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Does the brain's E:I balance really shape long-range temporal correlations? Lessons learned from 3T MRI.

Lydia Sochan¹, Jessica Archibald², Alexander Mark Weber^1,3,4

¹School of Biomedical Engineering, The University of British Columbia, Vancouver, BC, Canada.

Imaging Neuroscience (Cambridge, Mass.)

|June 3, 2026

Summary

This study found no correlation between brain signal complexity (Hurst exponent) and the excitatory/inhibitory (E:I) ratio in healthy adults. While signal complexity increased during movie watching, the E:I balance remained unchanged, suggesting H may not be a reliable proxy for E:I.

Keywords:

Hurst exponent complex systems criticality excitatory/inhibitory balance functional magnetic resonance imaging functional magnetic resonance spectroscopy long range temporal correlation magnetic resonance spectroscopy visual task

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Area of Science:

Neuroscience
Biophysics
Medical Imaging

Background:

The brain's excitatory/inhibitory (E:I) balance is crucial for optimal information processing and adaptability, potentially operating near a critical state.
The Hurst exponent (H), measuring signal complexity and temporal correlations, is hypothesized to reflect this critical brain state.
Disrupted E:I balance is implicated in various neurological and neuropsychiatric disorders, making reliable biomarkers essential.

Purpose of the Study:

To investigate the relationship between long-range temporal correlations in fMRI signals (Hurst exponent) and the excitatory/inhibitory (E:I) ratio in healthy adults.
To determine if the Hurst exponent (H) can serve as a practical in vivo biomarker for E:I balance.
To explore how task engagement (movie watching) affects H and E:I ratio.

Main Methods:

A multimodal 3T MRI study combining functional MRI (fMRI) and single-voxel magnetic resonance spectroscopy (MRS).
MRS measured glutamate and GABA concentrations to calculate the E:I ratio.
fMRI data analyzed for the Hurst exponent (H) of the blood oxygen level-dependent (BOLD) signal during rest and movie watching.

Main Results:

No significant correlation was found between the Hurst exponent (H) and the E:I ratio (glutamate/GABA).
The Hurst exponent (H) significantly increased during movie watching compared to rest.
The E:I ratio did not show significant changes between rest and movie watching conditions.

Conclusions:

The Hurst exponent (H) does not appear to be a reliable proxy for the excitatory/inhibitory (E:I) ratio in healthy adults under the studied conditions.
While H reflects changes in brain signal complexity with task engagement, it does not directly mirror the underlying neurochemical balance.
Further research with refined methodologies is needed to explore the relationship between brain dynamics and neurochemistry.