EEG Lempel-Ziv complexity varies with sleep stage, but does not seem to track dream experience

Following up on the authors’ prior work showing a correlation between posterior EEG complexity and the perceptual vividness of dream reports, this study attempted to replicate and extend those findings with a larger, pre-registered sample and improved methodology. Using 64-channel high-density EEG and a repeated awakening protocol, participants’ Lempel-Ziv complexity (LZC) was analyzed across sleep stages and paired with immediate verbal dream reports rated along a thought–perceptual axis.

The expected finding—that LZC decreases progressively from wakefulness through NREM stages—was replicated, confirming LZC as a robust discriminator of sleep depth. However, within NREM2, no significant difference in LZC was found between awakenings with and without dream reports. Furthermore, the previously reported positive correlation between posterior LZC and perceptual dream ratings did not reproduce in this pre-registered replication.

The results suggest that while complexity measures reliably index large-scale transitions in brain state (e.g., sleep depth), their sensitivity to graded variations in conscious content within a given state may be limited. The study illustrates the importance of pre-registration and replication in consciousness research, where exploratory findings can be difficult to generalize.

Recommended citation: Aamodt A, Sevenius Nilsen A, Markhus R, Kusztor A, HasanzadehMoghadam F, Kauppi N, Thürer B, Storm JF, Juel BE. (2023). "EEG Lempel-Ziv complexity varies with sleep stage, but does not seem to track dream experience." Frontiers in Human Neuroscience. 16:987714. https://doi.org/10.3389/fnhum.2022.987714
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