Transcranial Direct Current Stimulation in Voice and Speech Control: Scoping Review

The purpose of this review is to map the stimulated brain areas and summarize the available evidence on the application of transcranial direct current stimulation (tDCS) in the modulation of voice and speech, identifying its effects on vocal quality and vocal parameters, and relating the outcomes to the corresponding stimulated brain regions.

This review was guided by the research question: "What are the main effects of the application of tDCS on voice, as found in the literature?". The electronic databases searched were: PubMed, Web of Science, and the Virtual Health Library (VHL). The search key used was: Transcranial direct current stimulation AND Voice quality OR singing OR speech OR voice. The review used the PCC framework to structure the research question: Population (P): individuals with and without vocal complaints undergoing tDCS targeting aspects of voice and speech; study variables - concept (C): Brain areas and/or segments stimulated by tDCS, type of stimulation and intervention; outcomes - context (C): main effects on voice and/or speech and their relationships with brain areas and/or stimulated/inhibited segments.

The selection of studies was conducted independently by two reviewers through peer reading, considering publications available in full text in Portuguese, English, or Spanish. Studies that were literature reviews, monographs, dissertations, book chapters, consensus statements, supplements, or editor's comments were excluded.

Of the 919 studies, 459 duplicates were excluded. Based on abstract reading and the selection criteria, 18 studies were selected to be read in full text, and eight of them were included in this review. The areas of the brain that showed effects on vocal production and motor control of voice and speech with tDCS were the dorsolateral prefrontal cortex and the left ventral motor cortex. There was a modulation of speech motor learning with tDCS over the inferior parietal lobe, demonstrating that anodal tDCS provides improvements in vocal adaptation in the face of altered auditory feedback.

Anodal tDCS was associated with an improvement in motor adaptation to sensory changes, indicating its potential as a therapeutic tool to improve voice and speech performance in clinical environments.