Applications of Lesion-Deficit Mapping to Unresolved Problems in Aphasia

About This Grant

PROJECT SUMMARY A detailed model of how focal damage to the brain affects long-term language abilities would be of enormous value to clinicians trying to minimize language deficits from brain surgery or predict long-term recovery from strokes. The principal tool for creating such a model is lesion-deficit mapping (LDM), which generates statistical relations between language impairments and damage at each point in the brain. Advances in LDM methodology and neurobiological theories of language processing have enabled significant progress toward this goal, but many fundamental questions remain. This project addresses four such problems. The first concerns the phenomenon of accurate but slowed word processing in some people with aphasia, which could explain isolated sentence comprehension and production impairments in these individuals. This project will for the first time identify the neural correlates of this ubiquitous phenomenon using a novel LDM approach based on reaction time measurements during simple word comprehension and retrieval tasks. The second problem concerns the phenomenon of semantic access impairment, in which some people with aphasia appear to have adequate knowledge of word meaning but cannot selectively access this knowledge due to either excessive ‘competition’ from activation of similar word meanings or to inability to ‘control’ activation of word meanings. This project will be the first large-scale attempt to use LDM to define the lesion correlates of this syndrome and the first LDM study to compare distinct behavioral measures of competition and control. The third problem concerns new evidence for a selective deficit in comprehending noun-noun phrases such as ‘dog dish’ despite preserved comprehension of single words like ‘dog’ and ‘dish’. Understanding noun-noun phrases is thought to require computation of specific functional relationships between the first (modifier) and second (head) noun, and the new evidence suggests that such computations can be selectively impaired by focal brain damage. LDM will be used to identify the neural correlates of this novel type of comprehension impairment and to assess four distinct types of relational computations. The final problem concerns the inability of some people with aphasia to express coherent narratives (e.g., scene descriptions or stories) despite relatively preserved single-word retrieval. The lesion correlates of this deficit in higher-level semantic organization are unknown. The proposed approach will use a large language AI model (GPT4) to assess the coherence of scene descriptions and personal narratives produced by people with focal brain damage, then perform LDM using these scores. These studies will fill important gaps in our understanding of fundamental processing deficits in aphasia and introduce several novel and potentially powerful LDM methods.