Fragments of wake-like activity frame down-states of sleep slow oscillations in humans: New vistas for studying homeostatic processes during sleep

• Published in: International journal of psychophysiology Vol. 89; no. 2; pp. 151 - 157
• Main Authors: Menicucci, Danilo, Piarulli, Andrea, Allegrini, Paolo, Laurino, Marco, Mastorci, Francesca, Sebastiani, Laura, Bedini, Remo, Gemignani, Angelo
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.08.2013
• Subjects: Adolescent; Adult; Brain Waves - physiology; Cortex; Electroencephalography - methods; Gamma activity; Homeostasis - physiology; Humans; Male; Memory consolidation; NREM sleep; Sleep - physiology; Sleep bistability; Sleep slow oscillations; Sleep Stages - physiology; Spindles; Wakefulness - physiology; Young Adult; Sleep bistability; Memory consolidation; Gamma activity; NREM sleep; Sleep slow oscillations; Spindles
• ISSN: 0167-8760; 1872-7697
• DOI: 10.1016/j.ijpsycho.2013.01.014

During NREM sleep cortical activity corresponding to EEG fast rhythms (FRs>10Hz) is interrupted by fragments of neural stillness (down-states), responsible for the negative peak within sleep slow oscillation (SSO). Researchers still debate whether the down-states spontaneously occur or need an initial overshoot in fluctuating activity. Herein, we studied temporally-isolated SSO in healthy subjects in order to identify two distinct EEG markers defining a putative initial up-state: i) a significant positive deflection and ii) an associated FR increase, before the negative peak. We found a positive bump preceding the down-state, which is detectable already at the cortical SSO origin site, both during N2 and N3. This early positive deflection, concurrent with a broadband activation, is characterized by an increase of sigma activity (12–18Hz) from N2 to N3, while an opposite trend was observed for sigma activity crowning the up-state following the negative peak. Also, we found: (i) FR activations during up-states up to high gamma frequencies; (ii) depressed sigma activity in after-spindle recovery phase; and (iii) tightly coordinated activities between distinct bands (12–36Hz, ~70Hz, ~85Hz and 105–125Hz). The correlation between different bands suggested a common mechanism for sigma and gamma, and the pre-down-state activation associated with the initial bump suggested an activity ignition for down-state, whose intensity is dependent on sleep stage. In conclusion, we hypothesize that FR accompanying SSO could mark i) sleep homeostatic processes, such as the regulation/stabilization of sleep, counterbalancing the detrimental effects of continuous inputs from peripheries, and ii) neural mechanisms favoring the storage of information acquired during wakefulness. ► We examined the presence of a trigger inducing spontaneous down-states of NREM sleep. ► Sleep slow oscillation groups fast rhythms up to 150Hz in human NREM sleep. ► A leading fast rhythm increase anticipates the SSO onset.