Presentation

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Investigating word production errors in aphasia with large language models and intracortical microelectrode recordings

Poster Session D, Saturday, October 26, 10:30 am - 12:00 pm, Great Hall 3 and 4

Laura Schiffl¹, Lisa M. Held¹, Arthur Wagner², Bernhard Meyer², Jens Gempt³, Simon N. Jacob^1,2; ¹Translational Neurotechnology Laboratory, Department of Neurosurgery, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany, ²Department of Neurosurgery, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany, ³Department of Neurosurgery, Universitätsklinikum Hamburg-Eppendorf, Hamburg, Germany

Damage to cortical brain regions associated with the left-lateralized language network typically results in language disorders (aphasia). One of the most frequently observed chronic symptoms in patients with non-fluent aphasia is an impairment in word finding. Behavioral, electrophysiological, and neuroimaging evidence suggests that these difficulties can be attributed to failures in conceptualization, lexical retrieval, and phonological encoding rather than to sensory-motor deficits. However, current studies of the neural processes underlying word production errors in individuals with non-fluent aphasia lack the temporal and spatial resolution to disentangle the mechanisms at the single-neuron level. In particular, the role of the unimpaired right hemisphere of patients with left-hemispheric lesions in the rehabilitation of language functions remains a matter of debate. The extent to which right hemispheric regions support residual language functions, such as successful word production, in this group of patients requires further investigation. In this unique case study, we investigated the contribution of right-hemispheric brain regions homotopic to the left language system using large-scale single-unit recordings (N=256 intracortical microelectrodes covering the inferior frontal gyrus, middle frontal gyrus, angular gyrus, supramarginal gyrus) in a patient with non-fluent aphasia performing three language tasks (naming, repetition, comprehension). Over several months, the patient showed typical patterns of impairment, with reliably high performance in comprehension and repetition, but low performance in naming. Although stable over time, naming accuracy varied for individual words. This variation was predictable using logistic regression classifiers trained on speech and language embeddings from large language models (LLMs). The LLM embeddings effectively predicted the correctness of a trial. They also captured the relationships between target words and their output in speech, specifically the error type (e.g., semantic paraphasia, circumscription, onomatopoeia, phonological paraphasia, mixed paraphasia) and semantic distance. We found that individual neurons in all recorded brain areas exhibited firing rate changes that were task-, performance-, and region-specific, arguing for their involvement in the respective tasks. For example, contrasting neuronal responses in correct naming trials with responses in incorrect naming trials showed that single neurons, especially in MFG and IFG, exhibited variations in spiking activity that were correlated with the patient’s performance. Furthermore, neuronal activity in the SMG varied as a function of individual target words prior to speech onset in the naming task, but less so in the repetition and comprehension task. As repetition and naming involve similar articulatory processes but differ in word-finding demands, this result supports a role for the right SMG in lexical access and phonological encoding. In IFG, we also found an effect of the target word on neuronal activity before speech onset in naming trials. This is in line with the involvement of IFG in speech planning. By combining the strength of speech and language embeddings from LLMs with the superior spatial and temporal resolution of intracortical microelectrode recordings, this study will promote our understanding of the role of the right hemisphere in aphasia in general and the temporal and spatial signatures of the cognitive processes required to move from intention to articulation in particular.

Topic Areas: Disorders: Acquired, Methods