Effects of rTMS on Brain Activation in Aphasia

Aphasia, an acquired communication disorder, is a common consequence of left-hemisphere stroke. Persisting, or chronic, aphasia negatively impacts not only interpersonal communication but participation in activities of daily living, independence, and autonomy. It is also associated with higher rates of depression and lower quality of life. Therefore, examining factors and treatments that result in the best possible recovery of language function for individuals with chronic aphasia is of paramount importance. An intervention approach that has shown promise in early-phase research on treatment of chronic aphasia is repetitive transcranial magnetic stimulation (rTMS). rTMS is a non-invasive brain stimulation technique that can be used to focally modulate activity in targeted brain regions. Studies have shown that multiple sessions of 1Hz inhibitory rTMS applied to the right hemisphere Pars Triangularis (PTr) of people with chronic aphasia results in improved naming abilities. These improvements accrue over time, and may persist even after rTMS has ended. It has been proposed that rTMS induces this improvement by reducing the disruptive influence of compensatory activity in the right hemisphere PTr, allowing for recruitment of more efficient left hemisphere peri-lesional brain areas. However, existing neuroimaging evidence to support this hypothesis is insufficient. The goal of the proposed study is to investigate the neurological mechanisms underlying the effect of rTMS on naming performance in chronic aphasia. This will extend existing knowledge regarding hemispheric contributions to language recovery following stroke and elucidate how rTMS-induced neuroplasticity can be co-opted to encourage optimal reorganization. The study will also investigate a potential source of individual response variability to rTMS, one which can inform both candidate selection and optimal stimulation parameters. Sixteen participants will be enrolled, yielding a significantly larger sample size than previous studies that have examined changes in functional brain activation in response to rTMS (n = 1, 2). All participants will receive a sequence of 1200 pulses of 1 Hz rTMS to right hemisphere PTr across 10 daily sessions. Half of the participants will also receive a 6 Hz rTMS excitatory priming pulse sequence immediately prior to the 1 Hz sequence. This priming sequence ensures a consistent inhibitory response to the subsequent 1Hz rTMS and will permit an examination of state-dependent individual response variability. To evaluate the effect of rTMS over time, participants will undergo functional magnetic resonance (fMRI) scans at four time points: prior to initiation of rTMS ("baseline"), immediately following the first rTMS session ("post-rTMS"), following the conclusion of the rTMS series ("post-treatment") and at a 2-month follow-up visit. During the scans, participants will be asked to name pictures, and both patterns of regional naming-related activation and effective connectivity (directional causal influence between activated brain regions) will be evaluated at each time point. In addition, naming performance will be measured via standardized assessments at baseline, post-treatment, and follow-up. Changes in naming performance will be assessed over time, as a measure of rTMS effectiveness overall and between groups (priming sequence vs no priming sequence). In addition, changes in activation and effective connectivity will be correlated with naming improvement to assess the relative effectiveness of right hemisphere recruitment compared to left hemisphere peri-lesional recruitment. Overall increases in left hemisphere recruitment are hypothesized to result in greater improvements. However, the magnitude of left hemisphere recruitment due to rTMS is expected to depend on baseline levels of right hemisphere PTr activity. Results from this study will significantly improve the investigator's understanding of the effects of rTMS on stroke recovery. The results will also inform future studies evaluating rTMS as an adjunct to behavioral speech-language intervention, augmenting therapeutic gains from traditional aphasia treatment.