Towards a reliable neural biomarker for predicting response to non-invasive brain stimulation in the treatment of depression

Non -invasive brain stimulation (NIBS) approaches such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) are rapidly emerging as effective and well-tolerated treatments for depression. With the most recent head -to -head studies demonstrating equivalence in clinical efficacy between rTMS, tDCS and widely used pharmaceutical antidepressants, there is urgent need for a paradigm shift towards its inclusion as a lowcost, low-risk frontline treatment for depression. Here, we provide a narrative review outlining barriers currently impeding translation of NIBS approaches into large-scale clinical use, with a view to developing a neural biomarker that could provide early stratification of patients as potential responders or nonresponders. We describe how the TMS-evoked potential provides a marker of cortical excitability that could be used as a baseline predictor of whether the NIBS user will derive benefit from approaches tailored to achieve neuromodulation.