Electrophysiological Biomarkers in Accelerated TMS for Depression

Depression severity was assessed using the 17-item Hamilton Depression Rating Scale (HAMD-17) at baseline and within 1 week after completion of the accelerated iTBS course. The HAMD-17 is a clinician-administered scale; scores range from 0 to 52, with higher scores indicating more severe depression. Treatment response was defined as a 50% or greater reduction from baseline HAMD-17 score, and remission as a post-treatment HAMD-17 score of 7 or lower.

Heart rate was continuously sampled at 1-second intervals using a Polar H10 chest strap during every stimulation session. Heart rate deceleration (HRD) was calculated as the percentage reduction from the 1-minute pre-stimulation baseline heart rate to the mean stimulation heart rate, using the formula: HRD (%) = [(HR_baseline - HR_stimulation) / HR_baseline] x 100. HRD was computed across four parameters (all left-sided sessions, all right-sided sessions, highest-intensity left-sided sessions, highest-intensity right-sided sessions) and averaged per participant. Higher values indicate greater stimulation-induced heart rate deceleration.

Beck Depression Inventory (BDI), a self-report scale; total scores range from 0 to 63, with higher scores indicating more severe depression.

Beck Anxiety Inventory (BAI), a self-report scale; total scores range from 0 to 63, with higher scores indicating more severe anxiety.

Beck Scale for Suicide Ideation (BSS), a self-report scale; total scores range from 0 to 38, with higher scores indicating greater suicidal ideation.

Quick Inventory of Depressive Symptomatology, Self-Report (QIDS-SR16); total scores range from 0 to 27, with higher scores indicating more severe depression.

Clinical Global Impression - Severity (CGI-S); scores range from 1 (normal, not at all ill) to 7 (among the most extremely ill patients), with higher scores indicating greater illness severity.

17-item Hamilton Depression Rating Scale (HAMD-17) assessed 30 days after completion of treatment; total scores range from 0 to 52, with higher scores indicating more severe depression.

Absolute spectral power in the delta, theta, alpha, and beta frequency bands, derived from quantitative resting-state EEG (qEEG) analysis. Power was computed across standard electrode regions. Prefrontal theta cordance, a quantitative EEG measure combining absolute and relative theta-band power at prefrontal electrodes, derived from resting-state qEEG analysis.

Heart-brain coupling (HBC), a parameter quantifying the strength of stimulation-induced heart rate deceleration, computed as the mean oscillation power of cardiac rate at the frequency determined by the stimulation train interval (Dijkstra et al., 2023). HBC ranges from 0 to 1. Measurements were obtained in a dedicated session using a 10 Hz protocol (50 pulses per train, 5-second train duration, 15 trains, 11-second inter-train interval), recorded with a Polar H10 chest strap and the HeartBrainConnect application. HBC was assessed sequentially at four sites per participant: left dmPFC, left dorsolateral prefrontal cortex (DLPFC), right dmPFC and right DLPFC. For each site, stimulation began at 28% below target intensity and increased by 2% per train, yielding 15 HBC values that were averaged to produce a per-site HBC value.

Temporal parameters of the canonical resting-state EEG microstate classes (A, B, C, and D), including mean duration (milliseconds), occurrence (mean appearances per second), time coverage (percentage of recording time), and transition probabilities between classes. Parameters were derived from microstate segmentation of resting-state EEG recordings.