Invitation to Psychology

Chapter 9 Learning and Conditioning 321

paintings by those same two artists, they have

been able to tell the difference between them!

Sometimes an animal or person learns to re-

spond to a stimulus only when some other stimu-

lus, called a discriminative stimulus, is present. The

discriminative stimulus signals whether a response,

if made, will pay off. In a Skinner box contain-

ing a pigeon, a light may serve as a discriminative

stimulus for pecking at a circle. When the light is

on, pecking brings a reward; when it is off, pecking

is futile. Human behavior is controlled by many

discriminative stimuli, both verbal (“Store hours

are 9 to 5”) and nonverbal (traffic lights, doorbells,

the ring of your cell phone, other people’s facial

expressions). Learning to respond correctly when

such stimuli are present allows us to get through

the day efficiently and to get along with others.

Learning on Schedule. LO 9.11 When a re-

sponse is first acquired, learning is usually most

rapid if the response is reinforced each time it

occurs; this procedure is called continuous rein-

forcement. However, once a response has become

reliable, it will be more resistant to extinction if it

is rewarded on an intermittent (partial) schedule of

reinforcement, which involves reinforcing only some

responses, not all of them. Skinner (1956) happened

on this fact when he ran short of food pellets for his

rats and was forced to deliver reinforcers less often.

(Not all scientific discoveries are planned.) On in-

termittent schedules, a reinforcer is delivered only

after a certain number of responses occur or after a

certain amount of time has passed since a response

was last reinforced; these patterns affect the rate,

form, and timing of behavior. (The details are be-

yond the scope of this book.)

stimulus generaliza-

tion In operant condi-

tioning, the tendency for

a response that has been

reinforced (or punished)

in the presence of one

stimulus to occur (or be

suppressed) in the pres-

ence of similar stimuli.

stimulus discrimina-

tion In operant condi-

tioning, the tendency of a

response to occur in the

presence of one stimulus

but not in the presence

of similar stimuli that

differ from it on some

dimension.

discriminative stimu-

lus A stimulus that

signals when a particular

response is likely to be

followed by a certain type

of consequence.

continuous reinforce-

ment A reinforcement

schedule in which a par-

ticular response is always

reinforced.

intermittent (partial)

schedule of reinforce-

ment A reinforcement

schedule in which a

particular response is

sometimes but not always

reinforced.

Extinction. In operant conditioning, as in clas-
sical conditioning, extinction is a procedure that
causes a previously learned response to stop. In op-
erant conditioning, however, extinction takes place
when the reinforcer that maintained the response
is withheld or is no longer available. At first, there
may be a spurt of responding, but then the re-
sponses gradually taper off and eventually cease.
Suppose you put a coin in a vending machine
and get nothing back. You may throw in another
coin, or perhaps even two, but then you will prob-
ably stop trying. The next day, you may put in yet
another coin, an example of spontaneous recovery.
Eventually, however, you will give up on that ma-
chine. Your response will have been extinguished.

Stimulus generalization and Discrim ina-
tion. In operant conditioning, as in classical,
stimulus generalization may occur. That is, re-
sponses to a stimulus may generalize to other
stimuli that were not present during the original
learning situation but that resemble the original
stimulus in some way. For example, a pigeon that
has been trained to peck at a picture of a circle
may also peck at a slightly oval figure. But if you
wanted to train the bird to discriminate between
the two shapes, you would present both the circle
and the oval, giving reinforcers whenever the bird
pecked at the circle and withholding reinforcers
when it pecked at the oval. Eventually, stimulus
discrimination would occur. Pigeons, in fact, have
learned to make some extraordinary discrimina-
tions. They have learned to discriminate between
two paintings by different artists, such as Vincent
Van Gogh and Marc Chagall (Watanabe, 2001).
And then, when presented with a new pair of

extinction The weaken-

ing and eventual disap-

pearance of a learned

response; in operant con-

ditioning, it occurs when

a response is no longer

followed by a reinforcer.

Light

Bar

Food tray

Water

Light

Bar

Food tray

Water

FigURE 9.6 The Skinner Box
When a rat in a Skinner box presses a bar, a food pellet or drop of water is automatically released.
The photo shows Skinner training one of his subjects.