ABSTRACT
The latency of the P300 event-related potential is though to reflect the time it takes to conclude that a task-relevant stimulus has been presented, i.e. it is an index of cognitive processing time. Reaction time (RT) also reflects cognitive processing time, but additionally reflects the time taken physically to respond to the stimulus. If serial models of information processing are correct, then the P300 latency must be less than RT, and a positive correlation between the 2 measures is to be expected. We examined these hypotheses in 100 normal subjects aged from 18 to 92 years. The P300 component elicited via a 2-tone discrimination task, and RT to the task-relevant tones was measured simultaneously. The resulting correlation between these measures was weak (r = 0.26, p greater than 0.05), and there were instances in which RT preceded the P300 latency. These results are consistent with parallel rather than serial models of information processing. Fifteen patients with Alzheimer's disease were also examined. Three had an abnormally delayed P300 latency, and 7 had abnormally delayed RTs compared with age-matched controls. Simultaneous measurement of the P300 latency and RT may thus help resolve whether dysfunction is evident in neuronal networks concerned with stimulus evaluation or in those concerned with response execution.
