The functional locus of word-finding difficulties in Alzheimer's disease: insights from the picture-word interference task

Although many patients with Alzheimer's disease (AD) exhibit word¬finding difficulties, debate continues as to the cognitive origins of these problems. The aim of this study is to utilise the auditory picture-word interference paradigm, which involves naming a picture while ignoring an auditory distractor word, to gain insights into the nature of the word production difficulties of AD individuals and in particular to clarify where these difficulties may be localised within a two-stage theory of word production. We compared the performances of a group of 11 mild AD participants on an auditory picture-word interference task to that of older adult control participants. In the auditory picture-word interference task, pictures to be named were accompanied by distractors that were either semantically related, phonologically related or unrelated to the target, and these were presented at four different onsets ranging from -200 ms (before the target) to +400 ms (after the target). Recent models of this task have proposed that the semantic interference effect (slower naming latencies with semantic than unrelated distractors) reflects competitive processes during the lexical selection stage whereas semantic facilitation (faster naming latencies with semantic than unrelated distractors) reflects processes during semantic retrieval. It was thus hypothesised that should an AD participant primarily have a lexical selection impairment then they may show heightened semantic interference effects relative to controls, whereas if they primarily have a semantic impairment then they may show heightened semantic facilitation relative to controls or, in the presence of a severe semantic impairment, no effect of semantic relative to unrelated distractors. The findings suggested that the AD individuals in this study may have a lexical selection deficit. The AD participant group as a whole showed significantly greater semantic interference than controls on the picture-word interference task. For most of the AD participants, this semantic interference peaked at -200 ms (distractor before the picture), as was the case for controls. However, for the two AD participants with the most severely impaired naming (who were tested on an easier version of the task), the semantic interference effect peaked significantly later than for controls on the same task, at +200 ms (distractor after the picture). Further, analyses revealed that increased semantic interference effects (particularly at late presentation onsets) were related to poorer naming scores, even when comprehension or Mini Mental State Examination scores were taken into account, suggesting that the abnormally pronounced semantic effects reflect the participants' word production difficulties rather than more generalised impairments. The effects of phonologically related distractors were, for the most part, similar in AD and control participants, except that the two most severely naming-impaired AD participants showed an abnormally exaggerated phonological facilitation effect. The exaggerated phonological facilitation effect is also in keeping with a lexical selection deficit. Wider implications of our findings are discussed.