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

A 3T multimodal MR study of healthy adults (n = 18; 10 female; 21.3-53.4 years) was performed to investigate the relationship between functional magnetic resonance imaging (fMRI) long-range temporal correlations and excitatory/inhibitory balance (via single voxel magnetic resonance spectroscopy measurements of glutamate and $\gamma$ -aminobutyric acid (GABA)). The study objective was to determine whether the Hurst exponent (H)-an estimate of self-correlation and signal complexity-of the blood oxygen level-dependent signal was correlated with the excitatory-inhibitory (E:I) ratio. E:I has been proposed to serve as a control parameter for brain criticality-the theory that the brain operates near a critical point between order and disorder, optimizing information processing and adaptability-which H is believed to be a measure of. Thus, understanding whether H and E:I are indeed correlated would clarify this relationship. Moreover, findings in this domain have implications for neurological and neuropsychiatric conditions with disrupted E:I balance, such as autism, schizophrenia, and Alzheimer's disease. From a practical perspective, H is easier to accurately measure than E:I ratio at 3T MRI. If H can serve as a proxy for E:I, it may serve as a more practical clinical biomarker for this imbalance and for neuroscience research in general. The study collected functional MRI and magnetic resonance spectroscopy data during rest and movie watching. H and E:I (glutamate/GABA) were not found to be correlated. H was found to increase with movie watching compared with rest, while E:I did not change between conditions. This study represents the first attempt to investigate this connection in vivo in humans. We conclude that, at 3T and with our particular methodologies, no association was found. We end with lessons learned and suggestions for future research.