Age of acquisition and imageability ratings were collected for 2,645 words, including 892 verbs and 213 function words. Words that were ambiguous as to grammatical category were disambiguated: Verbs were shown in their infinitival form, and nouns (where appropriate) were preceded by the indefinite article (such as to crack and a crack). Subjects were speakers of British English selected from a wide age range, so that differences in the responses across age groups could be compared. Within the subset of early acquired noun/verb homonyms, the verb forms were rated as later acquired than the nouns, and the verb homonyms of high-imageability nouns were rated as significantly less imageable than their noun counterparts. A small number of words received significantly earlier or later age of acquisition ratings when the 20-40 years and 50-80 years age groups were compared. These tend to comprise words that have come to be used more frequently in recent years (either through technological advances or social change), or those that have fallen out of common usage. Regression analyses showed that although word length, familiarity, and concreteness make independent contributions to the age of acquisition measure, frequency and imageability are the most important predictors of rated age of acquisition.

This study examines the impact of progressive degeneration of conceptual knowledge on the content words used in connected speech elicited using the Cookie Theft picture description (Goodglass & Kaplan. 1983). We began with an analysis of control subjects' descriptions with regard to word types and their frequency and imageability. Because the impairment of conceptual knowledge in semantic dementia is graded by concept familiarity, we created a model of a standardized normal Cookie Theft description that was then progressively 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 imageability effect. Results from a longitudinal study. in which three patients with semantic dementia each described the Cookie Theft picture on three occasions during the progression of their disease, confirmed these predictions. An additional cross-sectional analysis, adding narratives from a larger number of cases, demonstrated that the decline in ability to produce suitable words for the picture description is closely related to the extent of semantic impairment as measured in tests of word comprehension and production. 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.

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