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DEVELOPMENT OF STIMULUS CONTROL
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Development of Stimulus Control under each Component of a Multiple Fixed-Ratio
Fixed-Interval Schedule of Reinforcement
Luis Otero-Valles
University of Florida
DEVELOPMENT OF STIMULUS CONTROL
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Development of Stimulus Control under each Component of a Multiple Fixed-Ratio
Fixed-Interval Schedule of Reinforcement
In behavioral research, many experiments use a multiple schedule of
reinforcement, but little research has been conducted on the development of
stimulus control under a multiple schedule. Stimulus control refers to the
relationship between an antecedent stimuli and a subject’s response. When some
aspect of the antecedent stimuli is altered and results in altered responding, then
stimulus control is present with respect to that stimuli’s variance (Eckerman, 1969).
Once a relationship is established between a stimulus and reinforcement, the
established stimulus becomes a signal of reinforcement contingencies and increases
the probability of responding in the presence of that stimulus (Williams, 1976). An
established stimulus alters the likelihood of responding when it makes it more likely
a particular event will result in reinforcement (McPherson & Osborne, 1986). In
previous literature, stimulus control has been observed when a subject emits a
particular response under a certain stimulus and a different response in the absence
of that stimulus. For example, if, in the presence of the houselight, a response on a
lever will produce a pellet, then stimulus control would be obtained when the rat
presses the lever only in the presence of the houselight and never when the
houselight is off. In this example the houselight is the discriminative stimulus, the
lever press is the response, and the pellet is the positively reinforcing consequence.
These events comprise a three-term contingency, which typically must be present
for the development of stimulus control.
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Morse and Skinner (1958), however, showed evidence of stimulus control in
pigeons by delivering food in the presence of one stimulus and not in the presence
of another regardless of response. Then, once the pigeons were required to respond
to receive the food as reinforcement, rates of responding were much higher under
the stimulus with which reinforcement was initially paired. The contingency
between the initial stimulus and reinforcement, independent of a response, was
sufficient to exert control over responding even when reinforcement depended on
responding. That is, responding was greater under a particular stimulus due to its
prior association with reinforcement. Thus, that particular stimulus exhibited
stimulus control by affecting the subject’s behavior under its presence and in its
absence (Morse & Skinner).
Additionally, McPherson and Osborne (1986) showed control of behavior by
a stimulus using a three-key discrete-trial procedure with pigeons. Under one
condition, illumination of the right key initiated the trial. A response on a right key
produced the lighting of a center key, leading to reinforcement when pecked. In a
second condition, illumination of the left key initiated the trial. Following a left key
response, a response had to occur on the right key followed by a response to the
center key to produce reinforcement. Thus, the illumination of the left key became a
conditional stimulus, making right key responding more likely to follow a left key
response. In the absence of illumination of the left key, responding on the right key
was promptly followed by a peck on the center key. That is, the control of the left-
key stimulus is seen through the left key’s influence on a pigeon’s right key response
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rate through signaling an increase probability of reinforcement (McPherson &
Osborne).
The clear presence of stimulus control has caused many researchers to be
interested in and observe its development. Eckerman (1969) believed that the
development of stimulus control could be traced to correlation of stimuli and
reinforcement probabilities. In other words, stimulus control developed if there was
a higher chance of reinforcement under a particular stimulus. Thus, the given
stimulus becomes a cue of reinforcement when present and a lack of reinforcement
during its absence (Williams, 1976). In fact, under contrasting stimuli in which
responses under one stimulus are reinforced and not under another have been
found to sharpen effect of stimulus control (Morse & Skinner, 1958).
Strong stimulus control tends to result in the occurrence of contrast or
discrimination, whereas weak stimulus control tends to result in induction or
generalization (Pear & Wilkie, 1971; Wertheim, 1965). Yarczower, Gollub, and
Dickson (1969) studied stimulus control in the presence of other cues also
competing over the response’s control. One group of pigeons was exposed to
discrimination training, in which a multiple variable-interval (VI) 30-s extinction
(EXT) schedule was in effect. In the presence of a 550-nm wavelength keylight,
responses were reinforced (S+) according to a VI 30-s schedule, and in the presence
of a 570-nm wavelength keylight, responding was not reinforced (S-). A second
group of pigeons only was exposed to the VI 30-s schedule in the presence of the
550 nm stimulus. Generalization of responding to a range of other wavelengths
then was tested. Pigeons that had been trained on the multiple VI EXT schedule,
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responded substantially less relative to the other group when wavelengths of 560
nm or greater were presented. The authors proposed that the control from
alternative cues may have weakened stimulus control when associated with the
reinforcement schedule. That is, for the pigeons trained on the single schedule,
responding tended not to come under the control of the wavelength (i.e., responding
generalized to greater wavelengths as well). These findings suggest that the use of a
multiple schedule tends to enhance the development of stimulus control.
The power of stimulus control further can be demonstrated by three-phase
experiment conducted by Wenrich (1963). He discovered that if an operant (lever
press) was brought under the control of a discriminative stimulus (light), the rats
emitted responses even when only a conditioned reinforcer (buzzer) was presented
and even when satiated via pre-session feeding. If the operant was not trained in
the presence of a discriminative stimulus, then these rats responded well below
baseline levels for the conditioned reinforcer and when satiated.
Powell (1973) found that deprivation increased response rate under an
extinction stimulus. Interestingly, the lower the existing level of stimulus control,
the greater the absolute rate of responding, contradicting previous literature.
Deprivation’s effect on performance has been known to diminish in correlation with
higher accuracy developed during training. Powell found accuracy of discrimination
to decline as deprivation increased. A possible explanation, suggested by Powell, for
the mixed data with respect to the strength of stimulus control producing contrast
or induction may be due to the broad definition of a “strong” and “weak” stimulus
control during baseline. The two-component multiple schedules often employed in
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these types of arrangements, designate one component in which reinforcement can
be earned (S+) and one component in which reinforcement is not available (S-).
Then discrimination indices are obtained by comparing responding under
experimental conditions (e.g., pre-session feeding) to baseline responding. Rather
than defining a “strong” and “weak” stimulus, Richards (1974) was interested in
differential stimulus control under each component of a multiple VI 1-min VI 1-min
schedule in which the delay to reinforcement varied across components (i.e.,
delayed versus immediate). Generalization probes were administered in which the
orientation of a line displayed on the key was rotated at several degrees relative to
baseline orientation of the line. The results demonstrated that stimulus control was
weakened when the reinforcer was delayed. Furthermore, when the magnitude of
the delayed reinforcer was increased from 1.5 s of food access to 4 s, stimulus
control was weakened even more. Richards noted it was plausible, however, that
the observed weakening of stimulus control was rather the failure of stimulus
control to develop. He suggested that the large magnitude of delayed reinforcement
slowed the development of stimulus control and that if baseline had been extended,
then weakening effects may not have been permanent, which could have resulted in
little to no difference across the magnitude conditions. Although Richards never
experimentally addressed that explanation, his results do indicate that, under
certain conditions, stimulus control may develop at different rates for each multiple-
schedule component. The explanation provided by Richards was consistent with
results reported by Van Houten and Rudolph (1972). They found the presence of a
salient, but irrelevant, antecedent stimulus (i.e., houselight illumination) inhibited
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the development of stimulus control. Additionally, they, too, suggested that
extended exposure to the contingencies may have allowed responding to come
under the control of the relevant stimulus (i.e., airflow in mph).
Whereas Richards (1974) was interested in the manipulation of
consequences, Perikel, Richelle, and Maurissen (1974), like Van Houten and
Rudolph (1972), examined the effects of antecedent manipulations of visual stimuli
as factors contributing to the development of stimulus control. Pigeons were studied
under both a one-key and a two-key procedure. In the one-key procedure, pecks on
the center key—illuminated green—resulted in reinforcement only after
presentation of a long duration stimulus (i.e., houselight illuminated red for 10 s)
but not after shorter-duration stimuli (range: 1 – 8 s). The two-key component
signaled reinforcement for both long- and short- duration stimuli, depending on the
side of responding: Left key pecks were reinforced after long- duration presentation
and right key pecks were reinforced after short-duration presentation.
Discriminative control was seen under both procedures. In the one-key procedure,
the probability of responding on the green key following a long-duration stimulus
was always above .75. The probability of responding following a short-duration
stimulus initially remained low but later increased to above .25 when the duration
of the stimulus presentation was increased beyond 6 s. When the short-duration
stimulus was 8 s, discriminative control was eliminated (i.e., equal probability of
responding following both short and long durations). The two-key procedure
yielded higher accuracy, measured in percent correct responses, as the duration of
the shorter stimulus was steadily increment by 1 s across conditions. That stimulus
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control developed at all under the one-key procedure was quite notable because
pecking green following either a long- or short-duration stimulus would have
allowed the pigeon to earn the maximum number of available reinforcers in the
session. The contingencies arranged for the two-key procedure, however, did
require that the pigeon learn a conditional discrimination (left versus right) in order
to earn 100% of available reinforcers.
One of the key factors that has been found to influence stimulus control has
been the schedule of reinforcement. The way in which reinforcers are arranged
engenders particular rates and patterns of responding, and Lattal (1986)
demonstrated that organisms are quite sensitive to even subtle differences. He
compared responding under a multiple schedule with responding under a mixed
schedule. In the multiple schedule VI and variable-time (VT) components were
signaled by distinct visual discriminative stimuli, whereas in the mixed schedule,
component changes were not present. Of course, one source of discriminative
control in the mixed schedule would be response-reinforcer dependence (i.e.,
differential consequences in the VI and VT components). During the multiple and
the mixed schedule, response rates under the VI component were higher than those
under the VT component. In order to obtain control by response-reinforcer
consequences, it was necessary for the component-durations to be relatively long.
Response rates were steadily higher in VI component relative to the VT component
in the multiple schedule compared to the difference in rates in the mixed schedule,
indicating that the discriminative stimuli played an important role in determining
behavior.
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Responding under a fixed-interval (FI) schedule necessarily is controlled by
both the discriminative stimulus and consequence. The first response to occur after
the interval elapses results in reinforcement. Any responses that occur earlier do
not affect the probability of reinforcement. Therefore, a “scalloped” pattern of
responding is typical of FI performance. Little responding occurs early in the
interval, with response rates accelerating towards the end of the interval as the
organism temporally approaches reinforcer availability (Wilkie, 1974; Zeiler, 1970).
Fixed-ratio (FR) schedules specify more about the contingencies of reinforcement
than do FI schedules. In an FR schedule, a specific number of responses must be
completed to produce a reinforcer (Zeiler, 1968). FR performance often is
characterized by a “break-and-run” pattern of responding. Generally, a period of
nonresponding is evident at the start of a ratio, followed by steady, rapid
responding until a reinforcer is achieved. The probability of reinforcement
proportionally depends on and increases with the number of responses emitted. A
comparison of stimulus control in FI and FR schedules only has been studied in the
context of a matching-to-sample task (Ferster, 1960). Ferster reported higher
accuracy when the FR schedule was in effect. There are no direct comparisons of
acquisition and maintenance of stimulus control under FI and FR schedules in a
free-operant procedure (e.g., a multiple schedule) reported in the literature.
Williams (1984) speculated that the decline of research on the development of
stimulus control in operant paradigms may have been due to a shift of interest in
studying stimulus control in respondent arrangements. Another possibility is that
in free-operant arrangements, the presence or absence of stimulus control is
DEVELOPMENT OF STIMULUS CONTROL
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revealed relatively easily by examination of cumulative-response records, so a lack
of interest in answering simple experimental questions could be to blame.
Answering “simple” questions with respect to stimulus control, could have
major implications in several important areas of behavioral research (e.g.,
behavioral momentum, behavioral economics, behavioral pharmacology). For
example, stimulus control interestingly has been shown to influence the effects of d-
amphetamine. Performance impairments that accompanied pre-session d-
amphetamine administration were minimized when stimulus control was strong
(Rees, Wood, & Laties, 1985). Similar effects were found when chlordiazepoxide
and phenobarbital were administered pre-session: Behavior under weak stimulus
control was more disrupted by drug effects (Thompson, 1975). That stimulus
control is able to modulate the effects of drugs underscores the importance of
identifying potential differential stimulus control, especially in different
components of a multiple schedule given that tolerance to drug effects also has been
shown to develop differentially depending on certain properties of each component
(e.g., Hoffman, Branch, & Sizemore, 1987).
The purpose of the present experiment was to assess the development of
stimulus control under each component of a multiple FI 5-min FR 30 schedule of
reinforcement. Behavior may be more sensitive to one set of contingencies over the
other due to the way the reinforcers are arranged. That is, behavior may come
under the control of the discriminative stimulus more quickly in a given component.
Whether responding comes under the control of discriminative stimuli differentially
is important with respect to understanding multiple schedule stimulus control.
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Furthermore, shedding light on the development of stimulus control also may help
shape future behavioral pharmacology experiments, especially those with aims to
identify behavioral determinants of acute and chronic drug effects.
Method
Subjects
Six experimentally naive adult male White Carneau pigeons (numbered 106,
113, 114, 400, 525, and 674) were obtained from Double-T Farms, Glenwood, Iowa.
The pigeons were maintained at 85% of their free-feeding body weight via post-
session feedings consisting of mixed grain and pellets. Outside of experimental
sessions, the pigeons were housed in individual home cages in a temperature- and
humidity-controlled colony room. The pigeons had free access to vitamin-enriched
water and health grit in their home cages.
Apparatus
The experiment was conducted using six pigeon operant-conditioning
chambers with interior dimensions of 35 x 30 x 35 cm. Three circular keys were
horizontally aligned on the front panel of the chamber. The keys measured 2cm in
diameter, were posited 5.5 cm apart, and were 23 cm from the chamber floor. Only
the center key was used in the present experiment. A minimum force of 0.098N was
required to activate the key. The activation of the center key produced a 30-ms
tone. A houselight was located 7 cm above the center key and 2 cm from the
chamber ceiling. An aperture measuring 5 cm by 5 cm was located 10 cm directly
below the center key and 11 cm from the chamber floor. When the hopper was
raised, the pigeon could access buckwheat, milo and hempseed through the
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aperture, which was illuminated during reinforcer presentations. All other lights in
the chamber darkened during reinforced presentations. A white noise at
approximately 95 dB was used in order to mask extraneous sounds in the rooms
containing the operant chambers. In an adjacent room, computers running EC-
BASIC software (Palya & Walter, 1993) controlled experimental events and
recorded data, and cumulative-response recorders provided live-time data
collection.
Procedure
Shaping. After the pigeons consistently approached and ate from the raised,
illuminated hopper, successive approximations to the target response were
reinforced in order to establish responding on the center key (illuminated white).
Once the pigeon reliably pecked the key, the following session consisted of an FR 1
schedule. The center key was illuminated white, responses to the center key
produced 3-s access to food, and the session lasted until the pigeon earned 60
reinforcers.
Multiple-schedule Procedure. The start of each session was signaled by a
5-min blackout. The FR component, signaled by a white key, always preceded the FI
component, signaled by a red key. A sequence of both components comprised a
block, and the session ended after 20 blocks. Completion of the FR requirement or
the first response after the interval for the FI elapsed produced 3-s access to food. If
the pigeon did not complete the FR requirement in 60 s or did not respond within
60 s after the FI elapsed, the component ended and no reinforcer was provided. A
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30-s blackout occurred between the FR and FI component, whether or not the
pigeon received a reinforcer.
The FR was gradually increased across session until the target FR of 30
responses was reached. Simultaneously, the FI was incremented by 1 min across
sessions until responding under the target FI of 5 minutes was achieved. Each ratio
and interval increase occurred only if the cumulative-response records and overall
response-rate data showed consistent responding within each session and
responding in all components appeared stable. The ratio schedule (FR 2, FR 5, FR 8,
FR 12, FR 20, FR 25, and FR 30) was increased each session for all pigeons, except
for Pigeon 400 for which FR 25 was excluded from the series. The interval schedule
(FI 1 min, FI 2 min, FI 3 min, FI 4 min, and FI 5 min) increased each session for all
pigeons. Thus, 6 sessions of training were conducted (5 sessions for Pigeon 400)
before the baseline schedule (a multiple FR 30 FI 5-min schedule of reinforcement)
was in effect. In this experiment, a total of 40 baseline sessions were conducted.
Results
Figures 1 through 6 show rate and latency data for individual pigeons. The
circles represent FR responding, and the triangles represent FI responding. The top
panels show two rate analyses. Overall rate was calculated by determining the
number of pecks emitted in the presence of the white (FR) or red (FI) key for the
whole session divided by the time the key was illuminated either white or red.
Average rate was calculated by determining the response rate in each of the 20 FR
component presentations and averaging them. Comparing overall rate and average
rate allowed us to analyze whether the session rate was representative of rates in
DEVELOPMENT OF STIMULUS CONTROL
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each component. Similarly, the bottom two panels display two latency analyses.
Total latency was calculated as the time before the first response was upon each
component presentation. Average latency refers to the average of 20 component
presentations. Phase-change lines separate training and baseline data.
Figure 1 shows Pigeon 106’s data. Stimulus control began to emerge as early
as the second and third sessions, as FR and FI responding diverged dramatically
even during training. That is, when the FR and FI values were increased, FR rates
increased, and FI latency increased. During baseline, overall and average rates
under FR remained much higher than those under FI. Towards the conclusion of
baseline, FR rates still appeared to be increasing. FR latency remained much lower
than FI latencies. Rates and latencies on session 25 were atypical relative to other
sessions. With the exception of session 25, FR rates were always greater than FI
rates and FR latencies were always shorter, with FR performance appearing
somewhat more variable than FI performance. Whole-session performance
represented average rates and latencies well.
Figure 2 shows rate and latency data for Pigeon 113. Stimulus control was
observed within the second and third sessions, with FR and FI responding diverging
in training. FR rates increased, and FI latencies increased. During baseline, overall
and average rates under FR were much higher than those under FI. Rates under FR
continued to increase even after baseline was established. FI latencies were much
higher than FR latency and continued to increase across sessions. FR rates were
always greater than FI rates and FI latencies were always greater than FR latencies,
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with FR performance being slightly more variable than FI performance. Whole-
session performance was representative of average rates and latencies.
Figure 3 shows data for Pigeon 114. Stimulus control was observed in
training. After training, in the first few sessions of baseline, FI rates and latencies
decreased to FR levels. The first few sessions of baseline were atypical in
comparison to other sessions, with FI rates and latencies dramatically increasing
shortly after and continued to increase across sessions. FR rates increased as FI
latencies increased. During baseline, overall and average rates under FR were much
higher than rates under FI. After baseline was established, FR rates continued to
increase across sessions. FR latencies were much lower than FI latencies. FR rates
were always greater than FI rates and, with the exception of the first few baseline
sessions, latencies under FI were always greater than latencies under FR. Whole-
session performance was representative of average rates and latencies.
Figure 4 shows data for Pigeon 400. Stimulus control was apparent from the
very first session, with notable difference observed between FR and FI responding
in training. However, responding began to decrease in the first few sessions of
baseline. The food-hopper time then was increased from 3 seconds to 5 (denoted by
the second phase-change line), given that 3 second reinforcement was not sufficient
to maintain responding. A dramatic recovery of performance was evident in the
very first session in which the hopper time was 5 s. The hopper time was kept at 5
seconds for the duration of the experiment. FR rates increased, and FI latencies
increased. During baseline, overall and average rates were higher than rates under
FI. FR latencies remained much lower than FI latencies. FR rates were always
DEVELOPMENT OF STIMULUS CONTROL
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greater than FI rates and FI latencies always greater than FR latencies, with FR
responding being somewhat more variable than FI responding. Whole-session
performance was representative of average rates and latencies.
Figure 5 shows data for Pigeon 525. Stimulus controlled began to appear
within the second and third session, with FR and FI responding drastically
diverging. FR rates increased, and FI latency increased. During baseline, overall and
average rates under FR remained much higher than those under FI. Rates under FR
drastically increased after training well into the conclusion of baseline, with
successively higher rates within each additional session. FR latency remained much
lower than FI latencies, with latencies steadily decreasing across sessions. FR rates
were always greater than FI rates and FR latencies were always shorter, with FR
performance appearing somewhat more variable than FI performance. Whole-
session performance represented average rates and latencies well.
Figure 6 shows data for Pigeon 674. Stimulus control was observed as early
as the second and third sessions, as FR and FI responding deviated greatly even
during training. FR rates increased, and FI latency increased. During baseline,
overall and average rates under FR were considerably higher than those under FI.
Towards the end of baseline, FR rates still appeared to be increasing and continued
to increase across sessions. FI rates remained unchanged throughout all sessions.
FR rates and latencies on the second to last session were atypical relative to other
sessions. With the exception of the second to last session, FR rates were always
greater than FI rates and FR latencies were always shorter, with FR performance
DEVELOPMENT OF STIMULUS CONTROL
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appearing somewhat more variable than FI performance. Whole-session
performance represented average rates and latencies well.
Figures 7 through 12 show rates and latencies for sequential component
presentations across the first five and final five sessions for all pigeons. That is, the
data points represent reinforcer-by-reinforcer responding. Depicting the data in
such a way allows for not only a detailed comparison from which the development
of stimulus control (from the beginning of the experiment to its completion) can be
examined, but also reveals whether performance in each component was stable and
representative of whole-session rates and latencies.
Figure 7 shows Pigeon 106’s data. Stimulus control was evident in that
differential responding in FR and FI components occurred in the first session. The
degree of stimulus control seemed to strengthen across sessions, however. FR rates
initially were high, and they continued to increase such that response rates in the
last five sessions were higher than those in the first five sessions. FI rates initially
were low and consistent in the first five sessions. By the last five sessions,
responding was much more variable, with lower responding in the final component
than responding in the very first component. FR latencies were continuously low
across all sessions. Latencies under FI were widely variable in the first five sessions.
By the last five sessions, however, latencies were high and consistent.
Figure 8 shows Pigeon 113’s data. Stimulus control was observed
through differential responding in FR and FI in the first session. However, stimulus
control appeared to strengthen across sessions. FR rates initially were high in the
first five sessions. High rates of responding were maintained well into the very last
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session of the experiment.. FI rates initially were widely variable in the first five
sessions. By the last five sessions, responding became much more variable, with
responding drastically fluctuating lower and higher than was observed in the first
five sessions. FR latencies were considerably low across all sessions, yet, became
slightly more variable in the last five sessions. Latencies under FI were widely
variable in the first five sessions. By the last five sessions, however, latencies were
much higher and consistent.
Figure 9 shows data for pigeon 114. Stimulus control emerged as differential
responding was observed between FR and FI components during the first session.
Yet, stimulus control was shown to strengthen across sessions. FI rates initially
were widely variable in the first five sessions. By the last five sessions, responding
was much more consistent, with lower responding in the final component than
responding in the very first component. FR rates initially were high and such
responding was maintained well into the final 5 sessions, which contained modestly
higher rates in than was observed in the first five sessions. FR latencies were
continuously low across all sessions. Latencies under FI were widely variable in the
first five sessions with disconnected high and low responding. By the last five
sessions, however, latencies were consistently high and with much less variance.
Figure 10 shows Pigeon 400’s data. Stimulus control was observed within
the first session through differential responding between FR and FI components. In
addition, strengthening of stimulus control was also observed across sessions. FR
rates initially were high, and they continued to increase such that response rates in
the last five sessions were higher than those in the first five sessions. FI rates
DEVELOPMENT OF STIMULUS CONTROL
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initially were low and consistent in the first five sessions. By the last five sessions,
responding was much more variable, with higher responding in the final component
than responding in the very first component. FR latencies were continuously low
across all sessions, with a slight decreasing pattern observed by the last five
sessions. Latencies under FI were widely variable in the first five sessions. By the
last five sessions, however, latencies were high and consistent.
Figure 11 shows Pigeon 525’s data. Stimulus control was evident in that
differential responding in FR and FI components occurred in the first session. The
degree of stimulus control seemed to strengthen across sessions, however. FR rates
initially were low, yet, they continued to increase such that response rates in the last
five sessions drastically higher than those in the first five sessions. FI rates initially
were low and consistent in the first five sessions. By the last five sessions,
responding was much more variable, with lower responding in the final component
than responding in the very first component. FR latencies initially were low, and
they continued to decrease such as latencies in the last five sessions were lower
than those in the first five sessions. Latencies under FI were widely variable in the
first five sessions. By the last five sessions, however, latencies were high and
consistent.
Figure 12 shows Pigeon 674’s data. Stimulus control became notable within
the first session though observed differential responding under FR and FI
components. However, stimulus control appeared to strengthen across sessions. FR
rates initially were high, and they continued to increase such that response rates in
the last five sessions were higher than those in the first five sessions. FI rates
DEVELOPMENT OF STIMULUS CONTROL
20
initially were low and slightly variable in the first five sessions. By the last five
sessions, responding was much more variable, with higher responding in the final
component than responding in the very first component. FR latencies were
continuously low across all sessions, with a slight decreasing trend observed by the
final five sessions. Latencies under FI were widely variable in the first five sessions.
By the last five sessions, however, latencies were high and consistent.
Figure 13 shows representative cumulative-response records from an
individual session for Pigeon 114 (top panel) and Pigeon 106 (bottom panel).
Vertical ticks represent individual responses, horizontal lines represent periods of
nonresponding, and diagonal ticks represent reinforcer delivers. Pigeon 114 had the
fastest rates and shortest latencies under FI, whereas Pigeon 106 had the slowest
rates and longest latencies under FI, with the pattern of responding resembling
typical FI performance, or a “scallop” pattern. For both pigeons, brief pausing
followed by rapid responding was evident in FR components. This figure shows how
FI performance can resemble behavioral patterns typical of an FR component for
individual subjects, although FI rates were always lower than FR rates.
Discussion
The purpose of the present experiment was to assess the development of
stimulus control under each component of a multiple FR 30 FI 5-min schedule of
reinforcement. Behavior may be more sensitive to one set of contingencies over the
other simply due to the manner in which reinforcers are arranged. That means that
behavior may come under the control of a discriminative stimulus more rapidly in a
one component than another. Whether responding comes under the control of
DEVELOPMENT OF STIMULUS CONTROL
21
discriminative stimuli differentially is important with respect to understanding
multiple-schedule stimulus control.
The results of this experiment showed stimulus control to take effect as early
the second to third sessions of training, through differential responding under FR
and FI components. However, differential responding increased across sessions
with FR rates and FI latencies surpassing initial responding by the end of the
experiment. Thus, stimulus control appeared to strengthen across sessions under
both FR and FI components. Furthermore, cumulative records showed that FI rates
might resemble behavioral patterns typically seen under FR components, for certain
subjects. As Fester (1960) described, FR performance often is characterized by a
“break-and-run” pattern of responding. Generally, a period of nonresponding is
evident at the start of a ratio, followed by steady, rapid responding until a reinforcer
is achieved. It appears, FI rates are able to mimic such steady responding until a
reinforcer is achieved in certain subjects. However, it is important to note that even
in the pigeons whose FI rates resembled FR, FR rates always surpassed FI rates.
These results were equally represented through whole-session performance as well
as average calculations of rates and latencies. That is, average session performance
reflected performance in each individual component.
The results of this experiment indicated that behavior came under control
rapidly under each component, with differential responding as early as the second
session of training. That is, under two varying stimuli, the subjects responded to the
signal of reinforcement contingencies and the likelihood of responding was altered
given the present contingencies almost immediately. FR rates increased at the same
DEVELOPMENT OF STIMULUS CONTROL
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pace as FI latencies increased. At the same time, FI rates and FR latencies remained
relatively stable, thus indicating that stimulus control not only emerged but also
strengthened at comparable rates under FR and FI components in a multiple
schedule. This was particularly notable in the high FR rates and FI latencies during
the final five sessions of the experiments, in comparison to the initial five sessions.
While latencies are in fact, an absence of responding, they are still indicative
of stimulus control. Latencies continued to increase across sessions under the FI-5-
min component because reinforcers were never available towards the beginning of
the interval. Morse and Skinner (1958) described that behavior comes under the
control of a stimulus when reinforcement is associated with its presence.
Interestingly, even though FR provided more frequent reinforcement than FI, FI
reinforcement rate not only was sufficient to allow behavior to come under stimulus
control but it was strong enough to allow stimulus control develop comparably to
FR stimulus control.
The results from this experiment suggest that performance averaged across
sessions is representative of performance within each individual component.
Knowing the average performance compares to performance within each
component provides greater accuracy of a subject’s behavior. For instance, average
performance may become misleading due to outlier performance under individual
components. Thus, knowing responding within individual components is well
represented by average calculation across sessions further solidifies subject data.
Overall, results from this experiment include that stimulus control not only
emerges but strengthens equally once the relationship between stimulus and
DEVELOPMENT OF STIMULUS CONTROL
23
reinforce is established, regardless of reinforce arrangement. These findings may be
applicable to researchers studying stimulus control under multiple schedules. For
instance, Powell (1973) claimed that the mixed data observed in his experiment
were caused by the broad definition of a “strong” and “weak” stimulus during
baseline. This experiment shows adequate acquisition and strengthening of stimulus
control under both FR and FI components, perhaps allowing further research to
study the strength and weakness of a stimulus based on the relationship established
between stimulus and reinforcer rather than the frequency of reinforcement.
Furthermore, bringing insight on the development of stimulus control also may help
shape future behavioral pharmacology experiments, especially those with aims to
identify behavioral determinants of acute and chronic drug effects. Additionally,
previous behavioral pharmacological studies have shown that average responding
does not appropriately represent performance in each individual component. The
fact that average and overall performances were represented in this experiment
brings to light the effect of pharmacological agents in average and overall
performance.
Basic schedule research, such as the present study, contributes by bringing
knowledge towards the control the environment exhibits on behavior.
Understanding how this power, deemed stimulus control, functions may bring
further knowledge towards environmental influences on behavior and its
development. While the mystery that is stimulus control is far from being solved, a
clearer picture of its emergence and development is observed with each passing
experiment. Behavioral schedule experiments still hold the capacity to bring insight
DEVELOPMENT OF STIMULUS CONTROL
24
towards behavioral mechanism and the intricate relationship it hold with
environmental influences through the observed effects of stimulus control under the
varying schedules of reinforcement, one can argue, we are all under.
DEVELOPMENT OF STIMULUS CONTROL
25
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DEVELOPMENT OF STIMULUS CONTROL
28
Figure Captions
Fig. 1. Overall, average, and total rates and latencies for Pigeon 106 across sessions. Circles
represented an FR component, triangles FI.
Fig. 2. Overall, average, and total rates and latencies for Pigeon 113 across sessions. Circles
represented an FR component, triangles FI.
Fig. 3. Overall, average, and total rates and latencies for Pigeon 114 across sessions. Circles
represented an FR component, triangles FI.
Fig. 4. Overall, average, and total rates and latencies for Pigeon 400 across sessions. Circles
represented an FR component, triangles FI.
Fig. 5. Overall, average, and total rates and latencies for Pigeon 525 across sessions. Circles
represented an FR component, triangles FI.
Fig. 6. Overall, average, and total rates and latencies for Pigeon 674 across sessions. Circles
represented an FR component, triangles FI.
Fig. 7. Rates and latencies for sequential component presentations across the first five and
final five sessions for Pigeon 106. Data points represent reinforcer-by-reinforcer
responding.
Fig. 8. Rates and latencies for sequential component presentations across the first five and
final five sessions for Pigeon 113. Data points represent reinforcer-by-reinforcer
responding.
Fig. 9. Rates and latencies for sequential component presentations across the first five and
final five sessions for Pigeon 114. Data points represent reinforcer-by-reinforcer
responding.
DEVELOPMENT OF STIMULUS CONTROL
29
Fig. 10. Rates and latencies for sequential component presentations across the first five and
final five sessions for Pigeon 400. Data points represent reinforcer-by-reinforcer
responding.
Fig. 11. Rates and latencies for sequential component presentations across the first five and
final five sessions for Pigeon 525. Data points represent reinforcer-by-reinforcer
responding.
Fig. 12. Rates and latencies for sequential component presentations across the first five and
final five sessions for Pigeon 674. Data points represent reinforcer-by-reinforcer
responding.
Fig. 13. Representative cumulative-response records for Pigeon 114 and Pigeon 106.
Vertical ticks represent pecks, horizontal lines represent periods of nonresponding,
and diagonal ticks represent delivery of reinforcement.
DEVELOPMENT OF STIMULUS CONTROL
30
Figure 1
Overall Rate
0 10 20 30 40
0
1
2
3
FI
FR
Average Latency
0 10 20 30 40
1
10
100
1000
106
Average Rate
0 10 20 30 40
0
1
2
3
Total Latency
0 10 20 30 40
10
100
1000
10000
Resp
on
ses p
er
seco
nd
Seco
nd
s
Sessions
DEVELOPMENT OF STIMULUS CONTROL
31
Figure 2
Overall Rate
0 10 20 30 40
0
1
2
3
FI
FR
Average Latency
0 10 20 30 40
1
10
100
1000
113
Total Latency
0 10 20 30 40
10
100
1000
10000
FR
FI
Average Rate
0 10 20 30 40
0
1
2
3
4
Seco
nd
sR
esp
on
ses p
er
seco
nd
DEVELOPMENT OF STIMULUS CONTROL
32
Figure 3
Overall Rate
0 10 20 30 40
0
1
2
3
Average Latency
0 10 20 30 40
1
10
100
1000
114
Total Latency
0 10 20 30 40
10
100
1000
10000
FR
FI
Average Rate
0 10 20 30 40
0
1
2
3
Resp
on
ses p
er
seco
nd
Seco
nd
s
Sessions
DEVELOPMENT OF STIMULUS CONTROL
33
Figure 4
Overall Rate
0 10 20 30 40
0
1
2
3
4
Average Latency
0 10 20 30 40
1
10
100
1000
400
Total Latency
0 10 20 30 40
10
100
1000
10000
Average Rate
0 10 20 30 40
0
1
2
3
4
FR
FI
Seco
nd
sR
esp
on
ses p
er
seco
nd
Sessions
DEVELOPMENT OF STIMULUS CONTROL
34
Figure 5
Overall Rate
0 10 20 30 40
0
1
2
3
Average Latency
0 10 20 30 40
1
10
100
1000
525
Total Latency
0 10 20 30 40
10
100
1000
10000
FR
FI
Average Rate
0 10 20 30 40
0
1
2
3
Seco
nd
sR
esp
on
ses p
er
seco
nd
Sessions
DEVELOPMENT OF STIMULUS CONTROL
35
Figure 6
Overall Rate
0 10 20 30 40
0
1
2
3
Average Latency
0 10 20 30 40
1
10
100
1000
674
Total Latency
0 10 20 30 40
10
100
1000
10000
FR
FI
Average Rate
0 10 20 30 40
0
1
2
3
Seco
nd
sR
esp
on
ses p
er
seco
nd
DEVELOPMENT OF STIMULUS CONTROL
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Figure 7
0 50 100 150 2000
1
2
3
4
0 50 100 150 2000.0
0.5
1.0
0 50 100 150 2000.1
1
10
100
0 50 100 150 2001
10
100
1000
Fixed Ratio Fixed Interval
Pecks p
er
sL
ate
ncy (
s)
Successive Componets
First 5 Last 5 First 5 Last 5
Pigeon 106
DEVELOPMENT OF STIMULUS CONTROL
37
Figure 8
DEVELOPMENT OF STIMULUS CONTROL
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Figure 9
DEVELOPMENT OF STIMULUS CONTROL
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Figure 10
DEVELOPMENT OF STIMULUS CONTROL
40
Figure 11
DEVELOPMENT OF STIMULUS CONTROL
41
Figure 12
DEVELOPMENT OF STIMULUS CONTROL
42
Figure 13