Natural selection: the impact of semantic impairment on lexical and object decision.

This study was designed to investigate the impact of semantic deficits on the recognition of words and objects as real/familiar. Two-alternative forced-choice tasks of lexical decision and object decision were each administered to a case series of patients with semantic dementia. In both tasks, the critical manipulation was whether the real word or object was more or less "natural" (i.e., typical of its domain) than the nonword or nonobject with which it was paired. For lexical decision, typicality of the words and nonwords was manipulated in terms of bigram and trigram frequencies of the letter strings. For object decision, high typicality in real and chimeric objects consisted in having only or mainly visual features that are standard for objects in that category. This manipulation of relative typicality of real and made-up stimuli exerted a dramatic influence on the patients' success in both lexical and object decision. The patients' strong tendency towards "natural selection" was further modulated by both the frequency/familiarity of the real words/objects and the degree of semantic degradation of the individual patients. This outcome is in line with the authors' model of semantic knowledge and the impact of its degradation on a wide range of cognitive behaviour.