Cortical excitation/inhibition ratios in patients with major depression treated with electroconvulsive therapy: an EEG analysis

Electroconvulsive therapy (ECT) is a highly effective treatment for severe, treatment-resistant depression, yet its precise neurophysiological mechanisms remain poorly understood. This study investigates whether the balance between cortical excitation and inhibition (E/I)—a fundamental property of neural network homeostasis—is altered by ECT and whether these changes correlate with clinical improvement.

The researchers analyzed resting-state EEG recordings from 28 patients with major depression, taken before and after a full course of ECT. They utilized a novel method to estimate functional E/I (fE/I) ratios based on critical brain dynamics, which relates spectral power to long-range temporal correlations. To control for natural test–retest variability, the study included an independent sample of 31 healthy controls with repeated EEG measurements. Clinical effectiveness was determined by changes in the Hamilton Depression Rating Scale (HDRS) scores, allowing the researchers to compare responders (≥50% reduction in symptoms) against non-responders.

At baseline, there were no significant differences in whole-brain or regional fE/I ratios between depressed patients and healthy controls. While the ECT course did not produce changes in fE/I ratios at the whole-brain level, a distinct pattern emerged in the frontal region. Specifically, patients who responded to treatment showed a significant increase in frontal fE/I ratios in the 12–28 Hz frequency band (beta rhythm) following the ECT course. This effect was not observed in non-responders or healthy controls. The authors suggest that this shift toward a more balanced E/I state in the frontal cortex may be a key mechanism through which ECT exerts its antidepressant effects.

These findings provide evidence that ECT may work by modulating the E/I balance in specific brain regions rather than globally. By identifying a potential biomarker for treatment response, this research offers a pathway toward better understanding the neurobiological underpinnings of ECT and potentially personalizing treatment protocols in the future.