Corrélats neuroanatomiques de l’apprentissage de séquences motrices chez les personnes jeunes et âgées

The learning of new motor skills is necessary for the performance of everyday activities throughout the lifespan. Memory consolidation refers to the process during which a newly acquired memory is transformed from an initial labile trace into a more stable and resistant form. Interestingly, there is now compelling evidence that sleep, and sleep spindles in particular, contribute to the consolidation of newly acquired motor sequences in young adults. Although a lot remains unknown regarding the effects of aging on this mnemonic process, behavioral studies have shown that while the initial learning phase is relatively spared in older participants, the latter tend to demonstrate an impairment in the consolidation of learned motor sequences. Work investigating the neural substrates mediating the different phases of motor sequence learning has mostly focused on the grey matter regions involved in this process, establishing that both cortico-cerebellar and cortico-striatal networks contribute to the initial learning stage, while consolidation and long term storage is mostly dependent on a distributed cortico-striatal circuit. Yet, until recently, very few studies had investigated the white matter substrates of motor sequence learning in healthy aging, and none had addressed how white matter differences may contribute to the age-related consolidation deficit. Moreover, no study had yet to investigate whether structural connectivity affects motor sequence consolidation in relationship with sleep spindles. To address these knowledge gaps, we set to characterize the white matter correlates of motor sequence learning and consolidation in healthy young and older adults, and assess the way sleep spindles interact with consolidation processes and this specific neural network. We achieved these goals with the use of diffusion weighted imaging in groups of young and older participants who were trained on an explicit finger sequence learning task before and after an afternoon nap, or an equivalent period of rest. First, our results extend the current knowledge of the neural processes underlying motor memory acquisition by showing that performance improvement relates to white matter characteristics in fiber tracts connecting motor related regions (ie. the internal capsule and the corpus callosum), and that this structure-behavior relationship is preserved across the adult life. Furthermore, our findings reveal that thalamo-cortical white matter indirectly affects motor memory consolidation through sleep spindles modulation in both young and older adults. Although still conjectural, these results suggest that age-related alterations in the white matter tracts known to be involved in sleep spindle expression may constitute one of the neural underpinnings of the motor memory consolidation impairment observed in the elderly. The present work has significant implications as a better understanding of the neural and physiological mechanisms underlying motor skill learning can eventually contribute to the development of interventions aimed to mitigate age-related declines in memory.

Keyphrases: consolidation, Motor sequence learning, White matter, Aging, Sleep, Diffusion weighted imaging, Polysomnography, Memory