Neurocognitive Mechanisms of Sentence Production Impairment in Aphasia

Post-stroke agrammatic aphasia (PSA-G) is characterized by a cluster of symptoms (fragmented sentences, errors in functional morphology, a dearth of verbs, and slow speech rate), yet extant theories and language interventions focus on individual symptoms. This single-symptom theoretical and intervention focus results in limited gains in functional communication. The long-term goal of this research is to improve the clinical effectiveness of interventions for PSA-G.

As a first step towards this goal, this project's objective is to advance the theoretical framework of PSA-G by addressing two critical gaps. The first gap is in the mechanistic understanding of how lexical, grammatical, motoric, and cognitive processes work together to enable fluent sentence production and how this breaks down in PSA-G. The second gap is in the understanding of neural mechanisms underlying how sentence production planning normally unfolds over time and what crucial spatiotemporal alterations give rise to PSA-G versus other variants of post-stroke aphasia with predominantly lexico-semantic deficits (PSA-LS). The central hypothesis is that agrammatic language production results from spatiotemporal alterations in the neural dynamics of morphosyntactic and phonomotor processes, causing a cumulative processing bottleneck at the point of articulatory planning. This Synergistic Processing Bottleneck Model of Agrammatism will be tested with two specific aims.

Specific Aim 1 will elucidate the relative contribution of syntactic and non-syntactic processes towards sentence production in aphasia by using speed metrics and a path modeling framework. The expected outcomes of this aim are an improved understanding of the extent to which delays in different linguistic processes underlying the agrammatic symptom cluster impair fluent sentence production in aphasia generally, and in PSA-G versus PSA-LS more specifically.

Specific Aim 2 will determine the neural mechanisms underlying sentence production across language deficit profiles. Magnetoencephalography (MEG) will be used to compare alterations in timecourse and functional connectivity of key perilesional and contralesional syntactic hubs across increasingly demanding morphosyntactic production tasks. The expected outcome of this aim is a spatiotemporally specified neural model of sentence production in neurotypical, PSA-G, and PSA-LS speakers.

The significance this research is that it will forward an empirically established multidimensional model of sentence production, which will lay the foundation for developing more targeted and effective language interventions for agrammatic aphasia. It will also contribute to a better understanding of agrammatism in neurodegenerative aphasias. The innovative aspects of this project include: a novel multidimensional theoretical framework that incorporates non-syntactic dimensions of phonomotor planning, processing capacity and speed, and neurophysiological dynamics; direct comparisons between PSA-G and PSA-LS groups; and MEG analysis of spoken language with simultaneous electromyographic measurement.