Stimulus Control - Stimulus Control in Classical Conditioning

Stimulus Control in Classical Conditioning

In Classical conditioning, when a conditional stimulus (CS) is repeatedly paired with an unconditional stimulus (UCS), the CS will come to elicit a conditional response (CR). In this way, the CR is a type of controlling stimulus. A standard procedure for establishing this relationship involves an arrangement such that the probability that the UCS is delivered, given the CS is presented, is equal to 1, and the probability that the UCS is delivered, given the CS is not presented, is equal to 0; P(UCS|CS)= 1 and P(UCS|~CS)=0. In other words, the UCS always follows the CS, and the UCS is never presented without the CS. The CS can be said to indicate the arrival of a UCS, and the likelihood that the CR occurs will increase when the CS is presented. When this occurs, the CR can be said to be under stimulus control.

An example experimental paradigm that illustrates this is similar to the arrangement used by Ivan Pavlov and involves measuring the amount of saliva a dog produces following a tone of a certain frequency. A tone of a certain frequency is played immediately before the dog receives some food, and after several pairings the tone will be sufficient to produce salivation. The presence of a tone increases the probability that the dog will salivate.

An important finding is that this CR will occur to variations of the CS that were not trained during the acquisition of the initial CS-CR contingency. For example, if the food was originally paired with a 500 Hz tone, the dog may also salivate (to a lesser degree) to a 450 Hz tone. The phenomenon in which a CR is elicited following a stimulus that was not trained in the initial arrangement is called respondent generalization.

This phenomenon can be experimentally verified in the following way: A tone of a certain frequency (e.g., 500 Hz) is paired repeatedly with food, until the salivary response (CR) is elicited following presentation of this frequency. Suppose that in response to the 500 Hz tone being presented (without the UCS) the dog will produce 1 ml of salivation. The experimenter could then vary the some characteristic (e.g., frequency) of the tone and measure the amount of salivation produced. For example, the experimenter could play a 600, 550, 500, 450, and 400 Hz tone, and then measure the amount of salivation produced as a result. The experimenter could then plot the amount of salivation produced as a function of the frequency of tone presented to generate a generalization gradient.

The particular “shape” of a generalization gradient can be altered by making adjustments to the schedule of reinforcement. A relatively “flat” gradient indicates respondent generalization, while a relatively “peaked” gradient indicates respondent discrimination. Respondent discrimination and respondent generalization can be conceptualized as two sides of the same coin: the more generalization that occurs, the less discrimination that occurs, and vice versa. While generalization refers to an organism displaying CR to untrained stimuli, discrimination refers to an organism not producing CR in response to untrained stimuli.

A sharper generalization gradient (respondent discrimination) can be produced through a series of explicit non-pairings. This procedure for producing respondent discrimination is very common. As before, a stimulus is presented and the UCS is delivered, this is called the CS+. Other stimuli are also presented, but do not receive reinforcement, they are called CS-. In this way, the CS-, are explicitly not paired with the UCS. From the example given above, over a series of trials, the 500 Hz frequency would be reinforced with food. Additionally, different frequency tones would be presented without food. Initially, the dog may show generalization to the different tones, but over a series of trials that dog will stop salivating to the different tones and will only salivate to the 500 Hz tone. If a new gradient were generated, it would be much sharper than the original.

It is important to note that this is not the only way to obtain stimulus control in Classical conditioning. Instead only presenting food in the presence of the 500 Hz tone, one could present a large quantity of food in the presence of the 500 Hz tone, and a smaller quantity of food in the presence of other tones. Alternatively, one could present a different type of stimulus, such as a shock or a drug, in the presence of other tones.

Describing generalization as occurring to stimuli that are similar to the original CS is a common mistake that should be avoided. Generalization itself is a measure of stimulus similarity. What is perceived as similar stimuli to humans may be perceived as dissimilar to another species (or may not be perceived at all). For example, homing pigeons are capable of magnetoreception, and they may have the ability to perceive the earth’s magnetic fields. Humans do not have this ability. Pigeons demonstrate discrimination between variations in magnetic fields, while humans do not.