Analysis of a sleep-dependent neuronal feedback loop: The slow-wave microcontinuity of the EEG

Increasing depth of sleep corresponds to an increasing gain in the neuronal feedback loops that generate the low-frequency (slow-wave) electroencephal ogram (EEG), We derived the maximum-likelihood estimator of the feedback ga in and applied it to quantify sleep depth, The estimator computes the fract ion (0%-100%) of the current slow wave which continues in the near-future ( 0.02 s later) EEG, Therefore, this percentage was dubbed slow-wave microcon tinuity (SW%). It is not affected by anatomical parameters such as skull th ickness, which can considerably bias the commonly used slow-wave power (SWP ). In our study, both of the estimators SW% and SWP were monitored throughout two nights in 22 subjects, Each subject took temazepam (a benzodiazepine) o n one of the two nights, Both estimators detected the effects of age, temaz epam, and time of night on sleep. Females were found to have twice the SWP of males, but no gender effect on SW% was found. This confirms earlier repo rts that gender affects SWP but not sleep depth. Subjectively assessed diff erences in sleep quality between the nights were correlated to differences in SW%, not in SWP. These results demonstrate that slow-wave microcontinuity, being based on a physiological model of sleep, reflects sleep depth more closely than SWP do es.