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The Behavior Analyst 1995, 18,

253

-269 No. 2 (Fall)

Tutorial
Stimulus Control: Part II

James A. Dinsmoor
Indiana University

The second part of my tutorial stresses the systematic importance of two parameters of discrimi-
nation training: (a) the magnitude of the physical difference between the positive and the negative
stimulus (disparity) and (b) the magnitude of the difference between the positive stimulus, in par-
ticular, and the background stimulation (salience). It then examines the role these variables play in
such complex phenomena as blocking and overshadowing, progressive discrimination training, and
the transfer of control by fading. It concludes by considering concept formation and imitation, which
are important forms of application, and recent work on equivalence relations.
Key words: stimulus control, disparity, salience, blocking, overshadowing, transfer, fading, con-

cept formation, imitation, equivalence relations

The first part of this tutorial dealt
with the basic principles that account
for the acquisition of stimulus control
under what is conventionally known as
discrimination training. Among the sa-
lient points that were raised were sug-
gestions that (a) control by antecedent
stimuli is just as important in operant
as it is in respondent behavior; (b) Pav-
lov’s conditional stimulus is a discrim-
inative stimulus; (c) stimulus general-
ization is not a behavioral process in-
dependent of and antagonistic to dis-
crimination but is simply another way
of describing control by antecedent
stimuli; and (d) the increases in control
that occur during discrimination train-
ing can be attributed to more frequent
and more prolonged observation of the
relevant stimuli accompanied, it is pre-
sumed, by concomitant changes in at-
tention.

Note that in conventional discrimi-
nation training, the subject is exposed
to repeated alternations between the
positive stimulus (S+), which is ac-
companied by a schedule of reinforce-
ment, and an alternative stimulus (S-),
which is not accompanied by rein-
forcement. This alternation establishes

Address correspondence to James A. Dins-
moor, Department of Psychology, Indiana Uni-
versity, Bloomington, Indiana 47405.

a positive correlation between the S+
and the primary reinforcer that selects
the one relevant stimulus out of the
many that impinge upon the organism
and transforms it into a conditioned re-
inforcer of observing behavior. Note
also that if the subject is exposed ex-
clusively to S+ training or to a single
period of S+ training followed by a
single period of S – training rather than
to a continued alternation, control is
less adequate (Honig, Thomas, & Gutt-
man, 1959; Yarczower & Switalski,
1969). Similarly, in a compound dis-
crimination like that studied by Blough
(1969), if one dimension is left contin-
uously at its positive value for a num-
ber of sessions, control by that dimen-
sion is reduced and control by the other
dimension enhanced; when the proce-
dure returns to the previous alternation
between positive and negative stimuli,
the discriminative performance returns
to its normal level.
The second part of my tutorial will

necessarily be less tightly integrated.
Taking off from the foundation laid in
Part I, it extends the treatment of stim-
ulus control to a discussion of two of
its most important parameters and to
several more complex patterns that
seem to hold special significance for
basic theorizing and practical applica-
tion.

253

254 JAMES A. DINSMOOR

STIMULUS PARAMETERS
There are a number of variables that

influence the rate at which the subject
learns to discriminate between stimuli
and the level of performance that will
ultimately be attained. However, some
of these, like the physical quality of the
stimulus (e.g., wavelength of light, en-
ergy of sound), the topography of the
response, and individual and species
differences among different subjects,
do not readily lend themselves to the
formulation of general laws of behav-
ior. Of greater interest from a system-
atic point of view are those parameters
that enter into a variety of behavioral
paradigms. There are two parameters,
in particular, that will appear and reap-
pear in subsequent accounts of topics
like overshadowing, blocking, the
easy-to-hard effect, fading, and con-
cept formation. These are (a) the mag-
nitude of the difference in physical
units between the positive and the neg-
ative stimulus, sometimes subsumed
by the phrase stimulus disparity, and
(b) the magnitude of the difference be-
tween the discriminative stimuli and
the background stimulation, subsumed
as stimulus salience.

Stimulus Disparity
Clearcut evidence for the role played

by the disparity between the two stim-
uli may be found in a series of early
studies, using rats as subjects, con-
ducted by Rosemary Pierrel and her as-
sociates at Brown University. Bar
pressing was reinforced on a variable-
interval schedule in the presence of the
positive stimulus but not in the pres-
ence of the negative stimulus. In Pier-
rel, Sherman, Blue, and Hegge (1970),
for example, the discriminative stimuli
were pulsed tones of 4 kHz that dif-
fered in intensity. The difference be-
tween the positive and the negative
stimulus was set at 10, 20, 30, or 40
dB, with the higher intensity serving as
the positive stimulus for half the
groups and as the negative stimulus for
the other half. Also, for half the ani-
mals, independently assigned, the low-

er intensity was set at 60 dB, with the
higher intensity determined by the
magnitude of the difference between
the two; for the other half, the higher
intensity was set at 100 dB, with the
lower intensity determined by the mag-
nitude of the difference. (Because there
was only one 60-100 group and one
100-60 group, there were 14 rather
than 16 groups in all, plus a special
control group.)

It seems obvious that very small
physical differences between the posi-
tive stimulus (SD or S+) and the neg-
ative stimulus (S^ or S-) must be dif-
ficult to discriminate. Up to some limit,
at least, larger differences should pro-
mote faster acquisition and a larger ul-
timate difference between the two per-
formances. This expectation is borne
out by a series of plots tracing the
course of the discrimination index
(multiplied by 10) as a function of the
hours of training. In all four panels of
Figure 1, the discrimination is slowest
to develop and attains the lowest final
level in groups for which the stimuli
differ by only 10 dB; groups for which
the difference is 20 dB do somewhat
better; groups for which the difference
is 30 or 40 dB differ less during ac-
quisition and tend to converge, but at
still higher levels of performance. With
pigeons as subjects, Hanson (1959)
found a similar relation between the
magnitude of the difference in wave-
length and the time required for the de-
velopment of a discrimination. Again
the slope of the function was steep at
small values but decreased as larger
values were approached.

Stimulus Salience

Further examples of the effects of
stimulus disparity would not be hard to
find, but the magnitude of the differ-
ence between the discriminative stim-
uli and their background stimulation
(salience) poses more of a problem.
This is a dimension that does not come
up for consideration within the older
response-strengthening-and-weakening
theories of discrimination learning.

STIMULUS CONTROL 255

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HOURS IN BLOCKS OF 8

Figure 1. Percentage of responding in SD (S+), multiplied by 10, as a function of hours of training.
Each point represents the mean of 4 rats during an 8-hr session. The stimuli for each group are
specified in decibels. (Reproduced from Pierrel et al., 1970; copyright Society for the Experimental
Analysis of Behavior.)

However, when investigators are guid-
ed by an interest in the role of observ-
ing or attention, it becomes a more
likely candidate. In my laboratory, we
became aware of the importance of sa-
lience while studying the effects of

stimulus disparity on the relative rates
of pecking two observing keys (Dins-
moor, Sears, & Dout, 1976). In our first
experiment, the birds pecked a key that
produced large increases or decreases
in illumination as discriminative stim-

256 JAMES A. DINSMOOR

uli at higher rates than they pecked a
key that produced smaller increases or
decreases in illumination. In our sec-
ond experiment, however, we varied
the magnitude of change for the posi-
tive and the negative stimuli indepen-
dently. We discovered that the positive
relation between magnitude of change
and rate of pecking stemmed entirely
from the positive stimulus: For the
negative stimulus, the function was
negative in slope. That is, larger
changes in stimulation reduced the
rate. On returning to the experimental
literature, we were led to the conclu-
sion that in most previous examina-
tions of stimulus difference, the mag-
nitude of the difference between the
two stimuli had been confounded with
the magnitude of the difference be-
tween the discriminative stimuli and
their background. That is, the disparity
betwen the stimuli had been confound-
ed with their salience.

At the time, we were not aware that
a study of discrimination in which the
salience of the stimuli had indeed been
varied independently of other factors
had already been conducted by John-
son (1970). A student of Cumming,
Johnson was interested in the role of
attention in discrimination learning,
and one of the parameters he had var-
ied, as part of a larger study of selec-
tive control, was the brightness of a
white line displayed on the pigeon’s
key. Pecks when the line was vertical
(S+) were reinforced on a random-in-
terval schedule, but pecks when the
line was horizontal (S-) were never
reinforced. The rate at which the birds
learned the discriminaton between
these two orientations was a function
of the brightness of the line.

Still blithely unaware of Johnson’s
study, Dinsmoor, Mueller, Martin, and
Bowe (1982) chose as their controlling
stimulus a black line bisecting a white
key. Pecks on the key produced grain
on a variable-interval schedule that al-
ternated with an extinction schedule.
Ordinarily, the line remained horizon-
tal in its alignment, regardless of which
of these schedules was operating. But

by depressing a low-lying “perch,”
similar to the conventional cross-bar
for rats, the pigeon produced stimuli
that were correlated with the compo-
nent schedules. During periods when
the variable-interval schedule was in
effect, the stimulus was, for two dif-
ferent groups, a clockwise tilt of 150 or
a clockwise tilt of 300 (S+); during pe-
riods when the grain was being with-
held, it was a counterclockwise tilt of
the same magnitude (S-). For half of
the subjects, then, the total difference
between the two tilts was 30°, and for
the other half, it was 600. These differ-
ences represented small and large dis-
parities, respectively, between the two
stimuli.
To vary the salience of the stimuli,

the experimenters used an arrangement
similar to that of Johnson (1970), re-
ducing the contrast between the black
line, regardless of its tilt, and the white
surround. This was accomplished by
the simultaneous lighting of two pro-
jection units, one of which displayed
an image of the line on the key and the
other of which produced only a uni-
form white field. Both lamps were re-
duced to 50% of their normal intensity.
Thus, the salience of the stimuli could
be manipulated without affecting their
disparity. When the image of the line
was projected at its maximal contrast
by a single projector cell, 7 of the 8
birds in that group learned both to hold
down the perch and to discriminate in
their rate of pecking between the re-
sulting stimuli. When the second cell
was lighted, the line, now gray, was
readily visible to the human eye and
presumably to that of the pigeon. Nev-
ertheless, only 1 of the 8 birds in the
other group gave any indication that its
behavior was influenced by the tilt of
the line. Obviously, the salience of the
stimuli was an important determinant
of the level of observing and of the
consequent level of discrimination.
The effects of disparity were not as
clear as those for salience, but in a sub-
sequent study, using an entirely differ-
ent design, Dinsmoor et al. (1983)
found that both of these dimensions af-

STIMULUS CONTROL 257

fected the rate at which pigeons pecked
an observing key.

SELECTIVE CONTROL

It was Pavlov (1927/1960, pp.
141ff.) who first called attention to the
reduction in the effectiveness of a stim-
ulus that sometimes occurred when
that stimulus was presented in tandem
with another stimulus. He found that
when two conditional stimuli were reg-
ularly presented together, in the same
temporal relation to each presentation
of the unconditional stimulus, one
member of the pair often came to as-
sume complete control over the
amount of saliva that was secreted, to
the partial or complete exclusion of the
other stimulus. That is, when tested
alone, the second stimulus was ineffec-
tive. On further examination, he found
that it was the relative intensity of the
two conditional stimuli (CSs) that de-
termined which of them prevailed and
the extent to which the effectiveness of
the less intense stimulus was reduced.
When the stimuli were of equal inten-
sity (e.g., two tones of equal loudness),
the phenomenon did not appear: The
two stimuli were followed by re-
sponses of equal magnitude. Pavlov
therefore spoke of the more intense CS
as ”obscuring” or “overshadowing”
the less intense CS.

Years later, at the Miami Symposium
on the Prediction of Behavior, Kamin
(1968) presented a series of experi-
mental comparisons that provided con-
vincing evidence not only for over-
shadowing but also for another phe-
nomenon, also based on the simulta-
neous presentation of two stimuli. In
this case, the dominance of one stim-
ulus over the other was not established
by its intensity but by prior training. If
the subject was trained first with a sin-
gle CS and later with a compound that
included the original and some other
stimulus, the added CS turned out to
be ineffective. Kamin spoke of the pri-
or training with the first stimulus as
“blocking” the subsequent acquisition
of control by the second stimulus. In

the same year, a set of experiments
published by Wagner, Logan, Haber-
landt, and Price (1968) implicated yet
a third variable, the consistency with
which each member of the stimulus
compound was followed by the uncon-
ditional stimulus (US). A CS that was
always followed by the US reduced the
effectiveness of a stimulus that was
followed by the US on only half of its
presentations.
The common thread running through

the loss of effectiveness in each of
these instances is that both stimuli are
regularly presented at the same time
and therefore in the same temporal lo-
cus with regard to the unconditional
stimulus. Other ways of describing the
relation between the two CSs are to say
that they covary, that they are con-
founded, or that they duplicate one an-
other. Cognitive psychologists often re-
fer to the second stimulus as “redun-
dant,” meaning that it provides no ad-
ditional information concerning the
arrival of the US (i.e., no improvement
in the prediction of the US over that
provided by the first stimulus). Both
stimuli convey the same meaning. In
courses on composition, the word re-
dundant is sometimes used to refer to
the type of error manifested by a
speaker who proclaims that “We’ve
won four straight in a row.” In a row
conveys the same information as
straight, and the repetition is jarring.
(Sometimes, however, a more remote
and less conspicuous redundancy is ef-
fective in expository writing.)

Blocking and overshadowing have
also been observed with operant be-
havior. The primary difference in pro-
cedure is that instead of the stimulus
control being based exclusively on the
presence versus the absence of a CS,
as in respondent conditioning, in op-
erant work the discrimination is rec-
ognized, and both a positive stimulus
and an explicit negative stimulus are
normally provided. In a study by vom
Saal and Jenkins (1970), for example,
pecking was initially reinforced in the
presence of green illumination of the
key but not in the presence of red; later

258 JAMES A. DINSMOOR

a tone and a noise were added to the
procedure, covarying with the first pair
of stimuli; and finally, control by the
tone and noise alone was compared
with control by those same stimuli in
a group of birds that had not been pre-
trained with the red and the green. The
prior training on the red-green discrim-
ination was found to have blocked the
acquisition of control by the tone and
the noise.

In a study of overshadowing, Miles
and Jenkins (1973) reinforced pecking
in the presence of a bright light and a
tone of 1000 Hz (S+) but not in the
presence of a light of lower intensity
and a white noise (S-). The intensity
of the second light varied from group
to group. In subsequent tests, the tone
had relatively little influence over the
responding by birds in the group that
had received the largest difference in
illumination (brightest light vs. total
darkness) but exerted much more con-
trol with birds that had been trained
with smaller differences in illumina-
tion. The easier the light discrimina-
tion, the more it overshadowed the
tone. The harder the light discrimina-
tion, the less it overshadowed the tone.

Note that in this experiment the vari-
able that determined the amount of
overshadowing was not the absolute
intensity of the S+ (which remained
the same from group to group) but the
size of the difference between that S+
and the corresponding S- (stimulus
disparity). This may have been the crit-
ical variable in the respondent work as
well, because the absolute intensity of
the CS (which was assumed to be the
relevant dimension) was always con-
founded with the magnitude of its dif-
ference from the absence of the CS
(disparity). Recognizing that the con-
ditional stimulus is a discriminative
stimulus enables us to formulate broad-
er, more general principles of behavior.

Blocking and overshadowing have
figured prominently in theoretical dis-
cussions about Pavlovian conditioning,
but at a more applied level they have
largely been ignored. I suspect that
their manifestations appear fairly fre-

quently in everyday life and that over
a period of time a good many illustra-
tions may be found. For the present,
one example that comes to mind is a
phenomenon, well known to social
psychologists, in which individual
members of an outgroup who share a
prominent physical characteristic are
perceived as “all looking alike.” (For
a recent list of citations, see Anthony,
Copper, & Mullen, 1992.) That is, for
each individual, the characteristic com-
mon to all members of the group over-
shadows the characteristics distinctive
of that one individual and makes dif-
ferent members of the group more dif-
ficult to discriminate.
Two major types of explanation

have been offered for blocking and
overshadowing. Rescorla and Wagner
(1972) have proposed an elegantly
simple mathematical model of Pavlov-
ian conditioning, suggested directly by
Kamin’s (1968) data, that accounts for
these phenomena in terms of a ceiling
on the level of associative strength.
When one stimulus is relatively intense
or is presented earlier in training, the
overall strength of conditioning based
on that stimulus approaches an asymp-
tote, and little additional conditioning
can occur to a second stimulus that is
less intense or is introduced later in
training. Theoretical descriptions of
this type (see also Revusky, 1971),
however, do not account for instances
in which overshadowing has been
demonstrated on the first trial of con-
ditioning (e.g., Mackintosh & Reese,
1979).
The other type of explanation that

has been popular in discussions of
blocking and overshadowing relates
the reduction in effectiveness to a fail-
ure to observe or to attend to the sec-
ond stimulus or pair of stimuli (e.g.,
Mackintosh, 1974, pp. 585ff.). Some
support for this interpretation may be
found in experiments on observing be-
havior. Two replications of such an ex-
periment were conducted in my labo-
ratory. In the first one, some of the data
were disrupted when we were required
to remove the birds from their usual

STIMULUS CONTROL 259

living quarters. Then the second one
became to a certain extent redundant
scientifically when a similar study was
published by Blanchard (1977). In our
experiments, which were somewhat
more complex than Blanchard’s, we
provided the pigeon with an observing
key that altered the tilt of a white line
displayed on a colored ground. During
periods when food was being pro-
grammed on a variable-interval sched-
ule, pecking the observing key inter-
mittently produced a shift in the ori-
entation of the line, rotating it 450 from
the horizontal; during periods when no
food was scheduled (extinction), the
shift was in the opposite direction.
Meanwhile, the colored backgrounds
switched back and forth on a schedule
that was unrelated to the schedules of
food delivery and the direction of the
rotation produced by pecking the ob-
serving key. On some sessions the al-
ternating colors differed by a large
amount, and on other sessions they dif-
fered by a small amount.

After the rate of pecking had stabi-
lized, however, we synchronized the
alternations of the colors with the al-
ternations between the variable-inter-
val schedule and extinction and with
the direction taken by the line. At this
point, the changes in the orientation of
the line produced by pecking the ob-
serving key became completely redun-
dant: Whenever such a change was
produced, its relation to the delivery of
food was exactly the same as that of a
color already displayed on the key. As
would be expected, the rate of pecking
the observing key in the presence of
one of the S – colors (pecking that pro-
duced only S- tilts) dropped precipi-
tously. Much more important was the
fact that the rate of pecking in the pres-
ence of one of the S+ colors also de-
clined. In other words, less observing
behavior was maintained by the line
tilts as conditioned reinforcers when
they covaried with another, more effec-
tive set of stimuli. Moreover, in accord
with conventional experiments on
overshadowing, when the colors that
differed by a large amount were dis-

played on the key, the rate of pecking
dropped more than when the colors
that differed by a small amount were
displayed.

At a broad and general level, then, a
model based on principles derived
from the study of observing can ac-
count for the effects of continued train-
ing, as well as for the overshadowing
sometimes noted on the initial trial. On
the other hand, it is difficult to see how
any model based on the observation of
or attention to one stimulus rather than
another could handle the results ob-
tained by Revusky (197 1), which mim-
ic blocking and overshadowing despite
the absence of any temporal overlap
between the two conditional stimuli.
Perhaps no single factor can account
for all of the data obtained in experi-
ments on blocking and overshadowing.

Progressive Discrimination

Another phenomenon that highlights
the importance of the magnitude of the
difference between the stimuli to be
discriminated (disparity) is known var-
iously as progressive discrimination
training, transfer along a continuum, or
the easy-to-hard effect. It has long
been recognized that the most efficient
way to train a subject to discriminate
between two stimuli that lie close to-
gether along the physical continuum is
to begin with stimuli that are farther
apart and then, in successive steps, to
close the gap (e.g., James, 1890, p.
515; Montessori, 1912, p. 184). In oth-
er words, initial training with easy
stimuli from the same dimension ac-
complishes more than an equal period
of training with the difficult pair on
which the subject is eventually to be
tested. In Pavlov’s laboratory, this
technique was employed to train a dog
to discriminate between a light gray
and a white circle and to train another
to discriminate between a fairly well-
rounded ellipse and a perfect circle
(Pavlov, 1927/1960, pp. 121ff.).
Even before bar-pressing or key-

pecking techniques came into common
use, systematic data were reported with

260 JAMES A. DINSMOOR

relatively crude procedures, typically
involving choices between test cards of
differing shades of gray. The study that
has been most widely cited, historical-
ly, is one reported by Lawrence (1952).
In his procedure, the rat was forced to
jump across a gap that separated the
starting box from a pair of goal com-
partments lined with cardboard in dif-
fering shades of gray. One group of
subjects was trained throughout with
two highly similar grays, the same as
those used in the final test; two groups
started with the lightest and the darkest
gray and were shifted to the final test
pair at different points in their training;
and finally, one group received its first
10 trials with the most disparate pair of
grays, the next 10 trials with a less dis-
parate pair, the third 10 trials with a
still less disparate pair, and its last 50
trials with the final test pair. All the
groups that began with the large dif-
ference performed better during their
last 50 trials than did the group that
began with the difficult discrimination.
The last group, which shifted in a se-
ries of steps, made the fewest errors of
all the groups. Lawrence suggested that
the easy stimuli facilitated later learn-
ing of the hard stimuli because they
helped “the animal to isolate function-
ally the relevant stimulus dimension
from all the other background and ir-
relevant cues” (Lawrence, 1952, p.
516). Both the magnitude of the dis-
parity between the positive and the
negative stimulus and the salience of
the positive stimulus, which is con-
founded with its disparity in many of
these studies, contribute to the effec-
tive reinforcement of relevant observ-
ing behavior (Dinsmoor et al., 1983).

Fading

In contrast to the research on block-
ing and overshadowing, which was
concerned exclusively with an eluci-
dation of the basic principles of con-
ditioning, the original impetus to re-
search on fading seems to have ema-
nated from attempts to fashion effec-
tive techniques for use in programmed

instruction. Early papers (Cook, 1960;
Skinner, 1958) describe a procedure
then known as “vanishing”: In succes-
sive steps, letters were eliminated from
words to be spelled by the subject,
words were deleted from passages to
be recited from memory, or labels for
various geographical features were re-
moved from a map. The subject con-
tinued to respond correctly, despite the
elimination of the original stimuli,
even when the only remaining cues
might be those generated by chains of
ongoing behavior.

Almost immediately, there was a
change in terminology, and the same
technique came to be known as “fad-
ing” (Holland, 1960). Then, not long
after that, investigators using nonhu-
man subjects began studying fading in
the conditioning laboratory. Using a
highly specialized training procedure,
Terrace (1963) had initially established
a discrimination between red and green
illumination of the pigeon’s key. Al-
though he did not point out the paral-
lel, the red and the green were equiv-
alent to the stimuli that were already
effective when a human trainee began
his or her program of instruction. Then
a vertical line was superimposed on the
red S + and a horizontal line on the
green S -. Gradually, in successive
steps, the brightness of the red and the
green was reduced, until the key be-
came totally dark except for the lines.
In other words, the red and the green
illumination were faded. As the sali-
ence of the colors was reduced, the
birds made less and less use of those
stimuli and gradually came to depend
on the direction taken by the line. In
two replications of this transfer proce-
dure, a total of 4 birds learned the ver-
tical-horizontal discrimination without
ever pecking in the presence of the S-
(i.e., without making a single error).

Other techniques, used as experi-
mental controls, proved to be far less
effective. For example, if the red and
the green were completely removed
from the key at the time that the ver-
tical and the horizontal lines were in-
troduced, so that there was no temporal

STIMULUS CONTROL 261

overlap between the two sets of stim-
uli, hundreds of S- responses occurred
before the birds learned the vertical-
horizontal discrimination. When the
two sets of stimuli overlapped in time
but nothing was done during this time
to reduce the effectiveness of the red
and the green, intermediate results
were obtained. Something was evi-
dently learned with regard to the lines
during the period when they were su-
perimposed on the colors, but not as
much. It was the gradual reduction in
the salience of the colors that forced
the birds in the fading group to switch
to the lines during the period when
both sets of stimuli were available. By
the time the colors were completely
gone, the birds had already learned to
discriminate accurately on the basis of
the lines. (For an alternative procedure
in which the original stimuli were de-
layed in onset rather than reduced in
salience, see Touchette, 1971.)

There is another parallel that was not
obvious at the time Terrace conducted
his work: His red and green illumina-
tion were also comparable to the pre-
trained or inherently more effective
stimuli subsequently used in operant
studies of blocking and overshadow-
ing. In light of this correspondence,
Terrace’s (1963) fading procedure can
be viewed as a method of overcoming
the overshadowing or undoing the
blocking of one pair of stimuli by an-
other. By reducing the influence of the
originally dominant pair, fading allows
the stimuli that have been blocked or
overshadowed to gain effectiveness.
The advantage offered by Terrace’s

(1963) fading procedure was that dur-
ing the prior training, highly effective
stimuli were available on the key. In
contrast to the lines, which were more
localized, the colors covered the entire
surface of the key, and anywhere the
pigeon might initially look while peck-
ing that key it was likely to see the
color. Early contact with a stimulus
correlated with reinforcement pro-
duced early acquisition and a high lev-
el of performance of the behavior by
which the bird observed that stimulus.

Later, when the lines were introduced,
the birds had already learned to look at
and attend to stimuli displayed on the
key. The lines were displayed in the
same general location as the colors
(i.e., on the surface of the key). Then,
as the salience of the colors was re-
duced, their value as reinforcers de-
clined (Dinsmoor et al., 1982, 1983)
and the bird shifted from observing the
colors to observing the lines.

Evidence confirming this interpreta-
tion comes from a study conducted by
Fields (1978). Again the pigeons were
pretrained to discriminate between red
and green illumination of the key.
Then white lines of differing orienta-
tion were superimposed on the red and
green backgrounds. Between blocks of
training trials, Fields inserted probe tri-
als on which he presented the same
white lines, but on a dark background.
These trials enabled him to track the
acquisition of the line discrimination
during the course of training.
The most revealing aspect of Fields’

(1978) procedure was that with one
group of subjects he faded both the red
and the green, with another group he
faded only the red (S+), leaving the
green at its original intensity, and with
a third group he faded only the green
(S-), leaving the red at its original in-
tensity. Interpretation of the difference
in results between the first two of these
groups is complex and need not con-
cern us here, but the interesting finding
occurred with the third group. Data
from a large number of experiments in-
dicate that it is the S+ that reinforces
observing (for a review of these data,
see Dinsmoor, 1983). Accordingly, an
interpretation in terms of the observing
paradigm predicts, uniquely I think,
that it is fading of the S+ that is re-
quired to transfer control of pecking
from the colors to the direction taken
by the lines. Fading of the S – should
have no effect. And that is the result
that Fields obtained. “Attenuation of
the S – alone … did not produce stim-
ulus control by the line-tilt dimension”
(Fields, 1978, p. 126). His Figure 1
showed no pecking to either S + or S –

262 JAMES A. DINSMOOR

probes when it was the green (S-) il-
lumination that was faded.

CONCEPT FORMATION

When, in everyday conversation,
people speak of someone “possessing”
a certain concept, it is taken for granted
that they are referring to some kind of
unobservable content or state of the
mind. But when we examine the mat-
ter, we find that the observation that
leads them to use the term concept is
that the person responds in the same
way to a set of objects or relations that
have some characteristic or set of char-
acteristics in common but does not re-
spond in that same way to other objects
or relations. All birds are called birds,
for example, but no snakes, squirrels,
or sheep. As Keller and Schoenfeld put
it, “Generalization within classes and
discrimination between classes” (1950,
p. 155).

If we can verbalize what it is that
the various instances have in common,
we may be able to define the concept,
but often this can be very difficult. Try
defining what is meant even by such
familiar categories as a “dog,” for ex-
ample, or a “human being.” Are you
sure you did not include wolves or
coyotes in the first instance or chim-
panzees in the second? Did you in-
clude feral children? Definition is sec-
ondary to the behavior: In essence, it
is a description of that behavior, al-
though it can provide in turn a rule for
increasing the accuracy of that behav-
ior or for passing it along to another
person (e.g., Skinner, 1969, pp. 121-
125, 136-142).

Laboratory research on the forma-
tion of concepts arose from an interest
in the processes of human thought and
has continued for many years in almost
complete isolation from research on
the formation of discriminations. The
general nature of the procedure is il-
lustrated by an experiment conducted
by Trabasso (1963). College students
were presented with drawings of flow-
ers, in a randomized sequence, and
were asked to classify each drawing by

calling it an A or a B. Four different
varieties of flower were used, and each
of these varied also in the shape of its
leaves, the angle of the leaves to the
stem, and the number of leaves on ei-
ther side. For most of the subjects, the
angle with the stem was the relevant
feature (i.e., the discriminative stimu-
lus), determining whether A or B was
the response to be reinforced. Trabasso
found that such strategies as holding
other features constant, rather than al-
lowing them to vary, and, as in ordi-
nary discrimination training, increasing
the difference between the positive and
the negative angles (i.e., disparity), or
increasing their difference from the
background stimulation by coloring
them red (i.e., salience) all led to faster
acquisition of the correct categoriza-
tion. These findings suggest that con-
cept formation is very similar to dis-
crimination training. In particular, the
effect of highlighting the angles to in-
crease their salience indicates that a
major part of the subject’s task is to
learn to observe the appropriate feature
within the field of stimulation.

Traditionally, as I have indicated,
psychologists have treated experiments
dealing with the formation of discrim-
inations and experiments dealing with
the formation of concepts as two sep-
arate and distinct fields of inquiry. But
the close relation between the two has
long been recognized by some writers
(e.g., Keller & Schoenfeld, 1950), and
whether the traditional distinction is
justified is open to question.

It is true that most of the work on
discrimination learning has been car-
ried out with rats, pigeons, and mon-
keys, whereas most of the experiments
on concept formation have been con-
ducted with human subjects. This re-
flects the differences in the historical
origins and rationales of the two types
of research. But there is no hard and
fast rule: Sometimes.humans are used
to study discrimination learning, and
sometimes other species are used to
study the acquisition of concepts.

Another common difference be-
tween the two types of experiment is

STIMULUS CONTROL 263

probably a result of the first. Because
an entire set of verbal responses can
readily and quickly be established with
members of the human species, human
subjects may be asked to acquire sev-
eral different concepts at the same
time. This practice enables the experi-
menter to collect more data from each
subject. When other species are used,
however, it takes much longer to estab-
lish each response topography, and the
use of any substantial number of cate-
gories becomes impractical, even in
those experiments designated as stud-
ies of concept formation. To respond or
not to respond is usually the question.
A third feature, which may also re-

late to the historic use of human sub-
jects, is that in experiments on concept
formation the distinction between the
positive and the negative stimulus is
often very subtle. Human subjects
learn simple, gross discriminations so
rapidly that variations in their perfor-
mance created by manipulating some
independent variable might be too
small to detect, and subtle differences
are necessary to collect meaningful
data. In this case, the historical feature
may have been extended to serve as an
implicit definition for classifying the
two types of research: Even when non-
human species are used, subtlety of the
properties distinguishing between the
positive and the negative instances
(e.g., Herrnstein, Loveland, & Cable,
1976) appears to be characteristic of
those experiments that are considered
to be examples of concept formation.
But this is a difference in a parametric
value, not a difference in the underly-
ing process.

Finally, the most meaningful differ-
ence between discrimination and con-
cept formation may be that in experi-
ments on discrimination, all aspects of
the environment other than the alter-
nation between the positive and the
negative stimulus are kept as uniform
as possible (“held constant”), so that
they will produce the least possible
fluctuation in the results. In experi-
ments on concept formation, by con-
trast, other aspects of the stimulus ob-

ject are deliberately varied. In this re-
spect, experiments on concept forma-
tion are more closely related to what is
colloquially called “real life.” They
provide us with a persuasive bridge be-
tween the stripped down simplicity of
the laboratory and the rich complexity
of the natural setting.

IMITATION

There is another form of stimulus
control that is not well understood but
that merits consideration because it is
believed to play an enormous role in
training the young of many species, in-
cluding our own, to behave like their
parents and other adults. The observa-
tion that leads us to speak of imitation
is that the stimulus setting the occasion
for a particular response or set of re-
sponses is the performance of approx-
imately the same pattern of behavior
by another individual, usually of the
same species. Such a correspondence
between two patterns of behavior is so
common that it is frequently assumed
that it must be the result of some in-
trinsic connection between the behav-
ior of the second organism and that of
the first. But not all behavior is imitat-
ed. In most cases in which imitation is
said to occur, the correspondence be-
tween the two sets of behavior appears
to be historical in origin. It is a product
of the contingencies of reinforcement.

It has sometimes been suggested
(e.g., Bandura, 1965) that imitation is
an alternative mode of learning, by
means of which a response can be es-
tablished without the use of reinforce-
ment (“no-trial learning”). In some
sense, at a global level, this may be
true, but it is not clear that any new
principle of behavior is required to ex-
plain Bandura’s results.

In exploring this issue, it may be
helpful to distinguish between a com-
plex pattern of physical activity like
swimming the breast stroke, riding a
bicycle, driving an automobile, or per-
forming a new dance step, and the con-
stituent movements that make it up
(see Guthrie, 1952, pp. 27-28, or Skin-

264 JAMES A. DINSMOOR

ner, 1953, p. 94). Modeling is akin to
verbal instruction, and, indeed, might
be considered another form of the same
process. If the constituent responses
are already a part of the individual’s
repertoire (by prior differentiation) and
some kind of imitative control has al-
ready been established, an appropriate
sequence or an effective combination
of responses can often be called forth
by the corresponding behavior of a
model. Sometimes this is all that is
needed. In other cases, it may still
serve as the initial step: Once some ap-
proximation to the appropriate behav-
ior has occurred, the overall pattern
can be maintained and can subsequent-
ly be refined by selective reinforce-
ment. But if the constituent responses
are not in the individual’s repertoire to
begin with, they cannot be established
simply by imitation. Who among us
has not struggled in vain to match the
sounds that have been demonstrated to
us by the teacher of a foreign language,
only to find that the closest approxi-
mations within our vocal repertoire
leave much to be desired? Who among
us has not tried to copy someone else’s
drawing, only to discover that much
more training was necessary before a
satisfactory replica could be produced?
The process by which the correspon-

dence between stimulus and response
is originally established was illustrated
more than half a century ago in an ex-
periment by Miller and Dollard (1940).
All training was conducted on a special
maze, shaped like the letter T. First,
several “leader” rats were trained to
discriminate between black cards and
white cards, half of them to enter the
arm of the maze with the black card
and half to enter the arm with the
white. The only function of these lead-
er rats, however, was to provide dis-
criminative stimuli for the rats that
were to follow them.
When the leader rats had learned to

turn in the direction indicated by the
appropriate card, they were used to
train the “follower” rats to imitate a
specific item of behavior. First, a leader
was placed in the start box at the be-

ginning of the stem, with the follower
in a second box immediately behind it.
When the leader was released from its
start box, it promptly ran to the choice
point at the intersection of the T and
turned in the designated direction. At
the end of the arm, it received its usual
allotment of food. The follower rat was
released immediately behind it. If the
follower rat turned in the same direc-
tion as its leader, its response was re-
inforced with food in a special recep-
tacle uncovered only after the leader
rat had passed over it. If the follower
rat turned in the other direction, how-
ever, its response was not reinforced.

For the follower rat, then, the leader
turning to the right was a discrimina-
tive stimulus for turning to the right,
and the leader turning to the left was a
discriminative stimulus for turning to
the left. As might be expected, the fol-
lower rats had no difficulty in master-
ing this discrimination. With respect to
this one response, their behavior
matched or imitated the behavior of
their leaders. However, in terms of
learning principles, there was nothing
unique about the resulting correspon-
dence between the behavior of the 2
animals. To demonstrate this point,
Miller and Dollard (1940) trained an-
other group of follower rats in a pattern
of stimulus control that was exactly the
opposite of imitation: Turn left when
the leader turns right and turn right
when the leader turns left. The two
groups learned their tasks with equal
facility. Following a partner in ball-
room dancing may provide an illustra-
tion of this reversed relationship, but
such a pattern of stimulus control is not
commonly reinforced in the outside
world.

Apparently there is more to imita-
tion, however, than establishing dis-
criminative control over a single re-
sponse. When a number of correspon-
dences have been reinforced between
the actions of an experimental subject
and the actions of a model, the corre-
spondence itself may become a gov-
erning factor in the relation between
the two actions, extending to new to-

STIMULUS CONTROL 265

pographies of behavior. Baer, Peterson,
and Sherman (1967) demonstrated this
with 3 profoundly retarded children
who originally showed no tendency to
imitate. In the beginning, the experi-
menters used manual guidance and se-
lective reinforcement with food to es-
tablish a series of physical actions like
raising the left arm, tapping a table, or
moving the arm in a circular motion,
each of which followed a like action
by the experimenter.
One of the things these authors

showed was that the behavioral corre-
spondence could itself be placed under
the control of some other stimulus in
what was therefore known as a condi-
tional discrimination. In their study, re-
inforcement of the child’s action was
conditional upon the prior presentation
of the verbal stimulus, “Do this,” fol-
lowed by a demonstration of the de-
sired response. Stimuli in the presence
of which correspondence is reinforced
determine when imitation will occur
and when it will not occur. In some
studies, the choice of a model has de-
termined when correspondence would
be reinforced.

In the Baer et al. (1967) study, 130
different responses were employed.
Eventually the children began to imi-
tate new actions, demonstrated for the
first time. Some of these were repeated
on a number of test trials, still without
reinforcement, interspersed among
training trials on which other actions
were reinforced. The performance of
these nonreinforced actions continued
to depend, however, on the general ten-
dency to perform responses demon-
strated by the experimenter. When re-
inforcement of the main body of re-
sponses was replaced by differential
reinforcement of other behavior, the
never-reinforced test actions also
dropped out. Both sets of behavior
were restored with restoration of the
original reinforcement schedule.

Similarity to the behavior of the
model may also serve as a conditioned
reinforcer. In another experiment, Lo-
vaas, Berberich, Perloff, and Schaeffer
(1966) reinforced the production of

English words by each of 2 autistic
children, in response to those same
words presented by an experimenter.
Then some Norwegian words were
slipped into the sequence, without re-
inforcement; with continued reinforce-
ment of the English words, the pronun-
ciation of the Norwegian words grad-
ually improved. These data suggest
that the increasing similarity between
the sounds produced by the child and
the sounds presented by the experi-
menter had itself become reinforcing,
leading to successive approximations
of those sounds by the children.

STIMULUS EQUIVALENCE

In his presidential address to the
Midwestern Psychological Associa-
tion, Skinner (1950) included brief re-
sumes of several pieces of work con-
ducted in his laboratory that were nev-
er subsequently reported in greater de-
tail. Among these was a procedure that
he called “matching to sample,” pre-
sented in an attempt to strip some of
the surplus meaning from what would
nowadays be called “cognitive” de-
scriptions of choice and other complex
patterns of behavior.
The pigeon was confronted with

three keys, arranged in a horizontal
row. First, on a given trial, the middle
key was illuminated with a sample col-
or, perhaps red or green. When the bird
pecked that key, the sample color was
extinguished and the keys on either
side were lighted with the comparison
colors. One of these colors was the
same as the sample, and the other was
different. In a strict matching-to-sam-
ple procedure, it was a peck on the
same-color key that was reinforced. In
what later came to be known as oddity
matching, it was a peck on the differ-
ent-color key that was reinforced. With
a delay introduced between the dark-
ening of the center key and the lighting
of the side keys, Skinner’s procedure
was widely used by more cognitively
oriented psychologists to analyze the
processes involved in responding to
past events (memory).

266 JAMES A. DINSMOOR

Years later, Sidman (1971, 1994)
adapted Skinner’s procedure to the task
of teaching a severely retarded boy to
read English text. Prior to the proce-
dure in question, this boy had already
learned correspondences in both direc-
tions between spoken words and pic-
tures. That is, he had learned to choose
the pictures that illustrated the words
presented as sample stimuli and to pro-
duce the correct vocal responses to
(name) the pictures. During the train-
ing procedure, the sample stimuli were
words spoken by the experimenter, and
the comparison stimuli were printed
words. Somewhat to Sidman’s surprise,
after his subject learned to choose the
printed word that corresponded to each
spoken word, not only was he able to
proceed in the opposite direction, pro-
ducing the correct oral responses to
printed text (reading aloud) but, with-
out further training, was able to choose
pictures corresponding to the printed
words (reading comprehension). Estab-
lishing an equivalence between the au-
ditory and the visual forms of a series
of words had extended the subject’s
ability to select the correct pictures
from the original spoken words to their
printed equivalents.

Struck by these initial findings, Sid-
man launched a major program of re-
search designed to explore their theo-
retical ramifications (Sidman, 1994).
Because there is now a burgeoning lit-
erature on the topic, it may be impor-
tant to consider some terminological
issues. First, Sidman objected to the
use of the term matching to sample as
a description of Skinner’s procedure,
on the grounds that this phrase implied
a generalized behavioral outcome that
did not necessarily result from that pro-
cedure. In its place, he substituted the
phrase conditional discrimination, first
applied by Cumming and Berryman
(1965). Generically, the term condi-
tional discrimination covers more than
the matching-to-sample paradigm (see
Yarczower, 1971), but in the Cumming
and Berryman usage an additional, ex-
traneous stimulus (e.g., a red sample or
a green sample) determines which of

the alternative stimuli (in this case, red
or green comparison stimuli) in a dis-
crimination is positive (i.e., followed
by reinforcement of the response). The
relation between comparison stimulus
and reinforcer is selected by or condi-
tional upon the nature of the sample
stimulus. When the sample stimulus is
red, pecking red is reinforced and
pecking green is not reinforced; when
the sample is green, pecking red is not
reinforced and pecking green is rein-
forced.

Sidman (1994) has also suggested
the use of the term conditional stimu-
lus to refer to the role of the sample
stimulus in this type of discrimination,
but such a usage could lead to termi-
nological confusion, because that term
is widely used in Pavlovian condition-
ing to refer to the stimulus that ac-
quires its effectiveness through its tem-
poral relation to the unconditional
stimulus. The terms instructional stim-
ulus (Cumming & Berryman, 1965) or
contextual stimulus (Sidman, 1994)
have also been suggested; the former
term has intuitive appeal, and the latter
term has the advantage of emphasizing
that stimuli and responses usually do
not pair off in a simplistic one-to-one
correspondence, as sometimes implied
in early writings by Watson (1919), for
example, but are characteristically re-
lated in a way that depends on what
other stimuli are present (context).

Although an exposition of the nature
of the procedure we are considering
has been greatly simplified by the use
of physically identical colors as ex-
emplars, in establishing equivalence
classes the experimenter utilizes stim-
uli that bear no necessary resemblance
to each other and establishes their cor-
respondence through the program of
training. A quantity of objects, for ex-
ample, and the corresponding numeral
begin as arbitrary, unrelated stimuli,
but the standard relations characteristic
of a given language can be established
through this type of training. Eight ob-
jects can be made equivalent to the
digit “8” and the binary number
“1000” and the sound “ate” and the

STIMULUS CONTROL 267

printed word “eight” and even the
Spanish word “ocho,” thus forming a
class, all of whose members may be
said for our purposes to be equivalent.
When they become members of the
same class, they are for many purposes
interchangeable. If we respond in the
same way to these physically very dif-
ferent stimuli, they may be said to have
the same meaning.

Sidman (1994) has laid out three cri-
teria, all of which are necessary to the
mathematical definition of an equiva-
lence relation among a set of stimuli.
Reflexivity requires that, without spe-
cific instruction or training, the subject
choose each stimulus in the list as a
comparison in reaction to that same
stimulus as a sample (i.e., generalized
identity matching). Symmetry requires
that the subject perform correctly when
the roles of sample and comparison
stimuli are reversed. Transitivity in-
volves three stimuli in a sequence.
Once “if a, then b” and “if b, then c”
have been established, then “if a, then
c” must emerge without further in-
struction or training.
The apparent significance of equiv-

alence classes for what are sometimes
known as higher mental processes is
currently attracting a considerable
amount of attention within the behav-
ior-analytic community. Quite a bit of
research has recently been reported, ac-
companied by a large amount of theo-
rizing. However, it remains a relatively
new field of investigation, still in flux,
and the available information does not
as yet permit us to be sure how it fits
into a broader, more systematic frame-
work.
One active program of research

stems from Fields and Verhave’s
(1987) analysis of the parameters that
define the internal structure of an
equivalence class. In a review of the
relevant literature, Fields, Adams, and
Verhave (1993) examined the effects of
two of these parameters. Directionality
refers to whether a stimulus serves as
a sample or a comparison in the pro-
cess of training. In several studies, this
relation was found to influence the

likelihood of class formation, the for-
mal characteristics of specific emergent
relations, and the degree of transfer be-
tween stimuli. Nodes are formally de-
fined as individual stimuli that are
linked by the program of training to
more than one other stimulus within
the particular equivalence class; they
may be thought of as intervening steps
or mediators of the relation between
stimuli that were not presented togeth-
er during the training. And nodal dis-
tance refers to the number of such
steps that lie between the stimuli that
are to be tested. In a number of studies,
the authors found, this parameter had a
consistent influence on the subjects’
performances on a number of different
types of test. A point of emphasis in
their review was that functions ac-
quired by one stimulus in an equiva-
lence class do not transfer equally, but
rather differentially, to other members
of the class. The relatedness of the
stimuli is affected by the directionality
of training and is an inverse function
of nodal distance.
The relation between the formation

of equivalence classes and earlier, sim-
pler forms of discrimination learning
remains the subject of wide-ranging
discussion. Still in dispute, for exam-
ple, is the question of whether emer-
gent relations and stimulus classes can
be demonstrated with nonhuman sub-
jects (see Schusterman & Kastak,
1993; Zentall & Urcuioli, 1993). Sid-
man (1994) has suggested that the for-
mation of equivalence classes through
conditional discrimination training
may not be reducible to or explicable
by other processes with which we are
already familiar but may be a wholly
independent phenomenon grounded in
the evolution of the human species. For
those of us in search of systematic laws
of behavior, this is not a very satisfying
solution. Hayes (1991), on the other
hand, has suggested that equivalence
classes can be traced to a preexperi-
mental history of training in relational
responding. At the present time, such
issues are far from being resolved.

268 JAMES A. DINSMOOR

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