Wernicke (1900, as cited in G. H. Eggert, 1977) suggested that semantic knowledge arises from the interaction of perceptual representations of objects and words. The authors present a parallel distributed processing implementation of this theory, in which semantic representations emerge from mechanisms that acquire the mappings between visual representations of objects and their verbal descriptions. To test the theory, they trained the model to associate names, verbal descriptions, and visual representations of objects. When its inputs and outputs are constructed to capture aspects of structure apparent in attribute-norming experiments, the model provides an intuitive account of semantic task performance. The authors then used the model to understand the structure of impaired performance in patients with selective and progressive impairments of conceptual knowledge. Data from 4 well-known semantic tasks revealed consistent patterns that find a ready explanation in the model. The relationship between the model and related theories of semantic representation is discussed.

The clinical presentation of patients with semantic dementia is dominated by anomia and poor verbal comprehension. Although a number of researchers have argued that these patients have impaired comprehension of non-verbal as well as verbal stimuli, the evidence for semantic deterioration is mainly derived from tasks that include some form of verbal input or output. Few studies have investigated semantic impairment using entirely non-verbal assessments and the few exceptions have been based on results from single cases ([3]: Breedin SD, Saffran EM, Coslett HB. Reversal of the concreteness effect in a patient with semantic dementia. Cognitive Neuropsychology 1994;11:617-660, [12]: Graham KS, Becker JT, Patterson K, Hodges JR. Lost for words: a case of primary progressive aphasia? In: Parkin A, editor. Case studies in the neuropsychology of memory, East Sussex: Lawrence Erlbaum, 1997. pp. 83-110, [21]: Lambon Ralph MA, Howard D. Gogi aphasia or semantic dementia? Simulating and assessing poor verbal comprehension in a case of progressive fluent aphasia. Cognitive Neuropsychology, (in-press). This study employed sound recognition and semantic association tasks to investigate the nature of the verbal and non-verbal comprehension deficit in 10 patients with semantic dementia. The patients were impaired on both verbal and non-verbal conditions of the assessments, and their accuracy on these tasks was directly related to their scores on a range of other tests requiring access to semantic memory. Further analyses revealed that performance was graded by concept and sound familiarity and, in addition, identified significant item consistency across the different conditions of the tasks. These results support the notion that the patients' deficits across all modalities were due to degradation within a single, central network of conceptual knowledge. There were also reliable differences between conditions. The sound-picture matching task proved to be more sensitive to semantic impairment than the word-picture matching equivalent, and the patients performed significantly better on the picture than word version of a semantic association test. We propose that these differences arise directly from the nature of the mapping between input modality and semantic memory. Words and sounds have an arbitrary relationship with meaning while pictures benefit from a degree of systematicity with conceptual knowledge about the object.

Although an insidious history of episodic memory difficulty is a typical presenting symptom of Alzheimer’s disease, detailed neuropsychological profiling frequently demonstrates deficits in other cognitive domains, including language. Previous studies from our group have shown that language changes may be reflected in connected speech production in the earliest stages of typical Alzheimer’s disease. The aim of the present study was to identify features of connected speech that could be used to examine longitudinal profiles of impairment in Alzheimer’s disease. Samples of connected speech were obtained from 15 former participants in a longitudinal cohort study of ageing and dementia, in whom Alzheimer’s disease was diagnosed during life and confirmed at post-mortem. All patients met clinical and neuropsychological criteria for mild cognitive impairment between 6 and 18 months before converting to a status of probable Alzheimer’s disease. In a subset of these patients neuropsychological data were available, both at the point of conversion to Alzheimer’s disease, and after disease severity had progressed from the mild to moderate stage. Connected speech samples from these patients were examined at later disease stages. Spoken language samples were obtained using the Cookie Theft picture description task. Samples were analysed using measures of syntactic complexity, lexical content, speech production, fluency and semantic content. Individual case analysis revealed that subtle changes in language were evident during the prodromal stages of Alzheimer’s disease, with two-thirds of patients with mild cognitive impairment showing significant but heterogeneous changes in connected speech. However, impairments at the mild cognitive impairment stage did not necessarily entail deficits at mild or moderate stages of disease, suggesting non-language influences on some aspects of performance. Subsequent examination of these measures revealed significant linear trends over the three stages of disease in syntactic complexity, semantic and lexical content. The findings suggest, first, that there is a progressive disruption in language integrity, detectable from the prodromal stage in a subset of patients with Alzheimer’s disease, and secondly that measures of semantic and lexical content and syntactic complexity best capture the global progression of linguistic impairment through the successive clinical stages of disease. The identification of disease-specific language impairment in prodromal Alzheimer’s disease could enhance clinicians’ ability to distinguish probable Alzheimer’s disease from changes attributable to ageing, while longitudinal assessment could provide a simple approach to disease monitoring in therapeutic trials.

Many cognitive psychological, computational, and neuropsychological approaches to the organisation of semantic memory have incorporated the idea that concepts are, at least partly, represented in terms of their fine-grained features. We asked 20 normal volunteers to provide properties of 64 concrete items, drawn from living and nonliving categories, by completing simple sentence stems (e.g., an owl is __, has __, can__). At a later date, the same participants rated the same concepts for prototypicality and familiarity. The features generated were classified as to type of knowledge (sensory, functional, or encyclopaedic), and also quantified with regard to both dominance (the number of participants specifying that property for that concept) and distinctiveness (the proportion of exemplars within a conceptual category of which that feature was considered characteristic). The results demonstrate that rated prototypicality is related to both the familiarity of the concept and its distance from the average of the exemplars within the same category (the category centroid). The feature database was also used to replicate, resolve, and extend a variety of previous observations on the structure of semantic representations. Specifically, the results of our analyses (1) resolve two conflicting claims regarding the relative ratio of sensory to other kinds of attributes in living vs. nonliving concepts; (2) offer new information regarding the types of features-across different domains-that distinguish concepts from their category coordinates; and (3) corroborate some previous claims of higher intercorrelations between features of living things than those of artefacts.