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Response of sleep slow oscillations to acoustic stimulation is evidenced by distinctive synchronization processes

Closed loop acoustic stimulation (CLAS) during sleep has shown to boost slow wave (SW) amplitude and spindle power. Moreover, sleep SW are suggested to be classified based on different processes of neuronal synchronization. Following this, different types of SW events may have distinct functional roles and be differentially affected by external stimuli. However, the SW synchronization processes affected by CLAS are not well understood. Here, we studied the effect of CLAS on the dissociation of SW events based on two features of neuronal synchronization in the EEG (topological spread and wave slope). We evaluated and classified individual SW events of fourteen healthy subjects during a CLAS stimulated (STM) and a control night (CNT). Three main categories of SW events were found denoting (C1) steep-slope SW with global spread, (C2) flat-slope waves with localized spread and homeostatic regulation, and (C3) multipeaked flat-slope events with global spread. Comparing between conditions, we found a consistent increase of event proportion and trough amplitudes for C1 events during the time of stimulation. Furthermore, we found similar increases in post-stimulus spectral power in θ, β and σ frequencies for CNT vs STIM condition independently of sleep stage or SW categories. However, topological analysis showed differentiated spatial dynamics in N2 and N3 for SW categories and the co-occurrence with spindle events. Our findings reveal the nature of post-stimulus SW and suggest that CLAS boosts SW amplitudes by increasing neuronal synchronization of wave troughs, leading thus the post-stimulus SW-spindle co-occurrence.