Phasic and tonic alertness in Alzheimer's disease.

Phasic alertness (the rapid mobilization of resources to process an expected stimulus) was examined in Alzheimer patients and normals by a choice RT task in which the stimulus was usually preceded by a warning signal. The time subjects needed to attain maximal phasic alertness was determined by varying the Stimulus Onset Asynchrony (SOA) between the warning and the stimulus. In comparison to trials without any warning, presentation of a warning signal reduced the RT of Alzheimer patients as much as it did that of normals. Similarly, the SOA necessary for maximal alertness was the same in Alzheimer patients and in normals. Maintenance of tonic alertness was investigated by examining how RT changed across a long period of continuous testing. Alzheimer patients and normals showed a similar rise in RT with increasing time on task. These results suggest that phasic and tonic alertness are relatively unimpaired by Alzheimer's disease.