| Motor memory consolidation refers to brain plasticity processes resulting in enduring long-term changes in the neural representations of the learned experiences. One of the paradigms used in the laboratory to study motor consolidation in both its behavioral and neuronal dimensions is the motor sequence learning task. The latter consists in executing the same series of implicitly or explicitly learned movements, and then in looking at the subsequent spontaneous improvement in performance after a period of time without additional practice. On one hand, recent studies have shown that in the case of explicit motor sequence learning, consolidation highly correlated with sleep, and more particularly with N-REM sleep spindles. Even though two types of spindles have been identified (fast and slow spindles), the role of these two sleep features in the consolidation of motor sequence learning is still unclear. In fact, only one study explored this role through artificially altered nights, showing an implication of fast spindles in this process. On the other hand, several functional imaging studies (using functional magnetic resonance imaging [fMRI] and positron emission tomography [PET] scans), have shown changes in the activity of the cortico-striatal system following the consolidation of an explicitly learned motor sequence. But to this day, no study has yet investigated the relationship between these brain functional changes and the sleep spindles characteristics occurring during the night following the experimental task. The objectives of this study were thus: 1) to determine, through polysomnographic recordings and correlation analysis, the contribution of the two spindle types (i.e. slow and fast) to the consolidation of a newly learned motor sequence task following an unaltered night of sleep, and 2) to explore through correlation analysis, the association between sleep spindles and neuronal changes that occur during consolidation of this motor skill. The results of our first study showed that fast, but not slow, sleep spindles play a role in the motor memory consolidation process. Indeed, statistical analyses revealed a significant increase in the density of fast spindles during the night following the motor sequence learning when compared to the control night. Furthermore, this increase in spindles correlated with the spontaneous gains in performance following sleep. Interestingly, the results of our second study revealed correlations between the amplitude of the spindles during the experimental night on the one hand, the amount of spontaneous gains in performance overnight as well as the changes in the BOLD signal within the cortico-striatal system on the other hand. Taken together, our results suggest a functional link between sleep spindles and both overnight gains in performance and brain correlates reflecting motor memory consolidation of a newly acquired sequence of movements. They also support the notion that fast spindles seem to play a more prominent role in this consolidation process, as they appear to help activate the cerebral network involved in it and thus to improve sleep-dependent motor memory consolidation.;Keywords: Motor consolidation, motor sequence, sleep, sleep spindles, slow spindles, fast spindles, EEG, fMRI... |
