Visual Training Induced Temporo‐Occipital Fast Sleep Spindle Clustering in Humans Revealed by Full‐Night <scp>HD</scp> ‐ <scp>EEG</scp> Recordings

This study investigates the impact of extensive visual procedural training on the temporal organisation of sleep spindles in healthy young adults. We selected 39 participants aged 16-20 and employed high-density electroencephalography to assess spindle characteristics during two full nights of sleep, with daytime practising in a contour integration task in between the two nights. We utilised linear mixed models (LMM) to comprehensively analyse the effects of age and training on basic, clustering- and rhythmicity-related spindle parameters. Our findings indicate no significant age effects in this age range, and no significant change between the two nights with respect to slow spindles. Fast spindles demonstrated a significant increase in density after training, and we observed significant changes in spindle clustering and rhythmicity parameters as well. Local spindle density, train density, the number of spindles within trains, the ratio of clustered spindles, and spindle duration in trains have increased, and inter-train interval decreased by the second night within the task-related temporo-occipital regions. These results might be interpreted in the context of sleep-dependent memory consolidation or potentially by homeostatic-related processes following extensive training. Moreover, here we illustrate that spindle reorganisation occurs not only in motor tasks but also in visual learning. The absence of age-related differences indicates that the reorganisation of spindles following training is a similar process in late adolescence and young adulthood. Our study emphasises the importance of spindle dynamics in procedural learning and suggests promising possibilities for future research into the neurophysiological basis of memory consolidation.