The “Edge-of-Chaos”: Brain Activity Underlying Consciousness
Post by Lani Cupo
The takeaway
Modeling the electrical brain activity underlying stages of consciousness reveals that the conscious brain exhibits activity poised at the “edge-of-chaos”, a critical point between stability and chaos. The loss of consciousness, such as with anaesthesia, corresponds with a transition away from the critical point, while psychedelics induce a state closer to the critical point.
What's the science?
As scientists seek to understand consciousness, they investigate patterns of electrical brain activity during various stages of consciousness, such as during generalized seizures, under anaesthesia, and after exposure to lysergic acid diethylamide (LSD). In doing so, they can examine the transition of brain activity from mathematically stable to chaotic dynamics. While previous research suggests that the conscious brain’s electrical activity exists at a critical point at the boundary of stability and chaos, it is yet unknown what phases (from a mathematical perspective) exist on either side of the critical point of wakefulness. This week in PNAS, Toker and colleagues sought to provide empirical evidence for the cortical dynamics (patterns of electrical brain activity) that underlie stages approaching consciousness and how these patterns relate to information richness.
How did they do it?
The researchers used a previously published model of low-frequency electrical activity in the brain reflecting cortical oscillations that allow for tuning of parameters associated with neuronal inhibition and excitation. By setting the parameters of the model based on the literature, the authors could simulate data for different brain states, including waking consciousness, generalized seizure, and anaesthesia, which have been previously validated in the model with acquired data. In the data they simulate, the authors assessed chaotic dynamics with the largest Lyapunov exponent, where a positive exponent corresponds with chaos and a negative exponent corresponds with periodicity (ordered pattern). The authors applied a modified 1-0 chaos test to this value where a value of 1 is assigned to chaotic systems and 0 to periodic systems. They also assessed the richness of information in the model with a measure known as the Lempel-Ziv complexity, which gives an estimate of the amount of non redundant information in a signal—in this case brain activity. The authors could then relate measures of chaos to measures of information richness, comparing between models of various states of consciousness. They also performed a “parameter sweep” where they simulated data with diverse parameters not corresponding with a specific brain state to explore the relationship between chaos and richness across the models. In addition to the simulated data for waking consciousness, generalized seizure, and anaesthesia, they also examined previously published data from two macaques and five humans during wakefulness, two macaques and three humans under anaesthesia, three humans experiencing generalized seizures, and 16 people after exposure to either saline or LSD.
What did they find?
The authors hypothesized that if they plotted the measure of information richness on the y-axis and a transition from periodicity to chaos on the x-axis of a plot, they would observe an inverse-U shaped curve, indicating that the richest information was correlated with the critical point at the edge-of-chaos, and information was lost as the system became more periodic or more chaotic. In line with their hypotheses, the authors found an inverse-U relationship between chaos and richness, with 0 on the x-axis representing the critical point of edge-of-chaos. Visualizing the simulated models for wakefulness, seizure, and anaesthesia reveals that wakefulness falls to the right of 0, towards the side representing chaos. This finding supports an old hypothesis that at a large scale, the brain’s electrodynamic system is at least weakly chaotic. Anaesthesia fell farther towards instability, but seizure fell on the periodic side of the graph, as predicted. Data from participants exposed to LSD suggests the psychedelic increases the information richness of the system and stabilizes it, moving the patterns of activity closer to the critical point when compared to consciousness.
What's the impact?
This study found a relationship between the chaotic dynamics of brain activity and the complexity of information in models representing the brain across different states of consciousness, suggesting wakefulness occupies a critical point between chaos and periodicity. The findings provide information to better understand states of consciousness, both in healthy brains (such as during sleep) and disorders related to consciousness.