The notion of reaction time in psychology is predicated essentially upon a information processing model of human thinking and cognition. According to the information processing model, cognition can be understood as a computational framework wherein a cognitive system receives information which it then stores internally (as 'representations'). The cognitive 'system' then manipulates those representations and executes actions that are judged to be an appropriate corrolary to the occuring of those representations.
The flow of cognition, according to this model, begins with sensation
of a stimulus, through an attentive stage involving perception and planning,
and continuing on to a response or action. Each stage in the process
takes some amount of time; the reaction time is the duration of
time it takes from occurance of an event (a stimulus) and the execution
of an action in response to that event.