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Eysenck's theory of incubation: An empirical test
Behar. Res. Thrr.
Vol. 20. pp. 329 to 338. 1982 reserved
Printed inGreatBritain. Allrights
Copyright
000%7967/82/040329-10803.00/0 Q 1982 Pergamon Press Ltd
EYSENCK’S THEORY OF INCUBATION: AN EMPIRICAL TEST* TERRENCE P. NICHOLAICHUK, LIONEL J. QUESNEL and ROBERT W. TAIT~ Psychology Department, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (Received 22
October
1981)
Summary-Eysenck’s incubation of fear hypothesis that states that repeated CS-alone presentations can yield an increase in measures of fear was tested by first giving rats either a paired or an unpaired presentation of a tone CS and either a strong (3.5 mA), weak (1.05 mA) or no-shock US and then 10 daily CS-alone presentations. Over the CS-alone trials, conditioned fear, as measured by duration of freezing, latency to escape and activity scores, extinguished, rather than incubated.
INTRODUCTION
Eysenck’s (1968, 1976, 1979) behavioral theory of neurosis is predicated on the assertion that neurotic behavior is both acquired and maintained through Pavlovian conditioning principles. Specifically, Eysenck postulated that environmental stimuli (CSs) that occurred in conjunction with an aversive unconditioned stimulus (US) acquire nocive responses (NRs) such as pain and fear. The acquisition of NRs is assumed to be rapid, and with a sufficiently traumatic US can occur with just a single CS-US pairing. Once acquired, some neurotic behaviors appear to intensify in the absence of traumatic stimuli (Eysenck, 1968, 1979). To account for the intensification, Eysenck postulated that postconditioning CS presentations affect the strength of the CS-NR association through two processes. The first process produces a decrement in the strength of CS-NR association, whereas the second produces an increment in the strength of the CS-NR association. Accordingly, the behavioral consequences of a CS-alone trial will be determined by the strongest of the two processes. Thus, when a series of CS-alone presentations occurs, extinction of NR occurs if the decremental process is stronger, but augmentation of NR occurs if the incremental process is stronger. The latter outcome is presumed to yield the intensification of neurotic behavior. The incremental process was labelled incubation by Eysenck (1968) and was hypothesized to result from the reinforcement of the CS-NR association by the drive stimuli produced by the elicitation of NR. If the NR conditioned to the CS is sufficiently strong, then the NR produced reinforcement will be stronger than CS-produced extinction effects and the CS-NR association will be strengthened. Thus, the observation of incubation will be determined by parameters that affect conditioned NR strength. Eysenck hypothesized that one of the determinants of the occurrence and strength of incubation was the strength of the US during conditioning. Since the magnitude of conditioned NRs is limited, in part, by the magnitude of the reaction to the US, the occurrence and degree of incubation was assumed to be a direct function of US intensity. While Eysenck concentrated his discussion on the contribution of US intensity, presumably, any conditioning manipulation that increases the strength of a CS-NR association will increase the probability that incubation will occur. In support of the incubation hypothesis, Eysenck (1968, 1979) has cited several conditioned fear studies that purportedly demonstrated either an increase in anxiety following repeated CS-alone presentations (Campbell et al., 1964: Dykman and Gantt, 1960; Lichtenstein. 19.50; Napalkov, 1963; Solomon and Wynne, 1953) or the presumed parametric control of the incubation process (e.g. Reynierse, 1966; Rohrbaugh and Riccio, 1970: Rohrbaugh et al., 1972: Silvestri et al., 1970). However, an analysis of the support* This experiment is based on a Masters thesis submitted Manitoba. t To whom all reprint requests should be addressed. 329
by T. P. Nicholaichuk
to the University
of
330
TERRENCE
P. NICHOLAICHUK
er al.
ing evidence (cf. Bersh, 1980; Kimmel, 1979; Nicholaichuk, 1978) suggests that the incubation hypothesis has not been empirically substantiated. The cited literature suffers from inadequate procedural descriptions (e.g. Napalkov, 1963) inadequate Pavlovian controls (e.g. Campbell et al., 1964; Lichtenstein, 1950; Napalkov, 1963) inadequate CS specification (e.g. Campbell et al., 1964; Dykman and Gantt, 1960; Lichtenstein, 1950), or other methodological inadequacies (e.g. Campbell et al., 1964: Rohrbaugh and Riccio, 1970). Results have been based on unsystematic observations (e.g. Dykman and Gantt, 1960; Lichtenstein, 1950) confounded by operant contingencies (e.g. Lichtenstein. 1950; Solomon and Wynne, 1953; Reynierse, 1966) or confounded by the increase in fear (e.g. Kamin, 1957) that occurs with the passage of time (e.g. Rohrbaugh er ul., 1972). And finally, incubation has been inferred from retardation of extinction (e.g. Silvestri ef al.. 1970) instead of the defined increase in magnitude of responding to CS-alone presentations. Since the incubation hypothesis is critical to Eysenck’s (1968, 1976, 1979) account of neurotic behavior, the present study was undertaken. to attempt to clearly document an incubation phenomenon. In order to maximize the conditions for the occurrence of incubation the following steps were taken. First, conditioning and testing took place in dissimilar chambers in order to minimize the contribution of nonspecific apparatus cues to behavioral control (cf. McAllister and McAllister, 1971). Second, rats were given a single pairing of a CS and an intense shock US. Eysenck’s (1968, 1976, 1979) model predicts that such pairing, if extremely traumatic, will be sufficient to produce incubation offear. Third, high- and low-shock intensities were presented to separate groups of subjects. Presumably incubation should be demonstrated with high-intensity shock. but extinction should be observed with the lower-intensity shock. Fourth, the animals were giveu a number of CS-alone trials to determine whether the fear response would incubate (increase in magnitude) or extinguish. Fifth, control groups were employed to separate the associative effects from the nonassociative effects of the shock US. Since incubation is presumed to be a consequence of an associative process, incubation should only occur when the CS and US are paired. And finally, since animals may respond to fear-evoking stimuli with one of several species specific responses (cf. Belles, 1971) and since the stimulus control of these responses has, as yet, not been fully delineated. a multivariate approach was taken in which freezing, escaping, and activity levels were recorded. By monitoring a range of behavioral responses, the opportunity of observing incubation effects should be maximized. METHOD Subjects
The Ss were 45 hooded rats, weighing 200-300 g, obtained from the University of Manitoba’s Dentistry Department breeding colony. They were housed in individual cages with food and water freely available. Apparatus
The apparatus consisted of two chambers: a conditioning chamber and a test chamber. Conditioning took place in a brushed aluminum Coulbourn Instruments Inc. Model E 10-10 operant chamber (30 x 24 cm and 29 cm in height) equipped with a house light, a grid floor and a 6.6cm diam. 8 R speaker. The grid floor was composed of 7 mm stainless steel bars 1.8 cm apart (center-to-center). Scrambled shock was delivered to the floor of the chamber by a Coulbourn Instruments Inc. Shock Generator. The conditioning chamber was housed in a sound- and light-attenuating fiberglass box equipped with an exhaust fan. The 60 x 30 x 20cm (I x w x h) test chamber. constructed of wood, was divided into two 30 x 30 cm sections by a wall. A door 7.0 cm wide and 7.0 cm high was centered at the base of the dividing wall. The floor and walls were painted white and an 8 R speaker was centered on the end wall opposite the door in each section of the chamber. A house light was located to the left of each speaker and centered 3.0 cm down
Eysenck’s theory of incubation: an empirical test
331
and 3.0 cm over from the top left-hand corner of each end wall. In addition, the floor of each section was divided into nine 10 x 1Ocm squares by black lines. The ceiling of the chamber was covered with clear Plexiglas. An adjustable mirror was mounted above and to the rear of the apparatus to permit unobtrusive observation of the Ss, and a piece of window screen was placed over the Plexiglas lid of the chamber to make the experimenter’s presence more difficult for the animals to detect. In addition, all observations took place in a darkened room. All stimulus deliveries and data collection were accomplished by means of Coulbourn Instruments Inc. programmable modules situated in an adjoining room. Procedure Subjects were randomly assigned to one of five groups (N = 9). The first group (3.5P) was given a single 4-set, white-noise CS of moderate intensity paired with a bsec 3.5 mA scrambled shock US at a forward interstimulus interval of 4 sec. The second group (1.05P) received the same CS parameters but the US was a 1.05 mA shock. The third group (CSa) was exposed to just the CS. The remaining two groups were explicitly unpaired groups (3SU and 1.05U) which differed only in the intensity of the US (3.5 and 1.05 mA, respectively). Four 5s of each unpaired control group received a single forward ordering of the CS and US separated by a 20-min interval. The other five Ss received a single backward ordering of the CS and US also separated by 20 min. Postexperimental analysis indicated that the presentation order of the unpaired CS and US was not a significant source of variance. ‘The treatment day consisted of a 30-min session in the conditioning apparatus for all groups. Fifteen minutes from the onset of the session, the paired and CS alone groups received the appropriate stimulation. For the unpaired groups, the initial stimulus (either the CS or the US) was delivered 5 min after placement in the chamber and the following stimulus (either the US or the CS, respectively) 20 min later. On the following day, each S was placed in the test chamber and allowed to habituate to the new environment. A measure of general activity was obtained for six consecutive 5-min intervals. The level of general activity was determined by counting the number of squares which both front paws entered. The number of times the rat crossed over to a different side of the chamber was also recorded. The rest of the experiment consisted of 10 daily test sessions. In each test session the rat was placed in one side (alternated daily) of the test chamber and after 5 min given a single 4-set CS-alone presentation that occurred on the side of the chamber occupied by the S. The session was terminated 2 min after the rat left or escaped the side of the chamber in which it received the CS. Subjects were returned to their home cages 2 min after the occurrence of the escape response, and the apparatus was washed out with a disinfectant solution before the next trial began. No upper limit was imposed on the duration of each test trial. During the test phase, a number of behaviors were recorded. Before the CS presentation the level of general activity and the number of crossovers were obtained. After CS onset, the duration of freezing, latency to escape and the level of activity were measured. Freezing was defined as a complete absence of movement characterized by abruptness of onset, wide open eyes and muscular rigidity. Latency to escape was defined as the time between CS onset and the moment the S placed both front paws through the door separating the two sides of the test chamber. The measure of general activity was defined in the same way as used on the habituation day. Since freezing and escaping are both presumed measures of fear (Bolles, 1971) and the amount of general activity is inversely related to fear (Walsh and Cummins, 1976) the three measures were employed to provide a fairly detailed assessment of the s’s reaction to the CS in order to maximize the possibility of observing an incubation effect. RESULTS The results of this investigation were organized and analyzed in the following manner. First, Analyses of Variance (ANOVA) were applied to measures of crossovers and activity
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TERRENCE P. NKHOLAICHUK
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taken during the habituation period. Second, measures of crossovers and activity recorded before the introduction of the CS on each of the 10 trials were subjected to analyses of variance. Third, between-group changes in the test phase were assessed by means of a Multivariate Analysis of Variance (MANOVA) using duration of freezing, latency to escape and activity after CS presentation as the dependent variables. Finally, univariate analyses of variance with orthogonal components for trend were employed to assess the changes in freezing, escape and activity across trials and groups. ~u~it~atiu~
day: crossovers
and uctivity
IQ the habituation
session the mean number of crossovers did not signi~cantly differ (F < 1.0) between groups. However, a two-way repeated measures analysis of variance indicated that the differences in the mean amount of activity between groups was significant C&4,40) = 2.65, P c 0.051. Posr-hoc orthogonal contrasts between groups indicated that the Group main effect resulted from the lower activity scores of the shocked groups (23.9 squares) relative to the mean activity score (37.5 squares) of the CS-alone control group [F(1,40) = 7.63, P < 0.011. In addition, the ANOVA revealed that the mean activity scores decreased over the six 5-min intervals of the habituation session [F(.5,200) = 113.42, P < O.OOl]. There were no interactions between groups and 5-min intervals. Test phase:
crossovers
and preCS activity
The mean number of crossovers during the preCS period over the 10 test trials for Groups 3SP, 3SU, l.O5P, 1.05U and CSa were 4.0, 6.4, 7.0, 9.5 and 8.6, respectively. A two-way repeated measures ANOVA yielded group differences in the number of crossovers [F(4,40) = 3.54, P c O.OS]. Post-hoc orthogonal contrasts between groups suggested that the Group effect reflects fewer number of crossovers made by groups receiving intense shock (Groups 3.5P and 3.5U) relative to groups which received less intense (Groups f.OSP and 1.05U) or no shock (Group CSa) at all CF(1.40) = 9.39. P < 0.011. Over the 10 test trials, the number of preCS crossovers gradually declined until trial 7. and then gradually increased again. This change in the number of crossovers produced a significant Trials effect C&9,630) = 2.95, f < O.Oi] in the ANOVA. The U-shape of the function was confirmed by a significant quadratic trend component [F(1,40) = 12.77, P K O.OOl] to the Trials effect. No other trend component was significant and there were no reliable Group x Trial interactions for the number of crossovers. The mean preCS activity scores over the 10 test trials for Groups 3.5P, 3.5U. l.O5P, 1.05U and CSa were 31.0, 47.9, 54.1, 64.4 and 59.3 squares, respectively. A two-way repeated measures ANOVA yielded significant Group differences [F(4,40) = 3.05, P < 0.051 in the amount of activity. Post-hoc orthogonal contrasts showed that Ss that received intense shock (i.e. Groups 3.5P and 3.5U) were less active than groups which received less intense (Groups 1.05P and 1.05U) or no shock (Group CSa) at all [F(1,40) = 8.63, P < 0.011. Over the 10 test trials, the mean amount of preCS activity decreased until trial 7, and then increased again. The U-shaped time course was confirmed by the ANOVA which yielded a Trials main effect [F(9,360) = 7.81, P < O.OOl] composed of only linear [F( 1,400)= 9.48, P < O.OOl], quadratic [F(1,4O) = 27.57, P c O.OOl] and quartic [F(1,40) = 8.85, P < O.Ol] trend components. The ANOVA also yielded a Groups x Trials interaction [F(4,40) = 1.62, P c O.OS] which contained only a significant quadratic component [F(4,40) = 5.10, P < 0.011. The quadratic interaction resulted from the U-shaped function of activity over trials for Groups 3.5P, 3SU and t.OSP, in contrast to the inverted U-shape function for Group 1.05U. and the lack of a quadratic component for Group CSa. Taken together the preCS crossover and activity measures suggest that the rats became less active to repeated exposures to the test situation. In addition. it would appear that rats that received the intense shock were less active than the others throughout training. However since there were no paired-unpaired differences, the lowered scores were a consequence of nonspecific effects of the intense shock.
333
Eysenck’s theory of incubation: an empirical test 1200
r •Jll CID
q
Latency uratlon
to
escape
of
freezing
Activity (righ;;:6qnd
1
15
.c
0 5
9 r
400 7
b
5
t
200
z 3
5”
I 3 5P
lO5P
Group
Fig. 1. Mean postCS duration of freezing, latency to escape and activity averaged over test days for Groups 3.5P, 3.5U, 1.05P. 1.05U and CSa. The abscissa for the escape and freezing measures is on the left-hand side of the figure, while the abscissa for activity is on the right-hand side.
Multivariate between-group comparisons for the postCS dependent measures
Hypotheses concerning specific differences between groups were investigated by means of a MANOVA applied to the dependent measures of duration of freezing, latency to escape, and level of activity following CS onset. The computer program used for all Multivariate analyses was Finn’s (1976) “MULTIVARIANCE, Univariate and Multivariate Analyses of Variance, Covariance and Regression. Version V, Release 3.” The means of each experimental group, collapsed across trials for freezing, escape, and activity are presented in Fig. 1. Duration of freezing and latency to escape were longest for Group 3.5P with Groups 3.5U, l.O5P, 1.05U and CSa following in approximate order of decreasing values. On the other hand, Group means on the activity variable appeared to follow no systematic pattern. Pearson’s product-moment correlations among the dependent variables indicated that freezing and escape were highly interdependent (r = 0.90) while activity and freezing (r = 0.09) and activity and escape latency (r = 0.20) were not. Multivariate contrasts were performed in order to determine where specific group differences lay. Group 3.5P was found to differ from Group 1.05P [F(3,38) = 8.63, P < O.OOl], Group 3.5U [F(3,38) = 4.47, P < 0.011 and Group CSa [F(3,38) = 3.38, P < 0.051. Group 1.05P differed from Group 1.05U [F(3,38) = 3.60, P c 0.051 and Group CSa [F(3,38) = 3.40, P < 0.051. Univariate F statistics for each of the comparisons are presented in Table 1. The table indicates that the largest univariate Fs were obtained with the freezing-dependent measure; that both the freezing and escape measures yielded significant values for all contrasts; and that the activity-dependent measure was significant on only one of the contrasts (3.5P vs l.OSP). Table 1 also contains the standardized discriminant function coefficients for each dependent measure on each contrast. It can be seen that the standardized discriminant function coefficient for freezing was two to three times the value for the next highest coefficient; and that the standardized discriminant function coeficient for latency to escape was substantially greater than the standardized discriminant function coefficient for amount of activity for three of the contrasts (3.5P vs 3.5U; 3.5P vs CSa; 1.05P vs CSri), but not for the remaining two (3.5P vs 1.05P; 1.05P vs 1.05U). Taken together, both the univariate F statistics and the discriminant function loadings indicated that freezing was the most heavily weighted variable with escape and activity in order of
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et al.
Table I. A summary of the univariate F statistics and standardized discriminant function coefficients associated with the multivariate contrasts Dependent measure
Contrast 35P vs 1.05P 3.5P vs 3.5u 3.5P vs CSa 1.05P vs 1.05u
1.05Pvs CSa
Freezing Escape Activity Freezing Escape Activity Freezing Escape Activity Freezing Escape Activity Freezing Escape Activity
F‘(1.40)
P
22.88 22.83 6.04 12.85 8.01 0.26 10.21 9.78 0.69 10.35 9.89 1.57 10.53 9.82 0.24
Standardized discriminant coefficient - 0.674 - 0.232 - 0.362 1.141 - 0.496 -0.163 - 0.699 -0.319 -0.021 - 0.743 -0.208 - 0.262 - 0.699 -0.319 -0.021
< 0.05
decreasing importance. The different durations of freezing behaviour, then, probably most parsimoniously explain the observed group differences in the MANOVA. In summary, inspection of the multivariate contrasts and Fig. 1 indicated that paired group performance (Groups 3.5P and 1.05P) significantly differed from their respective unpaired control contrasts (Groups 3.5U and 1.05U) and the unshocked control (Group CSa). Thus, conditioned fear was obtained. In addition, the indices for conditioned fear were greater for the high-intensity paired group (3.5P) relative to the low-intensity paired group (l.OSP). Thus, a shock-intensity effect was obtained. Univariate analyses of Trials and Trials x Groups interactions for the postCS dependent measures
Univariate, two-way, repeated measures ANOVAs with orthogonal components for trend were employed to examine the effects of Trials and Trials x Group interactions for freezing, escape and activity, respectively. Freezing. Figure 2 presents the main effect of Trials for each of the three dependent variables. The figure suggests that the mean amount of freezing increased over the first three days, and then systematically decreased to a level (97.4 set) that was substantially lower than the initial value (289.5 set). The graphic interpretation was confirmed by an ANOVA which contained a significant Trials effect [F(9,360) = 4.72, P < O.OOl] which significant linear [F(1,40) = 13.50, P < O.OOl] and quadratic resulted from 800
I-
-
Duration of freezing
.-
Latency
to
escape
Activity (right-hand
axis)
Trial Fig. 2. The mean postCS duration of freezing, latency to escape and activity as a function of trials.
Eysenck’s theory of incubation: an empirical test
I2
3
4
5
6
7
8
9
335
IO
Trio1
Fig. 3. The mean postCS duration of freezing (upper panel). latency to escape (middle panel) and activity (lower panel), for Groups 3.SP. 35-L l.OSP. l.OSU and CSa as a function of trials.
[F(1,40) = 4.47, P < O.OS]trend components. The ANOVA also yielded a Group x Trials interaction [F(3,360) = 1.63, P -cO.OSJ which was composed of only a significant linear trend component [F(4.40) = 4.18, P -c0.01). The Group x Trials interaction is depicted in the upper panel of Fig. 3. An examination of the panel shows that Groups 1.05U and CSa had very low levels of freezing that did not change appreciably over trials; that the duration of freezing for Groups 3.5P, 3.Y.J and 1.05P was much higher than for Groups 1.05I.J and CSa but after an initial increase over the first few trials, declined to the levels of Groups 1.05U and CSa; and that Group 3SP had the highest level of freezing and the sharpest decrease. Thus, the linear component to the Group x Trial interaction resulted from the flat functions for Groups 1.05U and CSa, the moderate declines for Groups 3.5U and l.OSP, and the steep decline for Group 3SP. It should also be noted that since the Group x Trial interaction did not contain a significant quadratic trend component, the quadratic trend component to the Trials main effect reflected the initial increase in the functions for Groups 3.5P, 3.5U and 1.05P. Escape. An examination of Fig. 2 reveals that the function for the Trials main effect for the latency to escape had the same form as the freezing measure. That is, after an initial rapid increase, the mean latency to escape decreased to a value (223.2 set) that was much lower than the initial value (432.2 set). Again the graphical interpretation was confirmed by the ANOVA which contained a significant Trials main effect [F(9,360) = 5.58, P c 0.001]composed of significant linear [F(1,40) = 14.98, P < 0.001 J quadratic [F(1,40) = 6.56, P -c0.013 and cubic [F(l,40) = 5.65, P < 0.05] trend components. The ANOVA also indicated that the Trials x Group interaction was significant f&36,360) = 2.10, P < O.OOl] and resulted from a significant linear [F(4,40) = 5.63,
336
TERRENCE P. NICHOLAICHUK
er d.
P < O.OOlJ trend component. The significant Trials x Group interaction is depicted in the middle panel of Fig. 3. The panel shows that the mean escape latencies for Groups 1.05U and CSa were short and did not change over trials. In contrast, the mean escape latencies for Groups 3SP, 3.5U and 1.05P were much longer, and after an initial increase. decreased to about the levels of Groups 1.05U and CSa. The flat functions over trials for Groups 1.05U and CSa. the gradual decline in escape latencies for Groups 3.5U and 1.05P and the large rate of decrease for Group 3.5P produced the linear interaction component. Again it must be noted that the Trial x Group interaction did not contain a quadratic component, and therefore, the quadratic component to the main effect must be due to the initial increase in escape latencies for Groups 3.5P. 3.5U and 1.05P. Activity. The Trials main effect for the activity measure is also shown in Fig. 2. Again an inverted U-shaped function was observed with the mean amount of activity showing a small rise which was followed by a very gradual and irregular decrease over trials. The changes in mean amount of activity over trials were not large enough to produce a Trials main effect [F(9,360) = 1.471 in an ANOVA but did yield significant linear [F(1,40) = 5.44, P -c 0.051 and quartic [F(l,40) = 4.37, P < 0.05] trend components. The lower panel of Fig. 3 illustrates the nonsignificant Trials x Groups interaction. The panel indicates that the mean amount of activity for each group was irregular. with the greatest fluctuations occurring over the first five days. The absence of significant effects for the activity dependent variable would appear to corroborate the low standardized discriminant function coefficients assigned to this variable (cf. Table 1).
DISCUSSION
The principle results of the experiment were as follows: (1) during the postCS period in the test phase, the MANOVA revealed that paired group performance was significantly higher than control levels, and, that the high-shock intensity group (3.5P) had significantly higher measures than the low-shock intensity paired group (1.05P): (2) the discriminant analysis indicated that differences between groups were primarily due to changes on the freezing and escape dependent measures; (3) during the postCS period in the test phase, the main effect of Trials for the freezing, escape and activity measures showed small initial increases in levels and subsequent large declines: and finally (4) for the freezing and escape measures, Group 3.5P exhibited a large decline, Groups 3.5U and 1.05P showed moderate declines, whereas Groups 1.05U and CSa displayed constant, low-level performance, over trials. The observation that paired groups differed from their respective control groups on the freezing and escape measures confirmed the establishment of conditioned fear with a single CS-US pairing. In addition, since Group 3.5P had significantly higher performance indicators than Group 1.05P, the experiment demonstrated the commonly observed (e.g. McAllister and McAllister, 1971; Moyer and Korn, 1966; Theios et ul., 1966; Zammit-Montebello et al., 1969) direct relation between the magnitude of conditioned fear and US intensity. Thus, the experiment met the criteria needed for the observation of incubation effects. If Eysenck’s hypothesis (1968, 1979) is correct, then the subjects receiving the intense shock should show increased indices of fear over repeated CS-alone trials. While the increase in freezing duration and escape latency for Group 3.5P over the initial few trials might be construed as support for the hypothesis, two facts mitigate against such an interpretation. First, if incubation were occurring, the reinforcement provided by NRs would be greater than the decremental effects due to extinction and performance indices should continue to increase or at least be maintained. However, contrary to expectations. over the last half of the experiment, performance indices for Group 3.5P decreased to control levels. Second. the incubation effect is stated to be a consequence of an associative effect between the CS and US. But in the present experiment. the early increases in freezing duration and escape latency were observed in both the associative (Group 3.5P) and nonassociative control (Group 3.5U). The observation of parallel func-
Eysenck’s theory of incubation: an empirical test
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for associative and nonassociative groups appears to preclude the application of Eysenck’s (1976, 1979) incubation of fear hypothesis. The failure to observe incubation effects in the present experiment might be due to three factors. First, Eysenck (1976, 1979) has asserted that one of the major determinants of incubation is individual subject characteristics. Hence, the failure to observe incubation might be due to the use of normative data that disguises the incubation occurring in a few subjects by averaging performance indices with subjects that show an extinction effect. However, an examination of individual protocols for all postCS response measures showed that only one subject showed a continued increase in freezing duration and escape latency. This subject was in the CS-alone group and, therefore, the behavioral changes can not be attributed to incubation effects. A second possibility is that the present intense US was not sufficiently traumatic to produce the incubation effect and thus, that the present experiment was not an adequate test of the theory. While this argument is a very real possibility, several factors mitigate against it. In the present experiment, the intense shock produced major associative (the substantial differences in postCS paired group performance relative to the effects observed in Groups 3.5U and l.O5P), and nonassociative (the suppression of activity measures for Group 3.5P and 3.5U on the habituation day, and during the preCS periods on test trials) effects. In comparison with other experiments, the intensity of shock used for Group 3SP was substantialiy higher than the intensity required to produce reliabie fear conditioning (e.g. Annau and Kamin. 1961), or the intensities employed to provide data that purports to support the incubation hypothesis (e.g. Reynierse, 1966; Rohrbaugh and Riccio, 1970). The within experiment effects and cross-experimental comparisons suggest that the 3.5 mA US of the present experiment was a strong aversive stimulus and that there was a reasonable expectation of observing an incubation effect. And finally, Eysenck (1976, 1979) has asserted that the probability of incubation occurring will be inversely related to the CS duration during testing. In the present experiment, a short CS duration that should maximize the occurrence of incubation’ was programmed, yet clear evidence of incubation was not observed. However, if the programmed CS was only a component of the effective CS, then a different interpretation, more consistent to Eysenck’s postulates, could be generated for the present results. Despite our attempts to minimize the role of contextual cues, it is possible that fear was conditioned to contextual cues during training and subsequently generalized to the cues of the testing apparatus. Such contextual conditioning could have occurred in both Groups 3.5P and 3SU. With these assumptions, the increase in the duration of freezing and latency to escape over the initial testing sessions would then indicate the occurrence of incubation to generalized CSs for both groups. The differences between groups would reflect the addition of the conditioned effects provided by the nominal CS-US pairing to the performance of Group 3SP. From Eysenck’s perspective (1976, 1979) the problem becomes accounting for the subsequent eIimination of the initial incubation effects and the soiution lies in the hypothesized role of CS duration. For, in the present experiment, the dominant response of the rats was long duration freezing which has the consequence of giving the subjects a long duration exposure to the hypothesized contextual CSs. Since in Eysenck’s formulation the strength of extinction processes is determined, in part, by the duration of CS-alone presentations, the initial incubation effects to the contextual CSs would be counteracted by the effects of extinction resulting from response-produced exposure to the contextual CSs. This would lead to the expectation of the observed decrease in response measures late in training. Collateral support for this post-hoc account can be obtained from an examination of the between group differences obtained in the preCS period. The principle observation was that Groups 3.5P and 3SU engaged in less activity and made fewer crossovers than did the other three groups. These observations suggest that the contextual cues were producing greater fear in the strong-shock groups. in addition, the failure to find differences between Groups 3.5P and 3.5U during the preCS periods would seem to support the postulated summative role of the nominal CS in postCS performance. Thus, by
tions
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switching the explanatory power of Eysenck’s (1976, 1979) model from the programmed CS to unspecified generalized contextual CSs, the testing results become consistent with Eysenck’s postulates. However, if fear was conditioned to contextual CSs during the training phase, then in the 30-min habituation session, the generalized contextual CSs should have yielded a shock intensity effect. No such effect was observed. There were no between group differences on the crossover measure, and only a shock-no shock difference on the activity measure. Accordingly, an interpretation based on hypothesized conditioned contextual CSs does not appear to provide a complete account of all the present results. The present failure to conclusively demonstrate an incubation of fear effect, coupled with the weakness of prior studies (cf. Bersh, 1980; Kimmel, 1979) leaves in doubt Eysenck’s (1968, 1976, 1979) reliance on the hypothesized incubation mechanism to account for the intensification of neurotic behavior in the absence of traumatic stimuli. However, it may well be that an incubation-like phenomenon is the product of situational and procedural variables as yet unknown. In the final analysis, Eysenck’s theory of incubation and neurosis can only be properly assessed in the light of repeated and varied investigations which attempt to ascertain the boundary conditions necessary for the reliable production of the phenomenon. Acknowledgements-This Canada Research Grant
work was supported A0312 to R. W. Tait.
by
Natural
Science
and
Engineering
Research
Council
of
REFERENCES ANNAU Z. and KAMIN L. J. (1961) The conditioned emotional response as a functton of intensity of the US. J. romp. physiol. Psycho/. 54, 428-432. BERSH P. J. (1980) Eysenck’s theory of incubation: a critical analysis. Behap. Res. Ther. 18. I l-17. BOLLES R. C. (1971) Species specific defense reactions. In Aversive Conditioning and Learning (Edited by BRUSH F. R.). Academic Press, New York. CAMPBELL B., SANDER~ON R. and LAVERTY S. (1964) Characteristics of a conditioned response in human subjects during extinction trials following a single traumatic conditioning trial. J. abnorm. sot. Psycho/. 68, 627-639. DYKMAN R. and GANTT W. (1960) A case of experimental neurosis and recovery tn relation to the orienting response. J. Psycho/. SO, 105-l 10. EYSENCK H. (1968) A theory of the incubation of anxiety/fear responses. BehaLl. Res. Ther. 6. 319-321. EYSENCK H. (1976) The learning theory model of neurosis-a new approach. Behac. Rex Ther. 14. 251-267. EYSENCK H. (1979) The conditioning model of neurosis. Behau. Brain Sci. 2, 155-199. FINN T. D. (1976) Multivariance: Uniuariate und Multivariate analysis of aariunce. coruriance, and regression. Version V. Release 3. National Educational Resources Inc., Chicago. KAMIN L. J. (1957) The retention of an incompletely learned avoidance response. J. camp. ph,vsiol. Psycho/. 50, 457-460. KIMMEL H. D. (1979) Eysenck’s model of neurotigenesis. Behau. Brain Sci. 2, 171-172. LICHTENSTEIN C. (1950) Studies of anxiety: 1. The productton of a feeding inhibttton in dogs, J. camp. physiol. Psychol. 43, 16-29. MCALLISTER W. R. and MCALLISTER D. E. (1971) Behavioral measurement of conditioned fear. In Arersiue Conditioning and Learning (Edited by BRUSH F. R.). Academic Press, New York. MOYER K. E. and KORN J. H. (1966) Effect of UCS intensity on the acquisition and extinction of a one-way avoidance response. Psychonom. Sci. 4, 12 l- 122. NAPALKOV A. (1963) Information process of the brain. In Progress in Brain Research, Vol. 2 (Edited by WEINER N. and SAFADE J.). Elsevier, Amsterdam. NICHOLAICHUKT. P. (1978) An investigation of Eysenck’s incubation of fear theory of anxiety and neurosis. Unpublished M.A. thesis. REYNIERSEJ. (1966) Effects of CS-only trials on resistance to extinction of an avoidance response. J. camp. physiol. Psycho/. 61, 156-158. ROHRBAUGH M. and RICCIO D. C. (1970) Paradoxical enhancement of learned fear. J. abnorm. Psvchol. 75, 210-216. ROHRBAUGH M., RICCIO D. C. and ARTHUR S. (1972) Paradoxical enhancement of conditioned suppression. Behav. Res. Ther. 10, 125-130. SILVESTRIR., ROHRBAUGHM. and RICCIO D. C. (1970) Conditions influencing the retention of learned fear m rats. Deul Psycho/. 2, 389-395. SOLOMONR. and WYNNE F. (1953) Traumatic avoidance learning. Psycho/. Monogr. 67, (4). THEIOSJ., LYNCH A. D. and LOWE W. F. (1966) Differential effects of shock intensity on one-way and shuttle avoidance conditioning. J. exp. Psycho/. 72, 294-299. WALSH R. N. and CUMMINS R. A. (1976) The open-field test: a critical review. Psychol. Bull. 83, 482-504. ZAMMIT-MONTEBELLOA.. BLACK M., MARQUIS H. A. and SUBOSKIM. D. (1969) Incubation of passive avoidance in rats: shock intensity and pretraining. J. camp. physiol. Psycho/. 6% 579-582.
Vol. 20. pp. 329 to 338. 1982 reserved
Printed inGreatBritain. Allrights
Copyright
000%7967/82/040329-10803.00/0 Q 1982 Pergamon Press Ltd
EYSENCK’S THEORY OF INCUBATION: AN EMPIRICAL TEST* TERRENCE P. NICHOLAICHUK, LIONEL J. QUESNEL and ROBERT W. TAIT~ Psychology Department, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (Received 22
October
1981)
Summary-Eysenck’s incubation of fear hypothesis that states that repeated CS-alone presentations can yield an increase in measures of fear was tested by first giving rats either a paired or an unpaired presentation of a tone CS and either a strong (3.5 mA), weak (1.05 mA) or no-shock US and then 10 daily CS-alone presentations. Over the CS-alone trials, conditioned fear, as measured by duration of freezing, latency to escape and activity scores, extinguished, rather than incubated.
INTRODUCTION
Eysenck’s (1968, 1976, 1979) behavioral theory of neurosis is predicated on the assertion that neurotic behavior is both acquired and maintained through Pavlovian conditioning principles. Specifically, Eysenck postulated that environmental stimuli (CSs) that occurred in conjunction with an aversive unconditioned stimulus (US) acquire nocive responses (NRs) such as pain and fear. The acquisition of NRs is assumed to be rapid, and with a sufficiently traumatic US can occur with just a single CS-US pairing. Once acquired, some neurotic behaviors appear to intensify in the absence of traumatic stimuli (Eysenck, 1968, 1979). To account for the intensification, Eysenck postulated that postconditioning CS presentations affect the strength of the CS-NR association through two processes. The first process produces a decrement in the strength of CS-NR association, whereas the second produces an increment in the strength of the CS-NR association. Accordingly, the behavioral consequences of a CS-alone trial will be determined by the strongest of the two processes. Thus, when a series of CS-alone presentations occurs, extinction of NR occurs if the decremental process is stronger, but augmentation of NR occurs if the incremental process is stronger. The latter outcome is presumed to yield the intensification of neurotic behavior. The incremental process was labelled incubation by Eysenck (1968) and was hypothesized to result from the reinforcement of the CS-NR association by the drive stimuli produced by the elicitation of NR. If the NR conditioned to the CS is sufficiently strong, then the NR produced reinforcement will be stronger than CS-produced extinction effects and the CS-NR association will be strengthened. Thus, the observation of incubation will be determined by parameters that affect conditioned NR strength. Eysenck hypothesized that one of the determinants of the occurrence and strength of incubation was the strength of the US during conditioning. Since the magnitude of conditioned NRs is limited, in part, by the magnitude of the reaction to the US, the occurrence and degree of incubation was assumed to be a direct function of US intensity. While Eysenck concentrated his discussion on the contribution of US intensity, presumably, any conditioning manipulation that increases the strength of a CS-NR association will increase the probability that incubation will occur. In support of the incubation hypothesis, Eysenck (1968, 1979) has cited several conditioned fear studies that purportedly demonstrated either an increase in anxiety following repeated CS-alone presentations (Campbell et al., 1964: Dykman and Gantt, 1960; Lichtenstein. 19.50; Napalkov, 1963; Solomon and Wynne, 1953) or the presumed parametric control of the incubation process (e.g. Reynierse, 1966; Rohrbaugh and Riccio, 1970: Rohrbaugh et al., 1972: Silvestri et al., 1970). However, an analysis of the support* This experiment is based on a Masters thesis submitted Manitoba. t To whom all reprint requests should be addressed. 329
by T. P. Nicholaichuk
to the University
of
330
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P. NICHOLAICHUK
er al.
ing evidence (cf. Bersh, 1980; Kimmel, 1979; Nicholaichuk, 1978) suggests that the incubation hypothesis has not been empirically substantiated. The cited literature suffers from inadequate procedural descriptions (e.g. Napalkov, 1963) inadequate Pavlovian controls (e.g. Campbell et al., 1964; Lichtenstein, 1950; Napalkov, 1963) inadequate CS specification (e.g. Campbell et al., 1964; Dykman and Gantt, 1960; Lichtenstein, 1950), or other methodological inadequacies (e.g. Campbell et al., 1964: Rohrbaugh and Riccio, 1970). Results have been based on unsystematic observations (e.g. Dykman and Gantt, 1960; Lichtenstein, 1950) confounded by operant contingencies (e.g. Lichtenstein. 1950; Solomon and Wynne, 1953; Reynierse, 1966) or confounded by the increase in fear (e.g. Kamin, 1957) that occurs with the passage of time (e.g. Rohrbaugh er ul., 1972). And finally, incubation has been inferred from retardation of extinction (e.g. Silvestri ef al.. 1970) instead of the defined increase in magnitude of responding to CS-alone presentations. Since the incubation hypothesis is critical to Eysenck’s (1968, 1976, 1979) account of neurotic behavior, the present study was undertaken. to attempt to clearly document an incubation phenomenon. In order to maximize the conditions for the occurrence of incubation the following steps were taken. First, conditioning and testing took place in dissimilar chambers in order to minimize the contribution of nonspecific apparatus cues to behavioral control (cf. McAllister and McAllister, 1971). Second, rats were given a single pairing of a CS and an intense shock US. Eysenck’s (1968, 1976, 1979) model predicts that such pairing, if extremely traumatic, will be sufficient to produce incubation offear. Third, high- and low-shock intensities were presented to separate groups of subjects. Presumably incubation should be demonstrated with high-intensity shock. but extinction should be observed with the lower-intensity shock. Fourth, the animals were giveu a number of CS-alone trials to determine whether the fear response would incubate (increase in magnitude) or extinguish. Fifth, control groups were employed to separate the associative effects from the nonassociative effects of the shock US. Since incubation is presumed to be a consequence of an associative process, incubation should only occur when the CS and US are paired. And finally, since animals may respond to fear-evoking stimuli with one of several species specific responses (cf. Belles, 1971) and since the stimulus control of these responses has, as yet, not been fully delineated. a multivariate approach was taken in which freezing, escaping, and activity levels were recorded. By monitoring a range of behavioral responses, the opportunity of observing incubation effects should be maximized. METHOD Subjects
The Ss were 45 hooded rats, weighing 200-300 g, obtained from the University of Manitoba’s Dentistry Department breeding colony. They were housed in individual cages with food and water freely available. Apparatus
The apparatus consisted of two chambers: a conditioning chamber and a test chamber. Conditioning took place in a brushed aluminum Coulbourn Instruments Inc. Model E 10-10 operant chamber (30 x 24 cm and 29 cm in height) equipped with a house light, a grid floor and a 6.6cm diam. 8 R speaker. The grid floor was composed of 7 mm stainless steel bars 1.8 cm apart (center-to-center). Scrambled shock was delivered to the floor of the chamber by a Coulbourn Instruments Inc. Shock Generator. The conditioning chamber was housed in a sound- and light-attenuating fiberglass box equipped with an exhaust fan. The 60 x 30 x 20cm (I x w x h) test chamber. constructed of wood, was divided into two 30 x 30 cm sections by a wall. A door 7.0 cm wide and 7.0 cm high was centered at the base of the dividing wall. The floor and walls were painted white and an 8 R speaker was centered on the end wall opposite the door in each section of the chamber. A house light was located to the left of each speaker and centered 3.0 cm down
Eysenck’s theory of incubation: an empirical test
331
and 3.0 cm over from the top left-hand corner of each end wall. In addition, the floor of each section was divided into nine 10 x 1Ocm squares by black lines. The ceiling of the chamber was covered with clear Plexiglas. An adjustable mirror was mounted above and to the rear of the apparatus to permit unobtrusive observation of the Ss, and a piece of window screen was placed over the Plexiglas lid of the chamber to make the experimenter’s presence more difficult for the animals to detect. In addition, all observations took place in a darkened room. All stimulus deliveries and data collection were accomplished by means of Coulbourn Instruments Inc. programmable modules situated in an adjoining room. Procedure Subjects were randomly assigned to one of five groups (N = 9). The first group (3.5P) was given a single 4-set, white-noise CS of moderate intensity paired with a bsec 3.5 mA scrambled shock US at a forward interstimulus interval of 4 sec. The second group (1.05P) received the same CS parameters but the US was a 1.05 mA shock. The third group (CSa) was exposed to just the CS. The remaining two groups were explicitly unpaired groups (3SU and 1.05U) which differed only in the intensity of the US (3.5 and 1.05 mA, respectively). Four 5s of each unpaired control group received a single forward ordering of the CS and US separated by a 20-min interval. The other five Ss received a single backward ordering of the CS and US also separated by 20 min. Postexperimental analysis indicated that the presentation order of the unpaired CS and US was not a significant source of variance. ‘The treatment day consisted of a 30-min session in the conditioning apparatus for all groups. Fifteen minutes from the onset of the session, the paired and CS alone groups received the appropriate stimulation. For the unpaired groups, the initial stimulus (either the CS or the US) was delivered 5 min after placement in the chamber and the following stimulus (either the US or the CS, respectively) 20 min later. On the following day, each S was placed in the test chamber and allowed to habituate to the new environment. A measure of general activity was obtained for six consecutive 5-min intervals. The level of general activity was determined by counting the number of squares which both front paws entered. The number of times the rat crossed over to a different side of the chamber was also recorded. The rest of the experiment consisted of 10 daily test sessions. In each test session the rat was placed in one side (alternated daily) of the test chamber and after 5 min given a single 4-set CS-alone presentation that occurred on the side of the chamber occupied by the S. The session was terminated 2 min after the rat left or escaped the side of the chamber in which it received the CS. Subjects were returned to their home cages 2 min after the occurrence of the escape response, and the apparatus was washed out with a disinfectant solution before the next trial began. No upper limit was imposed on the duration of each test trial. During the test phase, a number of behaviors were recorded. Before the CS presentation the level of general activity and the number of crossovers were obtained. After CS onset, the duration of freezing, latency to escape and the level of activity were measured. Freezing was defined as a complete absence of movement characterized by abruptness of onset, wide open eyes and muscular rigidity. Latency to escape was defined as the time between CS onset and the moment the S placed both front paws through the door separating the two sides of the test chamber. The measure of general activity was defined in the same way as used on the habituation day. Since freezing and escaping are both presumed measures of fear (Bolles, 1971) and the amount of general activity is inversely related to fear (Walsh and Cummins, 1976) the three measures were employed to provide a fairly detailed assessment of the s’s reaction to the CS in order to maximize the possibility of observing an incubation effect. RESULTS The results of this investigation were organized and analyzed in the following manner. First, Analyses of Variance (ANOVA) were applied to measures of crossovers and activity
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taken during the habituation period. Second, measures of crossovers and activity recorded before the introduction of the CS on each of the 10 trials were subjected to analyses of variance. Third, between-group changes in the test phase were assessed by means of a Multivariate Analysis of Variance (MANOVA) using duration of freezing, latency to escape and activity after CS presentation as the dependent variables. Finally, univariate analyses of variance with orthogonal components for trend were employed to assess the changes in freezing, escape and activity across trials and groups. ~u~it~atiu~
day: crossovers
and uctivity
IQ the habituation
session the mean number of crossovers did not signi~cantly differ (F < 1.0) between groups. However, a two-way repeated measures analysis of variance indicated that the differences in the mean amount of activity between groups was significant C&4,40) = 2.65, P c 0.051. Posr-hoc orthogonal contrasts between groups indicated that the Group main effect resulted from the lower activity scores of the shocked groups (23.9 squares) relative to the mean activity score (37.5 squares) of the CS-alone control group [F(1,40) = 7.63, P < 0.011. In addition, the ANOVA revealed that the mean activity scores decreased over the six 5-min intervals of the habituation session [F(.5,200) = 113.42, P < O.OOl]. There were no interactions between groups and 5-min intervals. Test phase:
crossovers
and preCS activity
The mean number of crossovers during the preCS period over the 10 test trials for Groups 3SP, 3SU, l.O5P, 1.05U and CSa were 4.0, 6.4, 7.0, 9.5 and 8.6, respectively. A two-way repeated measures ANOVA yielded group differences in the number of crossovers [F(4,40) = 3.54, P c O.OS]. Post-hoc orthogonal contrasts between groups suggested that the Group effect reflects fewer number of crossovers made by groups receiving intense shock (Groups 3.5P and 3.5U) relative to groups which received less intense (Groups f.OSP and 1.05U) or no shock (Group CSa) at all CF(1.40) = 9.39. P < 0.011. Over the 10 test trials, the number of preCS crossovers gradually declined until trial 7. and then gradually increased again. This change in the number of crossovers produced a significant Trials effect C&9,630) = 2.95, f < O.Oi] in the ANOVA. The U-shape of the function was confirmed by a significant quadratic trend component [F(1,40) = 12.77, P K O.OOl] to the Trials effect. No other trend component was significant and there were no reliable Group x Trial interactions for the number of crossovers. The mean preCS activity scores over the 10 test trials for Groups 3.5P, 3.5U. l.O5P, 1.05U and CSa were 31.0, 47.9, 54.1, 64.4 and 59.3 squares, respectively. A two-way repeated measures ANOVA yielded significant Group differences [F(4,40) = 3.05, P < 0.051 in the amount of activity. Post-hoc orthogonal contrasts showed that Ss that received intense shock (i.e. Groups 3.5P and 3.5U) were less active than groups which received less intense (Groups 1.05P and 1.05U) or no shock (Group CSa) at all [F(1,40) = 8.63, P < 0.011. Over the 10 test trials, the mean amount of preCS activity decreased until trial 7, and then increased again. The U-shaped time course was confirmed by the ANOVA which yielded a Trials main effect [F(9,360) = 7.81, P < O.OOl] composed of only linear [F( 1,400)= 9.48, P < O.OOl], quadratic [F(1,4O) = 27.57, P c O.OOl] and quartic [F(1,40) = 8.85, P < O.Ol] trend components. The ANOVA also yielded a Groups x Trials interaction [F(4,40) = 1.62, P c O.OS] which contained only a significant quadratic component [F(4,40) = 5.10, P < 0.011. The quadratic interaction resulted from the U-shaped function of activity over trials for Groups 3.5P, 3SU and t.OSP, in contrast to the inverted U-shape function for Group 1.05U. and the lack of a quadratic component for Group CSa. Taken together the preCS crossover and activity measures suggest that the rats became less active to repeated exposures to the test situation. In addition. it would appear that rats that received the intense shock were less active than the others throughout training. However since there were no paired-unpaired differences, the lowered scores were a consequence of nonspecific effects of the intense shock.
333
Eysenck’s theory of incubation: an empirical test 1200
r •Jll CID
q
Latency uratlon
to
escape
of
freezing
Activity (righ;;:6qnd
1
15
.c
0 5
9 r
400 7
b
5
t
200
z 3
5”
I 3 5P
lO5P
Group
Fig. 1. Mean postCS duration of freezing, latency to escape and activity averaged over test days for Groups 3.5P, 3.5U, 1.05P. 1.05U and CSa. The abscissa for the escape and freezing measures is on the left-hand side of the figure, while the abscissa for activity is on the right-hand side.
Multivariate between-group comparisons for the postCS dependent measures
Hypotheses concerning specific differences between groups were investigated by means of a MANOVA applied to the dependent measures of duration of freezing, latency to escape, and level of activity following CS onset. The computer program used for all Multivariate analyses was Finn’s (1976) “MULTIVARIANCE, Univariate and Multivariate Analyses of Variance, Covariance and Regression. Version V, Release 3.” The means of each experimental group, collapsed across trials for freezing, escape, and activity are presented in Fig. 1. Duration of freezing and latency to escape were longest for Group 3.5P with Groups 3.5U, l.O5P, 1.05U and CSa following in approximate order of decreasing values. On the other hand, Group means on the activity variable appeared to follow no systematic pattern. Pearson’s product-moment correlations among the dependent variables indicated that freezing and escape were highly interdependent (r = 0.90) while activity and freezing (r = 0.09) and activity and escape latency (r = 0.20) were not. Multivariate contrasts were performed in order to determine where specific group differences lay. Group 3.5P was found to differ from Group 1.05P [F(3,38) = 8.63, P < O.OOl], Group 3.5U [F(3,38) = 4.47, P < 0.011 and Group CSa [F(3,38) = 3.38, P < 0.051. Group 1.05P differed from Group 1.05U [F(3,38) = 3.60, P c 0.051 and Group CSa [F(3,38) = 3.40, P < 0.051. Univariate F statistics for each of the comparisons are presented in Table 1. The table indicates that the largest univariate Fs were obtained with the freezing-dependent measure; that both the freezing and escape measures yielded significant values for all contrasts; and that the activity-dependent measure was significant on only one of the contrasts (3.5P vs l.OSP). Table 1 also contains the standardized discriminant function coefficients for each dependent measure on each contrast. It can be seen that the standardized discriminant function coefficient for freezing was two to three times the value for the next highest coefficient; and that the standardized discriminant function coeficient for latency to escape was substantially greater than the standardized discriminant function coefficient for amount of activity for three of the contrasts (3.5P vs 3.5U; 3.5P vs CSa; 1.05P vs CSri), but not for the remaining two (3.5P vs 1.05P; 1.05P vs 1.05U). Taken together, both the univariate F statistics and the discriminant function loadings indicated that freezing was the most heavily weighted variable with escape and activity in order of
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Table I. A summary of the univariate F statistics and standardized discriminant function coefficients associated with the multivariate contrasts Dependent measure
Contrast 35P vs 1.05P 3.5P vs 3.5u 3.5P vs CSa 1.05P vs 1.05u
1.05Pvs CSa
Freezing Escape Activity Freezing Escape Activity Freezing Escape Activity Freezing Escape Activity Freezing Escape Activity
F‘(1.40)
P
22.88 22.83 6.04 12.85 8.01 0.26 10.21 9.78 0.69 10.35 9.89 1.57 10.53 9.82 0.24
Standardized discriminant coefficient - 0.674 - 0.232 - 0.362 1.141 - 0.496 -0.163 - 0.699 -0.319 -0.021 - 0.743 -0.208 - 0.262 - 0.699 -0.319 -0.021
< 0.05
decreasing importance. The different durations of freezing behaviour, then, probably most parsimoniously explain the observed group differences in the MANOVA. In summary, inspection of the multivariate contrasts and Fig. 1 indicated that paired group performance (Groups 3.5P and 1.05P) significantly differed from their respective unpaired control contrasts (Groups 3.5U and 1.05U) and the unshocked control (Group CSa). Thus, conditioned fear was obtained. In addition, the indices for conditioned fear were greater for the high-intensity paired group (3.5P) relative to the low-intensity paired group (l.OSP). Thus, a shock-intensity effect was obtained. Univariate analyses of Trials and Trials x Groups interactions for the postCS dependent measures
Univariate, two-way, repeated measures ANOVAs with orthogonal components for trend were employed to examine the effects of Trials and Trials x Group interactions for freezing, escape and activity, respectively. Freezing. Figure 2 presents the main effect of Trials for each of the three dependent variables. The figure suggests that the mean amount of freezing increased over the first three days, and then systematically decreased to a level (97.4 set) that was substantially lower than the initial value (289.5 set). The graphic interpretation was confirmed by an ANOVA which contained a significant Trials effect [F(9,360) = 4.72, P < O.OOl] which significant linear [F(1,40) = 13.50, P < O.OOl] and quadratic resulted from 800
I-
-
Duration of freezing
.-
Latency
to
escape
Activity (right-hand
axis)
Trial Fig. 2. The mean postCS duration of freezing, latency to escape and activity as a function of trials.
Eysenck’s theory of incubation: an empirical test
I2
3
4
5
6
7
8
9
335
IO
Trio1
Fig. 3. The mean postCS duration of freezing (upper panel). latency to escape (middle panel) and activity (lower panel), for Groups 3.SP. 35-L l.OSP. l.OSU and CSa as a function of trials.
[F(1,40) = 4.47, P < O.OS]trend components. The ANOVA also yielded a Group x Trials interaction [F(3,360) = 1.63, P -cO.OSJ which was composed of only a significant linear trend component [F(4.40) = 4.18, P -c0.01). The Group x Trials interaction is depicted in the upper panel of Fig. 3. An examination of the panel shows that Groups 1.05U and CSa had very low levels of freezing that did not change appreciably over trials; that the duration of freezing for Groups 3.5P, 3.Y.J and 1.05P was much higher than for Groups 1.05I.J and CSa but after an initial increase over the first few trials, declined to the levels of Groups 1.05U and CSa; and that Group 3SP had the highest level of freezing and the sharpest decrease. Thus, the linear component to the Group x Trial interaction resulted from the flat functions for Groups 1.05U and CSa, the moderate declines for Groups 3.5U and l.OSP, and the steep decline for Group 3SP. It should also be noted that since the Group x Trial interaction did not contain a significant quadratic trend component, the quadratic trend component to the Trials main effect reflected the initial increase in the functions for Groups 3.5P, 3.5U and 1.05P. Escape. An examination of Fig. 2 reveals that the function for the Trials main effect for the latency to escape had the same form as the freezing measure. That is, after an initial rapid increase, the mean latency to escape decreased to a value (223.2 set) that was much lower than the initial value (432.2 set). Again the graphical interpretation was confirmed by the ANOVA which contained a significant Trials main effect [F(9,360) = 5.58, P c 0.001]composed of significant linear [F(1,40) = 14.98, P < 0.001 J quadratic [F(1,40) = 6.56, P -c0.013 and cubic [F(l,40) = 5.65, P < 0.05] trend components. The ANOVA also indicated that the Trials x Group interaction was significant f&36,360) = 2.10, P < O.OOl] and resulted from a significant linear [F(4,40) = 5.63,
336
TERRENCE P. NICHOLAICHUK
er d.
P < O.OOlJ trend component. The significant Trials x Group interaction is depicted in the middle panel of Fig. 3. The panel shows that the mean escape latencies for Groups 1.05U and CSa were short and did not change over trials. In contrast, the mean escape latencies for Groups 3SP, 3.5U and 1.05P were much longer, and after an initial increase. decreased to about the levels of Groups 1.05U and CSa. The flat functions over trials for Groups 1.05U and CSa. the gradual decline in escape latencies for Groups 3.5U and 1.05P and the large rate of decrease for Group 3.5P produced the linear interaction component. Again it must be noted that the Trial x Group interaction did not contain a quadratic component, and therefore, the quadratic component to the main effect must be due to the initial increase in escape latencies for Groups 3.5P. 3.5U and 1.05P. Activity. The Trials main effect for the activity measure is also shown in Fig. 2. Again an inverted U-shaped function was observed with the mean amount of activity showing a small rise which was followed by a very gradual and irregular decrease over trials. The changes in mean amount of activity over trials were not large enough to produce a Trials main effect [F(9,360) = 1.471 in an ANOVA but did yield significant linear [F(1,40) = 5.44, P -c 0.051 and quartic [F(l,40) = 4.37, P < 0.05] trend components. The lower panel of Fig. 3 illustrates the nonsignificant Trials x Groups interaction. The panel indicates that the mean amount of activity for each group was irregular. with the greatest fluctuations occurring over the first five days. The absence of significant effects for the activity dependent variable would appear to corroborate the low standardized discriminant function coefficients assigned to this variable (cf. Table 1).
DISCUSSION
The principle results of the experiment were as follows: (1) during the postCS period in the test phase, the MANOVA revealed that paired group performance was significantly higher than control levels, and, that the high-shock intensity group (3.5P) had significantly higher measures than the low-shock intensity paired group (1.05P): (2) the discriminant analysis indicated that differences between groups were primarily due to changes on the freezing and escape dependent measures; (3) during the postCS period in the test phase, the main effect of Trials for the freezing, escape and activity measures showed small initial increases in levels and subsequent large declines: and finally (4) for the freezing and escape measures, Group 3.5P exhibited a large decline, Groups 3.5U and 1.05P showed moderate declines, whereas Groups 1.05U and CSa displayed constant, low-level performance, over trials. The observation that paired groups differed from their respective control groups on the freezing and escape measures confirmed the establishment of conditioned fear with a single CS-US pairing. In addition, since Group 3.5P had significantly higher performance indicators than Group 1.05P, the experiment demonstrated the commonly observed (e.g. McAllister and McAllister, 1971; Moyer and Korn, 1966; Theios et ul., 1966; Zammit-Montebello et al., 1969) direct relation between the magnitude of conditioned fear and US intensity. Thus, the experiment met the criteria needed for the observation of incubation effects. If Eysenck’s hypothesis (1968, 1979) is correct, then the subjects receiving the intense shock should show increased indices of fear over repeated CS-alone trials. While the increase in freezing duration and escape latency for Group 3.5P over the initial few trials might be construed as support for the hypothesis, two facts mitigate against such an interpretation. First, if incubation were occurring, the reinforcement provided by NRs would be greater than the decremental effects due to extinction and performance indices should continue to increase or at least be maintained. However, contrary to expectations. over the last half of the experiment, performance indices for Group 3.5P decreased to control levels. Second. the incubation effect is stated to be a consequence of an associative effect between the CS and US. But in the present experiment. the early increases in freezing duration and escape latency were observed in both the associative (Group 3.5P) and nonassociative control (Group 3.5U). The observation of parallel func-
Eysenck’s theory of incubation: an empirical test
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for associative and nonassociative groups appears to preclude the application of Eysenck’s (1976, 1979) incubation of fear hypothesis. The failure to observe incubation effects in the present experiment might be due to three factors. First, Eysenck (1976, 1979) has asserted that one of the major determinants of incubation is individual subject characteristics. Hence, the failure to observe incubation might be due to the use of normative data that disguises the incubation occurring in a few subjects by averaging performance indices with subjects that show an extinction effect. However, an examination of individual protocols for all postCS response measures showed that only one subject showed a continued increase in freezing duration and escape latency. This subject was in the CS-alone group and, therefore, the behavioral changes can not be attributed to incubation effects. A second possibility is that the present intense US was not sufficiently traumatic to produce the incubation effect and thus, that the present experiment was not an adequate test of the theory. While this argument is a very real possibility, several factors mitigate against it. In the present experiment, the intense shock produced major associative (the substantial differences in postCS paired group performance relative to the effects observed in Groups 3.5U and l.O5P), and nonassociative (the suppression of activity measures for Group 3.5P and 3.5U on the habituation day, and during the preCS periods on test trials) effects. In comparison with other experiments, the intensity of shock used for Group 3SP was substantialiy higher than the intensity required to produce reliabie fear conditioning (e.g. Annau and Kamin. 1961), or the intensities employed to provide data that purports to support the incubation hypothesis (e.g. Reynierse, 1966; Rohrbaugh and Riccio, 1970). The within experiment effects and cross-experimental comparisons suggest that the 3.5 mA US of the present experiment was a strong aversive stimulus and that there was a reasonable expectation of observing an incubation effect. And finally, Eysenck (1976, 1979) has asserted that the probability of incubation occurring will be inversely related to the CS duration during testing. In the present experiment, a short CS duration that should maximize the occurrence of incubation’ was programmed, yet clear evidence of incubation was not observed. However, if the programmed CS was only a component of the effective CS, then a different interpretation, more consistent to Eysenck’s postulates, could be generated for the present results. Despite our attempts to minimize the role of contextual cues, it is possible that fear was conditioned to contextual cues during training and subsequently generalized to the cues of the testing apparatus. Such contextual conditioning could have occurred in both Groups 3.5P and 3SU. With these assumptions, the increase in the duration of freezing and latency to escape over the initial testing sessions would then indicate the occurrence of incubation to generalized CSs for both groups. The differences between groups would reflect the addition of the conditioned effects provided by the nominal CS-US pairing to the performance of Group 3SP. From Eysenck’s perspective (1976, 1979) the problem becomes accounting for the subsequent eIimination of the initial incubation effects and the soiution lies in the hypothesized role of CS duration. For, in the present experiment, the dominant response of the rats was long duration freezing which has the consequence of giving the subjects a long duration exposure to the hypothesized contextual CSs. Since in Eysenck’s formulation the strength of extinction processes is determined, in part, by the duration of CS-alone presentations, the initial incubation effects to the contextual CSs would be counteracted by the effects of extinction resulting from response-produced exposure to the contextual CSs. This would lead to the expectation of the observed decrease in response measures late in training. Collateral support for this post-hoc account can be obtained from an examination of the between group differences obtained in the preCS period. The principle observation was that Groups 3.5P and 3SU engaged in less activity and made fewer crossovers than did the other three groups. These observations suggest that the contextual cues were producing greater fear in the strong-shock groups. in addition, the failure to find differences between Groups 3.5P and 3.5U during the preCS periods would seem to support the postulated summative role of the nominal CS in postCS performance. Thus, by
tions
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TERRENCE P. NICHOLAICHUK et al.
switching the explanatory power of Eysenck’s (1976, 1979) model from the programmed CS to unspecified generalized contextual CSs, the testing results become consistent with Eysenck’s postulates. However, if fear was conditioned to contextual CSs during the training phase, then in the 30-min habituation session, the generalized contextual CSs should have yielded a shock intensity effect. No such effect was observed. There were no between group differences on the crossover measure, and only a shock-no shock difference on the activity measure. Accordingly, an interpretation based on hypothesized conditioned contextual CSs does not appear to provide a complete account of all the present results. The present failure to conclusively demonstrate an incubation of fear effect, coupled with the weakness of prior studies (cf. Bersh, 1980; Kimmel, 1979) leaves in doubt Eysenck’s (1968, 1976, 1979) reliance on the hypothesized incubation mechanism to account for the intensification of neurotic behavior in the absence of traumatic stimuli. However, it may well be that an incubation-like phenomenon is the product of situational and procedural variables as yet unknown. In the final analysis, Eysenck’s theory of incubation and neurosis can only be properly assessed in the light of repeated and varied investigations which attempt to ascertain the boundary conditions necessary for the reliable production of the phenomenon. Acknowledgements-This Canada Research Grant
work was supported A0312 to R. W. Tait.
by
Natural
Science
and
Engineering
Research
Council
of
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