Simultaneous bimanual dynamics are learned without interference

Abstract

Dynamic learning in humans has been extensively studied using externally applied force fields to perturb movements of the arm. These studies have focused on unimanual learning in which a force field is applied to only one arm. Here we examine dynamic learning during bimanual movements. Specifically we examine learning of a force field in one arm when the other arm makes movements in a null field or in a force field. For both the dominant and non-dominant arms, the learning (change in performance over the exposure period) was the same regardless of whether the other arm moved in a force field, equivalent either in intrinsic or extrinsic coordinates, or moved in a null field. Moreover there were no significant differences in learning in these bimanual tasks compared to unimanual learning, when one arm experienced a force field and the other arm was at rest. Although the learning was the same, there was an overall increase in error for the non-dominant arm for all bimanual conditions compared to the unimanual condition. This increase in error was the result of bimanual movement alone and was present even in the initial training phase before any forces were introduced. We conclude that, during bimanual movements, the application of a force field to one arm neither interferes with nor facilitates simultaneous learning of a force field applied to the other arm.

Figure 1

Top view of experimental setup. The dots surrounding the central home positions represent potential target locations that the subject must reach to before returning to the home position.

Figure 2

A) Hand-paths of a typical subject on first exposure to a CCW force-field B) Hand-paths of the same subject in the final cycle of the exposure period. Hand-path is shown in grey with 8 radial target positions shown at their correct size (radius 1 cm) and position in dark grey. Catch-trial paths are shown in black. It can be seen that peak perpendicular distance (PD) decreases from early to late cycles, whilst PD increases for late catch-trials, indicating that learning and not co-contraction took place.

Figure 3

Performance during bimanual tasks with three different field types (extrinsic – green, intrinsic – red and null – yellow). A) shows PD for the right arm, B) shows PD for the left arm. PD was averaged over 7 trials per cycle and 6 subjects in the group, and is shown from 5 cycles before the onset of the field to 5 cycles after. The shaded areas show ±1 s.e. of the mean.

Figure 4

Performance when a single force field was applied during a bimanual movement task (yellow), when the other arm moved in a null field, compared with adaptation when a single force field was applied during a unimanual movement task (right arm – blue, left arm – cyan). A) shows PD for the right arm, B) shows PD for the left arm. The shaded areas show 1 s.e. of the mean.

Figure 5

Performance of the left arm (cyan) compared to the right arm (blue) during a unimanual task. The shaded areas show 1 s.e. of the mean.

Figure 6

Performance in the early and late epochs for the left and right arm for all groups. Early epoch performance was defined as the group average of the mean PD of the 2^nd and 3^rd cycle for each subject. Late epoch performance was defined as the group average of the mean PD of the 49^th and 50^th cycle for each subject.