Human Brain Mapping
January 1, 1995
Karl Friston, Giulio Tononi, Olaf Sporns et al.
114 citations
Neuronal interactions in the brain balance two opposing organizational principles: functional segregation, where specialized cortical areas exhibit relatively high entropy (unpredictable dynamics), and functional integration, where distributed influence across areas produces lower entropy overall. A measure of complexity is highest when small brain regions have high entropy on average relative to the whole system's entropy, equivalent to the average mutual information between small regions and the rest of the system. Applied to nonlinear simulations and fMRI data during photic stimulation, complexity peaked between high-dimensional chaotic behavior and low-dimensional orderly behavior—between asynchronous oscillations and global synchrony—confirming the hypothesis.
Nature Medicine
April 1, 2026
Manesh Girn, Manoj K. Doss, Leor Roseman et al.
8 citations
Psychedelic drugs are being studied again for their therapeutic potential, but how they change brain function is not well understood. By combining 11 brain-scanning datasets from five different psychedelics (psilocybin, LSD, mescaline, DMT, and ayahuasca) across three continents, researchers found a common pattern: increased communication between brain networks that handle high-level thinking (default, frontoparietal, and limbic) and those that handle sensory input (visual and somatomotor). Key deep-brain regions (thalamus, caudate, putamen) and the cerebellum also changed how they connect with sensorimotor networks. Contrary to some earlier studies, reductions in within-network connectivity were weak to moderate and varied by drug. These findings help resolve previous inconsistencies and provide a comprehensive map of how psychedelics alter large-scale brain organization.
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.