Abstract Normal young and elderly subjects and patients with very mild or mild dementia of the Alzheimer's type performed several reaction-time (RT) tasks tapping basic cognitive components of attentional and decision-making processes. The results of Experiment 1 suggested that processes common to Simple and Choice RT tasks were slowed with ageing but were not affected by Alzheimer's disease (AD) until dementia severity reached a mild degree. In contrast, decision-making processes were slowed by AD even in the very mildly demented patients. The results of Experiment 2 indicated that all groups benefitted from a warning signal, but the time course of increasing alertness varied across groups. Comparison of these experiments with the literature and with each other suggests that very mild AD subjects are abnormally dependent on the presence of a warning signal and, more speculatively, that the informative as well as alerting aspects of such signals play a critical role.
