Stimulus Generalization and Discrimination

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Classical Conditioning: Learning Signals and Associations
FIGURE 5 .3
Changes over Time in the Strength
of a Conditioned Response
Extinction
(UCS withheld)
Spontaneous
recovery
(CS again presented)
Strength of CR
As the conditioned stimulus (CS) and the
unconditioned stimulus (UCS) are repeatedly paired during initial conditioning, the
strength of the conditioned response (CR)
increases. If the CS is then repeatedly presented without the UCS, the CR weakens—
and eventually disappears—through a
process called extinction. If the CS is presented again later on, a weaker version of
the CR will reappear (Rescorla, 2004). This
phenomenon, called spontaneous recovery, is only temporary, though. Unless the
UCS is again paired with the CS, the recovered CR soon disappears.
Acquisition
(CS and UCS paired)
Extinction if
UCS again
withheld
Trials
Trials
Time delay
Conditioned Responses over Time:
Extinction and Spontaneous Recovery
classical conditioning A procedure in
which a neutral stimulus is paired with
a stimulus that triggers a reflexive response until the neutral stimulus alone
comes to trigger a similar response.
unconditioned stimulus (UCS) A stimulus that triggers a response without
conditioning.
unconditioned response (UCR) The
automatic, unlearned, reflexive reaction
to a stimulus.
conditioned stimulus (CS) An originally neutral stimulus that now triggers
a conditioned response.
conditioned response (CR) The response triggered by the conditioned
stimulus.
extinction The gradual disappearance
of a conditioned response.
reconditioning The relearning of a conditioned response following extinction.
spontaneous recovery The temporary
reappearance of a conditioned response after extinction.
stimulus generalization A process in
which a conditioned response is triggered by stimuli similar to the original
conditioned stimulus.
If a barking dog once bit you, you might have learned to feel distress whenever you
hear a dog’s bark. The more bad experiences you might have had with dogs, the stronger
will be your learned distress in response to barking sounds. In the language of classical conditioning, continued pairings of a conditioned stimulus (CS/bark) with an
unconditioned stimulus (UCS/bite) strengthen the conditioned response (CR/distress).
The curve on the left side of Figure 5.3 shows an example: Repeated associations of a
tone (CS) with meat powder (UCS) caused Pavlov’s dogs to increase their salivation
(CR) to the tone alone.
What if the tone (CS) is repeatedly sounded, but the meat powder (UCS) is no
longer given? As you might expect, if the unconditioned stimulus is not paired with
the conditioned stimulus at least now and then, the conditioned response will gradually disappear. This loss of the conditioned response is known as extinction (see the
center section of Figure 5.3). The term is not entirely accurate, though. Extinction suggests that, like the dinosaurs, the conditioned response has been wiped out, never to
return. In fact, though, if the CS (tone) and the UCS (meat powder) are again paired
after the conditioned response has been extinguished, that conditioned response will
return to its original strength after as few as one or two trials. This quick relearning of
a conditioned response after extinction is called reconditioning. Because reconditioning takes much less time than the original conditioning, extinction must not have
erased the association between the conditioned stimulus and the conditioned response
(Bouton, 1993, 2002).
The right side of Figure 5.3 provides more evidence for this conclusion. An extinguished conditioned response will temporarily reappear if, after some time has passed,
the conditioned stimulus occurs without the unconditioned stimulus. This is called
spontaneous recovery, the temporary reappearance of a conditioned response after
extinction (and without further CS-UCS pairings). In general, the longer the time
between extinction and the reappearance of the CS, the stronger the recovered conditioned response (Devenport, 1998). Even after many years, spontaneous recovery can
create a ripple of emotion—a conditioned response—when we hear a song or catch a
scent associated with a long-lost lover or a departed relative.
Stimulus Generalization and Discrimination
Once a conditioned stimulus is able to trigger a conditioned response, stimuli similar
to the conditioned stimulus will also trigger some version of that response. This phenomenon, called stimulus generalization, is illustrated by the fact that a person who
was bitten by one particular dog may now show some fear of all dogs. Usually, the