Non-Invasive Brain Stimulation to Investigate Language Production in Healthy Speakers: A Meta-Analysis

Abstract

Non-invasive brain stimulation (NIBS) has become a common method to study the interrelations between the brain and language functioning. This meta-analysis examined the efficacy of transcranial magnetic stimulation (TMS) and direct current stimulation (tDCS) in the study of language production in healthy volunteers. Forty-five effect sizes from 30 studies which investigated the effects of NIBS on picture naming or verbal fluency in healthy participants were meta-analysed. Further sub-analyses investigated potential influences of stimulation type, control, target site, task, online vs. offline application, and current density of the target electrode. Random effects modelling showed a small, but reliable effect of NIBS on language production. Subsequent analyses indicated larger weighted mean effect sizes for TMS as compared to tDCS studies. No statistical differences for the other sub-analyses were observed. We conclude that NIBS is a useful method for neuroscientific studies on language production in healthy volunteers.Keywords: language production, meta-analysis, picture naming, verbal fluency, TMS, tDCS IntroductionTranscranial magnetic (TMS) and direct current stimulation (tDCS) are non-invasive brain stimulation (NIBS) techniques that are increasingly used to investigate causal relationships between language functions and their underlying neuronal processes. The aim of this combined review and meta-analysis is to examine the efficacy and reliability of NIBS as an intervention method to study the neural correlates of language production in healthy volunteers. Prior meta-analyses on the effects of transcranial direct current stimulation (tDCS) on verbal fluency and picture naming have provided diverging results. Both Horvath, Forte, and Carter (2015) and Price, McAdams, Grossman, and Hamilton (2015) analysed performance changes in semantic production and word learning tasks, with the first finding no effect, but the latter reporting a reliable modulation of task performance. Furthermore, Westwood and Romani (2017) found no effect of tDCS on language production performance across production and reading tasks. Our present review offers an overview and metaanalysis of studies which measured changes in verbal fluency and picture-naming performance during or following the administration of tDCS or

Cerebellar tDCS does not modulate language processing performance in healthy individuals

“…It should be noted, however, that these analyses are based on one significant (i.e., from the lexical decision task) and one non-significant main effect of tDCS (i.e., from the sentence comprehension task). Interestingly though, the same pattern was found by Klaus and Schutter (2018), where the effect of tDCS on the left dorsolateral prefrontal cortex on three different tasks (picture naming, sentence comprehension, Flanker task) was tested. Both findings suggest that there are differences in the magnitude of the tDCS effect between individuals, adding to the existing literature on between-participant variability in response to tDCS (Cheeran et al, 2017;López-Alonso et al, 2015;Wiethoff et al, 2014).…”

“…In the motor cortex, anodal tDCS is thought to increase spontaneous neural firing, leading to performance improvement, whereas cathodal tDCS decreases cortical excitability, causing a performance decrement (Fernandes Medeiros et al, 2012;Nitsche et al, 2008). However, work focusing on brain areas outside of the motor cortex has suggested that this dichotomy may be less straight-forward for non-motor cortical regions (Brückner & Kammer, 2017;Klaus & Hartwigsen, 2020;Klaus & Schutter, 2018) including the cerebellum (Oldrati & Schutter, 2018).…”

“…Within the domain of language, numerous studies have applied tDCS over cortical language regions to investigate language processing (for meta-analyses, see Klaus & Schutter, 2018;A. R. Price et al, 2015;Westwood & Romani, 2017).…”

Verbal Fluency in Mild Alzheimer’s Disease: Transcranial Direct Current Stimulation over the Dorsolateral Prefrontal Cortex

“…The sample consisted of 40 patients, 13 males (32.5%) and 27 females (67.5%), with mean age of 73.17 ± 5.61 years (age range 62-80), mean education of 12.7 ± 3.58 years (education range [8][9][10][11][12][13][14][15][16][17][18].…”

“…NIBS is being increasingly used to study the connection between language processes and the underlying brain networks [8][9][10][11][12][13][14][15]. Just as an example, recent studies have reported that in healthy subjects inhibitory repetitive TMS (rTMS) over the right lateral frontal cortex significantly improves phonemic fluency performance [16], while excitatory tDCS over the left inferior frontal gyrus improves performance in naming, phonemic, and semantic fluency tasks [17][18][19][20].…”