A new study conducted by the Washington University in St. Louis, and published on 7 October 2019 in the journal Neuron, has confirmed the long-standing theory that our brains – as well as those of most, or all, freely behaving animals – are tuned to be as excitable as possible without tipping over into disorder.
“When neurons combine, they actively seek out a critical regime,” said Keith Hengen, Assistant Professor of Biology in Arts & Sciences, and lead author on the paper. “Our new work validates much of the theoretical interest in criticality and demonstrates that criticality is a hallmark of normally functioning networks”.
The key piece of evidence which lends support to the theory is the finding that criticality (a computational regime which optimises the processing of information) is actually a set point – regulated by a population of inhibitory neurons – rather than a mere inevitability inherent to neural systems.
Over the years, the idea that brains may be critical – originally proposed by theoretical physicists – was met with some controversy, stemming from the largely descriptive nature of most early work and the measurement of simple power laws, which may appear out of random noise, rather than something known as the exponent relation – the only true signature of criticality.
Thanks to the ability of track neural systems one neuron at a time, as well as techniques which allow neural activity to be recorded for over a week in each session (as compared to the standard recording time of about 30 min.), the research team confirmed that network dynamics in the visual cortex are, in fact, tuned to criticality.
Next, by blocking vision in one eye, the team showed that criticality was severely disrupted (even though individual neurons became suppressed by visual deprivation only a day later), and then re-emerged 24 hours afterwards.
“This is consistent with the theoretical physics that the critical regime is firing-rate independent,” explained Hengen. “It’s not about just the total number of spikes in the network, because the firing rate hasn’t changed at all at the very early part of deprivation — and yet the regime falls apart.”
In addition to serving theoretical purposes, the findings could have implications for further research into, and treatment of, neural disorders, such as Alzheimer’s, epilepsy, Rett Syndrome, autism, and schizophrenia, because all of the preceding have been recently tied to impaired homeostatic regulation, of which criticality is a type.