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Tracking errors during the three training and testing days. a Young subjects on day 1. Note that neither the assistance or no assistance group received guidance on day 1. b Young subjects on days 2 and 3. The assistance group received gradually reducing assistance from trials 18 to 31. Trials 38 and 39 were a retention test without assistance on day 3. c Old subjects on day 1 d Older subjects on days 2 and 3. Error bars in all plots show ±1 SD
