LIBRARY

Michigan $tatc
Universrty

 

This is to certify that the

thesis entitled
Conditioned Inhibition In An Operant Discrimination
As A Function 0f Component Duration

presented by
Robert Benjamin Young

has been accepted towards fulfillment
of the requirements for

Master '3 _degree in Psychology

I
Major professor

 

Date 8/19/1977

 

CONDITIONED INHIBITION IN AN OPERANT DISCRIMINATION AS

A FUNCTION OF COMPONENT DURATION

BY

ROBERT BENJAMIN YOUNG

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

MASTER OF ARTS

1977

ABSTRACT
Conditioned Inhibition in an Operant Discrimination as
A Function of Component Duration
by Robert Young

The effects of component duration on acquisition of a dis-
crimination and inhibition were assessed using a between subjects
design. Following five days of single stimulus training, four
groups of pigeons were given discrimination training between a
green key (3+) and a vertical line (8-). The durations of the
3+ and 8- components were either 15 or 120 seconds. These dura-
tions were combined factorially to produce the A groups of sub-
jects. Each subject received 30 alternations between 8+ and S-
for 25 days of discrimination training. After discrimination
training was completed, all subjects were given five days of single
stimulus training on a new 8+, a red key, and inhibition assoc-
iated with S- was measured by superimposing the S- on the red key
and the original S+.

The results showed that the groups with shorter S+ durations
and longer 5- durations produced higher response rates to 8+ at
the beginning of discrimination training. With extended training,
however, these differences disappeared. Superimposing the S- on
the S+ produced more suppression of responding in the groups trained
with the short S+ than in the groups trained with the long S+.
These differences, however, did not reach significance. No effect

of 8- duration could be found during inhibition testing.

ii

ACKNOWLEDGEMENTS

I would like to acknowledge the guidance and assistance
given me by my advisor, Dr. Mark Billing. Both his conceptual
advice and his editorial comments have been invaluable in the
production of this thesis.

I would also like to acknowledge the contributions of my
committee members, Drs. M. Ray Denny and James Zacks, and of
Mr. Steven Gitterman who freely contributed his time to help

produce the final copy of the paper.

iii

TABLE OF CONTENTS
page
LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . iv
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . 1
METHOD . . . . . . . . . . . . . . . . . . . . . . . . . 9
Subjects . . . . . . . . . . . . . . . . . . . . . . 9
Apparatus . . . . . . . . . . . . . . . . . . . . . 9
Procedure . . . . . . . . . . . . . . . . . . . . . 10
RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . 12
DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . 21

LIST OF REFERENCES . . . . . . . . . . . . . . . . . . . 27

iv

LIST OF FIGURES

Figure

Group mean response rates produced in the presence of
8+ during discrimination training and the last day of

single stimulus training. . . .

Group mean repsponse rates to 8+ during discrimination
training presented for the first, second, and third

hour of exposure to 8+. . . .

Individual response rates emitted in the presence of
the combined cue during the inhibition test which used

the red key as an 8+. . . .

Individual response rates emitted in the presence of
the combined cue during the inhibition test which used

the green key as an 5+. . . .

Group mean response rates produced in the presence of
the S+ stimulus when it was presented alone during the

two inhibition tests. . . .

Group mean suppression ratios obtained during the two

inhibition tests. . . .

page

13

15

17

19

2O

22

INTRODUCTION

A discrimination is formed when responses in the presence of
one stimulus (8+) are reinforced and responses in the presence of
another stimulus (8—) are extinguished. For dissimilar stimuli,
acquistion is complete when responding continues to occur in the
presence of the reinforced stimulus and no longer occurs in the
presence of the extinguished stimulus. Pavlov (1927) argued that
the reduction in responding to the extinguished stimulus occurred
through an active process he called conditioned or differential
inhibition. As evidence that the decrement in responding involved
an active process, he reported that when the positive and negative
stimuli were presented simultaneously in a compound, responding
to the postive stimulus was reduced or inhibited. He also found
that responding during the extinguished stimulus was increased or
disinhibited by the presentation of a novel stimulus.

In their analyses of instrumental discrimination learning,
Spence (1936, 1937) and Bull (19N3) also used the concept of
inhibition as a process which opposes excitation. However, Skinner
(1938) explained discrimination learning without appealing to a
concept of inhibition. He suggested that extinction is simply
a reduction in the probability of a response, not the growth of a
process which oppos.» the emission of a response. Discrimination
learning was proposed to be a modified form of extinction in which

responding is maintained in the presence of one stimulus through

reinforcement while it is extinguished in the presence of another
stimulus. According to Skinner, the stimulus associated with
extinction merely loses excitatory strength gained through general-
ization and does not become inhibitory. The response suppression
reported by Pavlov was explained by Skinner as an emotional by—
product of extinction which disrupted rather than suppressed
responding.

During the 1950's the operant approach to animal learning
blocked research into inhibitory phenomena. However, Guttman
and Kalish (1956) obtained generalization gradients from individual
subjects using Operant methodology. These gradients showed that
after intradimensional discrimination training between two stimuli
sufficiently close together on the same dimension, the point of
maximum responding in the gradient was no longer the stimulus
associated with reinforcement (8+), but a stimulus further removed
on the dimension from the stimulus associated with extinction (8-).
This phenomenon is called "peak shift." Peak shifts were predicted
by Spence (1937) in his theoretical paper on transposition. Spence's
theory uses an interaction of generalized excitation and inhibi-
tion to predict the shape of post-discrimination gradients.

Jenkins and Harrison (1962), Schwartzbaum and Kellicut (1962)
and Honig, Boneau, Burstein, and Pennypacker (1963) reported finding
U-shaped inhibitory gradients around an 3- after giving inter—
dimensional discrimination training between two stimuli on ortho-
gonal dimensions. These researchers argued that excitatory strength
generalized equally well to all stimuli on the S- dimension, while

inhibition generalized more strongly to those stimuli close to the

S— on the dimension. Thus, the most response strength was observed
at the stimuli on the dimension most removed from the S-. The
gradients seemed to confirm Spence's theory of inhibition, and
researchers began to use the peak shift and inhibitory gradients

as evidence for inhibition.

A "gradient test" of inhibition, however, involves a different
set of assumptions about inhibition than those proposed by Pavlov
(1927). A U-shaped inhibitory gradient only reflects a generalized
reduction in responding which decreases in strength at greater
distances from the original 3-. Since Skinner (1938) argued that
extinction as well as reinforcement could generalize, these gradients
do not discriminate between "passive" extinctive processes and
"active", suppressive, inhibitory processes. Brown and Jenkins
(1967) recognized this problem, and suggested that different tech-
niques could produce a more convincing demonstration of inhibition
than generalization gradients. In a procedure similar to Pavlov's,
Brown and Jenkins trained an operant discrimination in which the
S- was a combination of the 8+ and a second stimulus. They demon-
strated that the superimposition of this second stimulus on a
different S+ suppressed responding to that stimulus. They argued
that the suppression of responding to the new 8+ was evidence that
the S- stimulus controlled a tendency not to respond rather than
a mere reduction in excitation.

Hearst, Besley, and Farthing (1970) also expressed dissatis-
faction with gradient tests of inhibition, and argued that they do
not reflect the defining property of inhibition, a suppression of

excitation. Hearst et al. suggested that the minimum of an inhibi-

tory gradient could be neutral while stimuli further removed on
the dimension are progressively more excitatory. They proposed
that one or more tests be used to identify an inhibitory stimulus.
The first of these tests involved a procedure similar to that used
by Brown and Jenkins (1967). The S- is superimposed on the 8+,
and the amount of suppression produced relative to a neutral stimulus
is measured. The second test was a resistance to reinforcement
measure in which a CS- is paired with the reinforcer and the number
of trials required to condition a new response to the former CS-
is measured.

The problems inherent in using inhibitory gradients as a
measure of inhibition were highlighted in a study by Davis (1971).
Davis trained a discrimination between a green key that served as
8+ and a vertical line 3-, and then obtained generalization gradients
by superimposing different line angles on the S+. The response
rates of all subjects showed suppressed responding to the S+. How—
ever, only one animal produced a U-shaped inhibitory gradient around
8—. This study suggests that the presence of inhibition may not
always be reflected by generalization gradients.

The measures proposed by Hearst et a1. (1970) have an additional
advantage in that comparisions are possible between the amount
of suppression produced by an S- or the number of trials required
for conditioning. Although these measures have successfully been
used by researchers in Pavlovian conditioning to compare the amount
of inhibition produced by different conditioning procedures (0. f.
Wagner and Rescorla, 1972), they have more frequently been used

in operant conditioning literature as an adjunct to generalization

gradients. They have been used to show that the gradients actually
reflect an inhibitory stimulus rather than as a measure of the amount
of inhibition produced by differenct procedures. Consequently,
little is known about the determinants of inhibition in operant
discrimination learning.

The present study measured the suppression of conditioned
responding by an S- as a function of the duration of the stim-
ulus components used in discrimination training. The duration
of the 3+ and the S- was either 15 or 120 seconds. Thus four
groups were produced by this procedure: One with a 15 second
5+ and a 15 second 8-, one with a 120 second 8+ and 120 second
8- one with a 15 second 3+ and 120 second 8-, and one with a 120
second 8+ and a 15 second 3-. The number of alternations between
3+ and S- was held constant, so that the amount of experience with
reinforcement and extinction depended upon the duration of the
component.

Several investigators have suggested that increased experience
with inhibitory stimuli reduces inhibition. Beiderman (1968)
trained birds on two concurrent discriminations. In these dis-
criminations the 8+ and S- were present at the same time. Pecks
to the S+ key produced food; whereas, pecks to the 3- key produced
a time out. One of the 8+, 8- pairs occurred twice as frequently
as the other, and the two types of trials alternated randomly. On
test trials the birds were given a choice between the two S+ stimuli
or the two 8- stimuli. The subjects chose most frequently the S-
with which they had had the most experience during discrimination

training, and the S+ with which they had had the least experience.

Beiderman suggested that increased experience with an 8- may reduce
its inhibitory strength, and related the phenomenon shown in his
study to the overlearning reversal effect.

Terrace (1966) presented data showing that with increased
discrimination training inhibitory gradients became flatter and
behavioral contrast decreased. Terrace argued that the inhibitory
properties of the 8- had been reduced by the extended training
period. Hearst (1969), however, obtained gradients after longer
periods of training than used by Terrace, but found no tendency
for the gradients to flatten or for behavioral contrast to
decrease.

Terrace (1966), Hearst (1969), and Biederman (1968) manip-
ulated exposure to discriminative stimuli by increasing the num-
ber of stimulus presentations rather than by manipulating the
duration of each presentation. Only two investigators have man-
ipulated the duration of extinction predicted by an S-, and then
measured inhibition. Both of these investigators used a class-
ical conditioning paradigm.

Heisman and Litner (1971) presented a CS preceeding a shock-
free interval superimposed on a free operant avoidance schedule.
The shock-free interval was either one minute or five minutes
long. Although shocks occasionally occurred during the CS,
animals inhibited avoidance responding during the CS which
predicted the five minute shock-free interval, but not during
the CS which predicted the shorter extinction period. Pre-
sumably, the CS preceeding the longer interval had produced more

inhibition, and thus a greater suppression of the avoidance re-

sponse.

Hearst and Franklin (1977) used a similar design in which
free food was presented to pigeons, but a CS predicted a period
of no food delivery. Hearst and Franklin used withdrawl from
the key area when the CS was presented as a measure of inhibition.
They found no effect of the timeout period predicted by the CS
on either the amount of withdrawl from the key, or on the shape
of generalization gradients around the S-. The gradients did
differ, however, in that the gradient around the 8- associated
with the longest 3- period also produced the greatest number of
keypecking responses. Hearst and Franklin argued that the duration
of the food-free interval associated with a CS is not a powerful
determinant of inhibition.

Kamil and Davenport (1966) investigated the effect of component
duration on the acquisition of a discrimination. These investigators
trained discriminations in which the 8+ and S- alternated at dif-
ferent rates. They found that increasing the rate of alternation
to a rate over six times per minute precluded the formation of the
discrimination in rats. It is unclear, however, whether these
results were due to a lack of inhibition, or whether the subjects
were simply unable to discriminate the contingency changes when
the rate of alternation greatly exceeded the rate of reinforcement.

Several investigators have investigated the effect of com-
ponent duration on a second product of discrimination learning,
behavioral contrast. Behavioral contrast occurs when a decrease
in the incentive conditions of one component results in an increase

in response rate in the other component. Studies which have man-

ipulated component duration report that the amount of behavioral
contrast produced by a change in incentive is at least partially
determined by the duration of the components of the discrimination.
Taus and Hearst (1971) and Hilton and Clemens (1971) have shown
that increasing the duration of an 8- while the S+ duration
is held constant also increases the amount of behavioral contrast
produced. Two of the control groups run by Kodera and Rilling
(1975) made an analogous comparison in which the duration of the S-
was held constant at 60 seconds while the duration of the 8+ was
either three minutes or 60 seconds. The group with the longer S+
showed no behavioral contrast, while the group with the 60 second
S+ produced a large increase in rate to the unchanged component.
The design of the present study permitted the effect of both 8+
and 8- duration on the acquisition of the discrimination to be
observed. A replication of the results of Hilton and Clemens,
Taus and Hearst, and Kodera and Billing was expected. The groups
with the longer 8- durations and shorter S+ durations should show
higher response rates in the presence of 3+ during the acquisition
of the discrimilation.

The effect of component duration on inhibition is less easy
to predict. Since component duration has not been systematically
studied, it is unclear whether it affects behavior as an incentive
variable, as an amount of training variable, or in an unique,
independent way. Heisman and Litner (1971) and Hearst and Franklin
(1977) clearly assumed that the duration of a UCS free interval
predicted by a CS should be an incentive variable. This assumption

seemed to have been confirmed in the Heisman and Litner study

using an aversive reinforcer, but not in the analogous Hearst
and Franklin study which used an appetetive UCS.

The effect S+ duration should have on inhibition is even
less clear. If the duration of 8+ is an incentive variable, and
increased exposure and thus increased reinforcement in the pres-
cense of 8+ increases incentive, longer S+ durations should increase
the amount of inhibition produced by an 3-. Billing (1977) sug-
gested that increases in S+ incentive relativeto 8- increases
the probability of peak shift. Beiderman (1968), however, showed
that animals preferred S+ stimuli with which they had had less
experience. The present study was designed to eliminate some of

these questions.

METHOD

Subjects

Twenty four experimentally naive white Carneaux pigeons
were used. The subjects were individually housed in a constantly
illuminated colony room with free access to water and grit.
Apparatus

All birds were trained in a two key Lehigh Valley
Electronics operant conditioning chamber (LVE model 1519)
which was controlled by electromechanical programming equip-
ment located in an adjacent room. The left and center keys
were covered, and only the right key was used. During the
course of the experiment three stimuli were projected on

this key through an Industrial Electronics Engineers in line

1O

projector. Two of the stimuli, a green light with a peak
wavelength of 555nm and a red light with a peak wavelength
of 606nm, were produced through Kodak Hratten gelatin
filters #99 and #728 respectively. The third stimulus
was a 1/4 inch wide white vertical line which bisected a
dark surround.

Reinforcement consisted of 2.5 seconds of access to
University Mix grain. During reinforcement presentations
the hopper was illuminated by a white light. The chamber was
illuminated throughout each session by a 2.25 H. houselight.
A ventilating fan provided partial masking of external noise.
Procedure

All birds were first autoshaped using a procedure devel-
oped by Brown and Jenkins (1968). The CS+ was a green keylight
with a peak wavelength of 555 nm. During autopshaping the inter-
stimulus interval was six seconds, and the intertrial interval
was programmed using a constant probability variable interval
30 second schedule developed by Catania and Reynolds (1968).

On the first day after the autoshaped keypeck was con-
sistently established, the 8+ was constantly presented and
reinforcement was made contingent upon pecking. The first 15
keypecks were continuously reinforced, and the next 30 rein-
forcements were programmed on a constant probability 15 second
variable interval schedule. Thereafter all reinforcements were
programmed on a constant probability VI 30 second schedule. Each
animal recieved five days of single stimulus training on the VI

30 sec. schedule for 15 minutes each day.

11

Following the fifth day of single stimulus training, dis-
crimination training was begun. During S+ trials reinforcement
continued to be available on the VI 30 second schedule. No
reinforcement was available on 3- trials. 8+ and 8- trials
alternated, and each animal received 30 3+ and 8— trials each
day. The S- stimulus during discrimination training was a white
vertical line bisecting a dark key. The animals were randomly
assigned to four groups. For group 15-15 the duration of both 3+
and S- was 15 seconds. Group 15-120 had a 15 second 8+ and a 120
second 8-. Group 120-15 had a 120 second 8+ and a 15 second 8-,
and for group 120-120 the duration of both 8+ and S- was 120 seconds.

Following the last day of discrimination training, each
animal again received four days of nondifferential training on
the VI 30 second schedule. The stimulus, however, was a red
keylight with a peak wavelength of 606 nm. Each training session
lasted for 30 minutes.

Inhibition testing started after the last day of the second
phase of nondifferential training. Inhibition testing started
with a nine minute warmup on the V1 30 second schedule with the
red key. Following the warmup each of three stimuli were presented
three times for A5 seconds each time. The three test stimuli
were the 8-, the red light, and the S- superimposed on the red light.
Following the first nine test stimulus presentations, the animals
were reexposed to single stimulus training with the red light for
an additional 4.5 minutes. This was followed by nine more test
stimulus presentations, another “.5 minutes of nondifferential

reinforcement, and a final nine test stimulus presentations.

12

The stimuli in each block of test stimulus presentations
were ordered in a latin square, with the additional restriction
that no test stimulus ever immediately preceded itself. Thus
the red light alone was never the first test stimulus in a block
of testing. The second inhibition test was exactly the same as the
first except that the green training stimulus replaced the red

stimulus.

RESULTS

Figure 1 shows mean response rates of the four experimental
groups during discrimination training and on the last day of single
stimulus training. The response rates for the four groups were
approximately equal at the end of single stimulus training, and a
one-way analysis of variance found no significant differences be-
tween the groups at this stage of training, F < 1. Immediately
after transfer to the discrimination, however, differences in res-
ponse rates for the four groups developed. In general, birds with
short S+ durations (the birds from groups 15-15 and 15-120) produced
higher response rates than the birds with long S+ durations (the
birds from groups 120-15 and 120-120). The birds transferred to
discrimination training with long 8- durations (groups 15-120 and
120-120) also responded at a higher rate than the birds which re-
ceived training with short 8- durations. A 2x2x3 factorial
analysis of these data across the first three days of discrimin-
ation training showed a significant main effect for 8+ duration,

F(1,20) = 5.65, p < .05, but no significant main effect for S-

Figure 1. Group mean response rates produced in the presence
of 3+ during discrimination training and the last day
of single stimulus training. The square symbols rep-
resent the groups which were trained with a 120 second
S+ during discrimination training, and the circles rep—
resent the groups trained with a 15 second S+. The
filled symbols represent the groups trained with a 120
second 8- during discrimination training, and the open

symbols represent the groups trained with a 15 second 3-.

13

GROUP MEAN RESPONSES PER MIN TO 8+

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114

duration, F(1,20) = 2.6“, p > .1.

After the third day of discrimination training these differ-
ences in response rate disappeared, with the response rate of group
15-120 decreasing while the response rate of group 120-15 increased.
Average response rate over the first 2” days of discrimination
training was still highest for gr 1p 15-120, followed respectively
by groups 15-15, 120-15, and 120-120, but a 2x2x3 factorial analysis
of variance performed on these data in blocks of eight days showed
no significant main effect for either the duration of 8+, F < 1, or
the duration of S-, F < 1. None of the interactions were signif-
icant, and the only significant main effect indicated that responses
increased across blocks of days, F(2,A0) = 10.40, p < .01.

On each day of discrimination training the birds with the
short 8+ (groups 15-15 and 15-120) received 7 1/2 minutes of ex-
posure to 8+, while the groups with the long 8+ received 60 minutes
of exposure to 8+. The groups with the long 8+, therefore, received
approximately eight times the amount of daily reinforcement re-
ceived by the groups trained with the short 8+. Figure 2 shows
the response rate for each group when the groups were equalized
for the amount of exposure to 8+. Thus, the first hour of expo-
sure to 8+ reflects the first day of training for groups 120-15
and 120-120, but the first eight days of training for groups 15-15
and 15-120. These data indicate higher response rates for the
birds trained with the shorter 8+. The effect of 8- depended upon
the level of 8+ duration; there was very little difference between
the response rates produced by groups 15-15 and 15-120, but group

120-120 produced a higher response rate than group 120-15. A 2x

Figure 2.

Group mean response rates to 8+ during discrimination
training presented for the first, second, and third hour
of exposure to 8+. For the groups with the 15 second 8+,
eight days of training were required to produce one hour
of exposure to 8+. The groups with the 120 second 8+
received one hour of exposure to 8+ on each day of train-
ing. The square symbols represent the groups which were
trained with a 120 second 8+, and the circles represent
the groups trained with a 15 second 8+. The filled
symbols represent the groups trained with a 120 second
8-, and the open symbols represent the groups trained

with a 15 second 8-.

GROUP MEAN RESPONSES
PER MINUTE DURING 8+

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80

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AMOUNT OF EXPOSURE TO 5+

16

3x3 factorial analysis of variance performed on these data show—
ed a significant main effect for the duration of 8+, F(1,20) =
n.6, p < .05, but no significant main effect was found for the
duration of 8-, F < 1. Despite the differential effect of 8- at
the different 8+ values shown in figure 2, the 8+ x S- interac-
tion was not significant.

Two tests for inhibition were performed after the completion
of the 25 days of discrimination training. In the first test, the
8- was superimposed on a newly trained 8+, a red key. In the
second test the 8- was superimposed on the original 8+, the green
key. Figure 3 shows the response rate produced by each bird in
each experimental group in the presence of the red key - line
angle combination. Solid arrows were also used in this figure
to indicate the average response rate for each experimental group.
The lowest average response rate to this combination was produced
in group 15-120, followed by group 15-15, group 120-120, and finally
by group 120-15. Generally, the birds trained with a 15 second 8-
showed more inhibition than those trained with a 120 second 8+, and
fewer responses were emitted by the birds trained with a 120 second
8- than those trained with a 15 second S-. The difference in the
group averages, however, seemed to have been produced by a gradually
increased range of response rates rather than by a shift in the
response rates of all the birds. Each of the experimental groups
had several birds with very low response rates to the combined
cue, but the groups differed in the number of birds which produced
high reponse rates.

Figure A shows the individual and group mean response rates

Figure 3.

Individual response rates emitted in the presence of
the combined cue during the inhibition test which used
the red key as an 8+. The circles represent the res-
ponse rates of the birds trained with a 15 second 8+
during discrimination training, while the square sym-
bols respresent the response rates of the birds trained
with a 120 second 8+. The open symbols represent the
birds trained with a 15 second 8-, and the filled sym-
bols represent the birds trained with a 120 second 8-.
The arrows represent the mean response rate produced in

the presence of the combined cue for each group.

RESPONSES PER MINUTE TO
COMBINED CUE ON RED TEST

36
32
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24
20
16

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Is-Is 15-120 120-15 120-120
EXPERIMENTAL enoup

18

in the presence of the combined cue in the second test of inhibition.
In this test the 8— was superimposed on the original 8+, a green
key. The birds trained with the short 8+ again showed fewer responses
to the combined cue than the birds trained with the long 8+. The
effect of 8-, however, was different than the effect it showed
on the first test of inhibition. The groups trained with the long
8- had higher average response rates than the groups trained with
a short 8-. A single analysis of variance was performed on the
two tests of inhibition. The analysis showed only one main effect
approaching significance, the effect of 8+ duration, F (1,20) :
3.99, .1 > p > .05. No significant effect was found for the
duration of S- or for the type of inhibition test used. Despite
the reversal of the effect of 8— between tests, the 8- x test
interaction was not significant.

During the inhibition test, response rates to either the red
key or the green key alone were measured in extinction. Figure 5
shows the group mean response rates emitted in the presence of the
8+ during testing. This figure shows that response rates to the 8+
were higher for the groups trained with the 120 second 8+ than for
the groups which had the 15 second 8+ during discrimination training.
Separate analyses of variance were performed on the 8+ response
rate data of the two tests. These analyses showed that the main
effect of 8+ approached significance in the second inhibition test
using the green key, F (1,20) = 3.03, .1 > p > .05, but not for the
inhibition test with the red key, F < 1. The effect of 8- was
not significant in either test, F < 1. Since the only main effect

approaching significance in the analysis of response rates to the

Figure A.

Individual response rates emitted in the presence of

the combined cue during the inhibition test which used
the green key as an 8+. The circles represent the
response rates of the birds trained with a 15 second

8+ during discrimination training, while the square sym-
bols represent the response rates of the birds trained
with a 120 second 8+. The open symbols represent the
birds trained with a 15 second 8-, and the filled symbols
represent the birds trained with a 120 second 8-. The
arrows represent the mean response rate produced in the

presence of the combined cue for each group.

19

 

 

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Figure 5.

Group mean response rates produced in the presence

of the 8+ stimulus when it was presented alone during
the two inhibition tests. The data on the left rep-
resents response rate in the presence of the red 8+ on
the first inhibition test. The data on the right rep-
resents response rate to the green 8+ on the second
inhibition test. The square symbols represent the
response rates of the groups trained with a 120 second
8+ during discrimination training, and the circles
represent the mean response rates of the groups trained
with a 15 second 8+. The filled symbols represent the
mean response rates of the groups trained with a 120
second 8-, and the Open symbols represent the groups

trained with a 15 second S—.

GROUP MEAN RESPONSES

'PER‘ MINUTE DURING -S+

ISO ..

150 L
I4o -
I30 I-
120 I.
“O -
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so _

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:1 120 I5
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RED TEST ‘ GREEN TEST

21

combined cue was also the duration of 8+ during training, it was
possible that the higher rates shown by the birds in the 120 second
8+ groups may have been due to an overall higher tendency to respond.
To attempt to control for this problem, suppression ratios
were calculated for these tests in which the total number of responses
for each bird during the combined cue stimulus was divided by the
number of responses emitted in the presence of the 8+ alone. The
resulting group mean suppression ratios for the two tests are shown
in figure 6. These ratios show the same relationships between
groups that were shown in figures A and 5. In the inhibition test
using the red key, the group showing the most suppression was group
15-120 followed by groups 15-15, 120-120, and 120-15 respectively,
In the inhibition test using the green key, group 15-15 showed the
most suppression of responding to the combined cue, followed by
groups 15-120, 120-120, and 120-15 respectively. An analysis
of variance performed on these data showed a main effect for 8+
which approached significance, F(1,20) = 3.03, .1 > p > .05. No
significance was found for the effect of 8-, or any of the inter-

actions.

DISCUSSION

Immediately after transfer to discrimination training, dif-
ferences were produced in response rate that were consistent with
the results of Kodera and Billing (1976) and Taus and Hearst's
(1970) investigations of behavioral contrast. These studies showed

that birds transferred to discrimination training with 15 second 8+

Figure 6.

Group mean suppression ratios obtained during the two
inhibition tests. The suppression ratio was calculated
using the formula A / A + B. In this formula A equals
the number of responses emitted in the presence of the
combined cue, and B equals the number of responses
emitted in the presence of the 8+ during the inhbition
test. The square symbols represent the mean suppres-
sion ratios of the groups trained with a 120 second 8+,
and the circles represent the suppression ratios of the
groups trained with a 15 second 8+. The open symbols
represent the suppression ratios of the groups trained
with a 15 second 8-, and the filled symbols represent the

groups trained with a 120 second 8-.

22

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m.

 

GREEN TEST

RED TEST

23
durations showed a significantly larger increase in response rate
than birds transferred to discrimination training with 120 second
8+ durations. Birds transferred to discrimination training with
120 second 8- durations showed a larger increase in responding than
birds transferred to discrimination training with 15 second 8-,
although these differences were not significant.

The birds failed to maintain these differences in response
rate, however, after the third day of training. The major deter-
minant of this phenomenon may have been the equalization of the
number of alternations between 8+ and 8- in the present experiment.
Kodera and Rilling (1976) compared response rates to a 3 minute
and a 1 minute 8+ after equal amounts of exposure to the 8+ and
found higher response rates in the presence of the 1 minute stumulus.
Thus, in the Kodera and Rilling study, the groups with the 1 minute
8+ had three times the alternations between 8+ and 8- as the three
minute groups. The present study suggests that the number of
alternations between 8+ and 8- is a more powerful influence on
response rate than the amount of reinforcement in the presence of
8+. Hhen response rates were compared in the present study after
an equal amount of exposure to 8+, the groups with longer 8+ durations
and fewer alternations also showed lower response rates to 8+.

Hhen alternations were equalized, however, these differences dis-
appeared after the third day of training, so that the graph of
daily response rates shows no consistent differences between groups.
The data seem to suggest that birds given equal numbers of alter-
nations between 8+ and 8- learn to respond to 8+ equally well,

independently of the amount of experience with 8+ produced by each

2A

alternation.

Alternations between 8+ and 8- may have had this effect by
training the birds to reinitiate responding at the end of 8- periods.
Early in training, the birds frequently showed delay in responding
to the 8+. Later in training, however, responding was immediately
elicited by the 8+. Hhen the number of alternations between 8+ and
8- are not controlled, birds may be at different points in learning
to respond with short latencies to the introduction of the 8+.

Hhen response rates are averaged, this in turn may produce lower
rates of responding in the birds trained with fewer alternations
between 8+ and 8-.

Although Taus and Hearst (1970) reported that longer 8- in-
tervals produced more behavioral contrast, no significant effect
of 8- duration on response rate was ever discovered in the present
study. Response rate differences at the very beginning and end
of training, however, were consistent with the data reported by
Taus and Hearst. The failure to find significant differences in
the present study may have been due to the use of a different
type of 8- than that used by Taus and Hearst. Taus and Hearst
used a blackout procedure in which the box was completely dark-
ened during the 8- period. This procedure may have produced lar-
ger effects on responding to 8+ than the traditional discrimi-
nation procedure.

Several studies have shown that excitation is dependent upon
rate of reinforcement in both the presence and absence of a cue.
(c. f. Rescorla, 1968), and that inhibition shows a similar function;

increases in the rate of reinforcement in the abscence of a cue,

25

or decreases in the rate of reinforcement in the presence of a cue
serve to increase inhibition (Hagner & Rescorla, 1972). It was
hypothesized that the present study would show a similar phenomenon.
An 8- associated with a long absence of the opportunity to earn
reinforcement should differ in inhibition from an 8- associated
with a short period of non-reinforcement. Secondly, this effect
should be dependent upon the duration of 8+ since this deter-

mines the overall amount of opportunity to obtain reinforcement.

The duration of the 8+ had a more powerful effect on inhibi-
tion than the duration of 8-. The groups with the longer 8+ dura-
tions produced more responding to the combined cue than the groups
with the shorter 8+ duration. The groups with the longer 8+ also
Show less suppression of responding relative to their rates to 8+.
It seems unlikely that this phenomenon was caused by differences
in the excitatory strength of 8+. The 8- was superimposed on two
different cues in the inhibitory test. Hhile the birds trained with
different 8+ durations differed in response rate to the original
8+ in the test of inhibition, they did not differ in rate to the
newly trained cue. Nevertheless, differences in response rate to
the combined cue were found which were independent of the excitatory
cue used in the test. Suppression ratios, which include response
rate to 8+ alone as a baseline against which the reduction in
responding produced by the 8- is measured, also showed differences
in the level of suppression produced by different 8+ durations,
suggesting that differences in responding to the combined cue were
due to differences in inhibition and not in the excitatory strength

of 8+.

26

No effect was found for the duration of 8- in the inhibitory
test. Hearst and Franklin (1977) have also recently published data
suggesting that this is not a powerful variable in determining
inhibition. Hearst and Franklin used a classical conditioning
paradigm in which a C8 of a fixed duration signalled UCS-free
intervals of different durations. Movement away from the CS was
measured during inhibitory conditioning. No difference was found
in the amount of withdrawl produced by the different food-free
intervals. Generalization gradients of key-pecking were then
obtained around the CS'S. The gradients differed in absolute number
of responses emitted, but not in the slope of the gradient. Hearst
and Franklin also concluded that the duration of the UCS-free period
predicted by a CS- was not a powerful determinant of inhibition.

The present study showed little effect of the durations of an
8- during a discrimination on either the number of responses emitted
to the 8- during discrimination training, or the amount of inhibi-
tion produced in later testing. The duration of 8+ had a more
powerful effect; with equal amounts of exposure to 8+, lower resp-
onse rates were emitted when this exposure was concentrated in long
duration stimuli. This difference dissipated, however, when the
number of 8+ and 8- alternations were equalized. The groups with

the longer 8+ durations also showed less inhibition around 8-.

BIBLIOGRAPHY

27
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