Discussion
manual dexterity, task-specificity of motor learning, finger dexterity, sleep on motor skills acquisition, sleep-dependent learning, peg-in-hole
The main finding of the study was that AO + MI-training enhanced motor learning in terms of manual dexterity, especially when the training sessions were followed by early sleep. Specifically, participants undergoing AO + MI-training improved manual dexterity, but subjects performing AO + MI-training followed by early sleep revealed significantly larger improvements than those undergoing the same training at least 12 h before sleeping. These behavioral findings were supported by neurodynamical correlates during motor performance. Furthermore, sleep-dependent benefits were also detected at 1 month after the training end.Our findings agree with studies showing a role of sleep on motor skills acquisition and proved sleep benefits on motor learning, also when the training is delivered through the systematic observation and mental practice of motor tasks in the absence of executionWhen considering our study design in relation to obtained results, the potential influence of circadian rhythms on motor imagery ability deserves to be also considered. In particular, literature data adopted the temporal equivalence between imagined and executed movements as indicator of motor imagery accuracy, reporting that the ability of healthy subjects to internally simulate their own movements fluctuated through the dayIn the current study, greater enhancement in manual dexterity for AOMI-sleep group occurred at the level of the trained limb, similarly to previous studies showing a task-specificity of motor learning induced by observation in healthy subjectsThe kinematic assessment revealed manual dexterity improvements induced by AO + MI-training, which occurred at training end and persisted up to 1 month in AOMI-sleep and AOMI-control groups with no differences between the two groups. Improvements mainly occurred for total and removing NHPT times and during subphases requiring higher finger dexterity, such as peg-grasp and peg-in-hole. Similarly to functional results, these ameliorations were detected bilaterally. Improvements at the level of the untrained hand agree with studies showing an interlimb transfer effect from the dominant to non-dominant hand after a unilateral dexterity trainingAs anticipated, the opportunity to further enhance manual dexterity through immediate sleep after AO + MI-training was also supported by EEG results, showing greater mu rhythm desynchronization in frontal and parietal brain areas in AOMI-sleep group during NHPT with the left hand at the training end. The magnitude of mu rhythm suppression represents an index of sensorimotor system activation during movements planning and executionSome limitations of the current study need to be underlined. First, imagination abilities and the chronotype of participants were not investigated though specific outcome tools. However, the study had randomized design and these variables are expected to be equally distributed. Second, although participants’ adherence was monitored using daily message on a smartphone and a diary, AO + MIinterventions were unsupervised, and fluctuations of the attentive status might have influenced the training effects. Finally, sleep quantity and quality were monitored using the self-reported number of hours and a specific questionnaire. However, physiological investigations might have provided better sleep monitoring and neurophysiological correlates of sleep-dependent learning induced by AO + MI-training.