Evaluation of a Transcranial Stimulation With Direct Current on Language Disorders in Semantic Dementia

Within the spectrum of fronto-temporal lobar degeneration (FTLD) semantic dementia (SD) causes profound language dysfunction. SD damages semantic processing typically in the temporal poles (anterior temporal lobes, ATL). It is an early onset disease (often before 65 years of age) affecting about 4000 patients in France and for which no validated treatment is available. For several years a growing number of studies have explored the effects of transcranial stimulation (TCS) on aphasic patients following stroke. Several studies have targeted left-sided language areas and/or homotopical right-sided regions with excitatory or inhibitory TCS, respectively, according to the principle of inter-hemispheric inhibition. In addition, repetitive multi-day TCS has provided evidence for long-lasting language effects (\>6 months) presumably linked to stimulation-induced neuroplasticity. Such investigations have provided promising results and have demonstrated that the stimulation site is a determining factor by showing that stimulation of cortical areas belonging to the language network usually results in more convincing effects than stimulating areas outside that network. Despite these findings the use of TCS in degenerative language diseases, such as primary progressive aphasias including SD, has only been explored in few small cohort studies and, surprisingly, they have not targeted language-related cortices. This project proposes the application of multi-day repetitive TCS with direct current (tDCS) in a large population of SD patients (N=60). It is built on a exploratory investigation of our team which has used three single tDCS sessions in a double-blind sham-controlled study. Excitatory and inhibitory tDCS to the left and right temporal pole, respectively, demonstrated highly significant transient effects (20 min) on semantic processing in 12 SD patients, providing \'proof of concept\' and the rationale for this project. The aim here consists of using repetitive multi-day tDCS for a potential therapeutic outcome leading to long-lasting semantic improvement via neuroplasticity. The project is grounded on 2 hypotheses: i) tDCS to temporal poles (left-excitatory, right-inhibitory) reactivates semantic processing in SD, ii) repetitive tDCS during ten days could induce neuroplasticity and therapeutic language improvement.