly affected by whether they observed the fixed or random trial sequence.

Fig. 4.

Trajectories from experiment 1. A: a representative subject from each experimental condition. B: the mean of trials 1–8 (all directions, no catch trials) and for all subjects in each experimental condition.

Fig. 5.

Results from experiment 2. A: learning over bins of 16 trials. Although there were differences in performance initially, these differences were no longer significant by the end of training. Error bars represent SE. B: participants' initial response to the CWFF was significantly influenced by an interaction of force-field observation direction (CCWFF, CWFF) and information level (high error, high and low error, and low error).