Identifying target regions for non-invasive brain stimulation of the language network in primary progressive aphasia: A narrative review

Abstract

Primary progressive aphasia (PPA) is a heterogeneous neurodegenerative condition characterized by loss of language function. The present review discusses that Non-invasive brain stimulation (NIBS) holds the potential to benefit these patients by inducing neuroplasticity in language networks. Studies have also shown that neuroplasticity in the language networks is accompanied by behavioural gains irrespective of the degree of regional atrophy in PPA patients. However, the generalizability of NIBS in remediating speech deficits is currently restricted by inter-individual variability, lack of consistency, and understanding of the underlying improvement mechanism. Here, we highlight the organization of language networks in PPA patients by consolidating anatomical, functional, and tractography-based imaging studies. These imaging and brain-computer interface studies point out functional segregation of language processing in dorsal and ventral pathways that brace with the regional atrophy pattern seen in PPA patients. The dorsal pathway involved in the sequential processing of phonetics and syntax encompasses the neuronal network around the left posterior temporal-parietal region and inferior frontal gyrus. The ventral pathway is involved in the amalgamation of sensory input into semantics around the left middle and inferior temporal lobes, including the polar cortices. A separate speech production network is identified around the left inferior frontal gyrus and motor cortices dedicated to integrated motor articulation. Thus, the present review suggests selectively stimulating brain regions within these different speech processing networks to address specific speech deficits seen in PPA patients to maximize the benefits of functionality-specific NIBS.