Opportunities and obstacles in non-invasive brain stimulation

Abstract

Non-invasive brain stimulation (NIBS) is a complex and multifaceted approach to modulating brain activity and holds the potential for broad accessibility. This work discusses the mechanisms of the four distinct approaches to modulating brain activity non-invasively: electrical currents, magnetic fields, light, and ultrasound. We examine the dual stochastic and deterministic nature of brain activity and its implications for NIBS, highlighting the challenges posed by inter-individual variability, nebulous dose-response relationships, potential biases and neuroanatomical heterogeneity. Looking forward, we propose five areas of opportunity for future research: closed-loop stimulation, consistent stimulation of the intended target region, reducing bias, multimodal approaches, and strategies to address low sample sizes.

Keywords: FUS; TMS; TUS; brain stimulation; neurotechnology; tDCS; tES; tPBM.

Figure 1

Characteristics of the four non-invasive brain stimulation modalities: transcranial electrical stimulation (tES), transcranial magnetic stimulation (TMS), transcranial photobiomodulation (tPBM), and transcranial ultrasound stimulation (TUS). This figure outlines common subtypes, primary mechanisms, advantages, and limitations for each modality. Created with BioRender.com.

Figure 2

Illustration of the sources of variation that influence the efficacy of non-invasive brain stimulation. Variations in head and brain morphology, exogenous stimuli, neuron type and orientation, as well activity and stimulation parameters. Created with BioRender.com.