Cortical responses to transcranial magnetic stimulation during Non-Rapid Eye Movement Sleep

Transcranial magnetic stimulation coupled to high-density electroencephalography (TMS-EEG) allows for directly and non-invasively stimulating the brain and recording the subsequent cortical response. Previous TMS-EEG studies have shown clear-cut differences between conscious and unconscious conditions. When subjects are unconscious, as in non-rapid eye movement (NREM) sleep early in the night (stage N3), anesthesia or coma, TMS typically triggers a stereotypical and local slow-wave response. When they are conscious, as in normal wakefulness and rapid eye movement sleep (REM) sleep, brain responses to TMS are long-lasting, widespread, complex, and differentiated. In this study, we performed TMS-EEG during NREM sleep in stage 2 (N2) and N3 on 7 healthy participants. Brain activity was recorded using a 60-channel TMS-compatible EEG and single-pulse TMS was applied (up to 285 pulses per session) on the superior parietal cortex. After each TMS session (5 to 15 sessions per night), subjects were awakened to ask for a dream report. TMS-EEG responses were analyzed using the global mean field amplitude (GMFA), the perturbational complexity index (PCI), as well as time-frequency measures. Our results show that the TMS-EEG response during N3 is larger and slower than during N2 sleep. The GMFA is higher and PCI is lower in N3 as compared to N2 sleep. Moreover, N3 recordings showed activation at lower frequency bands after TMS as compared to N2 sleep. Additionally, TMS-EEG responses vary depending on whether subjects do or do not report a dream. TMS-EEG might provide valuable information for characterizing neurophysiological fluctuations and levels of consciousness within NREM sleep.