Mismatch negativity during attend and ignore conditions in Alzheimer's disease.

Mismatch negativities (MMNs) of the event-related potential to deviant tones and environmental sounds were recorded during active and ignore oddball sequences in young and elderly controls and patients with probable Alzheimer's disease (PAD). MMNs were smaller in the PAD waveforms compared to those of the controls, suggesting a degraded sensory memory trace in these subjects; however, under ignore conditions, environmental sounds elicited robust MMNs in the PAD group along with N2b and novelty P3 components in similar fashion to controls. As N2b and P3 are usually elicited by attended stimuli, these data suggest that in the PAD subjects, the highly deviant events involuntarily captured attention, perhaps reflecting the activation of an attentional switching mechanism. Because this passive switching is thought to reflect activation of a mechanism located in the frontal lobes, the data suggest that this putative frontal lobe mechanism is relatively intact in the early stages of the disease.