Evidence for plasticity in white-matter tracts of patients with chronic Broca's aphasia undergoing intense intonation-based speech therapy

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.

Figures

Figure 1

Absolute and relative fiber number of the right arcuate fasciculus (AF) before (gray bars) and after therapy (black bars) in all 6 participants.

Figure 2

Right AF in one patient with two scans before therapy and two scans after 75 sessions of Melodic Intonation Therapy. The scan-to-scan variability is minimal before therapy and a clear difference in the number of fibers and fiber volume can be seen comparing the AF before and after therapy.

Figure 3

CIU/min before (gray bars) and after therapy (black bars) for all 6 participants.

Figure 4

Correlation between absolute change in CIU/min and absolute change in number of AF fibers.