Picture comprehension in probable Alzheimer's disease

Patients with probable Alzheimer's disease (pAD) are frequently said to be compromised at a semantic level of picture comprehension. In order to test this hypothesis, pAD patients were asked to judge whether pictures are instances of a familiar superordinate category ("vegetable"). The targets (e.g., corn) varied in their representativeness, and the foils were either "coherent" items that were not target exemplars (e.g., apple, chair) or "anomalous" items displaying specific features that disqualified the stimuli as target exemplars (e.g., striped carrot). We also asked patients to judge a comparable set of words to determine whether difficulty appreciating pictures is material-specific. pAD patients were impaired at judging target pictures and a subset of picture foils, according to group-wide analyses, although they also encountered some difficulty judging the category membership of words. Individual patient analyses revealed that some patients were equally impaired at judging pictures and words. In particular, one subgroup experienced significant difficulty distinguishing coherent foils related to the target from true exemplars, and this occurred when the foils were presented as either pictures or words. An item-by-item analysis revealed highly concordant difficulty judging the same concepts expressed as either pictures or words in these patients. A material-specific deficit was evident in other pAD patients that resulted in selectively compromised picture judgments, including essentially all types of picture foils. We discuss these profiles of picture comprehension difficulty and conclude that deficits in understanding pictures in pAD may be due to semantic or perceptual types of impairment.