In psychology, a stimulus is an energy change (such as light or sound) which is registered by the senses. In behaviorism and related stimulus–response theories, a stimulus constitutes the basis for behavior, whereas it constitutes the basis for perception in perceptual psychology. In this context, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of sensory organs).
In contemporary experimental psychology the term stimulus is usually used to describe the event or object to which a response is measured. Thus, not everything that is presented to participants qualifies as stimulus: For example, a fixation cross is not said to be a stimulus, because it merely serves to center participants' gaze at the center of the screen. Also, it is uncommon to refer to longer events (e.g. the Trier Social Stress Test) as a stimulus, even if a response to such an event is measured.
In the second half of the 19th century, the term stimulus was coined in psychophysics by defining the field as the "scientific study of the relation between stimulus and sensation". This may have led James J. Gibson to conclude that "whatever could be controlled by an experimenter and applied to an observer could be thought of as a stimulus" in early psychological studies with humans, while around the same time, the term stimulus described anything eliciting a reflex in animal research.
The stimulus concept in behaviorism
The concept stimulus was essential to behaviorism and the behavioral theory of B. F. Skinner in particular. Within such a framework several kinds of stimuli have been distinguished (see also classical conditioning):
An eliciting stimulus was defined as a stimulus that precedes a certain behavior and thus causes a response. A discriminative stimulus in contrast increases the probability of a response to occur, but does not necessarily elicit the response. A reinforcing stimulus usually denoted a stimulus delivered after the response has already occurred; in psychological experiments it was often delivered on purpose to reinforce the behavior. Emotional stimuli were regarded as not eliciting a response. Instead, they were thought to modify the strength or vigor with which a behavior is carried out.
- "Stimulus". In: Richard L. Gregory (Ed.), The Oxford Companion to the Mind, Oxford, N.Y.: Oxford University Press.
- Gescheider, G. (1997). Psychophysics: the fundamentals (3rd ed.). Lawrence Erlbaum Associates. p. ix. ISBN 0-8058-2281-X.
- Gibson, James J. (1960): "The Concept of the Stimulus in Psychology". American Psychologist, 15, pp. 694–703, here p.694.
- Skinner, B. F. (1938). The Behavior of Organisms: An Experimental Analysis. New York.