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

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.

Breathwork-Induced Psychedelic Experiences Modulate Neural Dynamics

“…There is an increasing body of research showing that signal diversity is a robust neural marker that can distinguish between conscious states, such as sleep (Schartner et al, 2017; Aamodt et al, 2021; 2023; Pascovich et al, 2022), anaesthesia (Schartner et al, 2015; Varley et al, 2020b; Pal et al, 2020), and psychedelic administration (Schartner et al, 2017; Timmermann et al, 2019; Mediano et al, 2020; Timmermann et al, 2023b; Pascovich et al, 2022). Additionally, LZ complexity maps on to differences in the perceptual contents of consciousness in bistable paradigms (Canales-Johnson et al, 2020; 2023).…”

“…However, it remains difficult to disentangle the relationship between the contents of consciousness and neural signatures outside of strict task-based paradigms (e.g. Aamodt et al, 2021; 2023). Within the present study, we were able to reveal associations between nonlinear neural markers and subjective experience states during breathwork sessions.…”

Brain signal complexity and aperiodicity predict human corticospinal excitability

“…CTW is perhaps a more useful measure of cortical entropy than LZ for the purpose of inferring CSE. This is somewhat surprising, given that both entropy estimates are based on compression algorithms and have performed similarly on neural data in other studies (34, 62–64), and, furthermore, LZ is much more widely applied in human neuroscience across contexts ranging from studies of anesthetics (30, 31, 65, 66), sleep (31, 33, 67, 68), and pharmacological drug challenge (33, 69, 70), including effects on TMS-evoked potentials (71– 73). Nonetheless, from a purely computational perspective, there are strong reasons to believe CTW is a superior entropy estimator for time series data.…”

Sensory modality defines the relation between EEG Lempel–Ziv diversity and meaningfulness of a stimulus

“…In the present study, we did not include any subjective measures as we are not aware of tools which might allow the estimation of concepts such as temporal variability of experience caused by continuous naturalistic stimuli. So far, only studies investigating dream experiences 13 , 54 and psychedelics-induced states 18 , 21 , 22 , 25 have used subjective questionnaires to measure various dimensions of subjective experience (e.g., ego-dissolution, arousal, imagery) and reported that some of them were related to signal diversity 13 , 18 , 21 , 22 , 25 . Furthermore, several tools have been proposed to measure the contents of consciousness during the resting state, for instance the Amsterdam Resting State Questionnaire 55 , 56 , and future studies might relate estimates of such questionnaires to signal diversity.…”