Pavlovian conditioned inhibition of fear during shuttlebox avoidance behavior

LEARNING

AND

MOTIVATION

Pavlovian

(19’74)

5,

Conditioned Shuttlebox

424-447

Inhibition Avoidance

of Fear

During

Behavior1

R. G. M. MORRIS University

of Sussex

Three experiments explored the development and function of conditioned inhibition of fear during the acquisition and maintenance of shuttlebox avoidance behavior. The development of inhibition to an exteroceptive feedback stimulus was found to be a function of the number of successive avoidance responses to which the animal had been trained and of the duration of the intertrial interval, a parameter shown also to affect the rate of acquisition of avoidance learning. Master animals who learned the instrumental avoidance response, and yoked animals who did not, showed equivalent inhibitory fear conditioning in each experiment. The results of one experiment suggest that conditioned inhibition plays no important role in “protecting” fear conditioned to the discrete warning signal during avoidance maintenance. These data indicate that feedback stimuli develop their inhibitory properties by a Pavlovian process and that certain aspects of their function may, therefore, be readily understood within the framework of mediational two-process learning theory.

The purpose of this paper is to show that exteroceptive feedback stimuli develop powerful fear inhibiting properties whose function can be properly understood within the context of mediational two-process learning theory (Konorski, 1948; Mowrer, 1947; Rescorla & Solomon, 1967). Recent criticisms of two-process theory (Belles, 1970, 1971; D’Amato, 1969; Herrnstein, 1969; Morgan, 1968) have been partially based upon demonstrations that “warning-signal termination” may not be the powerful source of reinforcement it was once thought to be. One purely logical 1 Dedicated to the memory of Jerzy Konorski. This work was based on a D.Phil. thesis submitted to the University of Sussex in May 1973. I am grateful to N. S. Sutherland for his provision of facilities. Thanks are due also to A. H. Black, A. Dickinson, M. S. Halliday, E. M. Macphail, M. J. Morgan, J. B. Overmier and R. G. Weisman with whom I have had many valuable discussions about matters relating to this work. Thanks also to J. Scull for his patient statistical advice. Pilot work for Expt 1 was conducted at Queens University, Kingston, Canada during I971 under the direction of R. G. Weisman. Preparation of the manuscript was supported in part by the Addison Wheeler Fellowship. The author is presently the Addison Wheeler Fellow, Department of Psychology, University of Durham, South Road, Durham, DHl 3LE, United Kingdom. Copyright All rights

424 @ 1974 by Academic Press, Inc. of reproduction in any form reserved.

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difficulty with basing criticisms of two-process theory upon such evideuce is that it ignores the important theoretical distinction that must be drawn between the supposed necessity for a concept such as that of “fear-reduction” in the explanation of avoidance learning and the separate proposition that warning-signal termination acts as its source. The need to draw this distinction is demanded by W&man and LitncLr’s (1972) recent report that, in a one-way avoidance task, exteroceptive feedback stimuli become powerful inhibitors of fear. This demonstration seriously undermines these criticisms of two-process theory because it would appear to be no longer necessary for two-process theorists to argue that warning-signal termination is either necessary or sufficient for discriminated avoidance learning to occur. Already there exists a substantial literature attesting to the efficscy of feedback stimuli in increasing the rate of acquisition of avoidance responding (Belles, 1972a; Belles & Grossen, 1969, 1970a; Bower, Starr & Liuarowitz, 1965; D’Amato, Fazzaro & Et-kin, 1968; Dinsmoor & Sears, 1973: Keehn & Nakkash, 1959). However, the significant contribution of Weisman and Litner’s ( 1972) work is its clear implication that our understanding of the functional significance of feedback stimuli demands no uew principles of learning beyond those developed in recent studies of Pavlovian inhibitory conditioning ( LoLordo, 1969; Moscovitch & LoLordo, 1968; Rescorla, 1969a, 1969b; Weisman & Litner, 1969a, 1969b, 1971). Since the manner in which Pavlovian inhibitors of fear affect instrumental behavior can be explained by two-process learning theories (Grossen, 1971; Rescorla & Solomon, 1967), there is yet no reason to doubt that our understanding of the function of feedback stimuli can be readily incorporated within the framework of such theories. This view contrasts with that expressed, for example, by Bolles and Moot (1972) who belicvca that “some new concept is needed,” such as that of the feedback stimulus “bccause the current literature bristles with problems traditional two-factor theory cannot handle” ( p. 63). The purpose of the following experiments was to extend We&man alid Litner’s (1972) studies to the case of shuttlebox avoidance learning ( 1) to evaluate the generality of their findings in a different avoidance situation, and (2) to further investigate the conditions governing the development of conditioned inhibition. EXPERIMENT

1

Using a conditioned suppression technique, Kamin, Brimer, and Black (1963) have shown that fear conditioned to the warning stimulus in an avoidance task declines during avoidance maintenance. This result raises serious questions about the role of fear during avoidance maintenance.

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Before considering these questions, it is necessary to have evidence about the role of fear inhibiting stimuli at various stages of avoidance learning. The first study sought to provide such evidence. Conceptually, it is based on Kamin et aZ.‘s (1963) experiment, but the methodology is derived from Weisman and Litner’s (1972) work. Method Subjects. The subjects were 48 female hooded rats of the Lister strain weighing between 160 and 250 g. They were housed in pairs, a “masteryoke” pair always being housed together, and maintained on ad lib. food and water. More than 90 rats began the experiment but many were discarded for failing to reach a pretraining criterion described below. Apparutus. Two identical shuttleboxes and wheel-turn chambers were used. The side walls of the shuttleboxes were constructed of black acrylic plastic and measured 89.5 X 10.0 X 44.0 cm high. The two end walls and hinged ceiling were made of translucent white plastic. The floor consisted of 57 stainless steel bars 0.7 cm in diameter spaced 1.4 cm apart center to center. The entire construction tilted about the mid-point, the ends of the box moving spatially through a distance of 1.0 cm to open or close microswitches at each end. A response was recorded when the rat moved from one end of the box to the other. Behind each end wall and ceiling were lamps providing illumination at approximately 0.3 log ft-L. A loudspeaker was placed centrally in one side wall at a height of 30 cm from the floor. The wheel-turn chambers measured 25 X 25 X 25 cm. Three of the side walls were constructed of aluminium and the floor was made of 11 stainless steel bars, 0.7 cm in diameter spaced 1.4 cm apart. Part of one side wall, which was made of translucent white plastic, was hinged to allow access to the chamber. The wheel manipulandum was suspended horizontally on the front wall in which there was an opening 12.5 cm long x 9.8 cm high placed 5.3 cm above the grid floor. The wheel was constructed of 12 stainless steel bars, also 0.7 cm in diameter, mounted in circular aluminium end plates. It was 11.5 cm long X 8.8 cm in diameter. A lamp mounted behind the hinged door illuminated the chamber at approximately 0.3 log ft-L. A speaker was mounted externally on the ceiling. A 1000 Hz tone of 85 dB ( re. 0.002 dyns/cm*) could be delivered via the speakers. Illumination of the lamps served as the visual stimulus in each piece of apparatus. Electric shock was presented from Grason-Stadler scramblers ( Model 1064GS). Each apparatus unit was housed individually in sound insulated chambers. All stimulus presentations and measurements of responding were controlled by an on-line computer using a system developed at this laboratory ( Francis & Sutherland, 1970).

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Procedure. The procedure involved the following successive phases: Sidman avoidance pretraining; Pretest; Discriminated Shuttlebox avoidSidman-avoidance recovery; Transfer of Control ance/ yoked training; Test. Sidman-avoidance training: Following a 5-min adaptation period in the wheel-turn chambers, all subjects were trained to avoid shocks of 0.5-SW duration. Initially, these shocks were of l.O-mA intensity and presented at I-set intervals (onset-onset time). Whenever an animal made a response by turning the wheel through a quarter-turn, the train of shocks was interrupted for a 20-set shock-free period. Further responses during this interval also postponed shock for a further 20 sec. After 10 min of this training procedure, the shock intensity was reduced to 0.8 mA and the intershock interval raised from 1 to 5 sec. These shock parameters were left unchanged thereafter. Each daily session lasted 50 min. The rats were given daily sessions until they reached a pretraining criterion of less than 20 shocks during the last 40 min of the session. Rats failing to reach this criterion within 4 sessions were discarded. Pretest: The 48 rats which successfully met the pretraining criterion were then divided into groups which would subsequentIy be trained to different avoidance learning criteria in the shuttleboxes (Groups 1, 3, 9 and 27). They received identical training during the pretest phase. The rats were again given Sidman avoidance training in the wheel-turn chambers. After the first 10 min of the session, S-set tone and light stimuli were presented to assess their unconditioned effects upon Sidman avoidance rate. These stimuli were presented intermittently according to a Gellerman sequence at intervals of 85, 115, or 145 set (onset-onset time). For each stimulus presentation, records were taken of the number of responses occurring during each of three successive prestimulus 5-set periods, the stimulus 5-set periods and each of the three successive S-set poststimulus periods. Shocks which occurred during any of these seven successive S-set periods were also noted. The session continued until there had been 12 presentations of each stimulus for which no shock had occurred during any of the seven successive 5set periods of each stimulus presentation. Discriminated shuttIebox avoidance/yoked training: On the following day, a master for each pair of subjects was randomly chosen and trained to avoid shock in one shuttlebox. The remaining member of the pair-the yoked subject-was placed in the other shuttlebox and given exactly the same number, sequence and duration of warning signals, feedback stimuli and shocks as the master rat. The yoked rat had no control over the presentation of these stimuli. Each of the master subjects was trained to avoid using the “method of emergence” (Solomon & Brush, 1956); responses by the master rats during a lo-set interval prior to shock presen-

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tation resulted in immediate warning-signal termination, shock avoidance a.nd a lo-set feedback stimulus presentation. Failure to respond during this interval resulted in discontinuous shock, at an intensity of 0.8 mA, being presented (On-time 0.5 set, Off-time 1.5 set); responses at this time terminated the warning signal and interrupted the train of shocks but no feedback stimulus was given. Tone served as the warning-signal and light as the feedback stimulus for all subjects. Counterbalancing proved impossible in view of difficulties found in pilot work in avoidance training to light warning-signals. The groups were trained for as many trials as were necessary to complete criteria of 1, 3, 9 or 27 successive avoidance responses, given that at least one “escape trial” had occurred. The intertrial interval was 180 sec. Sidman avoidance recovery: The rats were returned to the wheel-turn chambers 24 hr later for a further Sidman avoidance session. No stimuli were presented during this session. Transfer of control test: The test session in the wheel-turn chambers began on the following day. The full Sidman avoidance schedule was in effect throughout this session. The tone and light stimuli were presented exactly as they had been in the Pretest session described above. The session terminated after 12 non-shock presentations of each stimulus and the patterns of response to these stimuli were recorded as described above. Results Sidmun avoidance: Baseline rates. The baseline rates of response during the Pretest and the Test session were stable at approximately 45 responses per min (rpm). An analysis of variance was conducted on the square-root transformation (to meet assumptions of homogeneity of variance) of the baseline rates of response during each of these sessions, across each of the three successive baseline 5set periods prior to the stimulus presentations. No significant differences were found between Groups, or between the master and yoked subjects of each Group. Neither Test-sessions nor Type of Stimulus to follow had a significant effect upon baseline rate either (AI1 Fs < 1). Shuttlebox avoidance performance. The master subjects of Groups 1, 3, 9, and 27 completed their training criteria in 12.3, 15.3, 26.6, and 57.8 trials respectively, performing 1, 3.8, 11.2, and 43.2 avoidance responses in SO doing. Kruskal-Wallis analyses of variance were conducted to see if the groups differed in their speed to complete these criteria. The four groups did not differ in the number of trials to complete a criterion of 1 avoidance response ( N( 3) = 0.10, p > O.(X), the three remaining groups did not differ in completing the criterion of three successive avoidance responses (H( 2) = 0.20, p > 0.05) and Groups 9 and 27 did not differ

COXDITIONED

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in completing the criterion of nine successive avoidance responses (H( I ) = 0.02, p > 0.05). Sidman avoidance: Analyses of responses to the stimuli. Further analyses were complicated by the observation that response rate changes elicited by the stimuli were often sustained for several seconds after their termination. Since it is proper that such sustained patterns of response bf, included in a full comparison of the different groups, measures of thei] responding were taken from the stimulus and the following two S-SW periods. A difference score, D, was calculated for each of these three periods given by D = A - B where A is the square-root of the rate of response for a particular 5-set period and B is the mean of the square-roots of the rates of response of each of the three prestimulus 5-set periods. The groups were compared on these D scores with the within-subjects factor Periods always referring to the scores derived from the successive stimulus, +5 and + 10 set periods. Separate Scheffb tests were conducted to compare the rate of response during or after the stimulus presentations with the baseline rate. Pretest. The tone appeared to elicit a small within-stimulus rate increase and a post-stimulus rate decrease as shown in Fig. 1, whilst the unconditioned effect of the light was confined to a post-stimulus rate increase. An analysis of variance of the effects of the tone upon the rate of Sidman avoidance responding showed neither Groups (F < 1) nor Yoking (F( 1,30) = 2.41, p > 0.10) effects. For purposes of clarity, the data have been collapsed across the Master/Yoke factor in Fig. 1. There was a significant change in rate across the three successive 5-set measurement periods ( F (2,80) = 7.69, p < 0.001) and subsequent Schefft! comparisons. conducted at the 10% level as recommended (L4yers, 1966, p. 3%). showed PRETEST +L

us (LIGHTI

-:I, -15

-10

-5 cs +5 40 85 5 SEC PERIODS

FIG. 1. Espt 1: The patterns of wheel-turn responding to presentations of the warning stimulus ( WS ) and feedback stimulus (FS) during the Pretest. Note the rate deceleration after WS offset and the rate acceleration after FS offset. The square-root difference score, D, is explained in the text.

430

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G. M.

MORRIS

the rate of response during the stimulus period to be elevated above baseline (criterion F = 4.70; F = 6.56) while that during the + 5set period was depressed (F = 7.79). A parallel anaIysis of the effects of the light also showed only a significant Periods effect ( Groups: F( 3,40) = 1.59, p > 0.20; Yoking: F < 1; Periods: F( 2,80) = 16.23, p < 0.001). Scheffe tests revealed that the rates of response during the +5- and +lO-see periods were elevated above baseline (Fs = 31.54 and 19.67 respectively) while the rate during the stimulus period itself did not significantly deviate from the baseline. Transfer of control test. Presentation of the warning-signal during the test session after the shuttlebox avoidance/yoked training elicited a sharp rate increase in all groups, with Group 3 showing a slightly larger mean increase than the other groups. These results are shown in Fig. 2. Some individual subjects reached momentary rates during the warning-signal in excess of 140 rpm although the more typical pattern was a doubling in rate from 45 to 90 rpm. The groups showed a graded differential response to presentation of the feedback stimulus. No differences between Master and Yoked rats were observed in their patterns of response to either stimulus. An analysis of variance of the effects of the warning-signal upon Sidman avoidance responding showed only a significant Periods effect (Groups: F(3,20) = 2.79, 0.10 > p > 0.05; Yoking: F < 1; Periods: F(2,40) = 12.47, p < 0.001). Scheffe comparisons indicated that rate during each of the three successive measurement periods was elevated above baseline (criterion F = 4.70; Fs = 116.29, 103.92, and 20.14 for the stimulus, +5and -I- IO-see periods, respectively). A quite different picture emerged from the analysis of the effects of the feedback stimulus. There were significant Groups (F( 3,20) = 4.48, TEST +L

FS (LIGHT)

5 StC

FIG. 2. Expt 1: The patterns of wheel-turn and FS during the Test Session. Note that arc immediate unlike WS offset.

PERIODS

responding to presentations the rate decreases induced

by

of the WS FS onset

CONDITIONED

INHIBITORS

ASD

AVOIDANCE

43 1

p < 0.025) and Periods (F( 2,40) = 10.43, p < 0.001) effects. The master and yoked subjects did not differ (F < 1). Subsequent orthogonal comparisons comparing the groups across all three measurement periods showed that Groups 9 and 27 responded more slowly during the feedback stimulus than Groups 1 and 3 ( F( 1,20) = 14.69, p < 0.005) while neither Groups 1 and 3, nor Groups 9 and 27 did themselves differ ( P’s < 1) . Separate Scheffe tests comparing the rate of response during the stimulus period with the baseline showed only Group 27 to be significantly lower (criterion F = 6.63; F = 9.94). Finally, a Spearman rank-order analysis of correlation revealed that the magnitude of the reduction in response rate during the feedback stimulus was significantly related to the number of feedback stimulus presentations that had occurred during avoidance/ yoked training (correlation coefficient = 0.43; t( 38) = 2.25, p < 0.025. one-tailed ) .

The results indicate that both conditioned excitation and conditioned inhibition of fear are developed during shuttlebox avoidance/yoked training and that they are conditioned to the warning and feedback stimuli respectively. There appears to be little change in the excitatory effect of the warning stimulus over the course of training studied. The inhibitory effect of the feedback stimulus develops monotonically as shown by ( 1) the graded pattern of response rate reductions of the groups to its presentation during the test session, and (2) the significant correlation between these rate reductions and the number of feedback stimulus presentations received during avoidance/yoked training. The absence of any differences between master and yoked subjects throughout testing strongly suggests that the present test procedures measure purely Pavlovian “events” that occur during discriminated avoidance behavior. Although the possibility that some superstitious operant conditioning did occur for the yoked subjects cannot be ruled out (See Black 1971 for a thorough review of this issue), the failure to find any differences between master and yoked rats is incompatible with an interpretation of transfer of control which appeals to mediating operant rt’sponses ( Trapold & Overmier, 1972). This indifference between master and yoked rats also indicates that control over warning-signal termination and feedback stimulus presentation has no influence on the conditioning of excitation or inhibition of fear. In respect of conditioned excitation, this result extends those of Brimer and Kamin (1963) and Payne (1972) in showing that indifference to control holds true across a wide range of training trials, The absence of any influence of control over feedback stimulus presentation upon inhibitory

432

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fear conditioning replicates Weisman and Litner’s (1972) results based on one-way avoidance and, together with their data, casts doubt upon Belles and Grossen’s (1970b) prediction that response-produced feedback stimuli would be more effective than noncontingent feedback stimuli. The present demonstration of a correlation between the level of avoidance training and the extent to which a feedback stimulus inhibits fear is consistent with Weisman and Litner’s (1972) contention that inhibitors of fear may be the main source of reinforcement for avoidance learning. However, the present results provide no basis for any exclusive emphasis upon inhibitory mediation since there was no indication that the warningsignal’s excitatory properties declined as training continued, a surprising result in view of several demonstrations that fear extinguishes as a consequence of nonreinforced exposure in both Pavlovian (Kalish, 1954; McAllister & McAllister, 1971) and avoidance learning paradigms (Kamin et al., 1963; Linden, 1969; Weisman & Litner, 1972). While differences in test sensitivity may be the main cause of this discrepancy, one aspect of the present procedure distinguishing it, for example, from Kamin et d’s (1963) procedure is the presence here of an inhibitory feedback stimulus during the later stages of training immediately after the nonreinforced warning-signal presentations on successful avoidance trials. Konorski (1948) and Soltysik and Kowalska (1960) have argued that a serially presented inhibitor “protects” fear conditioned to the warning-signal, presumably because the inhibitor blocks the nonreinforced exercise of the excitatory fear reaction which would otherwise continue for several seconds after warning-signal offset. While the present weight of evidence argues against the viability of this proposal (Black & Dalton, 1965; LoLordo & Rescorla, 1966; Seligman & Johnston, 1973), a further examination of this possibility in a separate experiment seemed worthwhile. EXPERIMENT

2

LoLordo and Rescorla’s (1966) transfer of control experiment conducted to test the Konorski/Soltysik protection hypothesis showed no difference in the rate of extinction of fear conditioned to two initially excitatory stimuli one of which was followed by an inhibitor of fear on each of 36 formal Pavlovian extinction trials. However, as the transfer test procedure used to monitor the fearfulness of the conditioned stimuli involved at least 80 presentations of each stimulus alone, any protection effect that did exist may have been masked by extinction, during these test sessions, of fear conditioned to the ostensibly protected conditioned stimulus. In addition to substantially reducing the ratio of transfer test trials to formal Pavlovian extinction trials, the present study differed from LoLordo and Rescorla’s (1966) experiment in attempting to study the putative

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protection process as it might actually occur during avoidance maintenance. TWO modifications of the procedures used in Expt 1 were introduced: First, a different strain of rats was used because of the excessive failure rate during Sidman avoidance pretraining in the previous experiment; second, the experiment began with the shuttlebox avoidance/yoked training phase and was then followed by the Sidman avoidance pretraining and test sessions. This reversal of the Pavlovian and instrumental training phases should have no untoward consequences upon the transfer results other than what Overmier and Leaf (1965) d escribe well as arising from “some correlated consequence of the order of training procedures, such as different retention intervals” ( p. 217). Method Suljjects. Thirty-two male rats of the RHA strain (Roman High Avoidance, Broadhurst & Bignami, 1965) were used as subjects. They weighed between 200-259 g and were aged approximately 60 days. All rats were housed in individual cages and maintained on ad lib. food and water. Apparatus. Both wheel-turn chambers and one of the two shuttleboxes used in Expt 1, and three identical Pavlovian conditioning chambers were used for this study. Each of the Pavlovian conditioning chambers was 25 cm square X 25 cm high. Three of the side walls were made of aluminium and painted matt black while the fourth wall was made of translucent white plastic behind which was a lamp illuminating the chamber at 0.3 log ft-L. A speaker was mounted on the hinged clear plastic ceiling. The floor consisted of 11 stainless steel bars, 0.7 cm in diameter spaced 1.4 cm apart. Electric shock was presented to the grid floor from Grason-Stadler scramblers. Procedure. The procedure involved the following successive phases: shuttlebox avoidance/ yoked training 1; Sidman avoidance pretraining; transfer of control test 1; shuttlebox avoidance/yoked training 2; transfer of control test 2. The basic design involved yoking three separate groups of subjects to a single group of master subjects, Comparisons between these yoked groups provide a test of whether an inhibitor of fear can protect fear conditioned to a nonreinforced excitatory stimulus immediately preceding it. Eight rats were placed in each of the four groups. The master subjects (Group M) were trained to avoid in the shuttleboxes exactly as described in the previous experiment excepting that (1) they were each run for exactly 61) trials, and (2) both avoidance and “escape” responses were followed by feedback stimulus presentations (as in Weisman & Litner, 1972). The yoked rats were placed in one of the three identical Pavlovian

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conditioning chambers and received the following training. Group Y received exactly the same sequence and duration of warning signals, feedback stimuli and shocks as their respective masters. Group RFS received exactly the same sequence and duration of warning signals and shocks at exactly the same intervals as their masters, but the feedback stimuli were delivered randomly according to a Variable Time 180 set schedule (range p444 see), with additional allowance made for the response latencies of the master subjects. This allowance necessitated starting Group RFS 30 min after the other groups. Group C was divided into two parts: Four subjects received the same sequence and duration of warningsignals and feedback stimuli as their masters but no shocks were delivered (Habituation control), while the other four subjects of Group C received the same sequence and duration of shocks as their masters but neither warning signals nor feedback stimuli ( Sensitization-control). Subsequent training sessions were identical to those of the previous experiment and run in the order described at the outset of this section. The second shuttlebox avoidance/yoked session was identical to the first, and the full Sidman avoidance schedule was in effect throughout each of the two identical transfer of control tests. The criterion of 12 no-shock presentations of each of the two test stimuli remained in force during both test sessions. Results Shuttlebox avoidance performance. The mean shuttlebox avoidance performance of Group M during sessions 1 and 5 is summarized in Table 1. Although the RHA rats used in this study showed a higher rate of anticipatory responding than the Lister rats of Expt 1 (as indicated by a comparison of the trial of the first avoidance response: Mann-Whitney U = 0, p < O.Ol), these RHA rats showed deteriorating performance toward the end of the first session. All but one subject in Group M showed poorer avoidance in the final block of trials than attained earlier in the session. However, this deterioration was not permanent as the avoidance per-

Mean

Shuttlebox Trial

Session 1 5

TABLE Performance

Avoidance

First avoidance 4.9 -

number

of

Fifth avoidance 12.4

1 of Group

Longest run of successive avoidance 23.6 25.9

M during Number

Sessions

1 and 5

of avoidances in each block of ten trials

1

2

3

4

6

6

4.4 7,s

7.8 8.8

8.3 8.5

8.6 8.8

7.4 8.5

5.6 9.1

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formance during the second shuttlebox avoidance session was stable between 80 and 90%. Sichmn-au&dance acquisition and test-session baseline performance. The differential treatment of the four groups in the first shuttlebox avoidance/yoked training session had no significant effect upon the shock rates ( Kruskal-Wallis H = 1.54, p > 0.50) or response rates (H = 2.60, p > 0.30) during the first Sidman avoidance session in the wheel-turn chambers on the following day. Thcb baseline rates of response of the groups did not differ during the two test sessions (F( 3,28) = 1.04, p > 0.20) though there was a slight decline in mean response rate from 66 rpm during Test 1 to 53 rpm during Test 2 (F( 1,28) = 7.06, p < 0.025). The mean baseline rate in the three 5set periods prior to the warning stimulus presentations (57 rpm) was slightly less than that prior to feedback stimulus presentations (61 rpm ) and the effect was significant (F( 1,28) = 6.48, p < 0.025). However, it is unlikely that this small difference could have contributed to the quite different patterns of response that occurred to these stimuli during the tat sessions. Transfer of control test. The changes in Sidman avoidance responding induced by test presentations of each stimulus are shown in Figs, 3 and 4. Groups M, Y and RFS all show rate elevations to presentations of the warning-signal during both Tests 1 and 2 but there is no indication of any differential response to the warning-signal by these groups. Responserate reductions to the feedback stimulus are apparent in Groups M and Y during both test sessions. An analysis of variance of the effects of the warning-signal upon Sidman avoidance responding gave significant Periods (F( 2,56) = 28.16, p <

Fx:. Groups

3. Expt 2: Wheel-turn responding during Test 1. Note the separation between hl, Y and RFS on the one hand and Group C on the other in response to the warning stimulus ( WS ) Onset of the feedback stimulus ( FS ) resulted in an immediate decline in rate in Groups hl and Y.

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TEST 2 E+~ ws (TONS

-15 -10 -5

+L Fs (LIGHT)

4

cs

+5

40 +15

FIG. 4. Expt 2: Wheel-turn responding shows a rate increase to warning-signal Groups M, Y and RFS in showing the parent in Test 1.

-15 -10 -5

during onset. biphasic

1

cs .5 +I0 +I5 5 SEC PERIODS

Test 2. Unlike Test 1, Group C now However, Group C still differs from response pattern to the WS first ap-

0.001) , Groups X Periods ( F( 650) = 3.22, p < 0.01) and Groups X Periods x Test-sessions (F( 656) = 2.61, p < 0.05) effects only. Subsequent Scheffe tests were conducted to ascertain the nature of these effects. The mean response rate of the combined Groups M, Y and RFS during the warning-signal 5-set period was significantly greater than that of Group C during Test 1 (Criterion F = 12.39; F = 14.56) but not during Test 2 (F = 2.23). Moreover, Groups M, Y and RFS showed a significant mean response rate increase above the baseline rate during this same warning-signal period in both Test 1 (Criterion F = 12.39: F = 44.36) and Test 2 (F = 83.32), while Group C only showed a rate elevation during this period in Test 2 only (Test 1: F < 1; Test 2: F = 12.58). Finally, trend analyses of the response rate changes during and after the warning-signal test period collapsed across the two test sessions showed that there was a significant linear decline in rate from the warning-signal to the + IO-set period confined to Groups M, Y and RFS ( F, I,( 1,56) = 32.83, p < 0.001; Fquad < 1) whereas Group C showed only a significant biphasic response pattern across the warning-signal, +5- and + lo-set periods (Flin < 1; Fauad( 156) = 18.05, p < 0.001). This last dissociation among the four groups indicates that the persistence in responding after the warning-signal in the test situation may be an extremely sensitive indicant of the presence of small excitatory effects. An analysis of variance of the effects of the feedback stimulus upon Sidman avoidance responding revealed significant Groups ( F ( 3,28) = 9.41, p < 0.001 ), Periods ( F( 256) = 23.09, p < 0.001) and Groups x Periods (F( 6,56) = 6.61, p < 0.001) effects. Perhaps the most striking feature of the results is the similarity between Test 1 and Test 2 of the patterns of response for the four groups: neither Test sessions, nor any

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interactions involving Test sessions, were significant. Dunnett’s test was used to compare the mean response rate of each group, collapsed across all three of the measurement periods of both Test sessions, with the mean performance of Group C: both Group M (d = 3.10, p < 0.01) and Group Y (d = 3.76, p < 0.005) differed significantly from Group C but Group RFS did not (d < 1). Finally, Scheffe tests were conducted to compare the mean response rate of each of the groups during the feedback stimulus period of both test sessions with the baseline rate. Groups M and Y showed significant rate reductions (Criterion F = 6.84; Fs = 47.75, 42.85. respectively) and Group RFS showed a significant rate elevation (F = 12.75). Group C showed no significant change in rate during this stimulus period (F < 1).

Discussion These data confirm and extend those of the preceding experiment in showing that the rate-reductions elicited by the feedback stimulus in Groups M and Y are conditioned effects and cannot, therefore, be accounted for in terms of habituation or sensitization. The results also show that the conditioned inhibitory properties of the feedback stimulus are unchanged after sixty further trials of well maintained shuttlebox avoidance behavior. A surprising feature of the results is that the warningsignal also appears to maintain its conditioned excitatory properties over the course of this more extended shuttlebox avoidance training. These data are inconsistant with the Konorski/Soltysik protection hypothesis. Neither the performance of the avoidance movement (Konorski, 1948) nor receipt of fear-inhibiting feedback stimulation after warning signal offset (Soltysik & Kowalska, 1960) by the subjects of Groups M and Y influenced the rate of extinction of conditioned excitation in Groups M, Y and RFS. However, because no extinction effect occurred at all, the absence of any differential extinction effect should only be seen as evidence that protection effects are unimportant during the initial stages of avoidance maintenance rather than as evidence that they do not occur. The persistence of conditioned excitatory effects in this experimental paradigm as compared with the conditioned suppression technique used by Kamin et al. (1963) and Linden (1969) may reflect, as noted above in the discussion of Expt 1, differences in test sensitivity. Some independent evidence bears on this issue: Scobie (1972, Expt 4) has shown that a weak excitatory stimulus whose presentation accelerates the rate of a free-operant shuttlebox avoidance response may not necessarily suppress the rate of appetitive bar-pressing, and Reberg (1972) has shown that compound summation tests upon an appetitive baseline are more sensitive than single-stimulus tests. Both studies point to an insensitivity in Kamin

438

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et aZ.‘s (1963) and Linden’s (1969) experiments which utilized single stimulus tests against appetitive baselines of bar-pressing. However, this discussion leaves open the question of why the results of Expt 1 and 2 differ from those of Weisman and Litner ( 1972) who, also using a wheelturning avoidance apparatus, found little evidence of conditioned excitation following one-way training. However, there is no guarantee, in their one-way procedure, that the tone warning-stimulus was actually attended to by the animals of their Groups SS: it may have been “overshadowed” by situational cues of the danger compartment. It would appear that in a task like the two-way shuttlebox where the animal must use the explicit warning signal controlled by the experimenter, this stimulus may maintain its fear exciting properties longer than is generally thought to be the case. EXPERIMENT

3

Demonstrations that conditioned fear inhibiting stimuli are developed during shuttlebox avoidance learning constitute unconvincing evidence that they play any important mediational role. It is possible to argue ( 1) that inhibition is inevitably conditioned to experimentally introduced feedback stimuli because sufficient conditions for its development are necessarily embedded within the avoidance learning paradigm, but (2) that it serves no function. On this view, inhibition is developed as a consequence of the animal learning to avoid rather than vice versa. One way to counter this argument is to show that parameters known to affect inhibitory fear conditioning also have some corollary effect upon the rate of avoidance learning. Unfortunately, any such simple demonstration would still be subject to the criticism that variation in the speed or pattern of avoidance acquisition was, in fact, directly influenced by the parameter in question and that this variation in the pattern of instrumental avoidance behavior in turn influenced the magnitude of Pavlovian inhibitory fear conditioning occurring during avoidance learning. Any adequate experimental design must control for this possibility. One parameter known to affect inhibitory fear conditioning is intertrial interval. Both Moscovitch and LoLordo (1968) and more recently, Weisman and Litner (1971) have shown that stimuli must be followed by a long, reliable shock-free interval for them to develop inhibitory properties, It is also known that lengthening intertrial interval increases the rate of acquisition of avoidance learning (Brush, 1962; Denny, 1971), and Weisman and Litner ( 1971) have suggested that this may be caused by direct effects of variation of intertrial interval upon inhibitory fear conditioning. The present experiment tests this prediction by means of a design which teases apart direct effects of intertrial upon inhibitory fear conditioning

CONDITIONED

INHIBITORS

AND

AVOIDANCE

139

from those mediated indirectly via differences in the speed or pattern of avoidance acquisition correlated with variation in intertrial interval. Two groups of rats were given shuttlebox avoidance training at either a 30- or 180-set intertrial interval, and two groups of rats were yoked to each of these master groups in the Pavlovian conditioning chambers. One group of each pair of yoked rats received a 30-set intertrial interval. the other received a 180-set intertrial interval. Comparison of one of these pairs of yoked groups with the other pair provides a measure of the influence of the pattern of avoidance acquisition upon inhibitory fear conditioning while comparisons within pairs provides a measure of any direct effect of intertrial interval. Three other control groups were also run as described below.

Method Subjects. The subjects were 72 male and female rats of the RHA strain. Apparatus. One shuttlebox, two wheel-turn chambers and three Pavlovian conditioning chambers were used as the warning-stimulus and onset of the lamps as the feedback stimulus for all groups. Procedure. The procedure followed invoIved the following successive phases: shuttlebox avoidance training, Sidman avoidance pretraining, transfer of control test. The subjects were randomly assigned to nine groups of eight rats with four rats of each sex per group. One group of master subjects (Group M180) was trained to avoid in the shuttlebox for 60 trials by exactly the same procedure as outlined earlier for Expt 2. Both avoidance and escape responses were followed by feedback stimulus presentations. Successive trials were separated by an intertrial interval of 180 sec. Three groups were yoked to this master group in the Pavlovian chambers: Group Y180/M180 received exactly the same sequence and duration of warning signals, feedback stimuli and shocks as the master, spaced at the same intertrial interval; Group Y30/ Ml80 received the same sequence and duration of shocks, warning and feedback stimuli but its trials were separated by an intertrial interval of 30 sec. This was done by recording the trial by trial latency of the master subjects and then retrieving it later and presenting the trials at a 30-set intertrial interval. The session for this group began 2 hr after the start of Group M18O’s session. The third yoked group (Group Y3O/N.US) also received the same sequence and duration of warning and feedback stimuli as Group Y30/ Ml80 but no shocks (Habituation control). The other group of master rats (Group M30) was trained to avoid in the shuttlebox at an intertrial interval of 30 sec. There were three groups yoked to this master group also: Group Y30/M30 received the same sequence and duration of warning and feedback stimuli and shocks at an

440

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MORRIS

intertrial interval of 30 set; Group YlSO/M30 received the same event sequence at an intertrial interval of 180 set; Group Y180/RFS received exactly the same sequence and duration of warning stimuli and shocks, but the feedback stimuli were presented randomly according to a VT 180 set schedule as in the preceding experiment (truly random control). The ninth and final group was a “Novelty” Control group ( Group N.C) . These rats remained in their home cage for session 1 of training. All rats were subsequently given two sessions of Sidman avoidance pretraining followed by a single transfer of control test. These sessions were conducted as described earlier: The full Sidman avoidance contingency was in effect throughout testing and the test session terminated only after 12 non-shock presentations of each of the two test stimuli. Results Shuttlebox avoidance behavior. The mean shuttlebox avoidance performance of the two groups of master rats is presented in Table 2. It is clear that both groups Iearned to avoid effectively with Group Ml80 showing rather more rapid acquisition during the early training trials. Mann-Whitney U tests confirmed this by showing that the groups did not differ in the number of trials taken to the first avoidance response ( U = 28.5, p > 0.36); that Group Ml80 took less trials to the arbitrary criterion of 5 avoidance responses both from the outset of training ( U = 10.5, p < 0.01) and from the trial of the first avoidance response (U = 14, p < 0.05); and finally, that Groups Ml80 and M30 did not differ in terms of their overall avoidance performance (U = 22, p > 0.16). These results may be conveniently summarized by saying that intertrial interval appears to have only affected the speed of the transition from escape to avoidance responding. The response rates of Groups Ml80 and M30 during the intertrial interval were 0.18 and 0.24 rpm, respectively. An anaIysis of variance of intertrial responding revealed no significant differences between Groups

Mean

Shuttlebox

Trial

Group Ml80 M30

TABLE 2 Performance of Groups

Avoidance

First avoidance 4.9 6.9

number

of

Fifth avoidance 12.1 20.9

M30

and Ml80

during

Nllmber

of avoidances in each block of ten trials

1

2

3

4

5

6

3.4 1.5

8.1 4.9

8.2 8.2

9.1 8.5

9.1

9.0

8.4

8.8

- ~

Session

Percent avoidance 78.3 66.5

1

CONDITIONED

INHIBITORS

AND

441

AVOIDANCE

(F < l), across Blocks (F < l), nor any interaction ( F( 5,70) = 1.48, p > 0.20). Sidrnan avoidance: Baseline rates of response. The mean baseline response rates of the nine groups varied between 60 and 90 rpm. NO significant Groups, Sex, Successive 5-set periods, or Type-of-CS-to-follow effects were found in the routine analysis of the square-root transformation of these rates. Sidmn avoidance test session: Patterns of response to the rcatnitzg signu2. The patterns of response to the warning-signal of each of the nine groups is shown in Fig. 5. Each of the seven groups for whom this stimulus was paired with shock showed equivalent response rate increases, with a clear difference emerging between these groups and Group Y30/ N.US, the habituation control group. Group N.C showed a rate increase during the warning stimulus period but this was not sustained into successive measurement periods. An analysis of variance of these data revealed significant Groups (F( 8,54) = 3.32, p < 0.005) and Periods (F( 2,108) = 29.3, p < 0.001) effects. Subsequent orthogonal comparisons showed that Groups Ml80 and M30 did not differ (F( 1,54) = 3.35, 0.10 > p > 0.05) and that the four Groups Y180/M180, Y180/M30, Y30/M180, Y30/M30 did not differ either (Fs < 1). However, comparisons of the three control groups showed that Group Yl8O/RFS, the only one of these three for whom the warning signal was paired with shock, responded faster to its presentation than did Groups Y30/N.US and N.C considered together (F(1,54) = 4.9s, p < 0.05), and that Group N.C responded faster than Group Y30/ N.US (F( 1,54) = 4.02, p < 0.05). These contrasts indicate that the TEST

WS (TONE)

-15

-10

-5

cs *5 .lO *15 5 SEC PERIODS

FIG. 5. Expt 3: The patterns of response to the warning-stimulus session. Rate increases are evident in each of the groups for whom was paired with shock and in Group N.C also.

during the test the warning signal

442

R.

TEST

Lw--

G.

M.

MORRIS

FS (UWT)

-Y 30iNUS

& 3 +--YltlO/RFS 3-4 t B- NC

-4

-v30/?.430 -15 -10 -5

1 cs +5 +lO .15 5 SEC PERIODS

FIG. 6. Expt 3: The patterns of response to the feedback-stimulus (FS) during the session. The intertrial interval received during shuttlebox avoidance/yoked training is the sole determinant of the rate decreases to the FS in the four yoked groups shown in the right-hand panel.

test

warning stimulus possesses some unconditioning excitatory properties (as was also seen in Expt 1) but that these habituate with repeated nonreinforcement. Sidmun avoidance test session: Patterns of response to th feedback stimulus. The patterns of response to the feedback stimulus are shown in Fig. 6. The data show a clear separation between Groups Ml80 and M30 and a substantial inhibitory effect in Groups Y180/M180 and Y180/M30the yoked groups which received the conditioning trials at an intertrial interval of 180 sec. An analysis of variance of these data revealed significant Groups ( F( 8,54) = 5.07, p < 0.001)) and Periods (F( 2,108) = 35.21, p < 0.001) effects and a Groups x Periods interaction of borderline significance (F( 16/108) = 1.76, p = 0.05). Orthogonal comparisons of the patterns of response summed across all three periods indicated that Group Ml80 responded slower than Group M30 ( F( 1,54) = 4.64, p < 0.05) ; that Groups Y180/M180 and Y180/M30 considered together differed significantly from Groups Y30/M180 and Y30/M30 (I?( 154) = 10.98, p < 0.005); and that Groups Y180/M180 and Y30/M180 did not differ from Y180/M30 and Y30lM18O (F < 1). Group N.C responded faster than Groups Y180/RFS and Y30/N.US (F( 1,54) = 7.35, p < 0.01) which in turn did not differ (F < 1). This latter pair of comparisons indicate that repeated presentations of the feedback stimulus reduced its unconditioned excitatory effects. Finally, Scheffk comparisons were conducted to compare the rates of response during the Feedback stimulus 5-set period with the baseline rate. Groups M180, Y180/M180, and Y180/M30 collectively showed a

CONDITIONED

INHIBITORS

AND

AVOIDANCE

-l43

significant rate reduction (criterion F = 14.16: F = 40.75); 22 of the 24 subjects showed the effect. However, Groups M30, Y30/M180, and M30/ M30 did not show a significant rate reduction (F = 11.25); only 15 of the 24 subjects responded below the baseline rate during the feedback stimulus presentations.

Discussion These data show that variation in intertrial interval has direct effects upon inhibitory fear conditioning in an instrumental situation. Inhibitory fear conditioning to the feedback stimulus only occurred when the interval between the trials received was long. Various control groups ensured that the effects observed were truly conditioned effects. Variation in intertrial interval was also correlated with a change in the speed of the transition from escape to avoidance responding but had no detectable indirect effect upon either excitatory or inhibitory fear conditioning. The absence of any indirect effect upon inhibitory fear conditioning is unlikely to be due to test insensitivity since the procedure is clearly sensitive to the direct effects of intertrial interval, These and other aspects of the data suggest that the existence of an inhibitory feedback stimulus may have been responsible for the rapid initial avoidance learning. First, the experiment has adequately teased apart the direct and indirect effects of intertrial so it is clear that the rapid initial avoidance learning did not cause the conditioning of inhibition Second, an explanation of the effect of intertrial interval in terms of intertrial interval responding (Pearl & Fitzgerald, 1966) is not supported because Groups M30 and Ml80 did not differ with respect to intertrial response rate. Third, Cicala, Masterson, and Kubitsky’s (1971) “initial response rate” hypothesis would appear to demand that Groups M30 and Ml80 should differ from the outset of training. In fact, these groups did not separate until after the first avoidance response. The data are, however, consistent with Weisman and Litner’s (1971, 1972) contention that intertrial interval has a direct effect upon the underlying Pavlovian inhibitory mediator which contributes to the reinforcement available for avoidance responding. This argument cannot be criticised on the grounds that the difference observed in the rates of acquisition of shuttlebox avoidance behavior by the two master groups was too small to be important. For, although the difference was small, it occurred at precisely that point in training thought to be critical by “cognitive” theories of avoidance learning (D’Amato, 1968; Morgan, 1968; Ritchie, 1951; Seligman & Johnston. 1973). In the most formally stated of these theories, Seligman and Johnston’s model, emphasis is placed upon the preference made by the animal

444

R.

G.

M.

MORRIS

between two anticipated outcomes-shock and nonshock-with the assumption made that only response-outcome expectancies are formed. The failure to recognize a possible role for event-event expectancies is a serious deficiency of Seligman and Johnston’s model as it now stands because it is thereby forced to account for transfer of control phenomena exclusively in terms of interactions between response- and non-response-outcome expectancies, an account which is logically identical to operant mediational accounts of transfer and which fails for the same reasons (Trapold & Overmier, 1972). However, the present data would be compatible with cognitive theories which do recognize the possibility of interactions between event-event and response-outcome expectancies (e.g., Bolles, 1972b). However, these results impose constraints upon the content of the representations formed of the experimental contingencies operative during the initial stages of acquisition. That is, in the language of cognitive theories, the demonstrated dependence of inhibitory conditioning upon intertrial interval implies that feedback stimuli actually embody knowledge about the nonarrival of shocks for some long period of time. The present results also relate to those of experiments which have studied the effects of presentation or removal of a feedback stimulus upon the acquisition or extinction of avoidance behavior respectively. Zerbolio (1968) found that rats would learn to avoid faster (slower) if trained to run towards (away) from a stimulus which had previously been associated with a long period of “non-shock confinement,” although he interpreted these findings within the framework of Relaxation theory (Denny, 1971). I n an acquisition study using a fairly long inter-trial interval (90 set), Bolles and Grossen ( 1969, Expt 3) found that introduction of a feedback stimulus ameliorated most of the deficit attributable to warning-signal termination delay, while Katzev and Hendersen (1971, Expt 1) using a short intertrial interval (mean 35 set, range 15-75 set) found that the presence or absence of a feedback stimulus during extinction had no effect on persistence. These results are all readily interpretable in terms of the present hypothesis that feedb’ack stimuli will have little or no effect upon performance unless they are inhibitors of fear. They will only be fear inhibitors when associated with long intertrial intervals. These experiments have shown that exteroceptive feedback stimuli develop fear inhibiting properties during shuttlebox avoidance learning. The development of these properties is independent of whether the animal is actually learning to avoid or passively receiving the equivalent noncontingent events. It is sensitive to the effects of intertrial interval during this learning task in exactly the same way as it is during conventional Pavlovian conditioning (Weisman & Litner, 1971). Although these experiments do not preclude the possibility that feedback stimuli may

CONDITIONED

develop other properties that do contribute to their effects less support the view that the understood within the context

INHIBITORS

AND

AVOIDASCE

445

are not revealed by transfer techniques but upon avoidance learning, the data neverthefunction of feedback stimuli can be properI) of mediational two-process learning theory. REFERENCES

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