Naming impairments in aphasia are typically targeted in therapy using semantic (word meaning based) and/or phonological (sound based) tasks. However, the relationship between the locus of language breakdown and response to different therapy tasks remains unclear, and knowledge of the neural mechanisms underlying treatment-induced recovery is scarce. The aim of this study was to examine brain activity and white matter integrity, associated with successful semantic versus phonological based treatments of word retrieval.
Methods: Eight aphasia participants received twelve naming treatment sessions, alternating between a phonological (PCA; Leonard, Rochon & Laird, 2008) and semantic task (SFA; Boyle & Coelho, 1995). Functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) was conducted pre and post-treatment for aphasia participants, and once for 14 age-matched controls. The fMRI task involved naming known or treatment items, with activations compared for correct post-treatment vs incorrect pre-treatment items. The number of white matter fibres and the tract integrity (generalised fractional anisotropy (GFA)) was measured in left and right arcuate fasiculus (AF), pre and post-treatment.
Results: Improvement was greater for PCA treatment, with 7 out of 8 improving significantly, compared to 4 out of 8 with SFA. Pre-treatment fMRI activity in the left caudate correlated with greater SFA treatment success, whereas the left supramarginal gyrus and right precuneus activity post-treatment correlated with greater PCA treatment success. Participants with aphasia had significantly fewer white matter fibres and lower mean GFA in the left AF pre-treatment compared to controls, which increased post-treatment to equivocate to controls. The mean GFA in the left AF positively correlated with maintenance of PCA treatment.
The results support previous studies that have found greater treatment outcome to be associated with left hemisphere cortical activity, and suggest that neuronal mechanisms are dependent upon treatment type. Successful treatment may improve the integrity of the white matter connecting language regions.