Transcranial Direct Current Stimulation (tDCS) of Wernicke's and Broca's Areas in Studies of Language Learning and Word Acquisition

Abstract

Language is a highly important yet poorly understood function of the human brain. While studies of brain activation patterns during language comprehension are abundant, what is often critically missing is causal evidence of brain areas' involvement in a particular linguistic function, not least due to the unique human nature of this ability and a shortage of neurophysiological tools to study causal relationships in the human brain noninvasively. Recent years have seen a rapid rise in the use of transcranial direct current stimulation (tDCS) of the human brain, an easy, inexpensive and safe noninvasive technique that can modulate the state of the stimulated brain area (putatively by shifting excitation/ inhibition thresholds), enabling a study of its particular contribution to specific functions. While mostly focusing on motor control, the use of tDCS is becoming more widespread in both basic and clinical research on higher cognitive functions, language included, but the procedures for its application remain variable. Here, we describe the use of tDCS in a psycholinguistic word-learning experiment. We present the techniques and procedures for application of cathodal and anodal stimulation of core language areas of Broca and Wernicke in the left hemisphere of the human brain, describe the procedures of creating balanced sets of psycholinguistic stimuli, a controlled yet naturalistic learning regime, and a comprehensive set of techniques to assess the learning outcomes and tDCS effects. As an example of tDCS application, we show that cathodal stimulation of Wernicke's area prior to a learning session can impact word learning efficiency. This impact is both present immediately after learning and, importantly, preserved over longer time after the physical effects of stimulation wear off, suggesting that tDCS can have long-term influence on linguistic storage and representations in the human brain.

Acquisition of concrete and abstract words is modulated by tDCS of Wernicke’s area

“…In total, 30 words (3 sets of 10) were chosen as a base for the experimental stimulus set to be modified via recombination to create novel pseudowords. These were assigned to abstract or concrete novel word categories and presented across the subject groups in a counterbalanced fashion to avoid any random effects of surface stimulus properties (see also Blagovechtchenski et al, for a more detailed methods description 33 ) . The base words all had eight letters/phonemes and consisted of three syllables with CVC–CV–CVC structure (where C stands for consonant and V for vowel).…”

“…Direct current of low intensity (1.5 mA) was delivered to the subjects’ skin through a pair of rubber electrodes covered with electrically conductive Geltek-Medica gel. The active electrode (5 × 5 cm) was placed on the scalp over Wernicke's area (CP5 electrode location according to the extended international EEG 10–20 system); the reference electrode (5 × 10 cm) was located at the base on the left side of the neck 33 . The chosen electrode sizes, current density, and duration of the stimulation were standard and optimal for producing sufficiently local stimulation 53 , 54 .…”

“…Effects of non-invasive brain stimulation on learning new words have been demonstrated in healthy individuals and for language re-acquisition after stroke-induced aphasia 28 – 30 . For instance, tDCS has been shown to enhance language learning performance 31 – 33 . A recent neurostimulation study 34 revealed that anodal tDCS over Wernicke's area in a word learning task significantly improved the accuracy and decreased latencies in a picture-naming task, while another experiment 28 showed faster and better associative verbal learning with the anodal tDCS over the posterior part of the left perisylvian area, compared to sham (placebo) condition.…”

“…Moreover, a recent tDCS study of language processing targeting Wernicke’s area demonstrated that both anodal and cathodal stimulation improved semantic processing in the lexical decision task in comparison with the sham condition, suggesting that cathodal inhibition, common for motor studies, can not be simply assumed to apply to the language function in the same way 45 . This effect can also be explained in terms of the hypothesis that the influence of tDCS makes the neural system sensitive to other stimuli by shifting the excitation thresholds up or down, rather than causes inhibition and excitation per se 33 , 46 – 48 . That makes exact predictions for the inhibitory and facilitatory effects of cathodal and anodal stimulation difficult and instead warrants experimental investigation of both polarities, which is what we have attempted in the present study.…”

Psycholinguistic Content of Complements in English and Ukrainian

“…The units of the language which denote the etiquette some scientists refer to the statements that regulate interpersonal relationships with a partner and, as a result, convey the mental state of the speaker, his/her attitude to certain events (Blagovechtchenski, Gnedykh, Kurmakaeva et. al., 2019).…”