SOURCE: Neuromethods. 202 (pp 339-365), 2023
DATE OF PUBLICATION: 2023
AUTHORS: Ferrucci R.; Ruggiero F.; Mameli F.; Bocci T.; Priori A.
ABSTRACT: tDCS is a noninvasive neuromodulation technique that induces changes in brain excitability and promotes cerebral plasticity. The device consists of a stimulator that delivers sustained direct subthreshold current (1.0-2.5 mA) through at least two connected electrodes placed on the scalp: the anode and the cathode, respectively connected to the positive and the negative pole of the battery. The stimulation administered is too weak to induce neuronal activity regardless of the afferent input, but still sufficient to alter neuron excitability and spontaneous firing rate. In particular, anodal stimulation results in excitation of the cerebral areas below the electrode by lowering GABA levels, while cathodic stimulation results in inhibition thereof by lowering glutamate levels. The consequences of this stimulation can last just a few minutes after stimulation (short-term effects affecting non-synaptic mechanisms) or up to several months (long-term effects mediated by synaptic modifications) and over time have been objectively assessed and monitored with other neurophysiological techniques such as TMS, EEG and ERPs, and MRI/MRS. Language performance is an example of a cognitive area that can be improved by stimulating the prefrontal and temporal regions of the brain in both healthy and aphasic individuals. Specifically, anodal tDCS in multiple areas of the left frontal lobe improved phonemic fluency, sentence production, naming performance, and language comprehension, while stimulation over Wernicke’s area increased learning speed and overall accuracy and decreased naming latencies in healthy subjects. In aphasic patients, repeated tDCS application over the frontal cortex resulted in
improvements in naming accuracy and fluency that lasted up to 6 and 2 months after treatment, respectively. Even stimulation of the posterior perisylvian region showed an improvement in naming accuracy and a reduction in naming latencies, as well as an improvement in auditory verbal comprehension. Overall, these results suggest that repetitive tDCS could be a promising tool in the rehabilitation of patients with language disorders, although further studies are needed to prove the potential and limitations of its effectiveness.