AN ELECTROPHYSIOLOGICAL STUDY OF SLEEP MODULATION IN RESPONSE TO MIRROR SYSTEM STIMULATION

Introduction Sleep is a physiological process involved in synaptic plasticity, memory consolidation, and homeostatic regulation. In particular, slow wave activity (SWA), defined as EEG activity within the 0.5-4 Hz frequency range during non-rapid eye movement (NREM) sleep, is considered one of the main electrophysiological markers of synaptic homeostasis and sleep-dependent plasticity. Studies demonstrated that motor learning tasks performed during wakefulness may induce local increases in SWA in the cortical regions involved in learning. However, it remains unclear whether action observation, by activating the mirror neuron system, may also modulate SWA during subsequent sleep. Objective The present study, performed in collaboration with CNR of Parma, aimed to investigate whether visual AO stimuli designed to activate the MNS could induce modifications in SWA during subsequent NREM sleep in healthy subjects. A secondary aim was to evaluate possible changes in sleep macrostructure between two experimental nights. Methods Twenty-four healthy volunteers underwent two consecutive nights of polysomnographic (PSG) recording. The first night served as a control condition (Night 1), whereas the second night (Night 2) was preceded by an observational task involving videos of goal-directed motor actions to activate the MNS. Continuous EEG recordings were acquired using a 19-channel montage according to the international 10-20 system. EEG preprocessing and quantitative analyses were performed using MATLAB and EEGLAB. Only NREM2 and NREM3 sleep stages were selected for analysis. Absolute delta power was calculated using Welch’s method, whereas relative power was analysed across the delta, theta, alpha, sigma, beta, and gamma frequency bands. Analyses were conducted at three levels: global, regional, and within a predefined MNS region of interest. Results The analysis did not reveal statistically significant increases in absolute delta power during Night 2 compared to Night 1 at the global, regional, or MNS levels. Instead, a slight reduction in absolute delta power was observed across most analyses. Relative power analysis showed a modest increase in relative delta power during Night 2, accompanied by reductions in the higher-frequency bands, particularly alpha and sigma activity; however, these findings did not remain significant after correction for multiple comparisons. Sleep macrostructure analysis showed a significant increase in rapid eye movement (REM) sleep percentage during Night 2, whereas no significant changes were observed in slow wave sleep (SWS). Discussion and conclusion Overall, the findings do not support the initial hypothesis that AO stimulation induces detectable increases in SWA during subsequent NREM sleep. Nevertheless, the observed spectral redistribution toward lower frequencies may suggest subtle sleep-related modulation following MNS activation. Methodological factors, including the use of a 19-channel EEG system and the observational nature of the stimulation, may have limited the sensitivity for detecting local SWA changes. Further studies using hd-EEG and alternative analytical approaches are needed to better clarify the relationship between AO, MNS activation, and sleep-dependent plasticity.