Addressing Phonological Deficit in Primary Progressive Aphasia

Abstract

Background: Individuals with primary progressive aphasia (PPA) are in a race against time as they contend with the rapid decline of communication ability. Research demonstrates that even in the presence of progressive neurodegeneration, some individuals with PPA can improve language function in response to behavioral intervention. In addition, a growing body of evidence suggests that transcranial direct current stimulation (tDCS) can potentiate the effects of speech-language treatment and improve generalization. Word retrieval deficits, which arise from disruption to critical components of the distributed language network, are ubiquitous in aphasia and the most common focus of behavioral treatment with tDCS. By contrast, relatively little attention has been directed toward improving underlying cognitive processes associated with focal cortical atrophy patterns characteristic of the recognized variants of PPA. In this study, we focused on remediation of phonological impairments that are prominent when left perisylvian regions are dysfunctional as observed in the logopenic and nonfluent/agrammatic variants of PPA. Specifically, we hypothesized that behavioral intervention intended to strengthen phonological manipulation skills and sound-letter correspondences—coupled with tDCS—has the potential to improve written language abilities. Methods: Twelve individuals with the logopenic or nonfluent/agrammatic variants of PPA and 24 neurotypical controls participated in this study. All participants completed structural and functional neuroimaging and comprehensive neuropsychological testing. Pre-treatment assessment revealed marked impairment of phonological skill as well as deficits in spoken and written language function associated with damage to left perisylvian regions in the PPA group. Six participants were randomized to receive phonological intervention with active tDCS in Phase 1, and six were randomized to receive treatment with sham stimulation. After a two-month break, they were crossed over to the other stimulation type for a second period of phonological treatment. Before and after each treatment phase, assessment data were collected to measure performance on measures of phonological skill, naming, single word spelling and reading, written narratives, and overall language performance. Follow-up testing was completed two months after the intervention ended. Results: Both the tDCS-first and sham-first groups exhibited significant improvement in sound blending and phonology-orthography transcoding ability after treatment. The sham-first group also showed improvements in spoken repetition after Phase 1 that were maintained over the course of the study. Generalization to text-level writing was observed only in the tDCS-first group. Specifically, this group showed impressive gains in spelling and the proportion of meaningful content included in their written narratives over the course of treatment, with performance at follow-up significantly surpassing pre-treatment levels. In addition, the tDCS-first group showed stronger maintenance of treatment gains after untreated epochs when compared to the sham-first group, whose performance showed a downward trend over time that was not significant. Conclusion: There is urgency to develop treatment protocols for individuals living with progressive language decline, and these data present compelling evidence regarding the impairment-based approach that targets compromised phonological skills in individuals with lvPPA and nfvPPA, especially when combined with tDCS as an adjuvant. Our study is the first to document that improved phonological skills resulted in better functional communication ability (text-level writing) relevant to the everyday lives of individuals living with PPA. In addition, our study contributes to a growing body of evidence demonstrating that tDCS is a safe intervention that has potential to improve speech-language treatment gains.