Why do people tend to overpredict pain? On the asymmetries between underpredictions and overpredictions of pain

Pergamon S0005-7967(96)00016-2

Behav. Res. Ther. Vol. 34, No. 7, pp. 545-554, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0005-7967/96 $15.00 + 0.00

WHY DO PEOPLE TEND TO OVERPREDICT PAIN? ON THE ASYMMETRIES BETWEEN U N D E R P R E D I C T I O N S AND OVERPREDICTIONS OF PAIN ARNOUD ARNTZ Department of Medical Psychology, University of Limburg, PO Box 616, 6200 MD Maastricht, The Netherlands (Received 22 January 1996)

Summary--After an underpredicted painful experience people tend to expect increased pain levels for a considerable time, despite disconfirmatory experiences. Underpredictions also tend to raise long-lasting fear and increased physiological responding. Overpredicted pain does not have such dramatic effects. What are the reasons for this asymmetry? Evidence for and against the hypothesis that underpredicted pain hurts more than correctly predicted pain, and that overpredictions result from a tendency to avoid the extra aversiveness of underpredictions, is reviewed. Based on recent experiments this explanation is rejected, and alternative explanations are discussed. It is reasoned that the most plausible explanation is that the organism automatically infers danger from an underprediction, because of the loss of predictability into the dangerous direction (i.e. more pain). Elevated expectancy and fear levels are the result of this. A modified stimulus-comparator model that accounts for the differential effects of both types of incorrect predictions is suggested. In contrast to previous models, such a model hypothesizes: (i) differential processing of under- and overpredictions; and (ii) different processes involved in the influence of expectations on subjective and non-subjective pain responses. Copyright © 1996 Elsevier Science Ltd

INTRODUCTION

Various studies have documented how people change their expectations of aversive experiences like pain and fear when an experience diverts from expectation (Arntz & van den Hout, 1988; Arntz, van Eck & Heijmans, 1990; Arntz, van den Hout, Lousberg & Schouten, 1990; Arntz, van den Hout, van den Berg & Meijboom, 1991; Arntz, Hildebrand & van den Hout, 1994; McCracken, Gross, Sorg & Edmands, 1993; Rachman & Levitt, 1985; Rachman & Lopatka, 1986, 1988; Rachman & Arntz, 1991; Telch, Ilai, Valentiner & Craske, 1994). If the experience exceeds the expectation (underprediction), future expectations are generally increased in a dramatic way. If the experience falls below the expectation (overprediction), future expectations are generally decreased, but in a less dramatic way than after an underprediction. One underpredicted experience of pain or fear generally leads to a strong raise in future predictions, with a certain resistance against correction (it takes a number of disconfirmatory experiences before the expectation returns to an accurate level), and also causes immediate and long-lasting elevations of physiological arousal, increased physiological impact of the aversive experience, anticipation anxiety, uncertainty about what intensities to expect, and avoidance (e.g. Arntz, van den Hout et al., 1991; Arntz, van Eck & de Jong, 1991; Rachman, 1994). In contrast, one overpredicted experience has relatively weak and short-lasting effects on these variables. Thus, there appears to be a marked asymmetry between the effects of an underprediction compared to the effects of an overprediction (Arntz, van den Hout et al., 1991; Rachman & Arntz, 1991; Rachman, 1994). So far, there is only limited understanding of the reasons for this asymmetry. One speculation is that the fear, avoidance and arousal effects result from elevated predictions (if more pain or fear is expected, it follows that subjective fear, physiological fear responses and avoidance are increased). Even if we take this explanation for granted, it still remains to be explained why pain (fear, etc.) predictions increase so dramatically after an underprediction, and are resistant to correction. Not only after an underprediction people tend to overpredict pain and fear. Subjects afraid of an aversive experience also tend to overpredict the amount of pain, aversiveness, or fear, that they will experience during confrontation with the feared stimulus. For instance, chronic low back 545

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patients afraid of pain tend to overpredict the amount of pain they will experience during a physiotherapeutic exercise (McCracken et al., 1993). Dental patients fearing dental treatment tend to overpredict the painfulness of dental treatment, and show marked resistance against correction by disconfirmatory experience (Arntz, van Eck et al., 1990). Anxiety patients tend to overpredict the amount of anxiety they will experience during confrontation with feared stimuli, and also show a slow change in this tendency, generally needing numerous disconfirmations (Rachman, 1994; Rachman & Bichard, 1988; Arntz et al., 1994). Thus, there seems to be a broad tendency to overpredict aversive events, not only after an underpredicted experience, but also in fearful people in general. UNDERPREDICTED PAIN HURTS MORE: AN EXPLANATION FOR THE ASYMMETRY One explanation for this asymmetry that has been suggested states that underpredicted pain hurts more than correctly predicted pain, and that overpredicted pain might even hurt less than correctly predicted pain (Arntz, van den Hout et al., 1991; Rachman & Arntz, 1991). For example Rachman and Arntz have put forward the hypothesis that "An underpredicted pain is experienced as being more aversive than a correctly or overpredicted pain" (Rachman & Arntz, 1991, p. 339). Similarly, underpredicted fear might be more anxiety evoking, or more aversive in a general sense, than correctly or overpredicted fear. By avoiding to make an underprediction, people might better prepare for the aversive experience, and suffer less. The safest way to avoid underpredictions is to make overpredictions. In short, the tendency to overpredict might result from the avoidance of more painful (aversive) underpredictions. Such a tendency might be strategic: a deliberate use of preparation for a coming aversive experience by expecting the worst; or automatic: the aversive consequences of making an underprediction result in extinction of these predictions, etc. Indeed, people sometimes seem to think that by a strategic use of pessimistic predictions the impact of an aversive event will be softened ("If one expects the worst, life can bring some aggreeable surprises"). However, such statements may also reflect attempts to explain automatic processes in socially acceptable ways to bystanders. THEORIES EXPLAINING D I F F E R E N T I A L IMPACT OF UNDER- AND OVERPREDICTED PAIN Several theoretical explanations for a pain-diminishing effect of overpredictions can be put forward. Stimulus-comparator models state that the impact of a stimulus is not only a function of its intensity, but also of the degree to which it matches with expectation. In other words, a process is assumed that compares expectation with input, and a match results in suppression of responding, whereas a mismatch results in increased responding (e.g. Gray, 1975, 1976, 1982, 1985; (~hman, 1979). But, these theories predict identical effects of under- and overpredictions, and not the observed asymmetry. A representational model capable of explaining the asymmetrical effects is Wagner's habituation model (Wagner, 1976, 1978, 1979, 1981). In this model a short-term memory process is hypothesized, which can temporarily contain a stimulus representation. Warning signals, like conditioned stimuli, are supposed to be capable of eliciting a representation of the stimulus before the stimulus occurs. The impact of the stimulus is reduced to the degree that it is already represented in short-term memory. Wagner has suggested that the short-term memory process in his theory can be equated with short-term memory as is known from memory theories (Atkinson & Shiffrin, 1968), and that controlled (strategic) process may play a role in creating stimulus representations in short-term memory (Wagner, 1978, p. 180; 1981, p. 6). However, his theory has been tested in animal research using habituation and conditioning designs, thus leaving this issue unanswered. If we assume that Wagner's suggestion is correct, it follows that conscious pain expectations (whether induced by information, experience, or the S's private reasoning) should be related to stimulus representation in short-term memory. It then follows from Wagner's model that an overprediction results in reduced responding compared to a correct prediction, and that an underprediction results in increased responding. In a similar vein others have, in less formal models, suggested that the impact of a stimulus is influenced by the contrast between expectation and stimulus: overestimation results in a negative contrast, inhibiting the impact of the stimulus;

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underestimation results in a positive contrast, increasing the impact of the stimulus (Arntz, van den Hout et al., 1991; Epstein & Clarke, 1970; Epstein, 1971). Lastly, it has been suggested that overpredicted pain results in reduced pain impact, because of the release of endogenous opioids associated with the high levels of pain expectation and fear (Arntz, van den Hout et al., 1991; Crombez, Baeyens & Eelen, 1994). However, since opioid release seems to be a relatively slow process, this explanation seems improbable for the findings of the experimental studies, which employed time frames too short to lead to differential opioid release (Janssen & Arntz, 1996). Given the theoretical possibility that underpredicted pain hurts more, and overpredicted pain hurts less than correctly predicted pain, what is the empirical evidence? EARLY STUDIES Some attempts have been made to directly test the hypothesis that a correct pain expectation reduces the painfulness of the stimulus, compared to underestimation. Rachman and Arntz (1991) reported evidence from naturally occurring matches, under- and overpredictions of pain, suggesting that underpredicted pain hurts more and is followed by a disruption of the habituation process, whereas overpredicted pain appeared to be less painful and to be followed by a fastened habituation of the subjective pain experience. But, because there had been no experimental control over the mismatches, the findings may be explained by other factors than the hypothesized effects of (mis-)matches. Two early experimental studies sought to test the effects of correct and incorrect pain expectations. In the first study (Epstein & Clarke, 1970), there was some evidence that physiological responses to the noxious stimulus (a loud noise burst) vary directly with anticipation, whereas experienced intensity is related to the contrast between expectation and experience: overestimated stimuli were rated as less intense than correctly estimated stimuli. The effects of underestimation were less clear, possibly due to a questionable success of the underestimation manipulation. In a second experiment, Epstein (1971) compared physiological and subjective habituation to two aversive stimuli, electric shock and loud noise, over 5 days. Epstein assumed that people have generally pessimistic expectations of electric shock, and overoptimistic expectations about noise. But, no direct evidence for the assumption that the shock was overpredicted and the noise underpredicted was reported. Nevertheless, Epstein concluded that in the early period self-fulfilling influences of expectancy were found (underpredicted stimuli reported as less aversive than they truly were, etc.), whereas in later periods contrast effects dominated (underpredicted stimuli reported to be more aversive than they truly were, etc.). In sum, some evidence for the hypothesis that overpredicted pain hurts less than underpredicted pain has been reported, but the evidence is meagre. Further doubt about the hypothesis results form a meta-analysis by Suls and Wan (1989), which found that pain-warnings generally do not influence pain reports. TWO RECENT

STUDIES WITH OPPOSITE

RESULTS

Two recent experimental studies attempted to test how pain expectations influence pain impact. The first study (Arntz, van den Hout et al., 1991) compared the immediate and long-term effects of correct, under- and overpredictions, induced by analogue information on a PC screen. Reports of pain anticipation and experience showed that the manipulation was successful. Overpredicted pain was not reported to be less painful than correctly predicted pain. Underpredicted pain was reported to be even less painful than correctly or overpredicted pain [t(39) = - 3.37, P < 0.002]. There were no long-term effects of experimentally induced mismatches on subjective pain reports, but mismatches influenced physiological pain impact, pain predictions, uncertainty, and anticipatory fear (subjective and physiological). As hypothesized, underpredictions had especially long-term negative effects on these variables. Thus, this study could not confirm the hypothesis that underpredicted pain hurts more, and overpredicted pain hurts less, than correctly predicted pain. The second recent study (Crombez et al., 1994) tested the influence of pain (vs no pain) information and temporal information on the impact of a painful heat stimulus. As to pain information, it was found that a pain warning had a beneficial effect compared to no pain warning on the subjective and physiological (SCR) impact of the heat stimulus [F(1,36) = 6.78, P < 0.013; F(1,33) = 6.37, P < 0.017]. Thus, if we equate no pain warning with the underprediction of pain,

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this study suggests that underprediction of pain has strong negative effects on the experience of pain. Thus, two recent studies demonstrated opposite effects of more or less comparable strength. How can this be explained? A major difference between the two studies is that in the Arntz et al. study the Ss had received a number of painful stimuli which were all preceded by (correct) visual information about the intensity, before an underprediction was induced; whereas in the Crombez et al. study Ss had no experience with information signals nor with the pain stimulus before the underprediction manipulation was done. Thus, the fact that Ss in the Arntz et al. study reported lower pain experiences than Ss receiving correct information may have to do with their experience with the low level signals. The low-level signal that was used for inducing the underprediction was comparable to several of the signals used before the experimental manipulation was done. As is suggested by Arntz, van den Hout et al. (1991, p. 557) in case of such strong external anchors the S might distort the report (or even the experience itself) into the direction of the signal. In that case, pain reports are a compromise between truly experienced pain and the level suggested by the external anchor. The fact that Ss in the overprediction condition did not report more pain than Ss in the correct information condition might have to do with the fact that for both of these two conditions the signaled and experienced level had never been experienced before: all preceding stimuli had been much weaker. Thus, the overprediction information did not refer to a clear anchor, so that distortion was less probable. Taken together, the differences between the procedures of the two studies suggest that a possible pain-increasing effect of underprediction might be better demonstrable under conditions where the underprediction-inducing information does not relate to clear anchors, so that it is less likely that pain reports are distorted into the direction of the information that suggests a low pain level to expect. Clearly, the Crombez et al. study using rather vague verbal information meets this requirement better than the Arntz et al. study. An attempt to replicate the study showing that underpredicted pain hurts more

We attempted to replicate the study by Crombez et al. (Arntz & Hopmans, 1996) using 40 healthy female volunteers with a mean age of 21.4 yr (SD 3.9). Subjects received a small remuneration for participating. Subjects were randomly allocated to one of two conditions: (1) pain warning; and (2) underprediction. During recruitment Ss were asked to participate in an experiment on attention. No reference to pain was made. At the start of the experiment Ss in the underprediction condition were told that they would receive tingling sensations on the left ankle during the attention task as a distractor. Subjects in the pain warning condition were told that they would receive a painful stimulation on the left ankle as a distractor. As in the study by Crombez et al. the attention task consisted of an auditory discrimination task. Short (100 msec) and long (200 msec) noises (45 dB white noise) had to be discriminated as quickly and correctly as possible, ignoring other stimuli. The noises were binaurally presented through a headphone. A two-button box connected to the computer was used to register the responses. After a baseline phase of 10 noises, the S was told that the discrimination task would be continued, but now with some distracting stimuli. Subjects in the pain warning condition were told that a mildly painful stimulation would be given on the ankle, that the stimulation would not be dangerous, and would have a duration of 2 sec. Subjects of the underprediction condition were told that a weak stimulation would be given on the ankle, which could cause some tingling sensations, would not be dangerous, and that the stimulation would have a duration of 2 sec. Subsequently Ss in both conditions were instructed to concentrate as much as possible on the discrimination task and to fill in a number of visual analogue scales (VASs) when the experimenter asked this through the intercom. The experimenter left the room, and a series of 20 noises started. Five times the noise was presented during the 2 sec painful electrical stimulation at the ankle (2.5 mA rectangular biphasic pulses of 20 msec). After each of the 5 pain stimulations the S filled in a number of VASs. Among the VASs, five asked for stimulus evaluations: experienced pain, tingles, stinging sensation; intensity; aversiveness. Factor analysis showed that at each stimulation only one factor could be extracted and that the five VASs loaded strongly on one factor (explained variances 67-82%). Consequently the five VASs were averaged (Cronbach alphas of the five stimulations: 0.88, 0.91, 0.92, 0.94, 0.94). A MANOVA trend analysis revealed a significant difference between the two

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Composite pain score 60

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Fig. 1. Mean composite subjective pain scores of five painful stimulations in two conditions: pain warning (the S was warned that the stimulus would be painful) and underprediction (the Ss were told that the sensation would be tingling). Pain warning resulted in significantly higher pain reports than the absence of a pain warning.

conditions on the mean composite pain scores [F(1,38) = 6.65, P = 0.014], and a difference in the linear trend [F(1,38)= 8.62, P = 0.006]. Figure 1 shows that the underprediction condition had lower composite pain scores (M = 40.0, SD 20.4) than the pain warning condition (M = 55.9, SD 18.5), and that the pain warning condition shows an increase in pain reports, whereas the underprediction condition shows a decrease. A separate analysis of the experienced pain VAS showed similar results. Another VAS asked for experienced distraction. As is shown in Fig. 2, the underprediction condition starts with higher distraction reports than the pain warning condition, whereas in the long run the pain warning condition reports more distraction by the pain stimulus than the underprediction group. The difference between conditions on the linear trend was highly significant [F(1,38) = 9.83, P = 0.003]. Other responses have not been analyzed yet, but the results strongly suggest that underpredicted pain is not experienced as more painful than (more or less) correctly predicted pain. Distraction reports suggest however that initially underpredicted pain might be more distracting than correctly predicted pain.

Distraction by pain stimulation 70 \ \

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Fig. 2. Mean subjective distraction scores during five pain stimulations in two conditions, pain warning and underprediction.

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Of the three recent studies aiming at a direct test of the hypothesis that underpredicted pain hurts more than correctly or overpredicted pain, one found the hypothesized effect, and two found evidence for the opposite effect (underprediction reduces the pain experience). Physiological measures also do not lead to an unequivocal conclusion, but here there is somewhat more evidence that underprediction results in increased responding. But, as has been mentioned before, the increased physiological impact of underpredicted pain probably reflects startle and fear responses, rather than pain responses (Arntz, van den Hout et al., 1991). The most recent study in our lab attempted to use the same methodology as the study by Crombez et al. (1994): in order to rule out the possibility that pain reports are distorted into the direction of unambiguous anchors, no pain experience was given before the first experimental trial, and manipulation of expectancy was done in a rather vague way, without referring to VASs used later to measure pain impact. Contrary to what we hypothesized, this did not appear to be the crucial factor. What are other explanations f o r the different findings? (1) Crombez et al. used heat and Arntz et al. electric stimulation as painful stimuli. But, though

some difference between the pain stimuli might account for the different findings, the conclusion that in general underpredicted pain does not hurt more than correctly predicted pain remains valid. (2) Crombez et al. appeared to have used a somewhat more intense pain stimulus than we did in our second study (transformed to comparable 0-100 scales mean composite pain scores were in the 51-65 and 40-56 range, respectively). Perhaps there is a sudden change from pain reducing to pain increasing effects of underprediction in the 45-55 range of this scale, though this seems highly unlikely given the findings by Arntz, van den Hout et al. (1991) in the 50--65 range that underpredicted pain did not hurt more than correctly predicted pain. (3) Finally, despite the significance levels in the three studies, it remains possible that there is simply no unequivocal effect of incorrect predictions on pain impact, especially not on the experience of pain. This conclusion is in line with the conclusion by Suls and Wan (1989) in their meta-analysis, that explicit pain-warning did not have an unequivocal effect on pain impact. In sum, it seems highly unlikely that, in general, underpredicted pain hurts more than correctly predicted pain. It is less evident what the effect of overprediction is, but so far the evidence does not suggest that overpredicted pain hurts less than correctly predicted pain. Given that it is highly unlikely that, in general, underpredicted pain hurts more than correctly predicted pain, it follows that this hypothesis cannot explain the asymmetrical effects of under- and overpredictions. What are alternative explanations for this asymmetry? Why has underpredicted pain stronger and longer-lasting effects than correctly and overpredicted pain? Strategic or automatic process?

Given the lack of pain increasing effects of underprediction, it seems unlikely that long-lasting overpredictions after an underprediction are the result of a strategic process, a deliberate attempt to avoid further underpredictions. Nevertheless, there are some reasons why a strategic process explanation might still hold: (1) It could be argued that pessimistic predictions are helpful in reducing impact of aversive events in more ambiguous domains (like the social domain), where evaluations are to a considerable extent constructed, than in laboratory pain. People might just use a strategy they found out to be effective in other domains in the laboratory situation, despite its ineffectivity there. On the other hand, our previous study (Arntz, van den Hout et al., 1991) as well as the later study (Arntz & Hopmans, 1996) demonstrated that pain reports can be distorted to a considerable degree in the direction of suggestive information. Thus, laboratory pain may not be so non-ambiguous as assumed. (2) Also in a social context, people might have learnt to use pessimistic strategies to prevent criticism by others in having been too optimistic or too naive (blaming the victim). On the other hand, it is not entirely clear why such a pessimistic strategy should function in the relatively private situation of the typical laboratory pain study.

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Although such explanations cannot entirely be ruled out, they seem somewhat far-fetched. Moreover, it can also be argued that pessimistic predictions are not helpful, but people tend to say or even believe that they are helpful in an attempt to explain an involuntary, automatic process (the generation of pessimistic expectations), that is otherwise difficult to understand and is difficult, if not impossible, to control. The asymmetrical effects of the different types of mismatches are most probably the result of automatic processes.

Automatic processes and their function Underprediction raises more and longer-lasting fear, uncertainty, overprediction and avoidance, than correctly predicted or overpredicted pain. It has been suggested (Arntz, van den Hout et al., 1991; Rachman & Arntz, 1991; Rachman, 1994) that overpredicted pain is more a 'non-event' than underpredicted pain, even if the objective pain level is the same. Cognitive research has demonstrated that people have more difficulties in processing 'non-events' than 'vivid and salient events', and that 'vivid and salient events' exert a disproportional influence on judgements under uncertainty (Alloy & Tabachnik, 1984; Nisbett & Ross, 1975; Tversky & Kahneman, 1974). The immediate processing and/or the retrievability in (declarative) memory might be influenced by the degree of 'vividness and salience'. It will be clear that the effects of stimulus intensity can be explained by this theory: obviously, more pain means more salience. But, this theory cannot explain why underpredicted pain is more salient than overpredicted pain of identical intensity. If we are to explain the asymmetry between under- and overpredictions in this framework, we have to assume that not only the intensity of the painful (fearful, aversive) stimulus determines salience, but also the degree that the stimulus exceeds expectation. Why would aversive events exceeding expectation be more salient than equally intensive events falling below expectation? It can be argued that underprediction means a loss of predictability in a dangerous direction, namely into the direction of more pain, whereas overprediction means a loss of predictability in a safe direction (Arntz, van den Hout et al., 1991). Loss of predictability above the predicted level means that, potentially, the stimulus might unpredictably become life-threatening. Loss of predictability in the range below what was predicted does not threaten life. Assuming that the organism processes information with priority to predict danger (in order to prepare for avoidance or fight/flight), it follows that an underprediction is more alarming and informative for survival than an overprediction. A stimulus-comparator model of asymmetrical processing of under- and overpredictions Stimulus-comparator theories seem to provide a useful basis for explaining the effects of incorrect pain predictions,/fthey are extended with an automatic process that asymmetrically processes underand overpredictions. A conservative bias seems to be built in into the system. So far, most stimulus-comparator theories hypothesize that symmetrical processing of both types of mismatches takes place (e.g. Gray, 1975; Rescorla & Wagner, 1972; Wagner, 1981), ignoring functional values of differential weighing of under- and overpredictions. Figure 3 sketches a possible stimulus-comparator model that asymmetrically processes under- and overpredictions. Two effects are distinguished: the immediate effects of a (mis)match (left) and the long-term (anticipatory) effects (right). The model describes that the immediate effects of an aversive stimulus (for appetitive stimuli a similar model could be designed) depend on three factors: (i) the intensity of the stimulus; (ii) the expectancy of the stimulus; and (iii) the degree that the stimulus exceeds expectation (for simplicity, it is assumed that correct matches and overpredictions are equivalent for immediate effects mediated by the comparator). In contrast to previous stimulus-comparator models, different processes for the subjective experience and the other variables are hypothesized*. The subjective experience of the stimulus is assumed to be a weighted mean of the stimulus intensity (aS; a being the weight >10; S the stimulus intensity) and the expectancy (bEg; b being the weight/> 0t; Ei the expected intensity at trial i). Thus, the model assumes that expectancy distorts the experience by partially determining it. The degree to which it distorts experience is determined by its weight b,

*In addition to intensity, other important characteristics of the stimulus could be processed in an identical way, e.g. duration of the stimulus (see Gray, 1975). #If the findings by Epstein and coworkers and by Crombez and coworkers are to be explained by this model, we must assume that the weight b can also be smaller than zero. It is still unclear which conditions are essential for this. BRT M/7--B

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Long-term Effects

Subjective Experience S'I

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Fig. 3. Modified stimulus-comparator model with differential processing of underpredictions and overpredictions; and differentprocesses involved in subjective and non-subjectiveresponses. and factors like (un-)certainty, believability of external or internal cues, etc. might play a role in determining b. Sympathetic responding, attention-diversion, flight/fight responses, and other responses are controlled by the intensity of the stimulus, but not directly by the stimulus expectation. Only if the stimulus exceeds the expected intensity an alarm signal is generated, and sympathetic responding, attention, and fight/flight responses are increased. If the stimulus matches expectation, or falls below it, safety is inferred, and these responses are inhibited. The more the organism builds a representation of the stimulus (by experience and/or information), the stronger the influence of the comparator on these responses is. Long-term effects are visualized to the right of Fig. 3. In case of an underprediction (S > Ei), the expectancy of the next stimulus (Ei+ ~) is increased to S' (Ei+~ = S'), S' being the experienced stimulus intensity. In case of a match or an overprediction, the expectation of the next stimulus is changed according to the formula Ee+ ~= (xS' + yE3/(x + y), x and y being positive weights (in case of a match this formula reduces to Ee+ l = S', because E~ = S'). Thus, the difference between an underprediction and an overprediction is, that in the first case the next expectation is completely raised to the level of the experienced pain stimulus, whereas in the latter case the next expectation is only partially reduced, being a weighed sum of the experienced stimulus and the previous expectation. Several factors might influence the constants x and y, with a larger y meaning more resistance against correction. The model might be expanded by assuming that an underprediction increases the weight y, thereby increasing resistance against disconfirmation. Other long term anticipatory responses, like anticipatory sympathetic responding, attentional processes, avoidance, etc. are hypothesized to be influenced by the outcome of the comparator and by the new expectancy. The first factor implies that anticipatory responding is larger after an underprediction than after a match/overprediction, independent of the expected intensity level, which has a separate influence on these anticipatory responses. As stated earlier, there are two main differences between this model and previous models: (i) there is not one unitary process assumed for all responses: there are important differences between

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processes involved in subjective a n d in o t h e r responses; (ii) the s t i m u l u s - c o m p a r a t o r influences o t h e r variables t h a n the subjective experience o f the stimulus in an a s y m m e t r i c a l way: u n d e r p r e dictions have different effects t h a n correct p r e d i c t i o n s a n d overpredictions. CONCLUSIONS It seems highly unlikely that people tend to o v e r p r e d i c t pain, a n d that u n d e r p r e d i c t i o n s have so d r a m a t i c effects, because u n d e r p r e d i c t e d p a i n hurts m o r e t h a n correctly p r e d i c t e d pain. There seems to be m o r e evidence t h a t the a s y m m e t r i c a l processing o f under- a n d o v e r p r e d i c t i o n s is the result o f the different m e a n i n g these two types o f m i s m a t c h e s have for survival. G i v e n an identical objective level, u n d e r p r e d i c t e d p a i n signals d a n g e r because o f loss o f predictability in a d a n g e r o u s direction ( a b o v e prediction: t o w a r d s m o r e pain); o v e r p r e d i c t i o n is o f far less significance because it signals d e v i a t i o n from e x p e c t a t i o n into a less d a n g e r o u s direction. The strong fear-evoking a n d pessimistic e x p e c t a n c y effects o f an u n d e r p r e d i c t i o n can be u n d e r s t o o d in terms o f their value for survival: they help to increase caution, avoidance, a n d p r e p a r a t i o n for fight/flight. The a s y m m e t rical effects o f under- a n d o v e r p r e d i c t i o n s can be i n c o r p o r a t e d in s t i m u l u s - c o m p a r a t o r models, p r o v i d e d that: (i) the c o m p a r a t o r gives a different signal after an u n d e r p r e d i c t i o n ( a l a r m signal) c o m p a r e d to after a m a t c h o r o v e r p r e d i c t i o n (safety); a n d (ii) the subjective experience o f p a i n [or o t h e r (aversive) experiences] is influenced in a n o t h e r way than other ( n o t a b l y physiological) responses, to a c c o u n t for the different influence o f expectancies on these two types o f variables. F u t u r e studies w o u l d be welcome to further test this class o f models, a n d to clarify why (implicit) u n d e r p r e d i c t i o n sometimes seems to increase, sometimes seems to decrease the experience o f pain. REFERENCES Alloy, L. B. & Tabachnik, N. (1984). 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