Adaptation of reaching movements to visuomotor transformations is generally thought to involve implicit or procedural learning. However, there is evidence that explicit or cognitive processes can also play a role (Redding and Wallace, 2006 ). For example, the early phase of adaptation to a visuomotor rotation appears to involve spatial working memory processes linked to mental rotation (Anguera et al., 2010 ). Since it is known that cognitive processes like mental rotation lead to larger reaction times (Georgopoulos and Massey, 1987 ), here we explored the relation between reaction time (RT) and reach error reduction. Two groups of subjects adapted their reaching movements to a 60° visuomotor rotation either without RT constraints or with RT limited to 350 ms. In the unconstrained group, we found that adaption rate varied widely across subjects and was strongly correlated with RT. Subjects who decreased hand direction error (DE) rapidly exhibited prolonged RTs whereas little RT cost was seen in subjects who decreased DE gradually. RTs were also correlated with after-effects seen when the visuomotor rotation was removed. Subjects with the longest RTs exhibited the smallest after-effects. In the RT constrained group, all subjects exhibited gradual DE adaptation and large after-effects, similar to the fast responders in the free group. These results suggest that adaptation to a visuomotor rotation can involve processes that produce faster error reductions without increasing after-effects, but at an expense of larger reaction times. Possible candidates are processes related to spatial working memory, and more specifically, to mental rotation.
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