Cortical long-range dynamics track excitation-inhibition balance across cognitive states and organizational principles

In the realm of systems neuroscience, a key inquiry pertains to the influence of the brain's excitation-inhibition (E/I) balance on the temporal patterns of neural activity. Can this relationship be non-invasively inferred from neural scaling properties? Researchers have now shown that measurements derived from magnetic resonance spectroscopy (MRS) of E/I balance are indeed reflected in the long-range temporal dynamics of the brain.

However, these dynamics are not uniform across the cortex but are instead structured and regulated by the brain's cytoarchitecture.

The organization of inhibitory circuits, ranging from agranular to granular cortex, plays a pivotal role in determining how accurately regional dynamics mirror the E/I balance. Crucially, this neurochemical signature is not static but varies depending on the cognitive state. During task engagement, the relationship between E/I balance and cortical dynamics undergoes a reversal, indicative of the brain traversing different dynamical regimes around a critical point.

Interestingly, this relationship extends beyond functional magnetic resonance imaging (fMRI) to encompass arousal-linked dynamics, which are indexed by changes in pupil diameter and microsaccades. This indicates that cortical state is a global property rather than a property confined to specific regions of the brain. The imprint of E/I balance on these long-range dynamics follows a clear pattern along the sensory-association axis at rest, but this pattern shifts during task engagement.

These findings provide robust evidence that E/I balance is a neurochemical quantity with structured, scalable signatures across the temporal and spatial organization of the human cortex. This relationship between E/I balance and the temporal structure of neural activity thus serves as a potent neurochemical indicator of the brain's cognitive state and organization.