Comprehension of long distance number agreement in probable Alzheimer's disease

Two cross-modal naming experiments examined the role of working memory in p rocessing sentences and discourses of various lengths. In Experiment 1, 10 memory impaired patients with probable Alzheimer's disease (AD) and 10 heal thy elderly control participants showed similar sensitivity to violations o f subject-verb number agreement in a short sentence condition and similar d egradation to this sensitivity in a long sentence condition. Performance in neither length condition correlated with performance on working memory tas ks, suggesting that the processes involved in interpreting a grammatical de pendency between adjacent and nonadjacent elements are different from those required in the working memory tasks. In Experiment 2, the same 10 AD pati ents were less sensitive than the 10 control participants to prounceanteced ent number agreement violations in a short discourse condition, but neither group was affected by additional length. In this experiment, performance i n both the short and long conditions correlated with working memory perform ance. These results show that grammatical and discourse dependencies pose d ifferent memory and processing demands, and that these differences are not simply due to differences in the amount of intervening material between dep endent words. The results also suggest that while the working memory defici ts characteristic of AD do not interfere with on-line grammatical processin g within sentences, they do compromise on-line discourse processing across sentences.