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The relationship between semantic memory and speech production: Neuropsychology, neuroanatomy, and a neural-net model
Lambon Ralph, M.A., Mcclelland, J.L., PATTERSON, K. & HODGES, J.R.
Higher Brain Function Research (Shitsugosho Kenkyu), 20 (2) 145-156
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The relationship between conceptual knowledge and object naming was investigated in a study that combined a computational model of single word production with longitudinal and cross-sectional data of patients with semantic dementia (a selective decline of semantic memory resulting from progressive temporal-lobe atrophy). Although all patients with semantic dementia have both anomia and impaired comprehension, previous reports had indicated two different longitudinal profiles: (a) a parallel decline in accuracy of naming and comprehension with frequent semantic naming errors, suggesting a purely semantic basis for the anomia; (b) a dramatic progressive anomia without commensurate decline in comprehension, which might suggest a mainly post-semantic source of the anomia. Results, described here, for 16 patients with semantic dementia reflected these two profiles, but with the following important caveats: (1) despite a few relatively extreme versions of one or other profile, the full set of cases formed a continuum in the extent of anomia for a given degree of degraded comprehension; (2) the degree of disparity between these two abilities was associated with relative asymmetry in laterality of atrophy: a parallel decline in comprehension and naming characterized patients with greater right-than left-temporal atrophy, while disproportionate anomia occurred with a predominance of atrophy in the left temporal lobe. These patterns were successfully reproduced in an implemented computational model of naming. This model incorporated semantic representations that were distributed across simulated left-and right-temporal regions but the semantic units on the left were more strongly connected to left-lateralized phonological representations. Bilateral but asymmetric damage to semantic units replicated the longitudinal patient profiles of naming relative to comprehension. On the basis of both the neuropsychological and computational evidence, we propose that semantic impairment alone can account for the full range of word production deficits observed in semantic dementia.