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Backward Chaining

Backward chaining is the secret of complex animal performances in movies and animal shows. Stimulus control is used in this technique. The name backward chaining comes from the fact that training starts with the last behavior in the to-be-learned sequence of behaviors. The trainer works backward from there.

Suppose a rat in a Skinner box learns to press a lever to get food pellets. Once the rat learns the bar-press, the experimenter can easily establish an S+. For example, the rat might be given a pellet for a bar press only when the green light is on. This makes the green light an S+.

How is backward chaining used to teach animals complex behavior sequences?

Here is the key to backward chaining. An S+ can function as a reinforcer for other behaviors. For example, if a green light (signifying that a bar-press will produce a food pellet) is illuminated only when the rat climbs the ladder, the rat will learn to climb the ladder in order to see the green light. After seeing the green light, the rat climbs off the ladder, presses the bar, and gets a food pellet.

In backward chaining this pattern is repeated many times to build up a chain of consecutive behaviors. If ladder climbing is reinforced by the green light only after a tone has been sounded, then soon the rat will wait for the tone before climbing the ladder. If the tone is sounded, the rat climbs the ladder and waits for the green light, then runs over presses the bar to get a food pellet when the green light turns on. If the tone is not sounded, nothing works; the rat can climb the ladder, but the green light does not turn on and the rat never gets a food pellet. So the tone comes to control the behavior of ladder climbing. The rat climbs the ladder only after the tone.

What dual function does the S+ serve, in backward chaining?

Now the tone functions as an S+, and it can be used as a reinforcer for another behavior. Perhaps you want to rat to crawl through a tunnel before it climbs the ladder. Put a little tunnel in the rat's cage, and as soon as the rat climbs through it, sound the tone that means "time to climb the ladder." The tone not only serves as an S+ for climbing the ladder; it also reinforces the rat for crawling through the tunnel.

One by one, you keep adding new requirements for the rat, building a chain of behaviors. You can see why the technique is called backward chaining. The chain of behaviors stretches backward from the final behavior (consuming the food pellet). Each S+ serves as a reinforcer for the previous behavior and a discriminative stimulus for the following behavior. Very complex chains can be built up this way.

Following is a sequence of S+s and behavioral responses learned by one prodigy, the star of a short movie titled, Barnabus: the Educated Rat.

S+ (light)

Response (climbing spiral staircase to platform)

S+ (drawbridge is visible)

Response (pushing down and crossing drawbridge)

S+ (ladder is visible)

Response (climbing ladder)

S+ (chain is accessible)

Response (pulling car by chain)

S+ (car near platform)

Response (entering car and pedaling through tunnel)

S+ (stairs are visible)

Response (climbing stairs)

S+ (tube is visible)

Response (going through tube)

S+ (elevator is accessible)

Response (entering elevator)

S+ (string attached to flag is accessible)

Response (pull the string, raising the flag)

S+ (elevator arrives at ground floor)

Response (exit the elevator)

S+ (buzzer sounds)

Response (bar press)

S+ (sound of food pellet)

Response: (consuming food pellet)

Why is backward chaining necessary to use, for moviemakers who want to use animals as stars?

Backward chaining is the only way to teach animals arbitrary sequences of behaviors. The word "arbitrary" allows for naturalistic exceptions such as route processing and nest building. Animals ranging from squirrels to jumping spiders can travel in complex, multi-part journeys to reach a goal, and common bird species go through a complex series of steps to build a nest. However, such behaviors are essential to the survival of the species and the ability to perform them has been shaped up by evolution. When animals in movies perform sequences of behavior that do not "come naturally," it is due to backward chaining.

What was discovered when backward chaining was used with Army technicians?

With humans, backward chaining is not necessarily helpful. In one study (Cox & Boren, 1965) United States Army researchers used backward chaining to teach technicians a 72-action procedure involving missile preparation. In other words, they started with the last movement (action #72) and worked backward. There was no advantage over the reverse procedure: starting with action #1 and working forward. Humans are unique in their ability to guide behavior with language, which allows long sequences of arbitrary behaviors to be learned by practicing under the guidance of a written or spoken plan.

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