Spectral slope and Lempel-Ziv complexity as robust markers of brain states during sleep and wakefulness

Spectral slope and Lempel-Ziv complexity are affected in many neurophysiological disorders and are modulated by sleep, anesthesia, and aging. Yet, few studies have explored the relationship between these two parameters. We evaluated the impact of sleep stage and task-engagement (resting, attention and memory) on spectral slope and Lempel-Ziv complexity in a narrow- (30 - 45Hz) and broadband (1 - 45Hz) frequency range in 28 healthy males (21.54 {+/-} 1.90 years) over three recordings. Only in the broadband range, the slope steepens and complexity decreases continuously from wakefulness to N3. However, REM sleep is best discriminated by the narrowband slope. Importantly, slope and complexity also differentiate between tasks during wakefulness. While the narrowband complexity decreases across tasks, the slope is flattening with task engagement in both frequency ranges. In general, broadband slope and complexity are strongly positively correlated, but we observe a dissociation between them in the narrowband range. Critically, only the narrowband slope is associated with better Go/Nogo task performance. Our results demonstrate that slope and complexity are both powerful indices of sleep depth, task engagement and cognitive performance. While the broadband range is better suited to discriminate between brain states, especially the narrowband slope is a unique marker of task performance.