NeuroImage
June 30, 2020
Thomas F. Varley, Robin Carhart‐Harris, Leor Roseman et al.
91 citations
Psychedelic drugs like psilocybin and LSD increase the fractal dimension of brain activity, suggesting that the brain moves toward a critical state between order and disorder. Using fMRI data from volunteers, the study tested two fractal measures: one for functional connectivity networks and one for BOLD time-series. Both drugs significantly increased the fractal dimension of functional connectivity networks. LSD also significantly increased the fractal dimension of BOLD signals, while psilocybin showed a non-significant trend in the same direction. Changes in the fractal dimension of BOLD signals were localized to brain areas in the dorsal attention network. These results indicate that psychedelic-induced changes in consciousness are associated with evolution toward a critical zone.
bioRxiv (Cold Spring Harbor Laboratory)
September 23, 2021
Pedro A. M. Mediano, Aleksi Ikkala, Rogier Kievit et al.
30 citations
preprint
Neural complexity measures, which can distinguish conscious from unconscious states, also detect meaningful fluctuations in conscious level during normal wakefulness. Using MEG and fMRI data from healthy adults, complexity decreased as participants became drowsy, validating the approach. Complexity changed within and between tasks, and higher complexity was associated with better performance and faster reaction times on an executive task. This offers a new way to explore the cognitive and neural basis of consciousness.
bioRxiv Preprint Server
April 4, 2020
Thomas F. Varley, Vanessa Denny, Olaf Sporns et al.
6 citations
preprint
The vividness of conscious experience is linked to brain dynamics. Propofol and ketamine, both anesthetics, produce different subjective states. This study examined how these drugs alter the structure of dynamic attractors reconstructed from electrical brain activity recorded from the cerebral cortex of two macaques. The awake condition showed the richest structure, visiting the most states with pronounced higher-order dynamics and the least deterministic activity. Propofol produced the most dissimilar dynamics, shifting to an impoverished, constrained, low-structure regime. Ketamine combined aspects of both: generally less complex than awake but well above propofol on almost all measures. These results offer deeper insights than typical point-measures of complexity.
bioRxiv Preprint Server
March 27, 2020
Thomas F. Varley, Olaf Sporns, Aina Puce et al.
3 citations
preprint
The brain may operate near a critical tipping point, a state thought necessary for consciousness and complex cognition. Using invasive ECoG recordings from a macaque transitioning between consciousness and unconsciousness under propofol and ketamine, the study found that propofol dramatically restricted the size and duration of neural avalanches and reduced the complexity of brain dynamics, while ketamine allowed more awake-like dynamics to persist. Despite these differences, all states showed some signs of persistent criticality when tested for exponent relations and universal shape-collapse, suggesting that maintenance of critical brain dynamics may be important for regulating conscious awareness.
arXiv Preprint Archive
October 31, 2023
Thomas F. Varley, Daniel Havert, Leandro Fosque et al.
The psychedelic DPT reversibly alters how information is processed in rat cortical tissue. Spontaneous neural firing became more random (higher entropy), and individual neurons stored information for shorter periods. Neural activity became less reversible, pushing the system away from equilibrium. Circuit structure shifted: overall information flow into each neuron decreased, but weak connections increased, blending integration and disintegration. Higher-order statistical synergy among groups of three neurons also dropped. These meso-scale effects offer a more detailed view of psychedelic action than whole-brain imaging provides.