No right to speak? The relationship between object naming and semantic impairment: Neuropsychological abstract evidence and a computational model

The processes required for object naming were addressed in a study of patie nts with semantic dementia (a selective decline of semantic memory resultin g from progressive temporal lobe atrophy) and in a computational model of s ingle-word production. Although all patients with semantic dementia are imp aired in both single-word production and comprehension, previous reports ha d indicated two different patterns: (a) a parallel decline in accuracy of n aming and comprehension with frequent semantic naming errors, suggesting a purely semantic basis for the anemia and (b) a dramatic progressive anemia without commensurate decline in comprehension, which might suggest a mainly postsemantic source of the anomia. Longitudinal data for 16 patients with semantic dementia reflected these two profiles, but with the following addi tional important specifications: (1) despite a few relatively extreme versi ons of one or other profile, the full set of cases formed a continuum in th e extent of anomia for a given degree of degraded comprehension; (2) the de gree of disparity between these two abilities was associated with relative asymmetry in laterality of atrophy: a parallel decline in the two measures characterized patients with greater right than left-temporal atrophy, while disproportionate anemia occurred with a predominance of atrophy in the lef t-temporal lobe. In an implemented computational model of naming, semantic representations 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. Asymmetric damage to semant ic units reproduced the longitudinal patient profiles of naming relative to comprehension. plus additional characteristics of che patients' naming per formance. On the basis of both the neuropsychological and computational evi dence, we propose that semantic impairment alone can account for the full r ange of word production deficits described here.