Physiological Psychology 1973.1'01.1 . .vo. 4.361·363

Periodic and aperiodic noise: The safety-signal hypothesis and noise aftereffects*

DAVID e. GLASS and MELVIN L. SNYDER n,e Ullil'ersity of Texas at Austill. Austill. Texas 78712

and

JEROME E. SINGER Stott Cllil'lmity of Sell' York at StOIlY Brook. StOIlY Brook. Sell' York 11790

Data from five previously published noise experiments were reanalyzed to test implications of Seligman's safety-signal hypothesis. Spontaneou5'fluctuation (SF) ooskin resistance, an index of emotional arousal, was measured during intervals between high-intensity noise trials. Two types of conditions were compared: Those where noise bursts occurred on an aperiodic schedule and those where they occurred periodically. The aperiodic group showed significantly more SFs than the periodic group, indicating that arousal was greater during exposure to an unpredictable stressor. However. there was no relationship between the magnitude of post noise performance deficits and number of SFs, even though the deficits were uniformly greater following aperiodic noise than periodic noise. The discrepancy between the two sets of data was explained in terms of a cognitive process involving perception of lack of environmental control and d -'creased task motivation.

Glass and Singer (1972) have shown greater deficits in task perfonnance following exposure to unpredictable aversive events than to predictable aversive events. This result was based on research in which predictability was manipulated in terms of signaled stimulation, as well as in terms of periodicity of repeated stimulation. Stress aftereffects were observed with a variety of stressors, including high-intensity noise and electric shock.

An explanation of these results might involve the notion of "informational cognitive control" (Furedy & Dobb, 197 I). Plediction reduces adverse aftereffects because knowing when to expect a stressor provides a measure of cognitive control over stimulation. It may also be that stressful situations become more distressing be­cause they lack a predictor of the absence of the aversive stimulus (Seligman, 1968). In any case. an aversive event has been shown to be less painful if the individual believes he can terminate or avoid it than if he has no control over its occurrence. A series of experiments by Glass and Singer (1972) demonstrate that perception of control. over an unpredictable stressor reduces the magnitude of adverse aftereffects. The belief that one can avoid or escape from noxious stimulation has ameliorative effects on subsequent task performance.

A more recent study by Geer and Maisel (1972) showed that the effects of perceived control are not simply determined by prediction. Ss who could predict the occurrence of a stressor. but could not terminate it,

*The research reported in this paper was made possible by grants from the National Science r oundation (GS-34329 and (;5-33216), Russell Sage Foundation, and the H'.'gg Foundation for \!ental Health. The idea for the analyses reported in this paper grew out of a conversation \\;th \Iartin E. P. Seligman. We also wish to thank the following people for their assistance in tabulation and analysis: Stephen Abney, \Iarc Bennett. James Cox. George MacConnell and Rena S. Sabrsula.

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showed higher GSR reactivity to that stimulus than Ss who also had control over its termination. This experiment fails to clarify the mechanisms involved in the effect. but it does suggest that the impact of prediction is not explicable primarily in terms of perceived control.

Several explanations have been proposed to account for reported differences in stress effects and aftereffects of predictable and unpredictable aversive events (cf. Seligman. Maier. & Solomon, 1'971: Furedy, 1970). We alluded earlier to one of the more compelling interpretations. namely, Seligman's (1968) safety-signal hypothesis. It argues that a S receiving a predictable aversive event is fearful only during the es and ues. AS receiving an unpredictable stressor is always in a state of fear. Therefore, we may expect a similar stress reaction to t:le LIeS, but different reactions during intertrial intervals (ITIs) when the unpredictable S is still afraid but the predictable S is relatively calm. Seligman (e.g., Seligman, Maier, & Solomon, 1971) reports a number of animal studies in support of this hypothesis. Price and Geer (1972) have shown that human Ss exposed to unpredictable stress exhibit nearly twice as many spontaneous skin conductance t1uctuations (SFs) during ITIs as do Ss exposed to predictable stress.

The safetY-Signal hypothesis was proposed as an explanation of differential stress effects during Signaled and unsignaled noxious stimulation. 1\lost of the experiments reported by Glass and Singer (1972) were concerned \vith periodic and aperiodic noise and noise aftereffects. However. cessation of periodic noise in this research may have served as a signal for an extended safety period. Ss exposed to periodic noise learned that the unwanted sOllnd occurred every minute for 9 sec. in which case they could feel relatively safe during the 51-sec ITIs. By contrast. unpredictable Ss were always in

362 GLASS AND SNYDER

Table 1 Average Total Number of SFs Corrected for Number of ITIs

Experimental Designation

R 1 R2 Easy Adapt of DA I Difficult Adapt of DA I DA II

Noise Condition*

Aperiodic ~oise

73.8 66.9 40 .5 53.4 76.9

Periodic Noise

54.2 47.0 41.0 38.7 74.6

-All cell means are based on 10 cases. except for the aperiodic condition of the R 1 experiment, which is based on 9 cases.

a state of apprehension. The intervals between noise bursts were random, as were the lengths of the bursts themselves, Previous analysis made it clear that GSR responding to the noise itself was no greater in aperiodic than in periodic noise conditions. However, it is entirely possible that SFs in the ITIs were reliably greater in the aperiodic treatment. Such an expectation is consistent with the findings of Price and Geer (1972).

This paper reports a re-analysis of skin resistance data collected in a Sample of experiments described in Glass and Singer (1972). These studies · showed that proofreading performance, tolerance for frustration , and ability to cope with a response-competition task were all impaired following exposure to unpredictable, in contrast to pre.dictable, high-intensity noise. Differential emotional arousal to the two types of noise stimuli could explain these differences in noise aftereffects. The SF measure has been successfully used as an index .of arousal (e .g., Price & Geer, 1973) and, unlike GSR responding to discrete stimuli, it is not confounded by other stimuli or responses (cf. Furedy, 1970; Furedy & Klajner, .1972). Accordingly, we predicted a greater number of SFs during ITIs in the aperiodic noise conditions than in the periodic noise conditions.

PROCEDURE

The five experiments in the analysis were designated as follows: Replications 1 and 2 (Rl , R2) in Chapter 4 of Glass and Singer (1972); the Easy-Adapt and Difficult-Adapt conditions l

of Differential Adaptation I (DAI) in Chapter 9 of the Glass-Singer monograph; and the Short lSI conditions of Differential Adaptation II (DAII) also reported in Chapter 9. In each of these studies, Ss were exposed to broadband noise bursts (typicaIIy 108 dBA, but in two cases 92 dBA) on either periodic or aperiodic schedules. The actual schedules varied from study to study and are described in Glass and Singer (1972). Both periodic and aperiodic Ss were usually presented with about 3\12 min of noise over a 20- to 25-min period.

Spontaneous fluctuation of skin resistance (SF) was defined as a decrease of at least 500 ohms occurring during an ITf. The IT! was calculated from the offset of noise to the onset of the next noise burst. Total IT! time was identical in the periodic and aperiodic conditions of each experiment, hence SFs were summed across ITIs for each condition within a given study. Each of the resulting scores was then divided by the number of ms for that experiment. This additional computation corrected

for varying numbers of ITIs between different experiments. The corrected total SF measure was the principal index of emotional arousal produced by periodic and aperiodic noise.

RESULTS

There was a resting level measurement period of 100 sec preceding the onset of the first noise burst in each experiment. Periodic and aperiodic groups did not differ from each other in the total number of SFs counted during this resting level period. When the two groups were compared on number of SFs during ITIs, the aperiodic condition showed mc:re SFs than the periodic condition in four of the five experiments. Table 1 presents the averages of the corrected SF totals for each experiment. Analysis of variance of these data revealed a reliable difference between experiments (F = 4.55, df = 4/89, P < .003) and the expected periodic-aperiodic effect (F = 3.62, df = 1/89 , p = .057).

The near significant result may actually underrepresent the magnitude of arousal differences between periodic and aperiodic conditions. Each III was not exactly the same duration for each rioise condition within an experiment, even though total III time was identical in both treatments. Another analysis was undertaken in which a single III was selected for each experiment. Selection of the interval was maqe so as to equate periodic and aperiodic conditions within the experiment for total amount of III, and for total noise time preceding the interval. An effort was also made to equate the two conditions for the duration of the noise burst that immediately preceded the selected ITI. Total number of SFs in the interval were counted, and these data subjected to a 2 by 5 analysis of variance. The periodic-aperiodic main effect was clearly significant at the .012 level (F = 6.45 , df = 1/89). The effect due to experimenfs was again signiflcant (F = 9.25, df = 4/89, P < .001). This latter result is certainly understandable in view of the fact that SF data were collected in different laboratories, by different Es, and at different times over the course of 4 years.

The greater number of SFs in the aperiodic than periodic noise conditions suggests that differential emotional arousal may be responsible for preViously reported differences in adverse aftereffects between these conditions (Glass & Singer, 1972). A stronger demonstration of this interpretation would be positive within-cell correlations between number of SFs and the magnitude of post noise performance deficits. However, a series of correlations within each noise condition for each experiment did not provide support for this prediction. Indeed, a sizable majority of the product moment coefficients showed nonsigniflcant inverse relationships between SFs and each of the aftereffect measures. (The median r was about .25.) Correlations for the periodic and aperiodic treatments, collapsed across

the five experiments. yielded similar nonsignificant results. A virtually identical pattern of correlational findings was observed when we combined treatment conditions within each experiment. It would appear that impaired proofreading performance and low tolerance for frustration are uncorrelated with the emotional arousal index in either periodic or aperiodic noise conditions.

DISCUSSION

The results reported in this paper indicate that aperiodic stressors produce greater emotional arousal than periodic stressors . This conclusion is entirely consistent with Seligman's (1968) safety-signal hypothesis, and replicates the findings of Price and Geer (I968). who showed more SFs in an unsignaled stress group than in a signaled stress- group. Total time in fear is presumably greater in unpredictable than in predictable conditions, and this difference would seem to account for the greater aversiveness of the former type of stimulation. Predicting a stressful event, either through signaling or temporal conditioning, reduces the emotional arousal of the context in which the event occurs. GSR responding to discrete noise stimuli does not distinguish predictable from unpredictable conditions (Glass & Singer. 1972, Chap. 3), but arousal to the total situation does provide a measure of the expected difference in aversiveness.

The SF results parallel the aftereffect differences reported in Glass and Singer (1972). However. correlations between the two sets of data did not reveal a consistent pattern of relationship. Greater arousal was unrelated to greater postnoise performance deficits, although there \vas a nonsignificant tendency toward an inverse relationship between the two variables. The sa f e t y-signal hypothesis may explain differential aversiveness of periodic and aperiodic stimulation. but it does not readily account for corresponding differences in aftereffects.

An alternative explanation attributes aftereffects to a cognitive process involving the perception of lack of environmental control. This view was discussed briefly in the introduction to this paper. It assumes that exposure to unpredictable stressors such as noise induces feelings of uncontrollability. Inability to predict the occurrence of a stressor leads the individual to label himself as helpless and give up efforts to cope with stressful stimulation. On the other hand, predictable aversive even ts provide a measure of "informational cont rol." which enhances rather than depresses the individual's motivation to deal with his environment. Unpredictable noise leads to greater adverse aftereffects than predictable noise. because the former type of stimulation results in feelings of helplessness and lack of task involvement. Following aperiodic noise. the

PERIODIC AND APERIODIC NOISE 363

individual tends to give up and does not try to perform on poststress t<Jsks.

This line of reasoning is, of course, largely conjecture. We do not have direct evidence in support of the assumption that predictability operates like perceived controllability. Indeed, Geer and Maisel (1972) have .shown that the effects of control cannot be accounted for in terms of a prediction-control confound. On the other hand, studies of controllable and uncontrollable stress in humans and animals (e.g., Glass & Si'lger, 1972 ; Seligman, Maier. & Solomon, 1971: Hiroto, 1973: Krantz, Glass, & Snyder. 1973) indicates that the latter leads to greater decrements in subsequent task performance. These effects may also occur irrespective of the degree of differential emotional arousal. Future research must explore the possibility of a common theoretic mechanism for aftereffects of unpredictable and uncontrollable aversive events.

REFERENCES

Furedy, J. J. Test of the preparatory-adaptive-response interpretation of aversive classical autonomic conditioning. Journal of Experimental Psychology, 1970, 84, 301-307.

Furedy, 1. J., & Klajner, F. Effect of signaling shock on unconfounded and validated autonomic indices of shock aversive ness. Journal of Experimental Psychology, 1972, 92, 313-318.

Furedy, J. J., & Dobb, A. N. Signaling un modifiable shocks: Limits on human informational cognitive control. Journal of Personality & Social Psychology, 1972,21, 111-115.

Geer, 1. H., & ~laisel, E. Evaluating the effects of the prediction-<:ontrol confound. Journal of Persinality & Social Psychology, 1972, 23,314-319.

Glass, D. C, & Singer, J. E. Urball strl!ss: Expuillll!nts olllloise alld social stressors. :>lew York: Academic Press, 1972.

Hiroto. D. S. Learned helplessness and locus of control. Journal of Experimental Psychology, 1973, in press.

KIantz, D. S., Glass, D. c., & Snyder, M. L. Helplessness, stress level, and the coronary prone behavior pattern. Journal of Experimental Social Psychology, 1973, in press.

Price. K. P. , & Geer, J. H. Predictable and unpredictable aversive events: Evidence for the safety-signal hypothesis. Psychonomic Science, 1972. 26, 215-216.

Seligman, M. E. P. Chronic fear produced by unpredictable electric shock. Journal of Comparative & Physiological Psychology, 1968,66,402-411.

Seligman. ~1. E. P., Maier. S. F., & Solomon, R. L. Unpredictable and uncontrollable aversive events. In F. R. Brush (Ed.), A l'ersil'<" cOllditioning and leaming. New York: Academic Press. 1971.

NOTE

1. These two conditions were actually part of the same experimental design. However, we treat them as separate studies in the reanalysis because noise aftereffects Df the predictable and unpredictable treatments within each condition were substantially different from one another (cf. Glass & Singer. 1972. Chap. 9).

(Received for publication September 17, 1973: accepteD September 27. 1973.)