Transcranial direct current stimulation (tDCS) and language

Abstract

Transcranial direct current stimulation (tDCS), a non-invasive neuromodulation technique inducing prolonged brain excitability changes and promoting cerebral plasticity, is a promising option for neurorehabilitation. Here, we review progress in research on tDCS and language functions and on the potential role of tDCS in the treatment of post-stroke aphasia. Currently available data suggest that tDCS over language-related brain areas can modulate linguistic abilities in healthy individuals and can improve language performance in patients with aphasia. Whether the results obtained in experimental conditions are functionally important for the quality of life of patients and their caregivers remains unclear. Despite the fact that important variables are yet to be determined, tDCS combined with rehabilitation techniques seems a promising therapeutic option for aphasia.

Keywords: Aphasia; Electrical Stimulation; Rehabilitation; Speech Therapy; Stroke.

Figure 1

The flowchart shows the criteria and key word search terms used to select papers from the PUBMED database. Twenty-one studies were selected, 10 for aphasic patients, 10 for healthy subjects and one study reported data from healthy subjects and patients. Irrelevant articles include papers on other pathologies, or using different techniques, or investigating functions different from language or are predictions based on computational models. tDCS, transcranial direct current stimulation.

Figure 2

Results obtained with frontal and temporal transcranial direct current stimulation (tDCS) in language tasks in healthy subjects. (A) Anodal tDCS, but not sham, applied over Broca's region increased phonemic and semantic fluency in 10 healthy subjects. Y axis: mean number of words. *Significant different error bars are standard error of the mean (SEM) (from Cattaneo et al, with permission). (B) Anodal tDCS over the right anterior temporal lobe significantly improved naming for people but not landmarks in 15 healthy subjects. Y axis: average percent accuracy for correct trials with long response times (>5 s) in the face condition and place condition. Note that face naming accuracy increased by 11%, from 27% in the sham condition to 38% after anodal tDCS to the right anterior temporal lobe. *Significant difference. Error bars are not reported (from Ross et al, with permission).

Figure 3

Results obtained with frontal and temporal transcranial direct current stimulation (tDCS) in language tasks in aphasic patients. (A) Cathodal tDCS over the left frontotemporal area significantly improved accuracy in the picture naming task in eight aphasic patients. Y axis: naming accuracy expressed by percentage variation from baseline. **Significant difference error bars are standard error of the mean (SEM). AtDCS, anodal tDCS; CtDCS, cathodal tDCS; StDCS, sham tDCS (from Monti et al, with permission). (B) Anodal tDCS applied over the right inferior frontal gyrus significantly decreased the total duration of utterances, the language variable that signifies an improvement in verbal fluency, in six patients with Broca's aphasia. Error bars are SEM (from Vines et al, with permission). (C) Cathodal tDCS over the right superior temporal gyrus significantly improved auditory-verbal comprehension in 21 patients. Y axis: auditory-verbal comprehension least squares means scores. *Significance difference error bars are the interval of confidence (CI). AVC, auditory verbal comprehension; LS, least squares (from You et al, with permission).

Figure 4

(A) A patient with aphasia during an experimental session with transcranial direct current stimulation (tDCS): the anodal electrode is placed over the (left) perilesional area and the cathodal over the contralateral hemisphere. tDCS is delivered during speech therapy through a special stimulating cap that allows a simple positioning of electrodes. (B) Schematic diagrams of brain locations where tDCS improved language in normal subjects (top) and in aphasic patients (bottom). Red circles represent anodal stimulation and black circles cathodal stimulation (active electrodes). Note, however, that the position of the reference electrode differed across different studies. This figure is only reproduced in colour in the online version and red circles are grey in the printed version.