Stimulus Control - Stimulus Control in Operant Conditioning

Stimulus Control in Operant Conditioning

In classical conditioning, the CS is an initially neutral stimulus that comes to play a direct role in indicating the occurrence of a UCS after a series of pairings. In operant conditioning, neutral stimuli do not play such a direct role. Instead, initially neutral stimuli come to signal the availability of reinforcement for producing certain behaviors. When a stimulus signals the availability of reinforcement it is called the SD, or discriminative stimulus. A discriminative stimulus is a type of controlling stimulus; its presence increases the probability that a behavior will occurs, because it reliably signals the availability of reinforcement. Another class of controlling stimuli in operant conditioning is called the extinction stimulus, or S∆. The extinction stimulus signals that certain behaviors will not receive reinforcement.

Stimulus control can be illustrated in operant procedures. A simple arrangement involves training a pigeon to peck a key with different frequencies depending on the presence of certain colors of a light. First, a baseline rate of behavior would be established by reinforcing the pigeon every time it pecks a key. As the pigeon is pressing the key, a light could be presented and only pecks made in the presence of the light would be reinforced. Although the light was initially neutral, it becomes the SD. Over a series of trials the light will be established as indicating the availability of food, and the light will increase the probability that key pecks occur. Similar procedures have been used to train pigeons to discriminate between paintings by Monet and Picasso. Other arrangements might involve different colored lights, with each light controlling different rates of behavior by having them programmed to different schedules of reinforcement. If a green light is associated with a VR 10 schedule and a red light is associated with a FI 20” schedule, the green light will control higher rates of behavior than the red light by providing higher rates of reinforcement.

Procedures for generating generalization gradients in operant conditioning are similar to those for used in Classical conditioning described above. Hanson (1959) conducted a study that illustrates generalization in operant conditioning. A control group of pigeons were placed in an arrangement involving a VI schedule where they received reinforcement for key presses when a 550 nm wavelength light was illuminated. After training the pigeon to peck the key in this way such that a steady rate of behavior was achieved, the frequency of the light was varied away from the 550 nm wavelength. These frequencies were not reinforced, but responses of the pigeons were recorded as the wavelength was varied. The greater the wavelength differed from the trained stimulus, the fewer responses were produced.

Additionally, there were four other experimental groups that received were placed in similar arrangements. The experimental difference being that, in addition to SD at 550 nm, they were presented with explicit non-pairings or S∆. Each of the four experimental groups was presented with S∆ at 555, 560, 570, or 590 nm wavelengths, respectively. The effect of this manipulation was to create a sharper generalization gradient. S∆ that were closer to the SD produced sharper generalization gradients. In this way, introducing S∆ in operant conditioning is similar to the effect of introducing CS- in Classical conditioning. However, there is another important phenomenon that occurs in operant conditioning as a result of this manipulation: introducing S∆ in this way caused the generalization gradients to shift away from the SD. This is called a peak shift, and in addition to the shift, the number of responses produced by the experimental groups at the peak actually increased above the control group.

It seems bizarre that the probability that the pigeon responds is higher at an untrained wavelength then originally trained SD. Based on an earlier theory involving inhibitory and excitatory gradients, Hanson proposed that this occurred as a sort of summation between excitatory and inhibitory stimulus gradients. However, the example of peak shifts described above may be an example of relational control where the pigeons were trained to peck the “greener” stimulus more frequently.