Forces that alter or interfere with perception – including state of attention, muscle tension, age, practice, distractions and physical fitness – affect reaction time. Because reaction time depends on the ability to perceive stimulus and respond, these factors affect the speed of responses and skew judgment about choosing between responses.
Much experimentation in reaction time studies human response to traffic and other rapidly changing situations. In those cases, subjects must first prove that they can perceive a stimulus and then react to it. A subject receives a rating according to his raw reaction times factored against a base state calculated from preparation for testing, life experience, age, physical fitness and state of attention.
Scientists commonly test to measure reaction times for four different types of reactions: reflex, simple, complex and discriminative. Because reflex reactions, such as eye blinks, are instinctive, they usually take the shortest time. Simple reactions, which also have short reaction times, are those to ordinary, everyday stimuli, such as the response to a traffic light turning from red to green. Complex reactions, where a subject has to choose among multiple responses, have longer reaction times and involve choosing the appropriate response based on experience, but without advanced planning. The most complex reactions with the longest reactions times are discriminative reactions, where subjects must choose from multiple responses which are not practiced or habitual.