The rise and fall of frequency and imageability: Noun and verb production in semantic dementia

This study examines the impact of progressive degeneration of conceptual kn owledge on the content words used in connected speech elicited using the Co okie Theft picture description (Goodglass & Kaplan, 1983). We began with an analysis of control subjects' descriptions with regard to word types and t heir frequency and imageability. Because the impairment of conceptual knowl edge in semantic dementia is graded by concept familiarity, we created a mo del of a standardized normal Cookie Theft description that was then progres sively degraded by the systematic removal of lower bands of word frequency, We drew two main predictions from this model: reduced availability of The lower bands of word frequency should result in (a) an apparent deficit for noun retrieval in relation to verb retrieval and (b) an apparent reverse im ageability effect. Results from a longitudinal study, in which three patien ts with semantic dementia each described the Cookie Theft picture on three occasions during the progression of their disease, confirmed these predicti ons. An additional cross-sectional analysis, adding narratives from a large r number of cases, demonstrated that the decline in ability to produce suit able words for the picture description is closely related to the extent of semantic impairment as measured in tests of word comprehension and producti on. Both verbs and nouns are affected by the degradation of semantic memory ; the fact that the impairment to noun production is manifested earlier and more catastrophically may be attributed to the relatively lower frequency of these terms, (C) 2000 Academic Press.