The analysis of neuropsychological disorders of lexical processing has provided important clues about the general organization of the lexical system and the internal structure of the processing components. Reports of patients with selective dysfunction of specific semantic categories such as abstract versus concrete words, living things versus inanimate objects, animals, fruits and vegetables, proper names and so forth, support the hypothesis that the neural organization of the semantic processing component is organized in these categories. There are reports of selective dysfunction of the grammatical categories noun and verb, suggesting that a dimension of lexical organization is the grammatical class of words. But the results reported in these studies have not provided unambiguous evidence concerning two fundamental questions about the nature and the locus of this organization within the lexical system. Is the noun-verb distinction represented in the semantic or in the phonological and orthographic lexicons? Is grammatical-class knowledge represented independently of lexical forms or is it represented separately and redundantly within each modality-specific lexicon? Here we report the performance of two brain-damaged subjects with modality-specific deficits restricted principally (H.W.) or virtually only (S.J.D) to verbs in oral and written production, respectively. The contrasting performance suggests that grammatical-class distinctions are redundantly represented in the phonological and orthographic output lexical components.

The processes required for object naming were addressed in a study of patients with semantic dementia (a selective decline of semantic memory resulting from progressive temporal lobe atrophy) and in a computational model of single-word production. Although all patients with semantic dementia are impaired in both single-word production and comprehension, previous reports had indicated two different patterns: (a) a parallel decline in accuracy of naming and comprehension, with frequent semantic naming errors, suggesting a purely semantic basis for the anomia and (b) a dramatic progressive anomia 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 additional important specifications: (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 the two measures characterized patients with greater right- than left-temporal atrophy, while disproportionate anomia occurred with a predominance of atrophy in the left-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 semantic units reproduced the longitudinal patient profiles of naming relative to comprehension, plus additional characteristics of the patients' naming performance. 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 described here.

PET activation studies identify significant local changes in regional cerebral blood flow (rCBF) in contrasts of behavioural tasks with control states, and these local changes identify net changes in local synaptic activity. A number of studies on word retrieval have all demonstrated left frontal (dorsolateral and medial) involvement in the task. However, there have been differences in the responses observed in the left temporal lobe, with variously a deactivation (significant decrease in rCBF), no response and an activation (significant increase in rCBF). In the four studies described here, we have examined word (verbs and nouns) retrieval contrasted with a number of different control states. The studies confirmed extensive activation of the left dorsolateral prefrontal cortex and, medially, the anterior cingulate cortex and the supplementary motor area (SMA). Activations of the left posterior temporal lobe and the inferior parietal lobe were consistently demonstrated when word retrieval was contrasted with a rest state. Contrasts with other single word tasks controlled out the activation in the perisylvian part of the left posterior temporal lobe, suggesting a role for this region in lexical processing. The left inferolateral temporal cortex and the posterior part of the inferior parietal lobe were only activated by word retrieval, particularly verbs. It is proposed that these activated regions reflect access to semantic fields.