In psychology, a stimulus is an energy pattern (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 the second half of the 19th century, the conception had been established by psychophysics, the "scientific study of the relation between stimulus and sensation", together with the notion of the reflex arc constituting a foundational concept of scientific psychology. While at this time "whatever could be controlled by an experimenter and applied to an observer could be thought of as a stimulus." In the context of perception, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of sensory organs).
Functions of stimuli 
Eliciting stimulus 
According to University of Iowa, eliciting stimulus is an important part of Pavlov's conditioning theory. It is defined as a change which is related to an anticipated future response. An example is stated to be having a piece of chocolate in our mouth (unconditioned stimulus) and have a later response of salivation (unconditioned response). Eliciting stimulus is the alteration in the environment which is associated with the state of the environment. The association with the environment comes from something which is directly affecting the organism.  The correlation with the stimulus and the response exists by having a response following or existing before the stimulus.  Change is also one factor of eliciting stimulus. Stimulus has a consistency in predicting generic behavior that has properties called continuous agents such as: location, intensity, quality and duration.  Change in one of these properties can either withdraw or stimulate the behavior.
Discriminative stimulus and the pseudo reflex 
Discriminative stimulus 
According to University of Iowa, discriminative stimulus affects the operating response because of anticipated or scheduled reinforcements which are or will be associated with the response. Discriminative stimulus is different from eliciting stimulus. Discriminative stimulus sets the ground for a response to occur. Even if the response does not occur, it does not depend on the discriminative stimulus. Once discriminative stimulus is present, it will lead other factors to response. Discriminative stimulus applies to operant behavior.
Pseudo reflex 
Pseudo reflex is the correlation between a stimulus and a response which appears to have a relationship. It is related to each other tenuously and involves relationship with other stimulus and response.
Reinforcing stimulus 
When an occurrence of a behavior that is followed is increasingly supplemented, a reinforcing stimulus occurs. This stimulus gets into action at the same time as the eliciting or discriminative stimulus. Its contributions in leading to a response is separate from eliciting and discriminative stimulus. Negatively reinforcing stimulus leads to a pseudo reflex. Termination of a negative reinforcement leads to a positive reinforcement. In a negative reinforcement, there will a superficial correlation with the stimulus and the response which makes it difficult to trace original properties.
Emotional stimulus 
Emotional stimulus is not eliciting. Its primary effect is to make a behavior stronger. Emotional stimulus is considered as the proportionality and the strength of an operation. Emotional stimulus is relevant to drive through specific responses in emotions. Drive and emotions are separate concepts which work together to explain emotional stimulus. They work together by recognizing the change in strength (emotion) and its functional relationship to an operation (drive). Changes in emotions affect the emotional stimulus and makes it very dynamic.
See also 
- "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.