Evidence for Plasticity in White‐Matter Tracts of Patients with Chronic Broca's Aphasia Undergoing Intense Intonation‐based Speech Therapy

TLDR

Recovery from aphasia can involve perilesional or homologous language regions, and for large left‑hemisphere lesions the right hemisphere—particularly the superior temporal, premotor, and primary motor areas linked by the arcuate fasciculus—may be the sole viable pathway, though this tract is less developed on the right. We tested whether an intonation‑based speech therapy (melodic intonation therapy) would lead to changes in white‑matter tracts, especially the arcuate fasciculus. The therapy was delivered intensively, with 75–80 daily sessions, and diffusion tensor imaging was used to assess changes in the arcuate fasciculus. Diffusion tensor imaging revealed a significant post‑treatment increase in arcuate fasciculus fiber count and volume in six patients, indicating that intensive melodic intonation therapy remodels the right‑hemisphere tract and may underlie the sustained clinical benefits observed.

Abstract

Recovery from aphasia can be achieved through recruitment of either perilesional brain regions in the affected hemisphere or homologous language regions in the nonlesional hemisphere. For patients with large left-hemisphere lesions, recovery through the right hemisphere may be the only possible path. The right-hemisphere regions most likely to play a role in this recovery process are the superior temporal lobe (important for auditory feedback control), premotor regions/posterior inferior frontal gyrus (important for planning and sequencing of motor actions and for auditory-motor mapping), and the primary motor cortex (important for execution of vocal motor actions). These regions are connected reciprocally via a major fiber tract called the arcuate fasciculus (AF), however, this tract is not as well developed in the right hemisphere as it is in the dominant left. We tested whether an intonation-based speech therapy (i.e., melodic intonation therapy [MIT]), which is typically administered in an intense fashion with 75-80 daily therapy sessions, would lead to changes in white-matter tracts, particularly the AF. Using diffusion tensor imaging (DTI), we found a significant increase in the number of AF fibers and AF volume comparing post- with pretreatment assessments in six patients that could not be attributed to scan-to-scan variability. This suggests that intense, long-term MIT leads to remodeling of the right AF and may provide an explanation for the sustained therapy effects that were seen in these six patients.