Psychedelics increase brain entropy

Psychedelics increase brain entropy & complexity

First to report: Lempel-Ziv complexity increases under psychedelics Schartner2017

Entropy analyses

Entropy of functional connectivity
- Carhart-Harris2014 → Shannon entropy of intranetwork synchrony & connectivity b/w specific set of ROIs from limbic/paralimbic
- Lebedev2015 → Shannon entropy of distribution of FC links b/w individual ROIs and 5 ROI groups calculated via community detection on placebo data
- Viol2017 → Shannon entropy of distribution of overall significant functional connections per ROI
Entropy of dynamics of BOLD signal
- Lebedev2016 → multiscale sample entropy of voxel-wise BOLD signals parcellated into 17 networks
- Varley2020 → Lempel-Ziv complexity (estimate of Shannon entropy rate) of BOLD signal per ROI
Entropy of dynamic functional connectivity
- Tagliazucchi2014 → Shannon entropy of binarized FCD patterns between four ROIs
- Barrett2020 → approximates diff. entropy of dist. of values of FCD between each pair of ROIs
- Kringelbach2020a → Shannon entropy rate of transitions between brain connectivity states
- Luppi2021 → network functional complexity of predominantly integrated & segregated substates (LSD)
  - Overall complexity increased, but due to primarily segregated state

Entropy findings in psychedelic neuroscience are most often reported as “brain entropy” but the methods used correspond to entirely different metrics

”width” of histogram (Shannon entropy)
unpredictability (entropy rate, Lempel-Ziv complexity, sample entropy)
Despite these differences, findings point toward same effect → increase of respective entropy estimate under psychedelics

Global increases in entropy, changes are greatest in parietal and occipital
Increased entropy correlates with intensity of subjective experience Timmermann2023

Brain entropy reliably decreases during states of reduced consciousness