Effect of multiple presentations of words on event-related potential and reaction time repetition effects in Alzheimer's patients and young and older controls

The hallmark symptom in probable Alzheimer's disease (PAD) is dramatic difficulty in storing and/or retrieving new information on tests of explicit or direct memory. However, in many studies of implicit or indirect memory, these same patients show repetition-priming magnitudes (i.e., facilitation of performance on the basis of previous experience) similar to that of normal controls. Recent studies of repetition priming have shown that PAD subjects have an intact event-related potential (ERP) repetition effect, which is thought to index indirect memory functioning. The present study was designed to test the effect of multiple repetitions of verbal stimuli on the ERPs of PAD patients. ERPs were recorded from 8 subjects with PAD, 8 age-matched elderly and 16 young healthy controls. Subjects were asked to make speeded but accurate choice responses to infrequently occurring animal words and frequently occurring nonanimal words, some of which repeated across three blocks of trials. All groups of subjects produced ERP activity that was more positive to repeated (i.e., old) than to new items, with no additional enhancement elicited by the third presentation. ERP enhancement to repeated items was associated with reaction time facilitation, which also showed no additional facilitation to the third presentation. Moreover, the scalp distribution of the repetition effect was similar in the PAD and control groups, suggesting that it emanated from similar brain tissue in the three groups. These results indicate that ERP and reaction time repetition-priming effects are relatively intact in subjects who are aging normally and in those with a diagnosis of 'mild' Alzheimer's disease.