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Abstract
Background: Non-invasive brain stimulation (NIBS) has been used as a probe to modulate cognitive functions in humans for the last 20 years. The two most commonly reported techniques are transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), which have both been used with different stimulation parameters to increase or decrease excitability in target cortical regions.
Aims: In the present review, we highlight recent developments in TMS and tDCS in language research in healthy individuals.
Main Contribution: We will first describe how tDCS and TMS have been employed to improve either performance of language tasks, or learning in the language domain (facilitatory brain stimulation). Then, we will show how these techniques were used as interference techniques (inhibitory brain stimulation) for understanding brain–behaviour interactions and to explore possible cause–effect links between altered activity in specific brain areas and particular behaviours. We will only review studies in healthy individuals, and first pilot trials in patient population with more general cognitive impairments (idiopathic Parkinson's Disease, Alzheimer's Disease). For tDCS studies in aphasia, see Holland and Crinion (Citation2012); for TMS studies in aphasia, see Medina et al. (Citation2012).
Conclusions: In the healthy brain, NIBS can be used to delineate the functional significance of a particular brain region for the respective language task under study. Here, new possibilities of combining TMS or tDCS with functional imaging techniques offer unique opportunities to not only address the functional significance of a brain area underlying the coil or electrode, but in addition allow testing NIBS effects on an entire network. In this setting, TMS, both in its single-pulse and its repetitive form, allows the modulation of a specific function within milliseconds (single-pulse) or the order of seconds (particularly high-frequency rTMS trains), while tDCS has the disadvantage of requiring longer stimulation times. On the other hand, the relatively easy use of tDCS in the magnetic resonance scanner environment will render simultaneous recording of brain function and activation much more accessible, thus offering novel avenues in the study of network effects of NIBS. For proof-of principle studies to improve language learning in the healthy brain, tDCS may be the most fruitful approach due to its easy applicability and excellent safety profile. Subsequent use of this technique in parallel to training protocols to improve re-acquisition of language functions aphasic patients offers exciting possibilities in the realm of neurorehabilitation.