Concurrent Multimodal Imaging Demonstrates That EEG-Based Excitation/Inhibition Balance Reflects Glutamate and GABA Balance

Abstract

Magnetic resonance spectroscopy (MRS)-derived excitation/inhibition (E/I) ratio is recognized as a valuable index of attentional modulation, the plasticity and stability of learning states, and disordered functioning. Inferring the E/I ratio from electroencephalography (EEG), however, offers greater accessibility and superior temporal resolution and therefore holds strong potential for advancing research on dynamic neural processes. Yet, the underlying neurochemical mechanisms contributing to changes in EEG-based E/I ratio remain unclear. In this study we used concurrent EEG and MRS to examine how the EEG-based E/I ratio correlates with the MRS-based E/I ratio in sleeping humans of both sexes ( n  = 15). The MRS-based E/I ratio was calculated as the ratio of Glx (glutamate + glutamine) to GABA + (GABA + coedited macromolecules) concentrations in early visual areas. We estimated 10 candidate EEG-based E/I indices using four algorithms across multiple spontaneous frequency bands from the occipital region. Uniquely, we quantified the associations between EEG- and MRS-based E/I ratios by separately analyzing between-subject and within-subject variations. We found that each EEG-based E/I algorithm showed reliable and positive associations with MRS-based E/I, particularly in the alpha band, which is known to play a key role in attentional modulation and in the plasticity and stability of learning states. These results highlight the potential of EEG-based E/I measures to serve as practical indices of neurochemical dynamics in early visual cortex.