Bimanual coordination with three hands: is the mirror hand of any help?

The mirror paradigm has been used extensively both as a research tool for studying kinesthesia in healthy individuals and as a therapeutic tool for improving recovery and/or alleviating symptoms in patients. The present study of healthy participants assessed the contribution of the mirror paradigm to motor control in a bimanual coordination task performed under sensorimotor disturbance conditions. In Experiment 1, the participants were required to produce symmetrical circles with both hands/arms at the same time. In Experiment 2, the task consisted of synchronous extension-flexion movements of both arms in the sagittal plane. These tasks were performed under four different visual conditions: (i) mirror vision (i.e. with the non-dominant arm reflected in a mirror--the third hand--and the dominant arm hidden), (ii) full vision (i.e. both arms visible), (iii) with only the non-dominant arm visible and (iv) with the eyes closed. In Experiments 1 and 2, sensorimotor disturbance was applied to the participant's dominant arm by co-vibrating antagonistic muscles (the biceps and the triceps). In the complex circle drawing task, bimanual performance was better in the mirror condition than when participants saw their non-dominant arm only. However, motor performance in the mirror vision condition was little better than in the eyes closed condition, regardless of whether or not sensorimotor disturbance was applied. In Experiment 2, there were no differences between the "eyes closed" and "mirror vision" conditions. Although mirror reflection of one arm has been shown to induce consistent, vivid, perceptual illusions (kinesthetic illusion), our results suggest that it is less effective in modulating motor behavior.