The resolution of lexical ambiguity with reference to context in dementia of the Alzheimer's type.

The present study investigated how a dementing illness such as Alzheimer's disease, might affect an individual's recourse to higher order contextual information in the access and integration of lexical material in on-line discourse comprehension. More specifically, the experiment investigated the priming of homophones in a discourse context, by use of a cross-modal lexical decision task, and compared the performances of a group of six subjects with mild to moderate dementia of the Alzheimer's type (DAT) with those of a matched control group. The subjects listened to 2-sentence paragraphs and performed a lexical decision on visually presented targets that followed ambiguous prime words (or homophones) at two inter-stimulus intervals (ISIs): 330 and 1000 msec. When the target was a word, it was either an associate of the prime word, a probable inference suggested by the discourse, or an unrelated word. The control subjects primed both the discourse-appropriate and discourse-inappropriate associate of the homophone at short (330 msec) ISIs (but not an appropriate inference word), a finding which supports the exhaustive access model of ambiguity resolution. As the ISI was lengthened to 1000 msec, however, the discourse-appropriate inference word was primed, and reflects the operation of attention-dependent integrative strategies. The subjects with DAT primed both appropriate associates and inference words at the short ISI. At ISI of 1000 msec, the DAT subjects primed the appropriate associate and showed substantial inhibition priming of the inappropriate associate. These results point to disturbances in the selective automatic activation of lexical material, and in the conscious integration and elaboration of lexical material in ongoing discourse comprehension in persons with DAT.