The left inferior frontal gyrus is causally involved in selective semantic retrieval: Evidence from tDCS in primary progressive aphasia

Abstract

Lesion and imaging studies have shown that the left inferior frontal gyrus (IFG) is involved in selective semantic retrieval of information from the temporal lobes. However, causal, i.e., interventional, evidence is sparse. In the present study we addressed this question by testing whether transcranial direct current stimulation (tDCS) over the left IFG in a group of individuals with primary progressive aphasia may improve semantic fluency, a task that relies to selective semantic retrieval. Semantic fluency improved significantly more in the tDCS vs. sham condition immediately post-treatment and improvement lasted up to 2 months. We further addressed the question of who will benefit most from such an intervention by testing possible demographic, clinical and functional connectivity variables that may predict the behavioral tDCS effect. We found that patients with stronger baseline functional connectivity between the subareas of the left IFG opercularis and triangularis, and between the middle temporal pole and superior temporal gyrus. were the most likely to benefit from tDCS over the left IFG. We thus provided causal evidence that the left IFG is the neural substrate of selective semantic retrieval and tDCS over the left IFG may improve semantic fluency in individuals with stronger baseline functional connectivity.

Transcranial direct current stimulation over the left inferior frontal gyrus improves sentence comprehension

“…Although still in its advent for neurodegenerative conditions, there is promise that tDCS may augment language therapy effects in PPA, and most tDCS studies have shown near-transfer effects, i.e., generalization to untrained items [14][15][16][17][18][19]. There is also evidence that tDCS effects over the left IFG generalize to untrained but related language tasks in PPA, such as generalization of a naming intervention to an untrained verbal fluency task [20,21]; we reasoned that this occurred because both tasks involve strategic retrieval and selection of the correct word amongst alternatives. However, far-transfer tDCS effects to untrained language functions and not only tasks, are reported in two small studies that showed a generic overall improvement.…”