Background: Current neurocognitive models of language function have been primarily built from evidence regarding object naming, and their hypothesized white-matter circuit mechanisms tend to be coarse grained. Methods: In this cross-sectional, observational study, we used novel correlational tractography to assess the white-matter circuit mechanism behind verb retrieval, measured through action picture-naming performance in adults with chronic aphasia. Results: The analysis identified tracts implicated in current neurocognitive dual-stream models of language function, including the left inferior fronto-occipital fasciculus, inferior longitudinal fasciculus, and arcuate fasciculus. However, the majority of tracts associated with verb retrieval were not ones included in dual-stream models of language function. Instead, they were projection pathways that connect frontal and parietal cortices to subcortical regions associated with motor functions, including the left corticothalamic pathway, frontopontine tract, parietopontine tract, corticostriatal pathway, and corticospinal tract. Conclusions: These results highlight that corticosubcortical projection pathways implicated in motor functions may be importantly related to language function. This finding is consistent with grounded accounts of cognition and may furthermore inform neurocognitive models. Impact statement This study suggests that in addition to traditional dual-stream language fiber tracts, the integrity of projection pathways that connect frontal and parietal cortices to subcortical motor regions may be critically associated with verb-retrieval impairments in adults with aphasia. This finding challenges neurological models of language function.
Keywords: action naming; aphasia; diffusion spectrum imaging; structural connectometry; white matter.