Handwriting capabilities are crucial for everyday life, and typically involve the dominant hand. These skills may be lost in case of stroke or amputation. As a consequence, these subjects may need to transfer them to their non-dominant hand. Here we investigate whether, in normal subjects, inter-manual transfer can be facilitated by training in a virtual environment. We focused on two types of assistance: visual (subjects could see the reference handwriting template on a computer screen), and visual+haptic. In this case, in addition to visual display, a hand-held robot generated forces that were directed either toward the reference template (path guidance) or toward the reference trajectory (trajectory guidance). The training protocol consisted of 3 assisted exercise sessions (about 1 hr each) on 3 consecutive days. Performance on the following day was tested to assess retention. We found that visual assistance is effective in reducing movement duration, but not in improving the shape error. In contrast, visual+haptic assistance results in a significant decrease in the shape error, which is retained one day after the end of the training. These results are consistent with the notion that visual guidance improves feedback control, whereas haptic assistance improves feedforward control.
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