The persistence of spatial interference after extended training in a bimanual drawing task

Abstract

Many studies of bimanual coordination have focused on the pervasive interference observed when people plan and produce non-symmetric movements. We investigated how the interference observed in one challenging bimanual task, simultaneously drawing non-symmetric three-sided squares (e.g., U and C), is modulated by practice. We assessed whether the benefits of practice were limited to the trained patterns or reflected the development of a more general ability for independently controlling movements of the two hands. We combined four orientations of a three-sided square, with one orientation assigned to each hand, to generate a set of 16 patterns. Participants were trained for six days with eight of the patterns. In the last two sessions, all 16 patterns were tested. The untrained patterns involved a shape that had not been practiced by one hand or a novel configuration of two practiced components. While a substantial reduction in inter-manual interference was observed over the extensive training period, participants remained much slower to plan incongruent shapes compared to congruent shapes. Incomplete generalization was observed when the new patterns were introduced. Planning time was shorter and accuracy higher for the trained patterns, but this effect was only observed in the first generalization session. There was little difference in performance between new patterns that involved an unpracticed shape or an unpracticed configuration. These results indicate that spatial interference was not eliminated with extensive practice. This persistent interference effect stands in contrast to the minimal interference observed when the gestures are conceptualized as a single action or do not involve the transformation of abstract spatial codes. The results suggest that a primary difficulty in bimanual drawing results from limitations in translating abstract goals into actions, a fundamental prerequisite for praxis.

Figure 1. The complete stimulus set and a representative velocity profile from one trial

(a) The stimulus set was formed by the factorial combination of the four orientations of 3-sided squares. Congruent patterns (no shading) were pairs involving translational or reflective symmetry; the remaining patterns were incongruent (shaded). Eight patterns were selected for training, subject to the constraints that three were congruent and one row was completely excluded. (b) The velocity profile from a trial in which the participant produced a congruent pattern. Similar to most trials, the movements of the two hands were closely synchronized for each of the three segments.

Figure 2. Planning times during the training phase

Planning time is defined as the sum of reaction time and the pauses that occurred prior to the start of the 2nd and 3rd segments. Participants became faster with practice, although there remained a substantial difference between incongruent patterns (black) compared to congruent patterns (grey). Error bars indicate +/− the standard error of the mean.

Figure 3. Planning times for an individual participant during the training phase

This participant showed the smallest congruence effect of all of the participants at the end of training. Nonetheless, there remained a substantial difference between the planning time for incongruent patterns (black) compared to congruent patterns (grey). Example trials are shown for each class of patterns at the beginning and end of training. The scale for all of the movements is indicated in the lower left example. Consistent with that observed in the group data, this participant showed a substantial reduction in planning time with little change in accuracy for the incongruent patterns..

Figure 4. Planning times during the generalization phase (Days 7 & 8)

An increase in planning time was found for the new patterns, especially on incongruent trials. This increase was similar for the novel shape and novel configurations. By the second day of practice, the costs associated with the new patterns are almost entirely eliminated. Error bars indicate +/− the standard error of the mean.