mowrer two factor avoidance theory

Instrumental Conditioning

Avoidance Conditioning

Avoidance occurs when an aversive event (typically a brief footshock in laboratory research with rats) that has been scheduled is cancelled as a result of the avoidance response. Avoidance behavior has puzzled theorists in the field of learning and behavior for some time -- since at least the 1940s when the first avoidance-conditioning studies were performed. In this paper I describe the development of method and theory in the study of avoidance. You will learn why the acquisition, maintainence, and extinction of avoidance behavior puzzled early theorists and how they attempted to solve those puzzles. As you will learn, these solutions often were closely tied to the invention of new avoidance procedures that called into question earlier explanations.

The Shuttlebox and the "Problem of Avoidance"

The earliest studies of avoidance used an apparatus known as a shuttlebox. It consists of a box divided into two compartments by a hurdle, over which a subject can jump to shuttle from one compartment to the other. [Later versions of the apparatus, like the one pictured at right, replace the hurdle with a doorway between compartments.] Shocks are delivered through the metal bars that form the floor of the box. The box also is outfitted with a lamp or speaker which can be used to provide signals. Subjects in the early studies were either laboratory rats or dogs, with the apparatus sized appropriately for the animal.

Two-Way Shuttlebox Avoidance

In the two-way shuttlebox avoidance procedure, the subject is placed in one of the two compartments. After a bit of time has passed, a stimulus (e.g., a tone) occurs. After the tone has been sounding for, say, 20 seconds, the shock is turned on, and the subject begins to run in response to the shock. Eventually it shuttles into the other compartment, an act that has two immediate consequences: (1) the shock ends, and (2) the tone ends. More time passes and the tone again sounds, and 20 seconds later the shock again begins, producing more running and, eventually, a second shuttle, returning the subject to the first compartment.
Very quickly the subject learns to orient toward the other compartment during the tone and quickly shuttles into it as soon as the shock occurs, thus terminating both shock and tone. The subject is now escaping both shock and tone. After more experience, however, something new happens: after orienting toward the other compartment during the tone, the subject shuttles into it before the shock has begun. This act has two consequences: (1) it terminates the tone, and (2) it cancels the shock that was programmed to occur after 20 seconds of tone. The subject has escaped from the tone, but it has avoided the programmed shock.
Notice that the subject can avoid shock by shuttling either way -- from the left to the right compartment or vice versa. For this reason the procedure is called "two-way avoidance."

The Problem of Avoidance

In the 1940s when research on avoidance conditioning was first being done, most behaviorists subscribed to a "stimulus-response" or "S-R" theory of behavior. According to this theory, all responses (behaviors) are elicited, reflex-fashion, by prior stimuli. If a rat in a maze learned to turn left at a certain choice-point, for example, it was assumed that the stimuli unique to that choice-point had through associative conditioning acquired the ability to elicit a left-turn response. Such responses were reinforced by stimuli that immediately followed the left turn and which, being on the way to the goal-box and food, were associated with reinforcement -- they had become conditioned reinforcers. Left-turning occurred because, in past trials, left-turning had been immediately reinforced by the immediate stimulus change.
But now, here in the shuttlebox, subjects were learning to shuttle, apparently in the absence or any immediate stimulus change that could serve as a reinforcer. There was no shock present before the shuttle, and there continued to be no shock present afterward. What, then, could be serving as the reinforcer? This question came to be known as "the problem of avoidance."

Mowrer's Solution to the Problem of Avoidance

Soon after the problem of avoidance surfaced, Hobart Mowrer proposed a solution that identified an immediate reinforcer for the avoidance response. According to Mowrer, during the early conditioning trials in the shuttlebox, subjects received many pairings between the tone and shock. These pairings constitute a typical classical conditioning procedure, with the tone serving as the initially neutral stimulus and the shock as an unconditioned stimulus arousing the emotional response of fear. After a few pairings of tone and shock, the previously neutral stimulus became a conditioned stimulus (CS), capable of producing a strong conditioned fear response.
Now that the tone produced strong fear, any response that terminated this conditioned stimulus would be strengthened through the process of negative reinforcement. What response terminates the CS? Shuttling to the other compartment! Thus, according to Mowrer's account, the subjects shuttle during the tone and before the shock begins, not to avoid the shock, but rather to escape the fear-inducing tone CS.
Note that Mowrer's account appeals to two processes or "factors." First, there is classical conditioning of fear to the tone, transforming it into an elicitor of strong fear. Second, there is instrumental conditioning of shuttling through negative reinforcement, the termination of the aversive and fear-evoking CS. Because it appeals to both classical and instrumental conditioning, the account is today known as Mowrer's two-factor theory of avoidance.

Another Problem: Extinction of Avoidance Responding

Mowrer's two-factor theory provided a possible answer to the question of what reinforces avoidance responses, but it did not solve a second "problem of avoidance," which is the extremely slow extinction of avoidance behavior. After a subject has received extensive avoidance training in the shuttlebox and is now avoiding nearly every programmed shock, the shocker is turned off. Now, even if the subject fails to shuttle during the tone, a shock still will not occur. Even so, subjects will continue shuttling for hours at a time. You can disconnect the shocker, pack it up, and put it away in a closet and subjects will continue to make avoidance responses.
Two-factor theory not only failed to explain why extinction of avoidance follows such a slow course, it predicted a phenomenon during extinction that was not observed to happen. In two-factor theory, successful acquisition of avoidance should lead to the extinction of conditioned fear to the CS. That is because, when the subject successfully avoids the shocks, the CS is no longer being paired with them -- which constitutes extinction of the conditioned response to the CS. As fear to the CS extinguishes, escape from the CS by jumping over the hurdle will no longer be reinforcing, and shuttling should extinguish. But this will then lead to further pairings between CS and shock, reinstating fear of the CS and making escape from the CS an effective reinforcer once again. This cycle of acquisition, extinction, reacquisition, extinction, etc. is not observed in shuttlebox avoidance.
To explain the slow extinction of avoidance, some theorists went so far as to suggest that the process of evolution might have selected for organisms whose avoidance responses are extremely resistant to extinction. In this view, continuing to avoid when it is no longer necessary may be far less costly to the organism than failing to avoid when avoidance is necessary (to avoid being eaten, for example). Slowness of extinction was said to be a special property of avoidance responses.
Later it was realized that the slowness of extinction of avoidance could be accounted for without appeal to any special characteristics of avoidance. Subjects learn when placed in the shuttlebox that they receive a shock at the end of the tone if they do not shuttle, and that they do not receive a shock at the end of the tone if they do shuttle. After a bit of practice, they almost always jump shuttle before the tone has reached its end. Consequently, when the shocker is disconnected, they have little opportunity to learn that the previous contingency between shuttling and shock-omission has changed. Tone-in-the-absence-of-shuttling still signals shock and, just as before, after a shuttle the tone ends and no shock follows. From the subject's perspective, nothing has changed and so avoidance behavior continues. Only after the subject has experienced a few failures to shuttle that were not followed by shock can it learn that there is no need to shuttle.