Stimuli are events in the environment that influence behavior. A single stimulus can serve many different functions. Listed below are several functions that a stimulus can serve.
A discriminative stimulus influences the occurrence of an operant response because of the contingencies of schedules of reinforcement or paradigms of reinforcement/punishment that are or have been associated with that response. Many authors further suggest that discriminative stimuli provide information to the organism, allowing it to respond appropriately in the presence of different stimuli. An observing response is sometimes necessary for presentation of the discriminative stimulus/stimuli.
For example, different individuals can serve as discriminative stimuli in a joke-telling situation. The jokes that you tell your priest are probably different from the jokes that you tell your best friend because of your past history of telling jokes to both people.
For instance, in the laboratory, a pigeon could be required in the presence of a steady chamber light to peck a key on a Fixed Interval schedule to produce food, whereas it could be required in the presence of a blinking chamber light to pull a chain on a Variable Ratio schedule to turn off a loud noise. The discriminative stimuli clarify the "rules of the game," making each prevailing three-term contingency unambiguous.
An eliciting stimulus is a change in the environment that is highly correlated with the occurrence of a later response.
An eliciting stimulus is an essential component of Pavlovian conditioning.
Placing the piece of chocolate into the mouth is said to elicit salivation.
Some stimuli may produce an emotional reaction which may influence the occurrence of behavior.
For example, a game of backgammon might be interrupted by news of the unexpected death of a famous politician.
A reinforcing stimulus is one that increases the occurrence of behaviors that it follows.
For instance, the receipt of a trophy may increase the chances of a young girl competing in a yearly road race.
A discriminative stimulus may have many identifiable attributes. Although we can readily observe the organism's response to the whole stimulus, it may not be clear exactly which attributes of the stimulus are controlling the behavior (see functional stimulus). The unanalyzed stimulus as a whole is said to be the nominal stimulus.
For example, your friend asks you to look at a passing sports car. It is not clear just what your friend wanted you to note about the car: its color, make, speed, location, driver, etc.
In the above example, your friend may have been particularly interested in the car's color.
We speak of stimulus control when a discriminative stimulus changes the likelihood of an operant response. The controlling relation between the discriminative stimulus and the operant response (what Skinner called attention) comes about because of the reinforcer/punisher that has followed the operant response in the presence of that discriminative stimulus. Thus, the three-term contingency lies at the root of stimulus control.