Mirror versus parallel bimanual reaching

Abstract

Background: In spite of their importance to everyday function, tasks that require both hands to work together such as lifting and carrying large objects have not been well studied and the full potential of how new technology might facilitate recovery remains unknown.

Methods: To help identify the best modes for self-teleoperated bimanual training, we used an advanced haptic/graphic environment to compare several modes of practice. In a 2-by-2 study, we compared mirror vs. parallel reaching movements, and also compared veridical display to one that transforms the right hand's cursor to the opposite side, reducing the area that the visual system has to monitor. Twenty healthy, right-handed subjects (5 in each group) practiced 200 movements. We hypothesized that parallel reaching movements would be the best performing, and attending to one visual area would reduce the task difficulty.

Results: The two-way comparison revealed that mirror movement times took an average 1.24 s longer to complete than parallel. Surprisingly, subjects' movement times moving to one target (attending to one visual area) also took an average of 1.66 s longer than subjects moving to two targets. For both hands, there was also a significant interaction effect, revealing the lowest errors for parallel movements moving to two targets (p < 0.001). This was the only group that began and maintained low errors throughout training.

Conclusion: Combined with other evidence, these results suggest that the most intuitive reaching performance can be observed with parallel movements with a veridical display (moving to two separate targets). These results point to the expected levels of challenge for these bimanual training modes, which could be used to advise therapy choices in self-neurorehabilitation.

Figure 1

Experimental setup (VRROOM).

Figure 2

Four different target locations per hand were presented in a random order with coming back to the home target (red) every time; Mirror (left) and Parallel (right); the arrows show the movement pattern in each group; the numbers represent the x, y and z coordination of the targets.

Figure 3

Group description (top); hand movement (dashed arrow), cursor movement (solid arrow). Sample learning curves for movement time (bottom); right hand (blue), left hand (black), bimanual (each color shows a different reaching direction), blocks used for data analysis (gray).

Figure 4

Parallel two target group is the clear winner for both movement time and error. Each column of dots represents a subject's 20 initial (lefthand) and 20 final (righthand) practice trials, with vertical lines indicating 95% confidence intervals. Change is indicated by diagonal lines (for subjects) and grey bars (group). Significance (solid lines), no significance (dash lines); subject (color).

Figure 5

Across practice, most subjects maintained left hand (red arrows) error while increasing speed in all groups. Right hand (green arrows) error was maintained only in groups reaching in parallel mode. The tail and head of each arrow represents a subject’s average speed/accuracy combinations at the beginning (trials 1–20) and end (trials 80–100) phases of practice.