Dynamic cortical involvement in implicit and explicit motor sequence learning - A PET study

We examined the dynamic involvement of different brain regions in implicit and explicit motor sequence learning using PET. In a serial reaction time t ask, subjects pressed each of four buttons with a different finger of the r ight hand in response to a visually presented number. Test sessions consist ed of 10 cycles of the same 10-item sequence. The effects of explicit and i mplicit learning were assessed separately using a different behavioural par ameter for each type of learning: correct recall of the test sequence for e xplicit learning and improvement of reaction time before the successful rec all of any component of the test sequence for implicit learning. Regional c erebral blood flow was measured repeatedly during the task, and a parametri c analysis was performed to identify brain regions in which activity was si gnificantly correlated with subjects' performances: i.e. with correct recal l of the test sequence or with reaction time. Explicit learning, shown as a positive correlation with the correct recall of the sequence, was associat ed with increased activity in the posterior parietal cortex, precuneus and premotor cortex bilaterally, also in the supplementary motor area (SMA) pre dominantly in the left anterior part, left thalamus, and right dorsolateral prefrontal cortex. In contrast, the reaction time showed a different patte rn of correlation during different learning phases. During the implicit lea rning phase, when the subjects were not aware of the sequence, improvement of the reaction time was associated with increased activity in the contrala teral primary sensorimotor cortex (SM1), During the explicit learning phase , the reaction time was significantly correlated with activity in a part of the frontoparietal network, During the post-learning phase, when the subje cts achieved all components of the sequence explicitly, the reaction time w as correlated with the activity in the ipsilateral SM1 and posterior part o f the SMA. These results show that different sets of cortical regions are d ynamically involved in implicit and explicit motor sequence learning.