The Effects of Transcranial Direct Current Stimulation over Broca’s Area on Grammatical and Verbal Ability of Bilinguals: Evidence from Hemoencepholography

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulatory method employed to enhance complex tasks including linguistic performance. The present investigation examined the possible effects of tDCS on English grammar competence and cognitive functioning in Persian-English bilinguals. tDCS is to change cortical and neural activities and modify different brain functions both in healthy subjects and aphasic patients. This method is employed to remediate cortical networks and their related neural oscillations with potential impacts on neurocognitive performance such as enhancing learning and acquisition of complex tasks through modulating synaptic activity and plasticity through neurotransmitters. 34 participants were randomly assigned to two arms, the control group with no intervention and the experimental group with either sham or real anodal tDCS over a key cortical area involved in grammar processing (FC5, based on the international 10-20 system) during two subsequent sessions 24 hours apart. All the participants were submitted to a series of English grammar tests after either no intervention or the tDCS sessions with their verbal ability profiled through Cambridge Brain Sciences-Cognitive Platform (CBS-CP) categorized into three higher-order cognitive components of reasoning, short-term memory and verbal ability. Hemoencephalography which is a non-invasive neuroimaging technique for detecting changes in brain metabolic activity reflected in localized cerebral blood flow was also recorded from the left frontoploar area in randomly selected subjects from the two arms during either no intervention or the sham/real tDCS and performance on 3 verbal ability trials of CBS-CP. Our findings proposed that FC5 anodal tDCS vs. sham improves the average total grammar score in Persian-English bilinguals by 5.2% (p=0.000). In terms of cognitive functions, significant differences were noted in different verbal ability tests, Digit span (p=0.030), Double trouble (p=0.000), and Grammatical reasoning (p=0.014). In the control group, no significant difference was reported between the grammatical and cognitive performance of the participants over the two sessions. Results of the present investigation demonstrated the effectiveness of tDCS in improving linguistic abilities, verbal ability as a sub-component of cognitive functions, and frontopolar optical density. Such enhancement in language performance and cognitive functions in the experimental group might be associated with increased frontopolar oxyhemoglobin optical density and cortical excitability potentiated by tDCS. Taken together, it can be stated that anodic polarization over FC5 (Broca’s area) may improve the mean grammatical and verbal ability scores in association with increased tDCS induced frontopolar oxyhemoglobin optical density, neuronal plasticity modulations and cortical excitability alterations. The results of the study may hold significance for individuals falling behind in foreign language learning as well as those with learning disability.

Keywords:

Bilinguals , Grammar , HEG , tDCS , Verbal ability