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

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.