Phasic visual alertness in Alzheimer's disease and ageing.

An individual's ability to see and react quickly to a target stimulus is enhanced if they are alerted to the arrival of this target by a stimulus that occurs just prior in time to it. This alerting effect is thought to occur due to a phasic increase in alertness mediated by noradrenergic activity. In Alzheimer's disease (AD) there is a dysfunction in the noradrenergic system resulting in a decrease in central levels of noradrenaline. We therefore predicted that patients with AD would not be able to benefit from the prior stimulus to the same extent as that seen in healthy older adults and thus would have a reduced or abolished alerting-effect. We measured reaction times to respond to a visual target that could be preceded (by 200 ms) by a visual alerting cue, in 17 patients with Alzheimer's disease, 19 age-matched controls and 13 younger controls. We found that the alerting cue significantly decreased the reaction times for both the young and old controls, but that this cue had no effect upon the reaction times for those with AD. This marked inability to increase phasic alertness in AD may contribute to the everyday problems faced by these patients, and may provide a simple tool to aid diagnosis and disease progression.