Serotonergic psychedelics LSD & psilocybin increase the fractal dimension of cortical brain activity in spatial and temporal domains
This fMRI study (2020) found that LSD and psilocybin increased the fractal dimension of cortical brain activity, which is suggested to serve as a direct measure to validate current theories of psychedelic neural mechanisms.
Authors
- Robin Carhart-Harris
- Leor Roseman
- Emmanuel Stamatakis
Published
Abstract
Psychedelic drugs, such as psilocybin and LSD, represent unique tools for researchers investigating the neural origins of consciousness. Currently, the most compelling theories of how psychedelics exert their effects is by increasing the complexity of brain activity and moving the system towards a critical point between order and disorder, creating more dynamic and complex patterns of neural activity. While the concept of criticality is of central importance to this theory, few of the published studies on psychedelics investigate it directly, testing instead related measures such as algorithmic complexity or Shannon entropy. We propose using the fractal dimension of functional activity in the brain as a measure of complexity since findings from physics suggest that as a system organizes towards criticality, it tends to take on a fractal structure. We tested two different measures of fractal dimension, one spatial and one temporal, using fMRI data from volunteers under the influence of both LSD and psilocybin. The first was the fractal dimension of cortical functional connectivity networks and the second was the fractal dimension of BOLD time series. In addition to the fractal measures, we used a well-established, non-fractal measure of signal complexity and show that they behave similarly. We were able to show that both psychedelic drugs significantly increased the fractal dimension of functional connectivity networks and that LSD significantly increased the fractal dimension of BOLD signals, with psilocybin showing a non-significant trend in the same direction. With both LSD and psilocybin, we were able to localize changes in the fractal dimension of BOLD signals to brain areas assigned to the dorsal-attention network. These results show that psychedelic drugs increase the fractal dimension of activity in the brain and we see this as an indicator that the changes in consciousness triggered by psychedelics are associated with evolution towards a critical zone.
Research Summary of 'Serotonergic psychedelics LSD & psilocybin increase the fractal dimension of cortical brain activity in spatial and temporal domains'
Introduction
Since around 2000 there has been renewed scientific interest in serotonergic psychedelics (for example LSD and psilocybin) both as potential therapeutics and as tools to probe consciousness. Earlier neuroimaging and electrophysiological work has shown increased signal complexity and reorganisation of functional connectivity under psychedelics, and this literature has been interpreted under the Entropic Brain Hypothesis (EBH): the idea that psychedelic states move the brain closer to a critical point between order and disorder, increasing entropy and flexibility of neural dynamics. However, most prior measures (Lempel–Ziv complexity, Shannon entropy, nodal entropy, etc.) quantify a movement along an order–randomness axis and do not directly test hallmarks of criticality such as scale-free or fractal structure. Varley and colleagues therefore propose assessing fractal character of brain activity as an additional index related to criticality, because many systems near critical points develop fractal geometry. The study applies two complementary fractal measures to fMRI data collected under LSD and psilocybin: a spatial/network fractal dimension estimated by a Compact Box-Burning box-counting algorithm on 1000-node functional connectivity graphs, and a temporal fractal dimension estimated by the Higuchi method applied to BOLD time-series. For convergent validity they also compute a standard non-fractal complexity measure (Lempel–Ziv complexity). The central question is whether these measures increase under psychedelics, consistent with a shift towards criticality as predicted by the EBH.
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Study Details
- Study Typeindividual
- Journal
- Compounds
- Topic
- Authors
- APA Citation
Varley, T. F., Carhart-Harris, R., Roseman, L., Menon, D. K., & Stamatakis, E. A. (2020). Serotonergic psychedelics LSD & psilocybin increase the fractal dimension of cortical brain activity in spatial and temporal domains. NeuroImage, 220, 117049. https://doi.org/10.1016/j.neuroimage.2020.117049
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