What is the dynamical regime of cerebral cortex?

TL;DR

This paper reviews evidence suggesting that the cerebral cortex operates in a loose balance regime where excitation and inhibition cancel each other but leave a net input comparable to the cancelling factors, influencing cortical dynamics.

Contribution

It clarifies the nature of excitation-inhibition balance in cortex, emphasizing the loose balance regime and its implications for neural dynamics and sensory processing.

Findings

Loose balance regime explains nonlinear cortical responses

Correlated variability persists under loose balance

Variability decreases with increased stimulus drive

Abstract

Many studies have shown that the excitation and inhibition received by cortical neurons remain roughly balanced across many conditions. A key question for understanding the dynamical regime of cortex is the nature of this balancing. Theorists have shown that network dynamics can yield systematic cancellation of most of a neuron's excitatory input by inhibition. We review a wide range of evidence pointing to this cancellation occurring in a regime in which the balance is loose, meaning that the net input remaining after cancellation of excitation and inhibition is comparable in size to the factors that cancel, rather than tight, meaning that the net input is very small relative to the cancelling factors. This choice of regime has important implications for cortical functional responses, as we describe: loose balance, but not tight balance, can yield many nonlinear population behaviors seen in sensory cortical neurons, allow the presence of correlated variability, and yield decrease of that variability with increasing external stimulus drive as observed across multiple cortical areas.