The term stimulus has many meanings; very generally, it is any occurrence (be it an external event, or anything perceived or thought) that causes a detectable response. Stimulus is often used with qualifying terms to further specify its meaning, for example, conditioned stimulus and neutral stimulus.
Various fields of study use the term stimulus in different ways. In psychology, it is most often used to describe energy forms that reach sense organs and cause a response. For example, the visual sense using the eyes responds to photic radiation or light. Because human sense organs respond to a limited number of energy forms, and even then to only limited amounts of that energy, some energy reaching the sense organs is not detected and does not cause a response. The energy reaching the sense organs but not causing a response may be deemed a stimulus to a physiologist, but for psychologists it would not be considered a stimulus unless it had been responded to or detected by the organism. A stimulus may also be an internal mental event that causes a response.
Stimulus is the primary term in stimulus-response theory, which refers to a number of learning theories that are theoretically based on conditioned bonds or associations between a stimulus and response. The associative bonds are formed through the repeated pairing of certain stimuli and certain responses. Most of these theories are also behavioristic in that they focus on behaviors and do not look at mental processes, and they see the environment as the most important determinant of human behavior. Indeed, these theories view the bond between stimulus and response as the basis of behavior and believe that psychology's primary goal should be to discover rules governing how stimuli and responses interact. The two dominant stimulus-response theories are classical and operant conditioning theories.
See also Perception.
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