The effects of shock intensity on fear incubation (enhancement): A preliminary investigation of Eysenck's theory

CWS-7967:X1 050413-06502.00~0 Copvr&rht 0 19x1 Perpamon Press Ltd

THE EFFECTS OF SHOCK INTENSITY ON FEAR INCUBATION (ENHANCEMENT): A PRELIMINARY INVESTIGATION OF EYSENCK’S THEORY THOMAS L. BOYD Divisionof Social and Behavioral Sciences, Untversity 171 University

Parkway,

(Rrccired

Aiken,

18 Nolemher

of South Carolina SC 29801. U.S.A.

at Aiken.

1980)

Summary-Rats were presented with three levels of shock intensity during acquisition followed by three levels of nonreinforced CS exposure during extinction (3 x 3) in the context of a one-way avoidance situation Extinction effects were directly dependent upon the diflerential shock-intensity levels which were delivered during acquisition. Avoidance behavior was sustained and. in fact. increased (incubation) for high-shock groups despite increased duration of nonreinforced CS exposure. These results were interpreted as support for Eysenck’s (1968) theory of fear incubation.

INTRODUCTION

The presentation of the conditioned stimulus (CS) in the absence of the aversive unconditioned stimulus (US) satisfies the procedural condition for Pavlovian extinction. One would expect the classically conditioned fear response to diminish during extinction procedures. Under certain circumstances. however. the strength of a classically conditioned fear response increases in the absence of further conditioning trials during the time in which the CS is presented without reinforcement (Sanderson et al.. 1962; Rohrbaugh and Riccio. 1970; Rohrbaugh et al., 1972). Eysenck (1968) referred to this paradoxical enhancement of fear as ‘incubation’. While fear incubation presents a critical theoretical dilemma, certain practical considerations apply as well. Behavioral models of psychopathology have described the development and maintenance of neurotic symptoms in terms of classical conditioning of fear to stimuli (Levis and Boyd. 1979). Accordingly. treatment often involves certain fear-reduction techniques. Most notable from the perspective of this research are those techniques which purport to use a basic Pavlovian extinction procedure (i.e. flooding and implosive therapy). The incubation phenomenon as described here necessarily has either dictated important procedural alterations of the extinction techniques (Rachman. 1969; Riccio and Silvestri, 1973); or, in some cases, investigators have abandoned such extinction procedures altogether as potentially harmful (Morganstern. 1973). To the extent that research will be able to provide the empirical boundary conditions of incubation with an eye toward a clearer theoretical understanding, more considered treatment decisions may be possible for behavioral disorders which are primarily due to fear. While a variety of theoretical explanations of fear incubation exist (e.g. Denny and Dmitruk, 1967; Gordon er al.. 1979; Miller and Levis, 1971; Quattlebaum, 1970; Rescorla. 1973; Watts. 1979). perhaps most cited is that of Eysenck (1968). Eysenck’s (1968) explanation stressed the role of ‘nocive’ or aversive response-produced properties of the fear CR in aversive conditioning. Eysenck noted that the presentation of the CS unaccompanied by the US in extinction always provokes a decrement in the strength of the CR. consistent with Pavlov’s (1927) notion of extinction. Eysenck also suggested, however. that following conditioning situations during which the CR would be expected to be exceptionally strong (e.g. when the US was strong). incubation should result. Presumably. the presence of highly aversive response-produced properties associated with the fear CR will serve to strengthen (reinforce) the CS-CR association, with the nocive properties of the CR mimicking the previous effects of the aversive US. If this reinforcement effect is 413

414

THOMAS L. BOYD

strong enough to offset the hypothesized decrement in CR strength due to the absence of the US, incubation will occur. Surprisingly few empirical efforts have been generated to test Eysenck’s (1968) theoretical account. One would expect a positive relationship between shock intensity and fear incubation with increased shock levels leading to greater degrees of fear incubation. As increased shock levels are expected to generate greater arousai levels and aversive tesponse characteristics, the effects of nonreinforced CS exposure (extinction) should be offset by the incubation process, as described by Eysenck (1968). The present research provides an attempt to test empirically this relationship with rats in a conditioned avoidance paradigm. METHOD Subjects

Subjects were 72, experimentally naive, male. Sprague-Dawley rats obtained commercially. They were adapted to the USC-Aiken colony at least two weeks prior to testing. At the start of the experiment they ranged in age from 87 to 135 days. Apparatus

The apparatus used was a Lafayette Instrument, One-Way Avoidance Box for rats (Model No. 85200). The apparatus was comprised of a ‘shock‘ chamber with inner dimensions: 20.5 cm wide, 23.5 cm long and 20.9 cm high; and an adjacent ‘safe’ chamber with inner dimensions: 20 cm wide, 12.3 cm long and 11.9 cm high. The safe chamber was raised 9.0cm from the shock-chamber floor. The shock-chamber floor was made of 0.5 cm stainless steel rods spaced 1.5 cm apart. By activating a 28 V motor. the back wall of the safe chamber traversed horizontally, ‘dumping’ the animal from the safe chamber on to the shock-chamber floor. A 4000 Hz tone produced by a Camden Instruments audio generator (Model 258) was delivered through a 10.5 cm speaker located at the center of the roof of the shock chamber. Ambient noise level measured by a Simpson (Model 886) sound-level meter was 60 _t 2 dB (Scale C). The tone raised this level to 70 + 2 dB. A BRS/LVE Shock Generator/Scrambler (SGS-004) delivered scrambled shock to the shock-chamber floor. Timing of the CS-US interval was done by a Camden Instruments, g-Bin Universal Timer (Model 249). Response latencies were recorded by a Lafayette Instrument Event Timer (Model 5710) and Sodeco print-out counter (Type PN107). Procedure

A complete factorial design was used with three levels of shock intensity by three levels of CS exposure duration (3 x 3). Each subject was given a 5-min period of adaptation to the apparatus prior to the start of the first trial. This period was followed immediately by acquisition training. During acquisition training, 72 subjects were divided equally into three groups of shock intensity: Group H was exposed to 73 V d.c. of shock; Group M was exposed to 55 V d.c. of shock; and Group L received 35 V d.c. of shock intensity as measured by a Sperry (Model SP-140) voltmeter at the grid. For each subject, the start of each acquisition trial was signaled by the dumping operation, automatically placing the subject on to the grid floor of the shock chamber. The back wall of the safe chamber remained in the ‘dumped’ position for 10 set, forcing the subject to remain in the shock chamber. At the end of this lo-set period, the back wall of the safe chamber retracted, allowing the subject to jump on to the safe platform. Simultaneously presented with this retraction was the tone onset. The CS-US interval was lOsec, with shock onset occurring at the 10th sec. The tone and shock stimuli remained on until the subject responded or until 30 set following shock onset. An escape response was defined as a jump-up response on to the safe compartment platform during tone and shock onset, while an avoidance response was a jump-up response during the

415

Effects of shock intensity on fear incubation

CS-US interval, prior to shock onset, Acquisition for each subject was defined as five consecutive avoidance responses. Immediateiy following acquisition, each subject was exposed to the forced exposure manipulation. An equal number of subjects from each level of shock intensity (S = 8) was placed into one of three nonreinforced CS exposure durations. Groups H-O, M-O and L-O received 0 set of nonreinforced CS exposure; Groups H-0.5, M-O.5 and L-O.5 received 30 set (0.5 min) of nonreinforced CS exposure; while Groups H-5, M-5 and L-5 each received 300 set (5 min) of nonreinforced. CS expasure. In order to control for the total amount of time and exposure to the background apparatus cues, all subjects were automati~alIy dumped on to the grid floor of the shock chamber at the start of the exposure manipulation, and were forced to remain on the grid ffoor for 360 set 16 minf. During this time interva1 subjects were exposed to their respective durations of CS exposure, with CS onset occurring after 165 set into the interval for Groups H-0.5, M-O.5 and L-0.5; and 30 set into the interval for Groups H-5, M-5 and L-5. Immediately following the 6-min exposure period, all subjects received regular extinction trials. During this procedure, trials were presented in a fashion identical to that described above for acquisition training with the exceptions that the CS was terminated at IOsec if the animaf did not respond and no shock was presented following the IOsec CS-US intervaI, Extinction was defined as five consecutive nonresponses within 20 seconds following CS offset. RESULTS The following acquisition indices were analyzed in the context of a 3 x 3 analysis of variance with three levels of shock intensity and three levels of exposure duration: (I) total number of escape trials; (2) mean shock duration on escape trials; (3) trial number of the first avoidance response: (41 total number of avoidance triats; (5) mean response Iatency of total avoidance trials; and (6) mean response latency of the last five consecutive avoidance trials. Table 1 presents the separate group mean data for these indices. For indices l-4, all main and interaction effects were nonsignificant. For index 5, a main effect of shack intensity was near the 0.05 level of significance with F(2.63) = 2.93, P < 0.06. The main effect of exposure duration and the interaction effect were nonsignificant. For index 6. a main effect of shock intensity was obtained with F(2,63) = 3.46. P < 0.04. This effect was due to slower avoidance tatencies for Group L-O.5 (mean = 3.36 secf refative to Groups M-O.5 (mean = 2.65 secf and I-I-O.5 (mean = 2.39 set) with specific comparisons of F(L63) = 4.93. P -c 0.05; and F(L63) = 9.32. P < 0.005; respectively. No further specific comparisons, main, or interaction effects were significant. Table 2 presents the mean total number of responses to extinction fallowing the forced-CS exposure manipulation. the mean longest series of consecutive avoidance responses. the mean avoidance latency and the mean avoidance latency of the first trial presentation for each group. Figures 1 and 2 display the mean total responses to extinction and the mean longest series of consecutive responses. A two-factor analysis of variance which was performed on the total number of responses obtained a significant main effect of both shock intenTable 1. Dependent variables of acquisition during the shock-intensity

manipulations

L-O

L-O.5

L-5

M-O

Group M-O.5

M-5

H-O

H-O.5

H-5

Mean total escape trials Mean shock duration Mean trial No. of Ist avoid

4.50 2.77

4.75 7.57

2.25 9.11

3.63 6.26

2.88 3.35

3.88 3.ff

5.63 2.92

3.50 4.06

3.25 3.28

response

3.‘ss

3.75

Mean total avoid. responses Mean total avoid. response latency Mean response latency on last 5 cons. avoidances

7.13

6.88

3.00 5.50

3.sg 5.38

3.25 5.50

3.63 6.25

4.88 6.63

3.63 6.25

3.50 5.75

2.72

3.50

2.63

2.51

2.71

2.47

2.14

2.44

2.50

2.19

3.36

2.62

2.52

2.65

2.38

2.73

2.39

2.36

416

THOMAS

Table 2. Dependent

variables

Mean responses to extinction Longest series of cons. responses Mean response latency Mean response latency of 1st trial

of extinction

L.

f%OYD

following

the forced-CS

exposure

manipulations

L-O

L-O.5

L-S

M-O

Group M-O.5

M-5

H-O

H-0.5

H-5

3.75 3.00 8.50

9.75 6.75 6.67

4.12 3.25 7.02

9.88 7.00 5.15

23.00 13.50 5.23

11.25 10.88 4.65

6.50 5.25 4.38

19.25 15.25 4.27

30.50 18.88 3.74

22.80

19.68

24.05

16.30

16.91

9.13

16.44

12.18

9.83

sity and exposure duration with F(2,63) = 5.46, P < 0.006; and F(2,63) = 4.00. P < 0.02; respectively. The interaction effect was close to significance levels with F(4.63) = 2.35. P < 0.06. It would appear from these findings that with increased levels of shock intensity, prolonged nonreinforced CS exposure leads to incubation This was confirmed by the following planned, nonorthogonal specific group comparisons being at or near the 0.05 level of confidence: Group H-O vs Group H-5, F(1.63) = 12.09. P < 0.001; Group H-O vs Group H-0.5, F(1,63) = 3.41, P -c0.10; and Group M-O vs Group M-0.5. F(1,63) = 3.61, P < 0.10. The observed interaction effect appears to be primarily a result of the increased level of responding for Group H-5 relative to Group M-5 with F(1,63) = 7.78, P c 0.01; and Group L-5, F(1,63) = 14.60, P < 0.001. The mean longest series of consecutive avoidance responses closely paralleled the above data (See Fig. 2) with obtained main effects of shock intensity, F(2,63) = 5.16, P -c0.008; and exposure duration, F(2,63) = 3.44, P < 0.038. The interaction effect was nonsignificant. Similarly, the following planned, nonorthogonal specific group comparisons were obtained: Group H-O vs Group H-5, F(l,63) = 7.85, P < 0.01; Group H-O vs Group H-0.5, F(1,63) = 4.23, P < 0.05;Group L-5 vs Group H-5, F(1,63) = 10.33. P < 0.005; and Group L-O.5 vs Group H-0.5, F(1,63) = 3.06, P < 0.10. As noted in Table 2 those subjects exposed to high shock levels during acquisition responded with a much faster mean avoidance latency during extinction than those with lower shock levels. This was confirmed by analysis of variance statistics with a main effect of shock intensity, F(2.63) = 6.36, P < 0.003. No further main or interaction effects were significant. A potentially more precise measure of the immediate effects of differen-

High shock

/

I

0

0.5

5

Expoaura condition, min Fig. 1. Mean total number of trials to extinction following forced-CS exposure high-, medium- and low-shock acquisition groups.

manipulations

for

Effects of shock

intensity

on fear incubation

417

20 -

High shock

5: i f

Is-

f

Li 8

z P

IO -

f

% 6

5-

f

Low shock

1

0

0.5

5

Exposum condition, min Fig. 2. Mean

longest

series of consecutive responses following forced-CS exposure for high-. medium- and low-shock acquisition groups.

manipulations

tial forced CS exposure was obtained by the mean avoidance latency of the first trial during regular extinction (see Table 2). Similarly only the main effect of shock intensity was significant with F(2.63) = 3.24, P < 0.04. Major differences appeared to be related to comparisons among groups exposed to the longest CS duration with Group L-5 vs M-5, F(l.66) = 4.39, P < 0.05; and Group L-5 vs H-5. F(L63) = 4.35, P < 0.05. No further specific comparisons were significant. DISCUSSION

The present experiment provided an empirical test of the relationship between CS exposure and shock intensity in the context of a one-way avoidance situation. Support for Eysenck’s (1968) theory of fear incubation was obtained. That is, the effects of nonreinforced CS exposure durations were dependent upon differential levels of shock intensities. Exposure to relatively higher shock levels during acquisition appeared to exacerbate the incubation phenomenon with avoidance behavior being sustained and, in fact increased despite increasing levels of nonreinforced CS exposure. For medium-shock level groups. intermediate (0.5-min) durations of nonreinforced CS exposure resulted in fear incubation (enhancement). while relatively more sustained CS exposure (5-min) had the expected effect of reducing avoidance behavior. For high-shock level groups, sustained CS exposure had the effect of further increasing avoidance behavior with no evidence for extinction within the parameters of the present experiment. These results are presented with the assumption that one-way avoidance behavior directly reflects levels of conditioned fear. An alternative interpretation may be provided in that the incubation phenomenon may be a reflection of a conditioned ‘freezing’ response of Groups M-O and H-O. Groups’ M-0.5, H-O.5 and H-5 avoidance behavior may not be evidence for fear enhancement. but rather Groups’ M-O and H-O avoidance behavior is reduced due to a conditioned freezing response acquired during forced apparatus exposure. As the degree of nonreinforced CS exposure is increased, fear extinction may have the effect of reducing the freezing behavior with the end result of increased avoidance behavior. This interpretation remains viable until further experimentation is completed with fear measures not as susceptible to such differential freezing effects (e.g. lick suppression). This alternative interpretation. however, is not supported by the latency data as Groups H-O and M-O did not significantly differ from Groups M-0.5,

418

THOMAS L. BOYD

H-O.5 and H-5 on mean total avoidance latency and mean avoidance latency of the first extinction trial. To the degree that the observed one-way avoidance behavior represents a realistic reflection of conditioned fear levels, the present experiment provides support for a critical theoretical point in Eysenck’s (1968) incubation theory. The need for a studied examination of initial fear levels in patients and their reaction to extinction techniques of psychotherapy seems apparent. ,4ckno~/edyemmrs-The present research was supported in part by a Research and Producttve Scholarship Grant awarded to the author and William J. House by the University of South Carolina. The author is grateful to William J. House who provided helpful comments.

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