Title Page
 Table of Contents
 List of Tables
 List of Figures
 Appendix: Basic paradigms relevant...
 Biographical sketch

Group Title: behavior of pigeons under free-operant schedules of shock avoidance and shock frequency reduction /
Title: The behavior of pigeons under free-operant schedules of shock avoidance and shock frequency reduction /
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00097491/00001
 Material Information
Title: The behavior of pigeons under free-operant schedules of shock avoidance and shock frequency reduction /
Physical Description: xiii, 119 leaves : ill. ; 28 cm.
Language: English
Creator: Jowaisas, Dennis Brian, 1939-
Publication Date: 1977
Copyright Date: 1977
Subject: Pigeons -- Behavior   ( lcsh )
Laboratory animals   ( lcsh )
Psychology thesis Ph. D   ( lcsh )
Dissertations, Academic -- Psychology -- UF   ( lcsh )
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 104-108.
Additional Physical Form: Also available on World Wide Web
Statement of Responsibility: by Dennis Brian Jowaisas.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00097491
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000186420
oclc - 03372638
notis - AAV3010


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Table of Contents
    Title Page
        Page i
        Page ii
        Page iii
    Table of Contents
        Page iv
    List of Tables
        Page v
    List of Figures
        Page vi
        Page vii
        Page viii
        Page ix
        Page x
        Page xi
        Page xii
        Page xiii
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    Appendix: Basic paradigms relevant to the study of avoidance conditioning
        Page 109
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    Biographical sketch
        Page 119
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        Page 121
Full Text







I dedicate this work to Dick Willis

who shared with me his joys, fears,

and professional expertise.


To Dr. E. F. Malagodi, who introduced me to

the experimental analysis of behavior and exposed

me to the contingencies of research; to Dr. Donald

A. Dewsbury and Marion Jowaisas whose patience and

encouragement enabled me to persevere; and to Anne

J. Bryan without whose help as both student and

colleague my laboratory and this study would not























Table Page

1 Sequence of Conditions in
Experiment I. . . . . . .. 35

2 Sequence of Conditions in
Experiment II . . . . . . 69

3 Conditional Probability of
Ineffective Responses . . . . 89


Figure Page

1 Mean response and shock rates for
the last 85 min of each 100 min
session, Pigeon D. Data collected
from consecutive sessions are
connected by a line. Arrows
indicate a daily session for which
data were lost due to counter
malfunctions. . . . . . .. 37

2 Mean response and shock rates for
the last 85 min of each 100 minm
session, Pigeon D. Consecutive
daily sessions are connected by
a line. . . . . . .. . 40

3 Conditional probabilities of a
response during 2 sec segments
of the R-S interval, Pigeon D.
All functions represent the mean
of the four sessions indicated
by the numbers in the upper center
of each set of coordinates. . . .. 43

4 Conditional probabilities of a
response during 2 sec segments
of the R-S interval, Pigeon D .... 45

5 Mean response and shock rates
for the last 85 min of each 100
min session, Pigeon N. Con-
secutive daily sessions are
connected by a line. . . . . 48

6 Mean response and shock rates for
the last 85 min of each 100 min
session. Consecutive daily
sessions are connected by a line. . 49

Figure Page

7 Conditional probabilities of a
response during 2'sec segments of
the R-S interval, Pigeon N. All
functions are based on the mean
of the four sessions indicated by
the numbers in the upper center
of each set of coordinates. . . ... 51

8 Conditional probabilities of a
response during 2 sec segments
of the R-S interval, Pigeon N . .... 54

9 Cumulative record of the entire
100 min session indicated for
Pigeon N. Each treadle press
moved the pen upward and slashes
denote shocks. Successive
segments have been collapsed
with the earliest segments at
the top. The arrow indicates
the end of the 15 min period
defined as warm-up. . . . . . 56

10 Cumulative record of the entire
100 min session indicated for
Pigeon N. Recording conven-
tions are as in Fig. 9. . . . . 57

11 Cumulative record of the entire
100 min session indicated for
Pigeon N. Recording conven-
tions are as in Fig. 9. . . .. 58

12 Cumulative record of the entire
100 min session indicated for
Pigeon N. Recording conven-
tions are as in Fig. 9. . . . .. 59

13 Cumulative record of the entire
100 min session indicated for
Pigeon N. Recording conven-
tions are as in Fig. 9. .. . . 60

14 Cumulative record of the entire
100 min session indicated for
Pigeon N. Recording conven-
tions are as in Fig. 9. . . . .. 61


Figure Page

15 Cumulative record for the entire
100 min session indicated for
Pigeon D. Each treadle press
moved the pen upward and slashes
indicate shocks. Successive
segments have been collapsed
with the earliest segments at
the top. The arrow indicates
the end of the 15 min period
defined as warm-up. . . . . .. 62

16 Cumulative record for the entire
100 min session indicated for
Pigeon D. Recording conven-
tions are as in Fig. 15 . . .. 63

17 Cumulative record for the entire
100 min session indicated for
Pigeon D. Recording conven-
tions are as in Fig. 15 . . .. 64

18 Cumulative record for the entire
100 min session indicated for
Pigeon D. Recording conven-
tions are as in Fig. 15 . . .. 65

19 Cumulative record for the entire
100 min session indicated for
Pigeon D. Recording conven-
tions are as in Fig. 15 . . .. . 66

20 Mean response and shock rates
for each 100 min session, Pigeon
S. For shock intensities in
effect for more than 20 sessions,
only the first and last 10
sessions data are shown. The
numbers in parentheses give the
total number of sessions at any
intensity. Consecutive daily
sessions are connected by a
line . . . . . . . 72

21 Mean response and shock rates
for each 100 min daily session
for Pigeon S. The arrow indicates
a session for which data were
lost due to a counter malfunction.
Other details of presentation are
as in Fig. 20 . . . . . . 74


Figure Page

22 Mean response and shock rates
for each 100 min session for
Pigeon C. Data from consecu-
tive daily sessions are
connected by a line. . .. . . . 78

23 Mean response and shock rates
for each 100 min daily session
for Pigeon C. Data from
consecutive daily sessions are
connected by a line. Data
from sessions indicated by the
open arrows were collected at
an intensity of 7.9 mA (probe
sessions) . . . . . . 80

24 Mean response and shock rates
for 100 min sessions for Pigeon
C. Consecutive daily sessions
are connected by a line, the
small dark arrow indicates a
session for which data were
lost due to a counter mal-
function and the open arrows
indicate probe sessions when
shock intensity was 7.9 mA. . . ... 83

25 Mean response and shock rates
for the last 5 sessions under
each shock intensity for Pigeon
S. The heavy lines above and
below each mean indicate plus
and minus one standard devia-
tion. The mean for the last
7.75 mA condition is based on
the 3 sessions immediately
preceding the death of
Pigeon S . . . . . . .. 85

Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy



Dennis Brian Jowaisas

June, 1977

Chairman: E. F. Malagodi

Major Department: Psychology

The importance of the species generality of

phenomena, especially for purposes of theoretical in-

terpretations, has been emphasized in recent years. This

report provides data which are currently unavailable

concerning the acquisition of temporal response gradients

during free-operant avoidance and extends the species

generality of the theoretically important shock-frequency

reduction procedure. Pigeons were exposed to two un-

signalled, free-operant shock schedules in which the

effective response was a treadle press. Experiment I

consisted of long-term exposure to Sidman's free-operant

avoidance schedule. The response-shock interval was 20

sec, the shock-shock interval was 5 sec and shock duration

was 0.3 sec throughout the experiment. One of the

pigeons in Experiment I performed under shock intensities

of 6 mA and 7 mA while the shock intensity for the second

pigeon was 6 mA for the entire experiment. Conditional

probabilities of responses during successive tenths of

the response-shock interval were collected for each of

the more than 120, 100-min sessions for each pigeon.

The results of Experiment I showed that the general

characteristics of avoidance performance of pigeons were

similar to those obtained in previous experiments with

rats under comparable schedules. Response rates were

typically higher, and shock rates typically lower, than

those obtained from rats. The conditional-probability

distributions showed that within 12 sessions, responses

with IRTs longer than 12 sec predominated and that

gradual emergence of high probabilities of responding at

longer IRTs occurred through the 100th session. The

most significant feature of the probability distribu-

tions was the rapid development of temporally-

discriminated responding and the stability of the

distribution functions across more than 100 sessions.

The development of efficient avoidance performance prior

to the appearance of temporal response gradients

provided support for a least-effortful-response inter-

pretation of the mechanism underlying these temporal

discrimination; this interpretation is opposed to the

Pavlovian mechanism suggested by two-factor theory.


In Experiment II, two pigeons were exposed to a shock-

frequency-reduction procedure in which treadle responses

reduced the overall frequency of shock from 9 per min to

3 per min for the period between a response and the next

scheduled shock. Following each shock, the probability

of shock remained at 9 per min until a response was

emitted. One of the pigeons in the second experiment

was exposed to several different shock intensities while

the other pigeon received occasional probe sessions in

which shock intensity was reduced from 10 mA to 7.9 mA

for one 100-min session. Shock duration was 0.3 sec

throughout the experiment. Both pigeons responded

under the shock-frequency reduction schedule and

performance was maintained for more than 170 sessions

of 100-min duration. For one pigeon, exposure to

increased shock intensities resulted in decreased shock

rates with the exception that average shock rates at the

end of two reexposures to an intensity of 7.75 mA were

consistently higher than each preceding exposure. This

was true whether reexposure followed a period when shock

intensity was 6 mA or 10 mA. There were no consistent

effects of shock intensity on response rates. For the

second pigeon, exposure to probes at the reduced shock

intensity produced decreased response rates and increased

shock rates when probe sessions were compared to the

preceding and succeeding sessions at the higher shock


intensity. In other respects, the performance of the

pigeons under the shock-frequency-reduction procedure

was similar to that of rats. It was concluded that the

process of acquisition and maintenance of responding

in pigeons under free-operant schedules of shock

avoidance and shock-frequency reduction is essentially

similar to that of rats. The contribution of the

activity-producing properties of shock to the acquisition

and maintenance of responding under shock-frequency

reduction procedures was discussed and the roles of the

form of the avoidance response and the characteristics

of the experimental space were considered.


Thomas Kuhn (1962), a physicist and historian

of science, provided an analysis of the structure of

scientific discovery in the physical sciences which proves

to be useful for understanding the historical development

of the methodology and theory in the study of avoidance.

According to Kuhn, "normal science," i.e., the particular

tradition of research within a discipline, emerges from

and is unified by paradigms. Paradigms are scientific

achievements which produce among the practitioners of a

discipline virtually universal agreement upon a model for

the conduct of research, interpretations of results, and

a framework for the construction of theory. Kuhn (1962)

notes that the adoption of an alternative paradigm does

not inevitably lead to a major upheaval within a disci-


These are episodes exemplified in their
most extreme and readily recognized form by the
advent of Copernicanism, Darwinism or Einstein-
ism in which a scientific community abandons
one time-honored way of regarding the world and
of pursing science in favor of some other,
usually incompatible, approach to its disci-
pline. I have argued ... that the historian
constantly encounters many far smaller but
structurally similar revolutionary episodes and
that they are central to scientific advance ...
To assimilate them the scientist must usually
rearrange the intellectual and manipulative
equipment he has previously relied upon, dis-
carding some elements of his prior practice

and belief while finding new significance
in and new relationships between many
others. (Kuhn, 1962, p. 42).

Seligman (1970) has recently questioned the generality

of the "laws of learning," Baum (1973) has challenged the

assumptions of the law of effect and several authors

(Bolles, 1975; Herrnstein, 1969; Rachlin, 1976) have re-

viewed the findings which suggest that conventional

theories of avoidance are inadequate. To varying extents,

all of these authors provide documentation for Kuhn's

thesis of the influence of paradigms upon a science,

exemplified in this case by the historical process which

characterized the study of avoidance. A brief review of

this historical context of the study of avoidance behavior

is necessary for an understanding of the importance of the

experiments to be reported.

For the first fifty years of this century, the

structure of learning theory in American psychology was

built upon the foundations of the work of Pavolv and

Thorndike. These men contributed the first paradigms

for the study of how behavior changes as a function of

experience and, to this day, conventional accounts of

learning stress the respondent conditioning model of

Pavlov and the instrumental conditioning model of

Thorndike. These models provided the methodology used to

study the acquisition and maintenance of behavior and were

widely held to represent the basic processes underlying

all learning. Both models were based on assumptions

derived from the philosophical school of British empiri-

cism which emphasized association of ideas by the mechan-

ism of temporal contiguity. Complex experiences were

explained through the associative linking of simple

sensations which occurred at the same time: the closer

in time and the more often these sensations occurred, the

stronger was the associative link.

For Pavlov, the explanation of his experiments on

salivary conditioning was the temporal contiguity of a

neutral stimulus with a stimulus which elicited a reflex.

With enough pairings of the stimuli, the previously neu-

tral stimulus came to evoke a conditional response, similar

in form to the reflex. The underlying mechanism was the

associative linking of the stimuli, as represented by

overlapping excitatory cortical fields triggered by the

external stimuli.

Thorndike clearly showed the influence of associa-

tionist theory in his statement of the law of effect:

Of several responses made to the same
situation, those which are accompanied by or
closely followed by satisfactions to the ani-
mal will, other things being equal, be more
firmly connected with the situation, so that,
when it recurs, they will be more likely to
occur; (Thorndike, 1911, p. 244)

Like Pavlov, Thorndike sought the mechanism for the law

of effect in physiological events: neurons and synapses

were conduction units and conductivity was altered by

experience. The major difference between the theories of

Pavlov and Thorndike was in the units that were linked,

stimulus-stimulus and stimulus-response, respectively.

Thorndike acknowledged the importance of the ways behavior

changed events by specifying "satisfying consequences" as

a necessity for associative linking but tended to concen-

trate on the stimulus "situation" with its implied

eliciting properties.

Research directed to the understanding of how behavior

changes as a function of experience provided support for

associative theories. The discovery that, in Pavlov's

paradigm, acquisition of the conditioned response depended

upon rather short delays (0.5 sec) between the conditioned

stimulus (CS) and the unconditioned stimulus (US) supported

the contiguity notion. Similarly, the finding that

acquisition of the conditioned response (CR) was retarded

or prevented by providing occasional presentations of the

CS or US alone bolstered confidence in the conventional


As noted by Herrnstein (1969), Pavlov did not dis-

tinguish the procedures used by V. M. Bekhterev from his

own; Hull's (1934) adaptation of Bekhterev's approach

made no mention of the manner in which the animal's

responses changed the experimental consequences.

An example of Bekhterev's procedure clearly shows

that Thorndike's paradigm was in use, rather than


A dog would be exposed to a sequence con-
sisting of some originally neutral stimulus,

followed by a painful electric shock to a fore-
paw. The reflex response to the shock was leg
flexion and the adaptive change was the occur-
rence of leg flexion as soon as the neutral
stimulus was presented. Superficially, the
differences between Pavlov's and Bekhterev's
discoveries was minor, concerning only the
physical response muscular instead of glan-
dular. (Herrnstein, 1969, p. 50)

Because of the similarity of the preshock signal to

Pavlov's CS, the similarity of the CR to the UR (flexion),

and because the preshock signal and the shock were pre-

sented in the same temporal pattern as the CS and US, the

important effects of the animal's responses were over-

looked, to wit, leg flexion eliminated shock delivery. In

essence, Bekhterev had invented the signalled avoidance

procedure: a response during a stimulus presentation pre-

vented delivery of an aversive event scheduled to occur in

the absence of responding. This arrangement differed from

the Pavlovian situation in which the animal's behavior in

no way controlled the sequence of stimuli presented by the


Brogden, Lipman, and Culler (1938, in Herrnstein, 1969)

were the first to challenge experimentally the conception

of avoidance conditioning as a var-iety of Pavlovian

conditioning and to support the view of avoidance as an

example of instrumental conditioning, based on Thorndike's

law of effect (see Appendix). The findings of Brogden et.

al. did not challenge the principle of contiguity, of course,

but clarified the place of the signalled avoidance

procedure, classifying it as an instrumental conditioning


Though few workers quarrelled with the new classi-

fication, Pavlovian conditioning, as one factor in the

acquisition and maintenance of avoidance responding, was

not abandoned. In a series of papers, Mowrer and his co-

workers developed a two-factor theory and applied it to

explanations of avoidance as well as appetitive condi-

tioning procedures (Rachlin, 1976). One appealing feature

of the theory was that it dealt with the troublesome

problem of why an organism would continue to perform

avoidance responses when the presumed reinforcer (satis-

fier) for such responses was no longer present, i.e., the

shock was avoided, and two -factor theory dealt with the

problem without rejecting the principle of contiguity.

Kuhn (1962) suggests that the typical response of scien-

tific theory builders to discrepant data is to expand

established theory rather than to re-examine assumptions

on which the theory is founded. Adding a second factor to

account for avoidance learning in order to maintain

Pavlovian conditioning (and contiguity) as some part of

the explanation is an example of the scientific conserv-

atism Kuhn described.

The two-factor theory explains avoidance responding

in the following way. Avoidance responding is typically

observed to proceed in two stages: (1) escape from shock

precedes (2) eventual avoidance of shock. In signalled

avoidance procedures a preshock signal occurs, then

shock, and finally a response which terminates both

*signal and shock. The temporal pairing of shock and

signal is said to produce fear or anxiety or "aversive-

ness" as a conditioned response within the organism as a

result of the first factor, Pavlovian conditioning. The

second stage in the development of avoidance of the shock

is due to the second factor, instrumental conditioning,

because responses terminate the conditioned aversive stim-

ulus, the preshock signal. Termination of the signal

reduces the conditioned drive state called fear and this

reduction is a negatively reinforcing event. The theory

fitted well with the propensity of theorists of the time

to postulate intervening variables which mediated the

organism's behavior and was generally accepted. The two-

factor theory reflected the paradigmatic influence of

Pavlov and Thorndike by positing two associative mecha-

nisms based on contiguity: the external pairing of stimulus

and shock and the internal pairing of fear reduction and


The next major procedural variation in avoidance

conditioning was Sidman's (1953) unsignalled procedure,

known as free-operant avoidance. In this procedure there

is no programmed preshock stimulus; shocks are simply

scheduled to occur at one of two intervals. In the

absence of a response, shocks occur at fixed, short

intervals known as the shock-shock (S-S) interval. If

a response occurs, a scheduled shock is postponed for a

specified interval, called the response-shock (R-S)

interval. Each response "resets" the R-S interval and

shock-free time cannot be accumulated: with an R-S

interval of 30 sec, shock is programmed to occur 30 sec

after the last response,regardless of the overall rate

of previous responses. Shocks are inescapable and of short

duration. Performance under this procedure is a joint

function of the length of the R-S and S-S intervals, the

limiting cases being the obvious ones of a zero R-S inter-

val, in which responses are in effect punished and a zero

S-S interval, essentially the procedure of escape. Moder-

ate rates of responding are typically acquired and main-

tained when the R-S interval is on the order of 30 sec and

the S-S interval is somewhat shorter.

At first glance, Sidman's (1953) free-operant avoid-

ance procedure would seem to pose a challenge to the two-

factor theory, for where is the signal with aversive

properties to be terminated by the response? Sidman was

quick to point out that there is a regularity in the

procedure, the fixed R-S and S-S intervals. A two-factor

adherent simply points to the sequence of internal stimuli

following a response, arguing that the passage of time

(equivalently, the progression of a regular, internal

stimulus sequence) provides an effective preshock stimulus.

Internal stimuli early in the sequence are rarely paired

with shock, while those later in the sequence are regu-

larly paired with shock. As a result, the response which

terminates these aversive stimuli is negatively

reinforced. Thus, the internal stimulus sequence, or

"clock," assumes the same status as the preshock signal

and the two-factor theory is applied accordingly.

Sidman (1954) examined the relative frequency of

various interresponse times (IRTs), the time between

successive responses, in order to evaluate the hypothesis

that internal stimulus sequences provided a preshock

stimulus, in the manner suggested by two-factor theory.

If responses late in the R-S interval terminated aversive

internal stimuli, then those responses would be differ-

entially reinforced. Early responses would tend to be

less frequent because they did not terminate stimuli

which were as aversive as those later in the internal

sequence, i.e., those closer in time to shock.

Sidman (1954) found no evidence for more frequent

responding in the later portions of the R-S interval

duration, in either the relative frequency of IRTs or the

cumulative frequency distribution of IRTs. He concluded

that there was no evidence of the predicted temporal

discrimination proposed to explain the acquisition and

maintenance of responding under the free-operant avoid-

ance schedule. Any discrimination which did develop,

argued Sidman, occurred only after prolonged training

and did not appear to be necessary for effective avoid-

ance performance.

Anger (1963) challenged Sidman's (1954) conclusions

on the basis of the statistical analysis. Anger

criticized the use of the relative frequency statistic for

description of the IRT distributions during the R-S

interval since, by.the definition of interresponse times,

there are more opportunities for responding in the first

segment of the interval than in later segments. Thus,

every response is followed by the opportunity to make

another response in the first segment of the interval

(and the opportunities are equal to the total number of

responses emitted) but the number of opportunities to

respond during the second segment is decreased by however

many observed responses occurred during the first segment,

and the number of opportunities to respond in the third

segment is decreased by the total of observed responses

in the first and second segments, etc.

For example, suppose an R-S interval of 10 seconds

was divided into 1 sec segments for the purpose of

analyzing the IRTs. Assume that during the time IRTs

were recorded, 100 responses were made, 10 in each 1 sec

segment of the R-S interval. The relative frequency

statistic for such an IRT distribution is 0.1 for each

of the 1 sec segments. Anger (1963) argued that the

relative frequency statistic does not reflect the

conditional nature of the procedure: if only 100 responses

occurred and if 80 of them were emitted at IRTs of 8 sec

or less, then there were only 20 opportunities to respond

at IRTs greater than 8 sec.

In the example above, 10 responses with IRTs of 9

sec did occur. Given that there were 20 opportunities to

respond with an IRT of 9 sec, and 10 such responses were

emitted, the conditional probability of a response at 9

sec is 10/20 or 0.5. Similarly, the conditional proba-

bility of a response with an IRT of 8 sec is equal to the

observed number of responses (10) divided by the number

of opportunities for response (100 minus the total number

of responses with IRTs of less than 8 sec: 10/100-70 =

0.33). Thus, for the data of the example, the relative

frequency statistic is 0.1 for all IRTs while the

conditional probability distribution shows that, given

an opportunity for a response with a long IRT, the proba-

bility is much higher that a response will occur: 0.5 at

9 sec, 0.33 at 8 sec, 0.25 at 7 sec, etc. Anger (1963)

proposed that, based on the argument above, the appro-

priate statistic for the evaluation of the temporal

discrimination during free-operant avoidance is the

IRT/Op (opportunity) statistic.

Anger (1963) reanalyzed the data presented by

Sidman (1954), substituting the IRT/Op statistic for the

relative frequency measure, and showed that the condi-

tional probability of responses in the later segments of

the R-S interval was consistently and considerably

greater than the probabilities of a response in early

segments, even during the early stages of acquisition.

Clearly, there was evidence for temporal discrimination

as an underlying process in the acquisition and mainte-

nance of responding under free-operant avoidance sched-

ules. Anger (1963) interpreted the results as support for

either of two views, the two-factor theory or the less-

effortful reduction in shock frequency, as explained below.

The first view postulates the reduction of condi-

tioned aversive stimuli, internal and unobservable, as the

reinforcing event and responses late in the interval are

said to produce a greater decrement in aversiveness than

do early responses. The second view (Sidman, 1962)

attributes avoidance to the reduction in shock rate. A few

spaced responses produce the equivalent amount of shock

frequency reduction as numerous short-latency responses.

Thus, spaced responding is less effortful.

Herrnstein and Hineline (1966) designed a pro-

cedure intended to provide the lacking evidence, a pro-

cedure which incorporated no fixed shock-free periods

following responses such as the R-S interval in free-

operant schedules. Their random-shock procedure elimi-

nated fixed relationships between the occurrence of

shock and any other aspects of the situation, providing,

instead, a statistical correlation between responses and

shock. Shock rate is reduced following a response and

reverts to the original rate after the next shock, which

is programmed to occur at varying intervals following a

response. The shock-free period following a response is,

on the average, longer than the shock-free periods

following any other point in time.

The parameters of the random-shock procedure which

determine response rate are the probabilities of shock

programmed by two independent shock distributions, the

postshock,and postresponse distributions. The postshock

distribution has a higher probability of shock than the

postresponse distribution. Following a shock, the high

probability distribution is in effect. A response serves

only to instate the lower probability shock distribution

and responses that follow a previous response have no

effect. The low probability distribution remains in

effect until a shock is delivered, at which time shocks

are again programmed to occur according to the high

probability distribution.

Herrnstein and Hineline (1966) showed that acqui-

sition and maintenance of responding occurred when the

probabilities of shock were 0.1 and 0.3 per 2 sec ( 3 and

9 per min) for the postresponse and postshock distribu-

tions, respectively. Unlike Sidman's (1954) results,

the conditional probabilities of a response across the

maximum intershock intervals showed no evidence of

temporal discrimination. Herrnstein and Hineline (1966)

concluded that a response-dependent change in the

amount of subsequent aversive stimulation appears to

be the sine qua non of avoidance conditioning" (Herrn-

stein and Hineline, 1966, p. 429). Only by postulating

"covert stimuli whose properties vary in concert with

the changes in shock rate" (Herrnstein and Hineline,

1966, p. 429) could a two-factor explanation of the

results be maintained. H-errnstein and Hineline proposed

that such an extension of the two-factor theory pre-

cluded the possibility of disproof of the theory, a

condition eschewed by theorists and deplored by histo-

rians of the scientific enterprise.

However, the major import of Herrnstein and Hine-

line's work is not the critique of two-factor theory but,

rather, the attention it focused on other variables, in-

herent in avoidance procedures, which may affect avoid-

ance performance. The emphasis upon contiguity expla-

nations of avoidance was shifted to analysis of the

contingent relationships between behavior and conse-

quences over periods of time. In their words,

We are familiar with theories that say
a response is influenced by its having termi-
nated an electric shock or by its having
terminated a conditioned stimulus associated
with shock. But we are unaccustomed to the
notion that a response can be influenced by
changing the rate of a stimulus, a change
that itself can be manifested only over some
period of time. (Herrnstein and Hineline,
1966, p. 429)

According to Kuhn's (1962) view, a scientific revolution

had occurred in the theory of avoidance conditioning;

abandoning the narrow concept of contiguity led to the

investigation of other underlying principles to account

for the phenomena.

Hineline (1970) soon provided data on another

variable affecting acquisition and maintenance

of avoidance responding. Noting that the Herrnstein and

Hineline's (1966) procedure confounded the average delay

to shock onset with the frequency of shock, Hineline

designed a trial-by-trial procedure whereby a response

delayed the onset of a single shock without changing the

overall frequency of shock. A trial consisted of a 20 sec

period which began with presentation of a retractable lever

to the rat. If no response was emitted in the first 8 sec

a shock was delivered, 2 sec later the lever was withdrawn,

and 10 sec after withdrawal, the lever was again presented

for another trial. If a response was emitted in the first

8 sec of a trial, the lever was withdrawn immediately and

the shock was not delivered until the 18th sec of the 20

sec cycle. Thus, responses delayed shock without changing

the shock frequency. All rats acquired the lever-pressing

response and typically responded on 85% of the trials.

Hineline (1970) performed a second experiment in

which responding resulted in a statistical increase in the

shock rate. The situation was similar to the first: shocks

occurred in the 8th sec of the cycle in the absence of a

response and 2 sec before the end of the cycle if a

response occurred. However, a response affected the dura-

tion of the cycle because the lever was retracted for 10

sec regardless of when the response occurred. Thus, a

response at 2 sec from the onset of the cycle still pro-

duced a delay before shock delivery but shortened the

cycle by 8 sec. Cycles were 20 sec long in the absence of

a response. Since every cycle produced a shock, delayed or

otherwise, shortening the cycyle produced more trials per

session and thereby increased shock frequency. No rat,

even if previously trained to lever press in an avoidance

situation, acquired and maintained responding under this

regimen. As Herrnstein (1969) points out in his review of

avoidance, "The difference between the two procedures

proved to be crucial one fully adequate, the other

totally inadequate" (Herrnstein, 1969, p. 65). And as

Herrnstein did not note, the "sine qua non" status of shock

frequency reduction in avoidance performance was chal-

lenged. Shock delay alone would maintain avoidance behav-


At this point in time it was clear that the two condi-

tions sufficient for acquisition and maintenance of avoid-

ance behavior were (1) shock frequency reduction combined

with (2) increased average delay to shock onset and the

delay of shock onset. Lambert, Bersh, Hineline, and Smith

(1973) attempted to isolate the effects upon avoidance

behavior of shock frequency reduction without shock delay.

Their procedure was essentially one of punished avoidance:

a response caused an immediate shock and eliminated five

shocks scheduled to occur in the absence of a response.

Lambert et al. (1973) also varied the presence or absence

of an escape contingency and the topography of the avoid-

ance response. It has long been recognized that the

opportunity for escape responses facilitates acquisition

of an avoidance response and Bolles (1970) has summarized

the evidence that the compatibility of the avoidance

response with the species-specific defense reactions (SSDR)

of the organism determines the ease of conditioning.

The results of Lambert, Bersh, Hineline,and Smith

(1973) revealed the interaction of all of these variables.

When the avoidance behavior was a shuttling response, com-

patible with the SSDRs of the rats, the procedure resulted

in acquisition and maintenance of responding without an

escape contingency. When a lever-press, incompatible with

SSDRs, was the avoidance response an escape contingency was

necessary for the maintenance of responding. For rats

trained with the escape contingency in effect, removal of

the contingency reduced avoidance rates and reintroduction

of escape increased the rates. A conservative interpreta-

tion of Lambert et al.'s (1973) results is that it is possi-

ble to produce avoidance responding under conditions of

shock frequency reduction without shock delay if the avoid-

ance response is high in the hierarchy of SSDRs (Bolles,

1970) of the organism. The variability of responding

across sessions suggests that punished avoidance behavior

was not strongly under control of shock frequency reduction.

at least with the parameters employed.

Further exposition of the role of shock delay in

avoidance performance resulted from the experiments of

Gardner and Lewis (1976) using a free-operant procedure in

which a response produced 10, 88,or 165 sec delays of shock

onset. In the absence of responses, shocks were delivered

according to a variable time schedule at an average rate

of two per min (VT 30 sec). A response produced a stimulus

change for 3 min and, depending upon the delay condition,

six shocks were delivered in the six sec beginning 10, 88,

or 165 sec after the response. The shock rate during the

3 min this response-dependent stimulus change was in effect

was thus equal to the rate under the VT 30 schedule of

shock delivery. At the end of 3 min in the response-

produced condition, the VT 30 sec shock schedule was again

imposed. Responses during the delay condition had no

programmed consequences.

After 30 hours of exposure, rats under the 88 sec de-

lay condition were spending about 80% of the time in the

response-produced (delayed shock) periods; rats in the 165

sec delay condition were spending 95%, while those in the

10 sec delay condition spent less than 20% of the time in

the delayed condition. Furthermore, the length of the de-

lay appeared to influence the patterning of responses: rats

in the longest delay condition responded most frequently

after the transition from the delayed to the VT 30 sec

condition, rats in the 88 sec delay condition responded

primarily after shock delivery,and rats in the shortest

delay condition showed neither pattern.

In a second experiment, Gardner and Lewis (1976)

demonstrated that when responses produced a condition of

156 sec delay and an increase in shock frequency,

responding was still acquired and maintained. Experi-

mental conditions were the same as in Experiment I. in most

respects: shocks were delivered on a VT 30 sec schedule in

the absence of responding, a response produced a delayed

shock-onset condition for 3 min and additional responses

during this condition had no programmed effects. The

important change in Experiment II. was the shock rate

during the delay period. Three groups of rats received

9, 12, or 18 shocks, delivered at 1 sec intervals be-

ginning 156 sec from the onset of the delayed condition.

Thus, for the three rats in each group, the delayed con-

dition produced an increase in shock frequency of 1.5, 2,

and 3 times the rate compared to the VT 30 sec condition.

All three rats in each of the first two groups spent

80-90% of the time in the delay condition in spite of the

increased shock density. One of the three rats in the

delay condition with a tripled shock frequency spent 90%

of its time in the delay condition but two other rats

rarely produced the delay condition. Those rats that pro-

duced the delayed, increased shock-frequency condition

responded primarily after the transitions from the delay

to the VT 30 sec condition. Responding was maintained for

60 hr under all conditions.

Three control groups, of 3 rats each, were exposed

to comparable conditions without the delay feature.

Shocks were delivered under a VT 30 sec schedule in the

absence of responding and under VT 20, 15, or 10 sec

schedules for the 3 min following a response. Under these

conditions, no rat showed response rates comparable to the

delayed-condition groups during the 60 hr of exposure to

the control condition. When the control conditions were

changed to those of the experimental group (delayed shock),

one rat in the 1.5 shock-rate increase, all three rats in

the 2.0 shock-rate increase, and one rat in the 3.0 shock-

rate increase condition produced the delay (and shock-

increase) condition more frequently and were spending 80%

or more of the time in the delayed condition by the end of

60 hr of additional training.

Obviously, Hineline's (1970) findings that increased

shock frequency prevents the development of responding

under shock-delay conditions depended upon the duration of

the delay. Gardner and Lewis' (1976) rats in the 10 sec

delay condition showed little responding even when shock

frequency did not increase. Differences in the two pro-

cedures are undoubtedly significant but the one specifi-

cally mentioned by Gardner and Lewis (1976) is the preven-

tion of postshock responding through the retraction of the

lever in Hineline's (1970) study. Nevertheless, it is

clear that rats will produce a condition of increased shock

frequency if the onset of the shocks is sufficiently

delayed. Under conditions of delay without shock frequency

increases, longer delays engender more responding and

affect the patterns, posttransition or postshock, of re-


Thus, the history of methods and theories of avoid-

ance conditioning shows an evolution from the original

procedure of Bekhterev, considered as an example of

Pavlovian motor conditioning for the first 40 years of this

century, through a period of analysis from the two-factor

viewpoint which combined Pavlovian and instrumental factors

to explain the phenomena, to the point where Herrnstein

(1969) reviewed the evidence for a shock frequency reduc-

tion explanation and challenged researchers to concentrate

on the observable variables which could be demonstrated to

affect the acquisition and maintenance of avoidance re-

sponding. Experiments by Herrnstein and Hineline (1966),

Hineline (1970), Lambert, Bersh, Hineline,and Smith (1973),

and Gardner and Lewis (1976) showed that response-contingent

reductions in shock frequency and response-produced delays

of shock are two variables sufficient to support acquisi-

tion and maintenance of avoidance responding and that shock

delay of sufficient duration may be more powerful a vari-

able than the shock frequency. The studies reviewed fall

generally within the methodological framework of the

experimental analysis of behavior. The analytic emphasis

is upon contingent and correlative relationships rather

than simple contiguous ones and covert associative pro-

cesses are disregarded in favor of observable behavior and

procedural events for explaining phenomena.

As Lambert, Bersh, Hineline,and Smith (1973) ac-

knowledged, other variables, such as species-specific

behaviors, interact with the procedures employed to study

avoidance behavior. The generality of results and theories

based on such results may be limited by considering "the

avoidance response" as an abstraction, to be arbitrarily

chosen to fit the experimenter's requirements. Seligman

(1970) has challenged the generality of the laws of

learning in a recent review. Like Herrnstein (1969), he

questioned the contiguity explanation of learning, mar-

shaling evidence that all events are not equally associable.

Seligman proposed that the evolutionary history of a par-

ticular species predisposes individuals of the species to

associate certain events, leaves them unprepared to asso-

ciate other events and contraprepared to associate still


Bolles (1970) has been the main investigator to

provide empirical evidence that the form of the avoidance

response has important consequences for the outcome of

avoidance procedures. Bolles' (Bolles, 1969; Bolles &

Riley, 1973) results clearly demonstrate that certain

responses selected as the avoidance response are rarely

conditioned while other responses are readily condition-

able. Bolles (1970, 1972) has made a strong argument for

the compatibility of the avoidance response with the

innate species-typical hierarchy of behaviors which com-

prise the organism's defensive repertoire. These species-

specific defense reactions (SSDRs) are prepotent responses

in the presence of aversive stimuli and tend to compete

with avoidance responses lower in the SSDR hierarchy. Thus,

lever pressing which decreases shock frequency is less

readily acquired than a wheel-turning response because

wheel-turning more closely resembles the rat's SSDR of

running. The difficulty of teaching pigeons to avoid

shock by pecking a key depends upon the reactions of

pigeons to sudden, intense shock running and flying -

which are incompatible with pecking (Rachlin, 1976, p.

357). Under conditions of gradually increasing intensities

of pulsed shock, pigeons tend to attack (peck) and can be

trained to avoid by key pecking. (Rachlin, 1969; Moraes

and Todorov, 1977)

The similarity of the thinking of Bolles (1970) and

Seligman (1970) to that of Herrnstein (1969) is their

concentration upon the observable variables which affect

an organism's performance under various conditions.

Bolles and Seligman have extended the consideration to

variables in the evolutionary history of the species.

Staddon and Simmelhag (1971) emphasized similar considera-

tions, pointing out that all behavior, and the mechanisms

by which it is changed, are part of biological inheritance

and that evolutionary mechanisms probably underlie both

the production and selection of behaviors in any labora-

tory situation. Herrnstein (1969) concentrated on the

observable and manipulable variables when evaluating

theoretical interpretations of avoidance.

This then is the background against which the

following experiments were conceived and conducted.

Pigeons were used as subjects in all experiments for

several reasons. First, relatively few systematic data

on the avoidance behavior of pigeons are available. Smith

and Keller (1970) were the first to demonstrate the acqui-

sition and maintenance of responding in pigeons under free-

operant schedules of shock avoidance. They used a small
chamber with a floor area of 68 in and treadle pressing

as the avoidance response. The R-S interval was 32 sec,

the S-S interval was 10 sec, shock duration was 0.25 sec

and the shock source was 6.2 vac. No details of the

resistance of the pigeons or series resistance of the

shock delivery circuit were provided. Consequently, no

accurate knowledge of shock intensity is possible.

Sessions were 90 min long.

Smith and Keller (1970) reported rapid acquisition of

the response and efficient shock avoidance performance for

all five of their birds within 20 sessions. Response rates

tended to be high for all subjects and shock rates were

approximately 0.05 to 0.10 per min, as estimated from

Figure 3, page 213. An analysis of the conditional

probability of a response as a function of the time since

the previous response was provided for the data from one

session for two birds. In general, the IRT/Op distribu-

tions were quite similar to those presented by Anger (1963)

for Sidman's (1954) rats: the probability of a response

increased as a function of the time since the last re-

sponse and response bursts accounted for the high proba-

bility of very short IRTs.

Foree and LoLordo (1970) trained pigeons under

signalled and unsignalled free-operant avoidance schedules

in which a treadle press was the avoidance response. The

R-S interval was 20 sec and in the signalled condition,

the last 10 sec of the R-S interval were accompanied by a

change in illumination from white to red. The chamber used

by Foree and LoLordo (1970) was larger in floor area than

Smith and Keller's (1970) and so was the surface of the

treadle. Shock duration was 0.25 sec, shocks consisted of

50 vac delivered to the base of the pigeons' wings via a

beaded chain. No resistance parameters were published so

no estimate of shock intensity is possible. Foree and

LoLordo (1970) demonstrated acquisition of responding in

most subjects although response rates were highly variable

both within and between birds in both conditions. The

restricted number of sessions (less than 20 for most

subjects) and the extreme range of shock rates between and

within birds relegates this study to the status of a

demonstration that avoidance responding under these con-

ditions is acquired by the majority of pigeons.

Klein and Rilling (1972) conducted a parametric study

of the effects of various R-S interval durations and

various shock intensities on the free-operant avoidance

performance of pigeons. Treadle pressing was the

avoidance response.

Klein and Rilling (1972) varied the R-S interval from

2.5 to 150 sec while shock duration (0.25 sec), shock

intensity (8mA) and the S-S interval (10 sec) were held

constant. Response rates decreased in all four pigeons as

the R-S interval increased beyond 7 sec, a finding that

replicated the functions described by Sidman (1953 b), using

rats, in a similar parametric study. Conditional probabil-

ity distributions of responding, averaged over the last

three sessions at each R-S interval duration, showed

evidence of temporal discrimination at 15 and 20 sec for

all birds and at 10 sec for three of the four birds.

In a second experiment, the R-S interval (20 sec), S-S

interval (5 sec) and shock duration (0.25 sec) were held

constant while each of four birds was exposed to shock

intensities of 2, 4, 8, and 16 mA in randomized orders. As

shock intensity increased from 2 to 8 mA response rates for

all four pigeons increased: beyond 8 mA, one bird showed

continued rate increases, two showed slight decreases, and

one bird died. These relationships between response rate

and shock intensity replicated the functions obtained by

Boren, Sidman and Herrnstein (1959) with rats.

Although Klein and Rilling's (1972) results represent

the single source of parametric data on free-operant

avoidance in pigeons, the shock rate stability criterion

they used bears examination. Their criterion was "five

percentage points or less difference in the mean percent-

age of S-S shocks avoided over the same two consecutive

blocks of three sessions" (Klein and Rilling, 1972, p. 296).

Most investigators, influenced by Sidman (1953 a, b), have

used the total number of shocks delivered, whether from

expiration of the R-S or S-S interval, to calculate shock

rates. Leander and Jowaisas (1971) observed very few S-S

shocks delivered to pigeons in the early stages of

exposure to free-operant avoidance schedules, a time when

shock and response rates were still changing. Previous

work with pigeons in my laboratory agrees with this

observation. Rats typically show similar avoidance of

S-S shocks well before overall shock rates become stable

and, in both pigeons and rats, response rates often

stabilize before overall shock rates do. Because of these

observations and the different shock rate criterion, Klein

and Rilling's (1972) results cannot be readily compared

to those using the more common shock rate measures.

For example, Figure 1 of Klein and Rilling's (1972)

study purports to show shock rate as a function of the R-S

interval duration: shock rate is said to decrease as the

R-S interval duration increases. A more accurate inter-

pretation is that the four pigeons rarely failed to

depress the treadle following a shock. No information on

the overall frequency of shock is contained in Klein and

Rilling's report even though shock rate is the primary

measure of avoidance performance under free-operant


Experiment I was also designed to provide data on

the stability of the behavior of pigeons as maintained by

a free-operant avoidance schedule and on the conditional

probability distribution o'f responses during acquisition;

that is, the development of temporal gradients in response

patterns. Knowledge of the concurrent or sequential

development of efficient shock avoidance and temporal

discrimination bears directly on the question raised by

Anger (1963) and the interpretation of Sidman (1962) con-

cerning conditional probability distributions, as dis-

cussed on page 12 herein. If temporal gradients of

responding develop after efficient avoidance responding

is apparent, then this feature of avoidance performance

is most parsimoniously interpreted as the result of

response effort or efficiency rather than due to the

Pavlovian component of a two-factor mechanism. No such

information for pigeons or rats currently exists in the


The second experiment is a replication of the random

shock procedure of Herrnstein and Hineline (1966). For a

procedure which marked such a dramatic departure from

conventional method and theory in the study of avoidance,

there is a surprising lack of information on the charac-

teristics of behavior engendered and maintained by this

procedure. In addition to the original report, Leander

(1973 a) provided the only other data on the procedure.

The rats used as subjects in both studies showed highly

variable response rates: shock rates were near the

minimum allowed by the parameters of the procedure (3

shocks per min). Neither Herrnstein and Hineline (1966)

nor Leander (1973 a) intended to study the characteristics

of behavior under these procedures: Herrnstein and

Hineline (1966) merely intended to demonstrate the possi-

bility of acquisition of a response under conditions

where shocks could not be eliminated and Leander (1973 a)

used the procedure to assess the generality of the effects

of food deprivation upon avoidance performance.

Herrnstein and Hineline (1966) did manipulate the

parameters of the two shock probability programs and found

that, in general, response rates were higher when

responses produced greater reductions in shock frequencies.

They also observed the course of extinction for one rat.

Extinction was defined as equality of shock probability

from the two shock distributions and the probabilities

for the distributions were 0.1/2 sec. The remarkable

feature of extinction was the persistence of responding;

over 17,000 min of exposure were required before re-

sponding was eliminated. Powell and Peck (1969) reported

similar effects upon the responding of rats with a history

of avoidance conditioning.

In addition to a replication with another species,

Experiment II represents an extension of the Herrnstein

and Hineline (1966) procedure in several ways. First,

the behavior of the two birds was studied for an extended

period of time in order to assess the stability of

response and shock rates; second, the effects of several

shock intensities upon the performance of one bird were

studied. If the Herrnstei'n and Hineline (1966) procedure

bears a similarity to previous work on shock-avoidance

behavior, then response and shock rates should vary as a

function of shock intensity (Leander, 1973 b). For a

second bird in Experiment II, shock intensity was oc-

casionally reduced for one session in order to observe

the effects of a short-term change in shock intensity.

Finally, much of the importance of the second experiment

reported herein lies in the additional information

provided about a procedure with major theoretical impli-

cations, information which is presently unavailable.




Subjects: One male feral pigeon (Pigeon D), captured as

a fledgling in the halls of the Psychology Department at

Oklahoma City University, and one two-year old male White

Carneaux pigeon (Pigeon N) obtained from Palmetto Pigeon

Plant, served. Between experimental sessions, the

pigeons were individually housed in large cages (Hirota,

1971) and had free access to mixed grain and water. The

colony room had a regulated 14 hr light/ 10 hr dark cycle.

Pigeon D had an extensive history of exposure to

simple FR, FI, mult FI FI, and mult FI FT appetitive

schedules and to schedules of signalled, discrete-trial,

and unsignalled, free-operant avoidance of electric shock.

The parameters of the unsignalled free-operant avoidance

schedule were identical to the parameters in Experiment I.

Pigeon N had been exposed to one hr of the shock-

frequency reduction schedule described by Herrnstein and

Hineline (1966). The session was ended when Pigeon N was

seen struggling in the experimental chamber: Pigeon N had

spastic paralysis of the lower legs and little ability to

control his left foot. The following day only spasticity

of the lower left leg was evident and after two weeks of

gradual recovery, no symptoms remained. His electrodes

were re-implanted and two days later Pigeon N was' exposed

to the conditions of Experiment I. No further symptoms

recurred during the course of the experiment.

Apparatus: A Plexiglas and metal chamber, 25.4 x 25.4 x

38.1 cm high, formed the experimental space. The floor was

a plastic grid of 1.2-cm squares and the ceiling was #2

hardware cloth with a 3 cm diameter hole for the shock

delivery cable to enter. The chamber door and the wall

opposite the door were of Plexiglas. Illumination of the

chamber was accomplished by lighting separate pairs of 24

vdc bulbs (GE 1812) covered with white, red, or green

translucent plastic and mounted outside the chamber on the

wall, 25.4 cm above the floor.

The manipulandum was a treadle similar to that used by

Smith and Keller (1970). The treadle, 12.7 cm long on the

side parallel to the wall and 10.2 cm wide, extended 8.5 cm

into the chamber from one metal wall, forming an angle of

approximately 36 degrees with the floor, through which it

extended. A piece of soft leather attached to the floor

and the face of the treadle prevented the pigeons from

catching their feet in the gap. A force of approximately

0.75 N applied to the center of the treadle closed the

attached microswitch and the switch closures defined the

response recorded on counters and a cumulative recorder.

A feedback relay was mounted behind the wall with the


The entire chamber was housed in a sound-attenuating,

ventilated enclosure located in the colony room. The

electromechanical equipment which controlled experimental

events and recorded responses was in a nearby room.

The shock source was a Layfayette 601-B shocker

equipped with selectable series resistors in 10K ohm values

from 10 to 990 K ohms and a continuously adjustable voltage

range of 775 VAC. Shocks were delivered to the pigeons

through the series resistors via an LVE mercury commutator

and a two-conductor jack terminating the cable which

entered the chamber through the ceiling.

Procedure: Both pigeons were implanted with stainless

steel electrodes around the pubis bones in the manner

described by Azrin (1959) and the connecting wires attached

to a Switchcraft two-conductor plug fastened by a wing

harness to the pigeon's back. Prior to each session the

pigeons were captured in a pitcher, weighed and the

resistance of their electrodes measured with a vacuum tube

volt meter. The polarity of the dc measuring-current was

alternated three times a sec by a hand-operated switch to

eliminate the effects of body capacitance on the measure-

ment. The reading was recorded and if it differed by 1 K

or more from the proceeding day's value or if it exceeded

5.5 K, the embedded portion of the electrode was rotated

out of the pigeon's flesh and scraped clean of any

accumulated deposits. In the absence of resistance changes,

the inspection and cleaning was done weekly.

With exceptions as noted, each pigeon was run daily

for 100 min. Each session began and ended with 5 min of

darkness and white illumination was correlated with the

operation of the shock avoidance schedule. Each closure of

the treadle microswitch during white illumination produced

the audible click of the feedback relay.

The R-S interval duration was 20 sec, the S-S interval

duration was 5 sec and shock duration was 0.3 sec through-

out the experiment. The treadle microswitch was disabled

for the 0.3 sec of shock delivery. As noted in Table 1,

the shock intensity was increased at Session 54 for Pigeon



For the first 15 min of each session responses and

shocks were counted separately so that the highly variable

period of responding (warm-up) that characterizes perform-

ance at the beginning of daily sessions (Powell, 1970)

could be evaluated. The response rates and shock rates

shown in Figures 1, 2, 5, and 6 are the mean rates during

the last 85 min of each session.

As can be seen in Figure 1, Pigeon D, with prior

experience under free-operant avoidance schedules, re-

ceived very few shocks after his first session. Although

shock rates were extremely stable during the first 20

sessions, Pigeon D's response rates showed a consistent

Table 1

Sequence of Conditions in Experiment I.

(mA) **


Pigeon N





Pigeon D





** R-S = 20, S-S = 5 and shock duration = 0.3 sec
throughout the experiment.


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crl) 10

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0 0

r- C:) .H


c) bOlr-l
p -H
C) 0.0

rO C) 0r

n 0
U) -P

bf 0
HL 'H 0

0 0


0. o

- -




C Cd,

0 0

C, N

0 ;

- r-

- A
Zr -\ -
If 0U, C



decreasing trend from 16 to 10 per min. Shock rates

remained stable from Sessions 21-33 when response rates

showed no trends and varied between 9 and 11 per min.

Sessions 34-41 were characterized by stable shock rates

and widely varying response rates (8-13 per min) and from

Sessions 42-68 responding varied from 9-14 responses per

min around a mean of 10.9 (standard deviation: 1.2) while

shock rates were still low but slightly more variable.

Response and shock rates remained in the same general

range from Sessions 69-76 (Figure 2) at which time Pigeon

D's right electrode was discovered imbedded in the skin

only, rather than looped around the pubis bone. The

electrode was reimplanted around the bone. The decreasing

response and increasing shock rates during Sessions 77-84

are attributed to dislocation of the left electrode from

the bone and a broken connection to the right electrode.

The electrodes were removed, reimplanted after five days

and Pigeon D was run again after a 12 day vacation.

The effects of the vacation are clearly seen in the

data from Session 86: response and shock rates are higher,

in the range of those from Session 1. From Sessions 87 to

96, response rates varied between 10.5 and 11.8 per min

and shock rates were stable at less than 0.1 per min.

Response rates then decreased by one per min and were more

variable while shock rates increased slightly and also

showed more variability: these changes persisted from

Session 97 through 124 when equipment malfunctions


0 C









Q) r


0 >.





*H H~



I- -

o-- -- -


c --------^--m
2 S.




precipitated the end of the experiment. The mean response

rate for the last five sessions was 10.46/min and the mean

shock rate was 0.084/min. The mean response and shock

rates for the proceeding five sessions were 9.88 and

0.096/min, respectively.

Conditional probabilities of a response (IRT/Op) in

successive 2-sec segments of the R-S interval for Pigeon

D are shown in Figure 3. The probability for the segment

ending at the 20th sec is not shown since the IRT/Op

statistic fixes that probability at 1.0. The functions

are an average over four sessions and presented for all

sessions from 1 to 44, in order to detect changes in

temporal discrimination. Since it is clear from the data

shown in Figure 3 that no major changes in the conditional

probability functions occurred after Session 40, beginning

with the 49th Session, the functions are shown for the

first four sessions of every 10 until the end of the

experiment (Figure 4).

The distribution for Sessions 1-4 indicates that

responding was equiprobable in all 2-sec segments of the

R-S interval; by Sessions 13-16 a temporal discrimination

had developed, as indicated by the sharp decrease in

response probability at 4 sec and monotonicly increasing

probabilities at 6, 8, and 10 sec. The function continued

to change through Sessions 37-40 at which point the

conditional probability was consistently highest at the 12

sec. Further sharpening of the temporal discrimination

was evident until the conditional probability of a


bl 4-)

(n 0 (n

bfl.0 0

= 04-)

0 Q)

oq 0)0

> .)

0i ) (n
r4 Z a)
ba~ .

.0l 0
ra H .
0 a0
*H -4 -4-
H >


' -H
* WI
bfl0 Wr
*H.0 0



. .^-

I ~I

I n I

. ^

, ^1



) (0 D t N C (0 tO

(riO/A>MI) aSNOdSKM V dO ALI'IlV2oHi 'IvrJOhLfONoD



U) B





W (0 .~ N










0 >





In I






SN %

SI -o




I 4




0 %

rr I

(do/uLM) RSNOdsad V dO XI'UIIgVgOyd qVNOIT,IGNO3




- U






O |0 W N

response was consistently highest at 14 and 16 sec by the

end of the experiment. However, the most striking feature

of these data is the similarity in the function from

Sessions 20-124. The acquisition of temporally-spaced

responding is quite rapid and stable for a pigeon with

previous avoidance experience.

Response and shock rates for all sessions of the

experiment for Pigeon N are shown in Figures 5 and 6.

Compared to Pigeon D, Pigeon N showed slower acquisition

of the response and higher response rates,and higher and

more variable shock rates throughout the experiment.

Pigeon N lacked the avoidance conditioning history of

Pigeon D. Response rates increased during the first 20

sessions, remained at 22-25 responses per min for 10

sessions and then decreased sharply to 18 per min at

Sessions 32-34.

During this same period, shock rates had declined

dramatically by Session 20 and remained at about 0.15 per

min through the 39th Session. From Sessions 38-53, both

response and shock rates increased and then decreased.

The conditional probability functions (Figure 7, Sessions

41-44, and 44-48) reflect this disruption of avoidance

behavior. No extraexperimental events were identified to

account for the sudden change in performance during this


Shock intensity was increased to 7 mA at Session 54

0 Q



Q) U)





0 0

w U)





Hr- -H-

e '

^ " "- - .

OW -


6- =

o -

. * I. . . .-
0 C 0 nn





4< ----


- 3I






~ \I '-
e( %

80 90 100 110 120

Fig. 6. Mean response and shock rates for the last 85
min of each 100 min session. Consecutive daily sessions
are connected by a line.




E 0l
U) .0 -P
a) a)
0 U)d

E 0.4
(1) 0

r.~ 0 1


0 -4-')


0a Q)


0 o1


M a) i

U O)
-4 4H CH
c'l 0(
0 *H
or -'t
.r U
*t U)C)

*C/ a) o

o 0a
bfl a)r C

~ 0



p I I


I In


5-- -- ---



1' I I __
-si -






I--- -

r, ,



r k


1 -


r- I I I

. I I I

..I ,

I 1
1 1 1"* *

/,) .INOd V O O 0 NO O










(0 (oN

and from Sessions 60-68 both shock and response rates were

relatively stable. The increases seen in Session 69 were

due to a broken electrode connection discovered in the

early part of the 70th Session. The dramatic increase in

rates during Session 77 was correlated with increased

shock rates in the other three pigeons run that day,

suggesting that some unidentified equipment malfunction

may have been the cause. Voltage measurements of the

shocker and resistance measurements on the cable revealed

no malfunction in the delivery system.

Prior to Session 80, Pigeon N was not run for five

days and the effects of the vacation (comparable to those

for Pigeon D in Figure 2, Session 86) can be seen in the

increased response and shock rates (Figure 6). Both rates

were variable until Session 111 at which time Pigeon N

displayed response rates from 16-18 per min and shock

rates from 0.1 to 0.2 per min until the end of the experi-

ment. The mean response rate for the last five sessions

was 16.62 and mean shock rate was 0.165. The response and

shock rates for the proceeding five sessions were 16.56

and 0.162.

The conditional probability distributions of

responding during the R-S interval for Pigeon N are shown

in Figures 7 and 8. Similar to the distributions for

Pigeon D, four session means were plotted until little

change was evident in the functions for 12 sessions. At















k r



Hr 4-













- --

A -



I'" I I


'-I 4

*> """









that point (Session 65), the conditional probability

functions are shown for the first four of every 10 sessions

until the end of the experiment.

Pigeon N showed a pattern of bursts of responding, as

seen in the cumulative records of Figures 9 through 14,

and these bursts, a consistent feature of this bird's

pattern throughout the experiment, account for the high

probability of a response in the first 2 sec. The

development of temporally-patterned responding was slower

to develop in Pigeon N: with the exception of the first

2 sec, responses were randomly distributed during the first

eight sessions. Some evidence of a discrimination can be

seen by Sessions 9-12 and the probability of a response

in the 14th, 16th, and 18th sec increased reliably from

Sessions 13-40. Increasing shock intensity had no

discernible effect upon IRTs. During the rest of the

experiment, changes in the distribution of responding

consisted of decreases in the probability of a response

during the second segment of the R-S interval. The

cumulative records as well as the overall decrease in

response rates, in the later stages of training suggest

that this feature of the probability distributions was

the result of a decline in the duration of bursts which

characterized Pigeon N's responding. This decline can be

seen by comparing Figures 10 and 14. The lack of bursts

of responding in the cumulative records of Pigeon D

(Figures 15 through 19) and the lower probability of a

r 2 min

Fig. 9. Cumulative record of the entire 100 min session
indicated for Pigeon N. Each treadle press moved the
pen upward and slashes denote shocks. Successive
segments have been collapsed with the earliest segments
at the top. The arrow indicates the end of the 15 min
period defined as warm-up.


2 min

Fig. 10. Cumulative record of the entire 100 min session
indicated for Pigeon N. Recording conventions are as
in Fig. 9.



2 min

Fig. 11. Cumulative record of the entire 100 min
session indicated for Pigeon N. Recording conventions
are as in Fig. 9.

SESSI ff 60

2 min

Fig. 12. Cumulative record of the entire 100 min
session indicated for Pigeon N. Recording
conventions are as in Fig. 9.



2 min

Fig. 13. Cumulative record of the entire 100 min
session indicated for Pigeon N. Recording
conventions are as in Fig. 9.



Fig. 14. Cumulative record of the entire 100 min
session indicated for Pigeon N. Recording
conventions are as in Fig. 9.



2 min

Fig. 15. Cumulative record for the entire 100 min session
indicated for Pigeon D. Each treadle press moved the pen
upward and slashes indicate shocks. Successive segments
have been collapsed with the earliest segments at the top.
The arrow indicates the end of the 15 min period defined
as warm-up.



/ 0


2 min

Fig. 16. Cumulative record for the entire 100 min
session indicated for Pigeon D. Recording
conventions are as in Fig. 15.



/ \

2 min

Fig. 17. Cumulative record for the entire
100 min session indicated for Pigeon D.
Recording conventions are as in Fig. 15.
/^ ^




2 min

Fig. 18. Cumulative record for the entire 100 min
session indicated for Pigeon D. Recording
conventions are as in Fig. 15.


2 min

Fig. 19. Cumulative record for the entire 100 min
session indicated for Pigeon D. Recording
conventions are as in Fig. 15.

response in the first segment of the conditional

probability distribution (Figures 3 and 4) support this

explanation. It should be noted that Pigeon N's response

bursts occurred in the absence of shock and thus represent

emitted rather than shock-elicited behavior.



Subjects: Two male White Carneaux pigeons, each about 2

years old, served. All care and housing conditions were

the same as in Experiment I.

Apparatus: The identical apparatus of Experiment I was

used throughout Experiment II.

Procedure: Implantation, weighing and resistance checking

followed the procedures of the first experiment. Each

pigeon was run daily for 100 min and each session began

and ended with 5 min of darkness. Illumination of the

pairs of bulbs behind the red and green translucent

plastic covers was correlated with the operation of the

random shock schedule devised and described by Herrnstein

and Hineline (1966). On this schedule, shock is delivered

independently of the behavior of the subject but the

frequency of the shock differs following responses.

Two independent shock distributions were programmed

on stepping switches, stepped every two sec by the same

clock. The high probability shock program was in effect

following a shock and remained in effect until a response

occurred. Shocks were delivered in a randomized sequence

with an average frequency of 9 per min while the high

probability program operated and at 3 per min while the

low probability program operated. When a shock was

delivered from the low probability program the high

probability shock program was reinstated. Thus, responses

following shocks had the effect of instating the low

probability shock program and responses following responses

had no programmed effects. The minimum number of shocks,

regardless of response rates, was fixed by the program at

approximately 3 per min.

Every treadle press produced an audible click of the

feedback relay except for the duration of a delivered shock

when the treadle microswitch was disabled. No programmed

stimuli accompanied either shock program. Shock duration

was 0.3 sec and the probabilities of shock for any two

sec period were 0.1 and 0.3 for the low and high shock

programs, respectively: these parameters remained fixed

for the course of the experiment. The point in the shock

distributions at which each daily session started was

varied unsystematically from day to day to destroy any

regularity in the shock sequences.

The sequence of experimental manipulations of shock

intensity for both pigeons is shown in Table 2. Initial

shock intensities were determined during the first session

by slowly increasing the intensity until the pigeon showed

Table 2

Sequence of Conditions in Experiment II.

















119, 122,
131, 141,


Total No.



( 1)






( 7)

( 8)

( 9)



( 5)
















bobbing and wing flapping of moderate intensity when

shocked. Repeated observations were made during the course

of a session to evaluate the pigeons' reactions to shock

and to check for adaptation. Increases in shock intensity

were made cautiously because of the paralysis inflicted on

Pigeon N in the first experiment and a similar accident

suffered by a mixed-breed pigeon intended as a sujbect in

this experiment. No symptoms of paralysis were ever

observed in Pigeons C and S during Experiment II.


The overall response and shock rates for both pigeons

are shown in Figures 20-24. The rates represent the total

number of responses in the session without regard for the

effectiveness of a response. Effective responses, i.e.,

those following a shock, varied over a narrow range of

275 to 300 responses in 100 min and the variation in rates

seen in the figures resulted from ineffective responses,

emitted while the low probability shock program was already

in effect. Overall shock rates were calculated without

regard for the program source, high or low, from which

they were delivered.

Pigeon S displayed a stable range of response rates

and increasing shock rates during the 13 sessions at a

shock intensity of 6.5 mA (Figure 20). Pigeon S's

observed reactions to shock during the 12th and 13th

0 4-' 4-)
Q) f-( U)
h > 0
S*H 0

.. bf uC)

C 0 1 ,

0) >- 0

-H -i U)
U) rC )-
) 0 S C)
0 ( *H- -

0 *C Ci

C) E-i
II 0 )

E -Q. >

0 WO U)

a-' E
CO 0>

o .0

-i U)
U) v0


d Od
C) -H cd z

0 0 1U

U) -i
0 (1)l ) C
a 4-H U)
O P 0 >C)

Cd 4-'

0H 0 (n
C 0 -r

0<( 0 r. 0
03 rO rO

1 S


S ,-

o -




a a a a S





.id r



a a I ,









0 m 0
N -

Sessions were of very low intensity and shock delivery

could not be reliably detected from his reaction. During

the first ten sessions at 7 mA Pigeon S's response rates

were variable and his shock rates decreased sharply after

the sixth Session at 7 mA. After 22 sessions at 7 mA

response rates varied around 16.5 responses per min, shock

rates were variable with a downward trend and Pigeon S

was again showing little reaction to shock.

During the 20 sessions in which shock intensity was

7.75 mA, shock rates were stable in the range of 3.05 to

3.2 per min and response rates showed a downward trend.

Decreasing the shock intensity to 6 mA produced an

immediate increase in shock rates while response rates

were somewhat lower and more variable.

In Figure 21 the rates for the last ten sessions at

6 mA presented in Figure 20 are repeated for ease of

comparison. Increasing shock intensity from 6.0 mA to

7.75 mA resulted in an immediate increase in responding

and little decrease in shock rates over four sessions.

Equipment problems necessitated a vacation and the effects

are clearly seen in the high response rates and low shock

rates during the next three sessions. Similar effects of

vacation from daily avoidance sessions were seen in

Experiment I. (Figure 2). By the end of 46 sessions at

7.75 mA, response rates were more stable and slightly

higher than those of the last sessions at 6 mA. Shock


- - - -

-- -

\ **-

c --

- ---^



.. .. * .. 1*- t

.^''- -

0--- ____

I I 3 I I I a q I.




E 6


E ^ -




Mn o



rates were stable and equal to those under the 6 mA

condition and slightly higher than shock rates at the end

of the first exposure to 7.75 mA.

A change to 10 mA shock intensity produced elevated

response rates and sharply decreased shock rates in the

first session. Shock rates remained low during the 19

sessions at 10 mA: shock rates were often at the minimum

rate dictated by the procedure. Response rates were

generally higher than those at lower shock intensities but

were decreasing at the end of 19 sessions. Since response

rates can be expected to be variable due to the lack of

explicit contingencies except the correlative shock re-

duction effected by a response following shock and since

no further reduction in shock rates was possible, an

attempt was made to replicate the effects of shock inten-

sity at 7.75 mA upon shock rates (or alternatively, on

effective response rates). Following the exposure to

6 mA, Pigeon S's rate of effective responses had not

increased (shock rate did not decrease) upon re-

introduction to 7.75 mA. Effective response rate at the

second exposure to 7.75 mA was thus lower (shock rates

increased) than those of the first 7.75 mA condition.

The third exposure to a shock intensity of 7.75 mA

resulted in increased shock rates, higher than those at

any time except the beginning of the experiment. Response

rates decreased to the lowest values observed during the

experiment. After seven sessions equipment problems pre-

vented daily sessions and Pigeon S died within two hr after

the third session following the vacation. Cumulative

records of the last session showed no obvious disruptions

of responding but Pigeon S was bleeding from the anus when

removed from the chamber. A crude autopsy indicated that

he had suffered a hemorrhage of the intestinal wall.

The overall response rates and shock rates for all

sessions for Pigeon C are shown in Figures 22 through 24.

Pigeon C showed a very high tolerance for shock as

indicated by his reaction to shock. In general, response

rates and shock rates varied considerably (Figure 22) at

intensities from 7 to 9 mA (Sessions 1-66).

The effects of increasing shock intensity to 10 mA

can be seen in Figure 23. Response rates stabilized

during Sessions 72-83 and shock rates declined. At

Session 87 Pigeon C showed increased response rates and

his shock rates decreased to near the minimum possible

rate of 3 per min. Following a period of decreasing

response and increasing shock rates (Sessions 99-106,

Figure 23) Pigeon C's electrodes were removed in order to

treat an infection which had developed in the area of the

right electrode.

Upon return from the 10 day vacation and following

the vacation effect of increased response and decreased

shock rates observed throughout these experiments, a









0 co

*H 'H
0 E

a 0
(A p


bO U
.rf -



S---... ---- o

L '..


91V -----------1
oo a



-I -


o H
-H r-i r
U) d
(1) 4-)
U) ci

E 0)

0 a

od a, c

0U) ;-

U) r.

U) (1)
) E C

(1~) U)

00 *
0 > >U)
._ '-4 .0 U)
U) C)
-j U)

S04-) (0)
cdj U) cd '

01) 0 -4 ;A
Un 0'd

a 0o

U) ;- U)E

Z' 4-) U) t-
cd dU)

zP- U)o0


rz44 4 U)1
0 d
*0cn 0)
*H HcOj

L kl..l

ec ...- (

- -




- I I I I

-- - - -
_l CO ___..- -- '*>

.i e



I -I
Be ,r-"


s -^::-C





- I I


period of relatively stable rates ensued and was disrupted

by two procedural errors which resulted in exposure to a

shock intensity of 7.9 mA (Sessions 119 and 122). At this

lower shock intensity response rates were lower and shock

rates were higher than in the proceeding and succeeding

sessions. Since Pigeon C seemed rather unreactive to

shock and because of our experience with the damaging

effects of shock intensities higher than 10 mA, and because

the data from Pigeon S suggested that adaptation to higher

shock intensity may result from extended exposure to

varied shock intensities, a series of intended probes (as

opposed to the unintended probes in Sessions 119 and 122)

at 7.9 mA was carried out.

The first intended probe (third exposure to 7.9 mA)

occurred in Session 131 (Figure 23) and resulted in a

reduced response rate and sharply increased shock rate.

Following that probe, response rates were high for the

next five sessions and shock rates were low. The probe at

Session 141 produced a decreased response rate and little

effect upon shock rate (Figure 24). It should be noted

that a counter malfunction resulted in the loss of data

from the following session, Session 142. A comparison of

the cumulative records for Sessions 142 and 140 revealed

no dramatic differences.

The last probe at Session 155 was programmed to

follow a stable period of response and shock rates. The

U m



U) r-q Q) a
Z Cd 4-) Q)
0 E
*H C/)

&r- U) -

U~) Q) 0
C d 0,Q

0 rl = r

Q) -P U)

0 o 0
-4- 0

(1)-P U)
4-) 0 (1 1
cd *H U

cr( Q)~ &

0 0 Cd 0

Ci) -

rU *4-)
~ &~ c~


0 H

U) U) 0

U) 0
o1 >-.* 0
~-r-I C) $-A
d Q) Cd


*H) C/) 0)

-r-, C ) 0~
4-) (1 -
-4-) (H
N 0 d C
c Q*H c)
(1 0 -
U) -lq
b.0r. 0 7
0 Z =

u Cr) C e

-- .-. -


I I I <

0 u0
--**- - -r- -

.J ^ (___

n - - c
-c w,

0E~ -
'7/ -
p .0W



mean rate of responding for the three sessions (152, 153,

154) prior to the probe was 11.11 per min and differed by

0.5% from the mean response rate of 11.17 for the three

proceeding sessions (149, 150, 151). The mean shock

rates for these three-session blocks were 3.16 and 3.09

per min (a difference of 2.2%), respectively. The

response rate during the probe session was 9.6 per min and

the shock rate was 3.32 per min. Following the last probe,

shock rates remained stable and at the lower possible

limits for 10 sessions and response rates showed wide

variations. There followed a series of equipment problems

and the variability in rates associated with vacations

from daily sessions. The experiment was terminated when

infection around the electrode implantation area developed

and the electrodes consistently showed encrustation and

discharge of fluids from the entry site.

A summary of the results of shock intensity

manipulations for Pigeon S is presented in Figure 25. The

mean response and shock rates for the last five sessions

at each intensity are shown and the line through each data

point represents the limits of plus and minus one standard

deviation from the mean. As can be seen in Figure 25,

increasing the shock intensity from 6.5 mA to 7 mA, and

from 7 mA to 7.75 mA, resulted in decreased shock rates.

The first exposure to a lower shock intensity, 6 mA,

produced an increase in shocks. The first reexposure to








6.5 7 7.75 6 7.75 10


7.75 7.75

Fig. 25. Mean response and shock rates for the last 5
sessions under each shock intensity for Pigeon S. The
heavy lines above and below each mean indicate plus
and minus one standard deviation. The mean for the
last 7.75 mA condition is based on the 3 sessions
immediately preceding the death of Pigeon S.


7.75 mA produced no change in the shock rate whereas

increasing shock intensity to 10 mA did decrease the shock

rate. When Pigeon S was subjected to 7.75 mA, shock rates

increased compared to the rate under 10 mA and the shock

rate was higher than at either previous exposure to 7.75

mA. The last mean is the average of the last three

sessions following a vacation from daily sessions and prior

to Pigeon S's death. Shock rate fell to levels similar to

those at the first exposure to 7.75 mA and the 10 mA

condition. Whether this low average rate was the result

of the vacation effect (Clay-Findley, 1971; Experiment I.

of the present study) or of increased aversive properties

of shock due to a pathology resulting in death at the end

of the third session cannot be determined. What is clear

is that the effects of changes in shock intensity upon

shock rates under the Herrnstein and Hineline (1966)

procedure may be partially irreversible. Initial

exposures to increased intensities of 7.0, 7.75, and 10 mA

produced decreased shock rates. Increasing the intensity

from 6.0 to 7.75 mA (second exposure) did not and shock

rates during the second exposure were higher than under

the first. The third return to the 7.75 mA intensity

followed exposure to 10 mA and produced the highest

terminal shock rates of any of the shock intensity con-

ditions. Thus, the three exposures to 7.75 mA produced

successively greater mean shock rates each time, even

though two of these changes followed conditions of

lower shock intensity.

To further assess the effects of the probes

conducted with Pigeon C at reduced shock intensity, the

response and shock rates of the probe sessions were

compared with the rates from the sessions immediately

preceding and succeeding the probe. Again, the probe

data from Session 131 were omitted from the comparison

because of their unusual value. A t-test for

correlated means was employed to test the hypothesis

that the mean difference (2.02 responses per min) in the

response rates of the immediately preceding sessions and

the probe sessions was a chance occurrence. The hypo-

thesis was rejected: t(3) = 5.01, p > .05. For the

corresponding comparison of probe rates and those of

immediately succeeding sessions the null hypothesis

was again rejected: t(3) = 5.69, p .05. The mean

difference for the latter comparison was 1.56 responses

per min. The comparisons of mean shock rate differences

were not significant in either case.

Under the random shock procedure used in this

experiment, the conditional probability distributions

of a response following a response (IRT/Op) should be

an essentially flat function because the only responses

which effect a change in shock frequency are those

immediately following a shock. Thus, except for the

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