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

Arnfinn Aamodt; André Sevenius Nilsen; Rune Markhus; Anikó Kusztor; Fatemeh HasanzadehMoghadam; Nils Kauppi; Benjamin Thürer; Johan Frederik Storm; Bjørn Erik Juel

doi:10.3389/fnhum.2022.987714

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

Front Hum Neurosci. 2023 Jan 10:16:987714. doi: 10.3389/fnhum.2022.987714. eCollection 2022.

Authors

Arnfinn Aamodt¹, André Sevenius Nilsen¹, Rune Markhus², Anikó Kusztor^{1

3}, Fatemeh HasanzadehMoghadam¹, Nils Kauppi¹, Benjamin Thürer¹, Johan Frederik Storm¹, Bjørn Erik Juel¹

Affiliations

¹ Brain Signalling Lab, Division of Physiology, Faculty of Medicine, Institute of Basic Medical Sciences, University of Oslo, Oslo, Norway.
² National Centre for Epilepsy, Oslo University Hospital, Oslo, Norway.
³ School of Psychological Sciences, Monash University, Clayton, VIC, Australia.

Abstract

In a recent electroencephalography (EEG) sleep study inspired by complexity theories of consciousness, we found that multi-channel signal diversity progressively decreased from wakefulness to slow wave sleep, but failed to find any significant difference between dreaming and non-dreaming awakenings within the same sleep stage (NREM2). However, we did find that multi-channel Lempel-Ziv complexity (LZC) measured over the posterior cortex increased with more perceptual ratings of NREM2 dream experience along a thought-perceptual axis. In this follow-up study, we re-tested our previous findings, using a slightly different approach. Partial sleep-deprivation was followed by evening sleep experiments, with repeated awakenings and immediate dream reports. Participants reported whether they had been dreaming, and were asked to rate how diverse, vivid, perceptual, and thought-like the contents of their dreams were. High density (64 channel) EEG was recorded throughout the experiment, and mean single-channel LZC was calculated for each 30 s sleep epoch. LZC progressively decreased with depth of non-REM sleep. Surprisingly, estimated marginal mean LZC was slightly higher for NREM1 than for wakefulness, but the difference did not remain significant after adjusting for multiple comparisons. We found no significant difference in LZC between dream and non-dream awakenings, nor any significant relationship between LZC and subjective ratings of dream experience, within the same sleep stage (NREM2). The failure to reproduce our own previous finding of a positive correlation between posterior LZC and more perceptual dream experiences, or to find any other correlation between brain signal complexity and subjective experience within NREM2 sleep, raises the question of whether EEG LZC is really a reliable correlate of richness of experience as such, within the same sleep stage.

Keywords: EEG; complexity; consciousness; dreaming; sleep.

Grants and funding

This project was supported by the Norwegian Research Council’s FRIMEDBIO (Grant No. 262950) and the European Union’s Horizon 2020 Framework Programme for Research and Innovation under the Specific Grant Agreements No. 785907 (Human Brain Project SGA2 and SGA3).