43
Pain, 46 (1991) 43-51 0 1991 Elsevier Science Publishers ADONIS 030439599100152C
B.V. 0304-3959/91/$03.50
PAIN 01801
Can anxiety help us tolerate pain? Mustafa Al Absi and Paul D. Rokke Dept. of Psychology, North Dakota (Received
10 August
1990, revision
State lJniLwsi@, Fargo, ND 58105 (U.S.A.)
received
21 November
1990, accepted
7 December
1990)
It was hypothesized that anxiety which is relevant to the source of pain exacerbates pain, whereas Summary anxiety which is irrelevant to the source of pain reduces the experience of pain. Female subjects were given either high or low anxiety provoking information about a cold pressor task (relevant anxiety) or high or low anxiety provoking information about a potential shock (irrelevant anxiety). Subjects were then exposed to the cold pressor. The results demonstrated that subjects who were highly anxious about the cold pressor reported experiencing the most pain. Subjects who were highly anxious about the shock reported the least pain and reported significantly less pain than subjects who were highly anxious about the cold pressor. These findings clearly demonstrate that the relationship between anxiety and pain is not always positive or unidirectional. Key words: Anxiety; Pain
Introduction
The relationship between anxiety and pain perception has been the topic of several scholarly works [7,15,21,36,40,41]. Although the overwhelming majority of studies have demonstrated that increased levels of anxiety are associated with increased pain reports [11,20,28,30,32,36,37], the nature of the relationship between these constructs is not without controversy. Both theoretical and empirical accounts have suggested that anxiety and pain are not necessarily positively associated. From a theoretical perspective it has been proposed that autonomic arousal in the presence of threat (the flight or fight response) will take precedence over pain and may inhibit the experience of pain [5,39]. When applied to humans, the processing of information relevant to the painful situation may also play an important role. Malow [231 conducted a signal detection analysis of pain perception in the presence of an anxiety inducing manipulation. He found that anxiety reduced the subject’s ability to discriminate among different stimu-
Correspondence to: Paul D. Rokke, Ph.D., Department of Psychology, North Dakota State University, Fargo, ND 58105-5075, U.S.A. Tel.: (701) 237-8626.
lus intensities. Subjects who evidenced both verbal and physiological signs of anxiety also showed a decrease in their bias to report the perceived sensations as painful. A different aspect of cognition was studied by Nisbett and Schacter [31]. In this study, subjects in a low state of fear who attributed signs of arousal to a placebo reported electrical shock to be less painful and tolerated higher levels of shock than subjects who were unable to attribute their arousal to an external source. This difference was not found for subjects in a high fear condition in which expectations were manipulated by telling the subjects that the shock would be very painful. Consistent with Schacter’s attributional formulation of arousal states, Weisenberg et al. [421 proposed that anxiety which is relevant to the pain inducing stimulus will exacerbate pain whereas anxiety that is irrelevant to pain would reduce the experience of pain. In fact, it could be argued that much of the literature supporting a positive correlation between anxiety and pain has been studied in a context where anxiety is related to the painful stimulus. For example, Dworkin and Chen 1131 found that when the tooth pulp of subjects was electrically stimulated in a setting that is typically associated with oral pain (the dentist’s office) they reported more pain than when the same measure was obtained in the laboratory. Likewise, Klepac et al. [19] found
J-l
that subjects who were dental phobic were no different than controls in their tolerance to arm shock, but reported tooth shock to be much more painful than non-phobic subjects. The typical study on the relationship between anxiety and pain has focused on differences in the pain reports of subjects who score either high or low on measures of trait anxiety [ 11,12.24,32.36]. In these latter cases, we could infer that individuals who are prone to experience anxiety in a wide variety of circumstances are also likely to experience more anxiety about pain in situations involving pain. The importance of anxiety that is relevant to the painful stimulus can also be inferred from several studies which show that pain tolerance can be increased by using some manipulation to reduce anxiety. In this case it can often be assumed that the anxiety which is reduced is that which is related to the source of pain. Perceived control over the painful stimulation may reduce pain because it reduces anxiety about the painful stimulus [6,35,38]. Providing information about medical procedures and their consequences reduces one’s anxiety about the procedures [2,7,10,17]. Finally, relaxation training is often used to help individuals cope with a particular painful stimulus and thus reduces relevant anxiety [ l-3,9,19,22,33.43]. Two studies have been conducted which specifically examine the role of irrelevant anxiety in pain perception [8,42]. Weisenberg et al. [42] found that subjects who were made to be anxious about an irrelevant learning task obtained the lowest galvanic skin rcsponse (GSR) in reaction to the presentation of shock. GSR in this case was used as an indirect measure of pain. There was also a trend, though non-significant, for subjects who were anxious about both relevant and irrelevant features of the pain situation to report the most pain with subjects who were anxious about the learning task reporting the least. Subjects who were anxious about the pain alone fell in between these conditions. The mean pain ratings were not reported for subjects in the low anxiety conditions. Thus, the degree of pain attenuation is not known. In fact, anxicty ratings were positively correlated with pain ratings. An alternative interpretation of the data could be that irrelevant anxiety increased pain perception over no anxiety, but not to the degree that relevant anxiety did. Cornwall and Donderi [8] examined a similar set of hypotheses. Young male subjects were exposed to finger pressure pain following instructions warning them about the pain and the potential dangers of the pressure stimulator (relevant anxiety), instructions warning them of a stressful interview that was to take place immediately following the pain induction (irrelevant anxiety), or simple procedural instructions. These 3 conditions did not differ from each other in pain tolerance, threshold. or self-report measures of pain obtained after the pain induction. Pain ratings ob-
tamed during the pressure stimulation showed that both anxiety conditions reported more pain than the instructions alone condition. The two anxiety conditions also responded with higher heart rates and stress ratings during the pain trial than the instructions alone condition. Cornwall and Donderi argued that these data do not support Weisenberg’s hypothesis about relevant and irrclcvant anxiety, but rather are more consistent with a perceptual disruption theory [S]. This theory suggests that anxiety, whether relevant or irrelevant, ought to interfere with one’s ability to discriminate among diffcrent levels of pain intensity. In addition, reports of pain ought to increase since the painful stimulation is likely to be the most salient aspect of experience and be attended to more closely. Although it is true that the two anxiety conditions in this study reported more stress and pain (on 1 of 5 measures) than the no anxiety instructions condition. the lack of a specific manipulation check makes it impossible to make any definite conclusions about the appropriateness of either the attribution hypothesis or perceptual disruption hypothesis. The extent to which subjects were concerned about either the pain or the stressful interview is not known. It is quite plausible that, after being initially aroused about the prospect of undergoing a stressful interview, subjects in the irrelevant anxiety condition promptly forgot about the interview and shifted the focus of their attention and concern to the unavoidable painful stimulation. The present study was conducted to further examine the influence of anxiety on pain perception. Subjects were exposed to a cold pressor task in a 2 X 2 experimental design, with an added control condition. Either high or low levels of anxiety were induced through an instructional manipulation which either presented information about the cold pressor (relevant anxiety) or the potential of receiving an electric shock during the cold pressor trial (irrelevant anxiety). A fifth condition in which subjects did not receive any pretrial instructions was also included. Care was taken to ensure that high and equivalent levels of anxiety were produced in both the relevant and irrelevant conditions and to verify that subjects were differentially concerned about the two sources of anxiety. It was hypothesized that subjects who were highly anxious about an electric shock, something irrelevant to the pain stimulus. would report experiencing the least amount of pain. In contrast it was expected that subjects who were highly anxious about the cold pressor would report experiencing the most pain. Although differences between these conditions could potentially be explained by a number of different mechanisms. the primary purpose OC this study was to demonstrate that increased levels of anxiety are not necessarily associated with increased pain perception.
45
One hundred female subjects were recruited from undergraduate psychology classes. Subjects ranged in age from 18 to 41 years with a mean of 20.4 years. As a benefit of participation subjects were given credit to be used toward their course grades. Six subjects with medical problems which could have been complicated by placing a hand in cold water (e.g., high blood pressure, Reynaud’s disease) were excluded from participation. Apparatus
The cold pressor was used to induce pain. This apparatus consisted of a 2 gallon insulated pail that was filled with crushed ice and water. A wire mesh cylinder was used to hold the ice around the periphery of the pail and keep it from coming in contact with the subject’s hand. The water was continuously circulated by an aquarium pump. The temperature of the water was maintained at 0-1°C. This was monitored by a mercury thermometer. A Grass Model 7B polygraph was used to measure skin resistance. Ten millimeter Beckman Ag/AgCl electrodes were attached to the thenar and hypothenar eminences of the palm of the non-preferred hand. A Grass 7Pl DC amplifier, attached to a Grass Model 7DAC driver amplifier, supplied a constant current of 10 PA to obtain skin resistance levels. A Grason-Stadler model 700 shock generator was placed on a low table next to the subject. This black metal box was clearly labeled as a shock generator and had switches indicating the duration and intensity of shocks as well as a red label with a skull and cross bones printed next to a message which stated, “Caution: when switch is in this position shock output is potentially dangerous.” A large electrode with concentric metal rings was attached to a wire coming from the back of this box. Although the bulbs on the front panel of the shock generator were lighted when the power switch was on, the equipment was not functional and served only as prop for the experimental manipuiation. No electric shocks were delivered to any subject. Measures manipulation checks. The State-Trait
Anxiety Inventory (STAI, Trait scale) [34] was used to assess the subjects’ level of trait anxiety. This was used to explore any differential susceptibility to situationally induced anxiety. Previously, it was reported that those with high levels of trait anxiety tend to be more responsive to situationaIly induced anxiety [14], The anxiety scale of the Multiple Affect Adjective Check List (MAACL) 144) was used to assess state
anxiety. This instrument consisted of 21 items, 10 positive adjectives and 11 negative adjectives. Baseline skin resistance was taken during the 50 set immediately before the taped instructions containing the anxiety manipulation. Skin resistance levels were monitored again during the 60 set immediately following the taped instructions. For analysis the highest and lowest skin resistance levels during each of these periods were averaged to obtain single baseline and postinstruction skin resistance levels. The reciprocals of these scores was then multiplied by 1000 to yield skin conductance levels. Two additional rating scales served as manipulation checks. Subjects were asked to try to remember the time while their hand was in the water and answer the following questions: To what extent were you anxious about the cold pressor? To what extent were you anxious about the possibility of getting electric shock? These questions were rated on a scale from 1 to 100 with 1 meaning “not at all anxious” and 100 meaning “very anxious.” Pain measures. A rating scale similar to that used by Johnson [16] was used to assess pain. This scale ranged from 0 (not at all painful) to 100 (extremely painful). Subjects were asked to report a number from 0 to 100 which described their current level of pain. In addition, the adjective portion of the McGill Pain Questionnaire (MPQ) [29] was also used. This instrument consists of 20 categories of words used to describe pain. The subject was to select each adjective that accurately described her pain at its worst during the coid pressor trial. The total pain rating index score was used for analysis. Experimental conditions
The design of this study was a 2 (level of anxiety: high and low) X 2 (source of anxiety: cold pressor and expected shock) between-group factorial design with an added control condition. Subjects in the high anxiety about the cold pressor condition were told that “many people get very anxious, become frightened, and report experiencing a lot of stress when they are exposed to the cold water. This stressor produces a great deal of physiological arousal that includes dramatic increases in blood pressure, respiration, and heart rate. This is why we asked you about any medical conditions that you may have. While you may realize that these autonomic responses are the body’s natural way of Ietting you know that danger is present, please be assured that no permanent tissue damage will result.” Subjects in the high anxiety about the expected shock condition were told, “While your hand is in the water, there is a high probability that you will receive a quite painful electric shock through this electrode” (attached to the non-preferred forearm). The instructions to these subjects continued exactly as the instruc-
tions stated above with the word “shock” substituted for the words “cold water.” Subjects in the low anxiety conditions were told that the cold pressor though discomforting was often tolerated without significant distress and that the shock would be “nothing more than a tingle or tickle.” Subjects in the neutral condition did not receive any particular information and were asked simply to sit quietly. AH instructions were prerecorded on audiocassette tapes.
Subjects were tested individually in a single session. Following informed consent subjects were randomly assigned to 1 of the 5 conditions. Electrodes for measuring skin resistance were attached to the paIm of each subject’s non-preferred hand. Another electrode from the shock generator was aIso attached to each subject’s non-preferred forearm. The shock generator was in plain view for subjects in the two expected shock conditions, but was covered by a towel in the other conditions. No comments about the purpose of the electrode were made to subjects in the conditions where the instructions were focused on the cold pressor or to subjects in the control condition. Following the attachment of electrodes subjects were asked to sit and relax for 5 min. This time was used to calibrate the polygraph and to take the baseline readings of the physiological measure. Subjects then listened to an audiocassette with the manipulation instructions about the procedures they should follow. ImmediateIy after the taped instructions terminated. the physiological measure was taken for 60 sec. Subjects were then asked to complete the MAACL. They were exposed to the cold pressor for 2 min. During the cold pressor, subjects were asked to report their pain after 15 SK, 30 set, 60 set, 90 set, and 120 set from the beginning. Immediately after making the final pain report subjects were asked to remove their hand from the water. Subjects then completed the McGill Pain Questionnaire, the State-Trait Anxiety Inventory (Trait scale), and the manipulation checks.
Results Munipulation
check
Previous research had indicated that subjects with higher levels of trait anxiety were more susceptible to situational anxiety and may be more likely to report higher levels of pain than those with lower levels of trait anxiety. To check for a non-equivalent distribution of the level of trait anxiety across the 5 conditions an analysis of variance was conducted on the STAI-Trait scores. A significant main effect of condition was found, E’ (4, 94) = 3.19, P = 0.01. The means ranged from 35.5
to 41.8 with the subjects rccciving information about ;I potential shock reporting the lowest means ancl the subjects receiving information about the cold prossor having the highest means. The STAI-Trait scores were atso positively correlated with the MPQ scores (I’ = 0.33. P < 0.01) and the pain ratings (rx ranged from I). 13. P = 0.10. to 0.20, P = 0.02). Because there was an unequal distribution of trait anxiety across conditions and because trait anxiety was correlated with our primary dependent measures, it was decided that including trait anxiety as an independent variable in ail further analyses would help to increase the statistical precision of the experiment. That is, WC could rcducr: the error variance due to individual differcnccs. thcreb) increasing statistical power. and also examine the potential interactions between trait anxiety and our manipulations [26]. The independent variable wa.s created by dividing the subject sample in half at the median score (35) on the STAI-Trait. A 2 (level of anxiety) >c 2 (source of anxiety) j< 2 (Icvel of trait anxiety) analysis of covariance was conducted on the post-instruction skin conductance scores using the baseline skin conductance scores as the covariate. Equipment difficulties reduced the total number of subjects with complete data for this analysis to 6X. A significant main effect for level of anxiety was found, F (I, 59) L 4.87, P = 0.03. Subjects in the high anxiety conditions had higher adjusted mean skin conductance scores (M = 12.5) than subjects in low anxiety conditions (M = 10.2). The adjusted mean skin conductance level of the control group was X.X (N = 15). A significant interaction between source of’ anxiety and trait anxiety was also found, F t 1, 3) = 4.77. f’ -= 0.03. Because specific predictions were not made for intcractions of trait anxiety and source of anxiety, post hoc analyses for this and the following manipulation checks should be interpreted in light of the total number of possible comparisons among means. Using the Tukey correction, the critical value for F’, with alpha set at 0.05, is 6.99. (Please notc that although our interprctations have been adjusted to reflect this more stringent criterion for calling a difference significant the I’ values reported are not corrected.) Thus, for subjects who were given instructions about a potential shock. there wah a trend for subjects with high trait anxiety to have higher adjusted mean skin conductance \corcs (M 1 1.4) than subjects with Iow trait anxiety (M ==0.41, b ( I, 32) = 4.09, 1’ = 0.05. No difference was found bctween subjects with high trait anxiety (adjusted M = 10.3) and subjects with low trait anxiety (adjusted M = 12.4) when they received instructions focusing on the cold pressor, F (1. 30) = l.S.5, I’ = 0.22. No other cffects were significant. A 2 (level of anxiety) X 2 (source of anxiety) X 2 (level of trait anxiety) analysis of variance was conducted on the MAACL. anxiety scale scorcx. Kegard-
47
less of the level of anxiety, subjects who were given information about the possibility of receiving an electric shock reported higher scores on the MAACL (M = 12.2, SD. = 4.0) than those who were given information about the cold pressor task (M = 9.2, S.D. = 2.31, F (1, 73) = 5.48, P = 0.02. No other effects were significant. The mean of the control condition was 9.3, S.D. = 3.9. A 2 (level of anxiety) X 2 (source of anxiety) X 2 (level of trait anxiety) analysis of variance was conducted on each of the manipulation check ratings. The first question was about the degree to which subjects were anxious about the cold pressor. A significant interaction between level of anxiety and source of anxiety was found, F (1, 73) = 7.95, P = 0.01. Table I depicts the 2-way interaction between level of anxiety and source of anxiety. It can be seen that in the high anxiety conditions subjects given information about the cold pressor reported higher anxiety about the cold pressor than subjects given information about the shock, F (1, 37) = 6.84, P < 0.01. The two low anxiety conditions did not differ from each other in their reported levels of anxiety about the cold pressor, F (1, 38) = 1.60, P = 0.21. There was also a significant 3-way interaction among level of anxiety, source of anxiety, and trait anxiety, F (1, 73) = 9.33, P < 0.01. Post hoc analyses revealed that the 2-way interaction for low trait anxious subjects was significant, F (1, 35) = 17.38, P < 0.001, and was similar in pattern to the overall 2-way interaction. Subjects with high trait anxiety were equally anxious about the cold pressor regardless of the condition they were in, all Fs < 1.0. The second question asked subjects to rate the degree to which they were anxious about the possibility of getting an electrical shock. Significant main effects were found for level of anxiety, F (1, 73) = 8.46, P < 0.01, and source of anxiety, F (1, 73) = 9.72,P < 0.01. The level of anxiety by source of anxiety interaction
TABLE
1
MEAN RATINGS (and S.D.s) OF ANXIETY ABOUT THE COLD PRESSOR AND THE STOCK FOR EACH CONDITION Level of anxiety
P
Source of anxiety Cold
Shock
Exi;rent of anxiety abour the cold pressor Low N High N Control N
57.4 (30.0) 20 69.3 (23.5) 20 56.6 (20.0) IX
Extent of anxiety about clcctrical shock 19.2 (26.6) Low 20.9 (28.4) High 21.4 (29.3) Control
68.4 (24.2) 20 51.7 (26.5) 21
27.6 (22.Y) 54.9 (39.1)
0.21 0.01
0.29
0.01
was also significant, F (1, 73) = 5.10, P = 0.02. Table I also shows that subjects in the high anxiety conditions were much more anxious about receiving a shock when they were told about the possibility of shock than when they were told about the cold pressor, F (1,39) = 10.03, P < 0.01. Subjects in the two low anxiety conditions did not differ from each other in terms of how much they were anxious about the shock, F (1, 38) = 1.15, P = 0.29. There was also a significant 3-way interaction among level of anxiety, source of anxiety, and trait anxiety, F (1, 73) = 9.93, P < 0.01. Post hoc analyses to examine the 3-way interaction consisted of conducting a 2 (level of anxiety) X 2 (source of anxiety) analysis of variance at each level of trait anxiety. Among low trait anxious subjects, those who received information about the shock tended to report the most anxiety about the shock, F (1, 35) = 5.38, P = 0.02. Among subjects who reported high trait anxiety, the main effect for level of anxiety was significant, F (1, 38) = 11.02, P < 0.01. The effect for source of anxiety was not, F (1, 381= 4.29, P < 0.05. For high trait anxious subjects, the 2-way interaction was also significant, F (1, 38) = 16.25, P < 0.001, and mirrored the overall 2-way interaction except that the response to anxiety provoking information about the shock was exaggerated. In summary, these data demonstrate the efficacy of the manipulation. Subjects in the two high anxiety conditions were more anxious than subjects in the two low anxiety conditions as evidenced by increased skin conductance levels. Although the self-reports of anxiety on the MAACL did not differentiate between the high and low anxiety conditions, subjects in the two shock conditions reported higher levels of anxiety than subjects in the two cold pressor conditions. This finding is important for testing our primary hypothesis. Because we were predicting that anxiety about an irrelevant event would lead to a decrease in pain perception, it is good to know that subjects who were in the irrelevant condition were anxious. The ratings on the manipulation check questions indicated that all subjects were somewhat anxious about the cold pressor, but those who were supposed to be more anxious about the cold pressor reported being the most anxious. Subjects who were supposed to become anxious about the potential shock were anxious about the shock and subjects in the other conditions were relatively unconcerned about the possibility of getting shocked. Pain measures
Three subjects from 3 different conditions were not able to continue through the 2 min of exposure to the cold pressor. All of these subjects rated their pain level as being 100 before they removed their hands. It was assumed that the remaining unreported ratings would have been at the maximum level of pain and a score of 100 was assigned to each rating that was missed.
It will be recalled that we had hypothesized that subjects who were anxious about the cold pressor would report the most pain while, in comparison. subjects who were anxious about the shock would report significantly less pain. Therefore, we expected significant interactions between the level of anxiety and focus of anxiety and were especially interested in the direct comparison of the two high anxiety conditions. In addition, we were interested in testing whether high anxiety about an irrelevant event diminished pain in comparison to the no instruction control condition. The analyses reflect our interest in these planned comparisons. For the sake of brevity and clarity only those effects in the following analyses that achieved a significance level of 0.05 or less will be reported. A 2 (level of anxiety) x 2 (source of anxiety) x 2 (level of trait anxiety) x 5 (time of rating) mixed design analysis of variance was conducted on the pain ratings during the cold pressor trial. The time of rating was treated as a within-subjects variable. The mean pain ratings for each condition at each rating period are graphed in Fig. I. A significant main effect of time was found, F (4. 296) = 235.59, P < 0.001. The longer subjects left their hands in the water the higher their ratings of pain became. None of the other main effects were significant, all Fs < 1. A significant interaction
100
90
.5
I? z
60
Hi-Anx-Cold 50
E 2 E a
HcAnx-Shock 40
Low-An&old 30
TAB1.E MEAN
II PAIN
RATINGS
OF ANXIETY
AND
‘Vofc: numbs ratings
AND
in parentheses
averaged
over
MPQ
SOURCE
SCORES
FOR
EACII
i_I:VEI
OF ANXIETY are htandard
5 measurement
devtatlons.
periods.
MPQ
Rating
= pain
= McGill
Pain
Oue\twnnairc. ,\nwict>
Rating
LWd
Source
I .OH
Cold
53.2 (2
Shock
57.4 (20.8)
High
ix
MPO
1.O)
27.3
( I 1.7)
xl.8
(9.7)
20 ‘0
(‘old
5Y.l (17.2)
32.0 (IO.71
71I
Shock
40.0 (22.7)
22.5
‘I
Control
53.1 (19.0)
27.7 (1 1.3)
(9.7,
I ‘J
_
between level of anxiety and source of anxiety was found, F (1, 73) = 4.08, P = 0.04. Table 11 illustrates this interaction by reporting the overall means of the pain ratings collapsed across the 5 rating periods. Further analyses showed that subjects in the high anxietyabout-shock condition reported less pain than subjects in the high anxiety-about-cold condition, F (1, 37) = 4.09, P = 0.05. The two low anxiety conditions did not differ from each other, F < 1.0. Though the mean pain rating of the high anxiety-about-shock condition was lower than the control condition, the difference was not statistically significant, F < I .O. A 2 (level of anxiety) X 2 (source of anxiety) X 2 (level of trait anxiety) analysis of variance was conducted on the total score of the MPQ. The means and standard deviations for the MPQ scores are depicted in Table Il. A significant interaction between level of anxiety and source of anxiety was found, F (1, 74) = 5.09, P = 0.02. The results of further analyses showed a significant difference between the two high anxiety conditions, F (1, 39) = 8.74, P < 0.1. Subjects who were anxious about the cold pressor task reported more pain than those who were anxious about the potential shock. No significant difference was found between the low anxiety conditions, F < 1. An ANOVA was also conducted to test for a difference between the high anxiety-about-the shock condition and the control condition. Though the means were again in the predicted direction. the difference did not reach statistical significance, F (1, 38) = 2.43, P = 0.12.
Low-Anx-Shk
-
First
Second
Third
Rating Fig. I. Mean
pain ratings
Fourth
Control
Fifth
Period
for each condition
at each rating
period
Discussion Much of the literature on the relationship between anxiety and pain has indicated that these constructs arc positively correlated. There are, however, theoretical and empirical accounts which question the veridicality of a simple unidirectional positive association [5,21,25,
49
311. When examining the literature, it appears that evidence which supports a positive relationship between pain and anxiety has been obtained in contexts in which anxiety is relevant to the experienced pain. The present study was conducted to test a hypothesis forwarded by Weisenberg et al. [42]. This hypothesis could be stated in two forms. The strong form would predict that irrelevant anxiety should reduce pain. The weak form would simply predict that anxiety will not increase pain perception, even though, according to some theorists, it should. Prior to discussing the results, we should caution that the data derived from healthy young adults in the laboratory may not readily generalize to clinically induced pain or to other populations. The results of this study demonstrate that anxiety has a differential impact on pain perception depending on whether or not anxiety is relevant to the pain. It was found that subjects who either received anxiety provoking instructions about a potential electric shock (irrelevant anxiety) or about the cold pressor task (relevant anxiety) were equally anxious and significantly more anxious than subjects in the low anxiety conditions. In spite of high levels of anxiety, subjects in the irrelevant anxiety condition reported less pain when exposed to a cold pressor than those who received anxiety provoking instructions relevant to the cold pressor. The mean pain reports of subjects who were exposed to the irrelevant anxiety condition were lower than those of subjects in either of the low anxiety conditions and a no instruction control condition, though the differences did not attain statistical significance. Thus, we are unable to conclude that irrelevant anxiety reduces the experience of pain in comparison to low anxiety. The weak version of our hypothesis, however, was confirmed. We can say with certainty that increased anxiety is not necessarily associated with increased pain. Previous attempts to test this hypothesis may not have been as successful because of the difficulty in manipulating clearly different levels and sources of anxiety. Weisenberg et al. [42] used a learning task in which false negative feedback was presented to the subjects while they were exposed to a painful stimulus. The authors reported that this task may not have generated a level of anxiety equivalent to the fear of shock induction. Nor do we know if the feedback lights for the learning task sufficiently captured the attention of the subjects while they were experiencing the shock. Likewise, Cornwall and Donderi [S] did not utilize any measures to confirm whether or not subjects were at all concerned about the stressful interview that was to follow the pain tolerance trial. These studies point to some of the inherent challenges in investigating the relationship between anxiety and pain perception. Because the presentation of a painful stimulus may itself be anxiety provoking, it is
not enough to show that experimental conditions do not differ in levels of anxiety during the induction of pain. It is also necessary to demonstrate that subjects are aroused by different concerns and, if possible, to have some independent means for verifying that subjects in relevant and irrelevant conditions are differentially attending to the pain and the irrelevant task. We found that it was difficult to get subjects to maintain their focus of attention on the alternative stimulus when confronting a continuous pain like cold pressor pain. The cold pressor task was painful to the point where it was difficult for many of the subjects to think about anything other than the sensations in their hand. During the debriefing several subjects reported that they forgot all about the potential electric shock when they were confronted with the cold pressor task. Another test of this hypothesis may benefit from an anxiety induction which maintains the subject’s attention by making continued threats or anxiety inducing demands over a specified period of time and by using a pain stimulus that is intermittent, thereby allowing the subject to continue to focus attention on the irrelevant events. Further research should be directed toward understanding why irrelevant anxiety does not increase pain. Basing their ideas primarily on animal research, Belles and Fanselow [5] have proposed that fear and pain are associated with different and incompatible motivational systems. They postulate the presence of a physiological mechanism whereby fear inhibits pain. Although this model may very well apply to humans, we believe that either the form of anxiety which is represented in this study is a different construct from fear or otherwise there must be some cognitive mechanism which directs the influence of arousal. Otherwise, how else could we explain the differential pain reports of subjects who are equally aroused. This logic leads us to consider other possible cognitive interpretations. The subjects in the two high anxiety conditions were anxious. If skin conductance and the MAACL were to represent separate and additive components of anxiety, it might even be argued that subjects in the high anxiety about shock condition were more anxious than subjects in the high anxiety about the cold pressor condition. Yet, subjects in the shock condition reported significantly less pain than those in the cold pressor condition. The difference in pain reports between these conditions cannot be accounted for solely on the basis of their relative levels of anxiety. Therefore we must wonder about the potential mechanisms responsible for changing the perception of pain in the presence of anxiety. There may be several plausible explanations for these findings. Telling subjects that they would receive a shock to the arm may have served to distract them from the painful sensations in the hand [27]. To the
extent that subjects are aroused by a distracter they may be motivated to attend more closely to the distracter and stimuli related to it (i.e., though the difference was not statistically significant subjects who were highly anxious about the shock reported lower mean levels of pain than subjects who were less anxious about the shock). It could be argued that although subjects who were told about a potential shock were aroused, the arousal was not a functional part of the schema used to process stimulation from the cold prcsser. The painful stimuli would have been processed by a more objective schema and was therefore less distressing [21]. Finally, the data presented in this study could be explained by an attributional model [31]. If anxiety is attributed to the pain experience. then it may exacerbate this experience. When subjects attribute their arousal to a stimulus that is not related to the source of pain they will be less concerned about the pain and be more tolerant of it. These potential explanations arc not necessarily incompatible with each other. All of them are plausible and are ultimately testable. In summary we would like to return to the primary question posed by this investigation, “Can anxiety help us tolerate pain?” Based on the present data we are unable to conclude that increasing anxiety about an unrelated event was helpful. Subjects who were anxious about receiving an electric shock were not differcnt from less anxious subjects in their pain reports. From an applied perspective, if a patient were not anxious we should leave well enough alone. We certainly would not advocate making individuals anxious about irrelevant events in order to reduce their expericncc of pain. However, if WC were to ask. “Does anxiety necessarily increase pain?,” our answer. contrary to what many previous authors have stated, is definitely no. If an individual wcrc anxious about an upcoming surgery or about a medically unexplainable pain in the back, rather than reducing anxiety, an alternative approach to treatment might be to help that individual redirect the focus of his or her concerns or by making alternative attributions for the arousal. Regardless of any applied or causal speculations we might make, these data demonstrate that the relationship between anxiety and pain perception is not a unidirectional one.
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Pain, 46 (1991) 43-51 0 1991 Elsevier Science Publishers ADONIS 030439599100152C
B.V. 0304-3959/91/$03.50
PAIN 01801
Can anxiety help us tolerate pain? Mustafa Al Absi and Paul D. Rokke Dept. of Psychology, North Dakota (Received
10 August
1990, revision
State lJniLwsi@, Fargo, ND 58105 (U.S.A.)
received
21 November
1990, accepted
7 December
1990)
It was hypothesized that anxiety which is relevant to the source of pain exacerbates pain, whereas Summary anxiety which is irrelevant to the source of pain reduces the experience of pain. Female subjects were given either high or low anxiety provoking information about a cold pressor task (relevant anxiety) or high or low anxiety provoking information about a potential shock (irrelevant anxiety). Subjects were then exposed to the cold pressor. The results demonstrated that subjects who were highly anxious about the cold pressor reported experiencing the most pain. Subjects who were highly anxious about the shock reported the least pain and reported significantly less pain than subjects who were highly anxious about the cold pressor. These findings clearly demonstrate that the relationship between anxiety and pain is not always positive or unidirectional. Key words: Anxiety; Pain
Introduction
The relationship between anxiety and pain perception has been the topic of several scholarly works [7,15,21,36,40,41]. Although the overwhelming majority of studies have demonstrated that increased levels of anxiety are associated with increased pain reports [11,20,28,30,32,36,37], the nature of the relationship between these constructs is not without controversy. Both theoretical and empirical accounts have suggested that anxiety and pain are not necessarily positively associated. From a theoretical perspective it has been proposed that autonomic arousal in the presence of threat (the flight or fight response) will take precedence over pain and may inhibit the experience of pain [5,39]. When applied to humans, the processing of information relevant to the painful situation may also play an important role. Malow [231 conducted a signal detection analysis of pain perception in the presence of an anxiety inducing manipulation. He found that anxiety reduced the subject’s ability to discriminate among different stimu-
Correspondence to: Paul D. Rokke, Ph.D., Department of Psychology, North Dakota State University, Fargo, ND 58105-5075, U.S.A. Tel.: (701) 237-8626.
lus intensities. Subjects who evidenced both verbal and physiological signs of anxiety also showed a decrease in their bias to report the perceived sensations as painful. A different aspect of cognition was studied by Nisbett and Schacter [31]. In this study, subjects in a low state of fear who attributed signs of arousal to a placebo reported electrical shock to be less painful and tolerated higher levels of shock than subjects who were unable to attribute their arousal to an external source. This difference was not found for subjects in a high fear condition in which expectations were manipulated by telling the subjects that the shock would be very painful. Consistent with Schacter’s attributional formulation of arousal states, Weisenberg et al. [421 proposed that anxiety which is relevant to the pain inducing stimulus will exacerbate pain whereas anxiety that is irrelevant to pain would reduce the experience of pain. In fact, it could be argued that much of the literature supporting a positive correlation between anxiety and pain has been studied in a context where anxiety is related to the painful stimulus. For example, Dworkin and Chen 1131 found that when the tooth pulp of subjects was electrically stimulated in a setting that is typically associated with oral pain (the dentist’s office) they reported more pain than when the same measure was obtained in the laboratory. Likewise, Klepac et al. [19] found
J-l
that subjects who were dental phobic were no different than controls in their tolerance to arm shock, but reported tooth shock to be much more painful than non-phobic subjects. The typical study on the relationship between anxiety and pain has focused on differences in the pain reports of subjects who score either high or low on measures of trait anxiety [ 11,12.24,32.36]. In these latter cases, we could infer that individuals who are prone to experience anxiety in a wide variety of circumstances are also likely to experience more anxiety about pain in situations involving pain. The importance of anxiety that is relevant to the painful stimulus can also be inferred from several studies which show that pain tolerance can be increased by using some manipulation to reduce anxiety. In this case it can often be assumed that the anxiety which is reduced is that which is related to the source of pain. Perceived control over the painful stimulation may reduce pain because it reduces anxiety about the painful stimulus [6,35,38]. Providing information about medical procedures and their consequences reduces one’s anxiety about the procedures [2,7,10,17]. Finally, relaxation training is often used to help individuals cope with a particular painful stimulus and thus reduces relevant anxiety [ l-3,9,19,22,33.43]. Two studies have been conducted which specifically examine the role of irrelevant anxiety in pain perception [8,42]. Weisenberg et al. [42] found that subjects who were made to be anxious about an irrelevant learning task obtained the lowest galvanic skin rcsponse (GSR) in reaction to the presentation of shock. GSR in this case was used as an indirect measure of pain. There was also a trend, though non-significant, for subjects who were anxious about both relevant and irrelevant features of the pain situation to report the most pain with subjects who were anxious about the learning task reporting the least. Subjects who were anxious about the pain alone fell in between these conditions. The mean pain ratings were not reported for subjects in the low anxiety conditions. Thus, the degree of pain attenuation is not known. In fact, anxicty ratings were positively correlated with pain ratings. An alternative interpretation of the data could be that irrelevant anxiety increased pain perception over no anxiety, but not to the degree that relevant anxiety did. Cornwall and Donderi [8] examined a similar set of hypotheses. Young male subjects were exposed to finger pressure pain following instructions warning them about the pain and the potential dangers of the pressure stimulator (relevant anxiety), instructions warning them of a stressful interview that was to take place immediately following the pain induction (irrelevant anxiety), or simple procedural instructions. These 3 conditions did not differ from each other in pain tolerance, threshold. or self-report measures of pain obtained after the pain induction. Pain ratings ob-
tamed during the pressure stimulation showed that both anxiety conditions reported more pain than the instructions alone condition. The two anxiety conditions also responded with higher heart rates and stress ratings during the pain trial than the instructions alone condition. Cornwall and Donderi argued that these data do not support Weisenberg’s hypothesis about relevant and irrclcvant anxiety, but rather are more consistent with a perceptual disruption theory [S]. This theory suggests that anxiety, whether relevant or irrelevant, ought to interfere with one’s ability to discriminate among diffcrent levels of pain intensity. In addition, reports of pain ought to increase since the painful stimulation is likely to be the most salient aspect of experience and be attended to more closely. Although it is true that the two anxiety conditions in this study reported more stress and pain (on 1 of 5 measures) than the no anxiety instructions condition. the lack of a specific manipulation check makes it impossible to make any definite conclusions about the appropriateness of either the attribution hypothesis or perceptual disruption hypothesis. The extent to which subjects were concerned about either the pain or the stressful interview is not known. It is quite plausible that, after being initially aroused about the prospect of undergoing a stressful interview, subjects in the irrelevant anxiety condition promptly forgot about the interview and shifted the focus of their attention and concern to the unavoidable painful stimulation. The present study was conducted to further examine the influence of anxiety on pain perception. Subjects were exposed to a cold pressor task in a 2 X 2 experimental design, with an added control condition. Either high or low levels of anxiety were induced through an instructional manipulation which either presented information about the cold pressor (relevant anxiety) or the potential of receiving an electric shock during the cold pressor trial (irrelevant anxiety). A fifth condition in which subjects did not receive any pretrial instructions was also included. Care was taken to ensure that high and equivalent levels of anxiety were produced in both the relevant and irrelevant conditions and to verify that subjects were differentially concerned about the two sources of anxiety. It was hypothesized that subjects who were highly anxious about an electric shock, something irrelevant to the pain stimulus. would report experiencing the least amount of pain. In contrast it was expected that subjects who were highly anxious about the cold pressor would report experiencing the most pain. Although differences between these conditions could potentially be explained by a number of different mechanisms. the primary purpose OC this study was to demonstrate that increased levels of anxiety are not necessarily associated with increased pain perception.
45
One hundred female subjects were recruited from undergraduate psychology classes. Subjects ranged in age from 18 to 41 years with a mean of 20.4 years. As a benefit of participation subjects were given credit to be used toward their course grades. Six subjects with medical problems which could have been complicated by placing a hand in cold water (e.g., high blood pressure, Reynaud’s disease) were excluded from participation. Apparatus
The cold pressor was used to induce pain. This apparatus consisted of a 2 gallon insulated pail that was filled with crushed ice and water. A wire mesh cylinder was used to hold the ice around the periphery of the pail and keep it from coming in contact with the subject’s hand. The water was continuously circulated by an aquarium pump. The temperature of the water was maintained at 0-1°C. This was monitored by a mercury thermometer. A Grass Model 7B polygraph was used to measure skin resistance. Ten millimeter Beckman Ag/AgCl electrodes were attached to the thenar and hypothenar eminences of the palm of the non-preferred hand. A Grass 7Pl DC amplifier, attached to a Grass Model 7DAC driver amplifier, supplied a constant current of 10 PA to obtain skin resistance levels. A Grason-Stadler model 700 shock generator was placed on a low table next to the subject. This black metal box was clearly labeled as a shock generator and had switches indicating the duration and intensity of shocks as well as a red label with a skull and cross bones printed next to a message which stated, “Caution: when switch is in this position shock output is potentially dangerous.” A large electrode with concentric metal rings was attached to a wire coming from the back of this box. Although the bulbs on the front panel of the shock generator were lighted when the power switch was on, the equipment was not functional and served only as prop for the experimental manipuiation. No electric shocks were delivered to any subject. Measures manipulation checks. The State-Trait
Anxiety Inventory (STAI, Trait scale) [34] was used to assess the subjects’ level of trait anxiety. This was used to explore any differential susceptibility to situationally induced anxiety. Previously, it was reported that those with high levels of trait anxiety tend to be more responsive to situationaIly induced anxiety [14], The anxiety scale of the Multiple Affect Adjective Check List (MAACL) 144) was used to assess state
anxiety. This instrument consisted of 21 items, 10 positive adjectives and 11 negative adjectives. Baseline skin resistance was taken during the 50 set immediately before the taped instructions containing the anxiety manipulation. Skin resistance levels were monitored again during the 60 set immediately following the taped instructions. For analysis the highest and lowest skin resistance levels during each of these periods were averaged to obtain single baseline and postinstruction skin resistance levels. The reciprocals of these scores was then multiplied by 1000 to yield skin conductance levels. Two additional rating scales served as manipulation checks. Subjects were asked to try to remember the time while their hand was in the water and answer the following questions: To what extent were you anxious about the cold pressor? To what extent were you anxious about the possibility of getting electric shock? These questions were rated on a scale from 1 to 100 with 1 meaning “not at all anxious” and 100 meaning “very anxious.” Pain measures. A rating scale similar to that used by Johnson [16] was used to assess pain. This scale ranged from 0 (not at all painful) to 100 (extremely painful). Subjects were asked to report a number from 0 to 100 which described their current level of pain. In addition, the adjective portion of the McGill Pain Questionnaire (MPQ) [29] was also used. This instrument consists of 20 categories of words used to describe pain. The subject was to select each adjective that accurately described her pain at its worst during the coid pressor trial. The total pain rating index score was used for analysis. Experimental conditions
The design of this study was a 2 (level of anxiety: high and low) X 2 (source of anxiety: cold pressor and expected shock) between-group factorial design with an added control condition. Subjects in the high anxiety about the cold pressor condition were told that “many people get very anxious, become frightened, and report experiencing a lot of stress when they are exposed to the cold water. This stressor produces a great deal of physiological arousal that includes dramatic increases in blood pressure, respiration, and heart rate. This is why we asked you about any medical conditions that you may have. While you may realize that these autonomic responses are the body’s natural way of Ietting you know that danger is present, please be assured that no permanent tissue damage will result.” Subjects in the high anxiety about the expected shock condition were told, “While your hand is in the water, there is a high probability that you will receive a quite painful electric shock through this electrode” (attached to the non-preferred forearm). The instructions to these subjects continued exactly as the instruc-
tions stated above with the word “shock” substituted for the words “cold water.” Subjects in the low anxiety conditions were told that the cold pressor though discomforting was often tolerated without significant distress and that the shock would be “nothing more than a tingle or tickle.” Subjects in the neutral condition did not receive any particular information and were asked simply to sit quietly. AH instructions were prerecorded on audiocassette tapes.
Subjects were tested individually in a single session. Following informed consent subjects were randomly assigned to 1 of the 5 conditions. Electrodes for measuring skin resistance were attached to the paIm of each subject’s non-preferred hand. Another electrode from the shock generator was aIso attached to each subject’s non-preferred forearm. The shock generator was in plain view for subjects in the two expected shock conditions, but was covered by a towel in the other conditions. No comments about the purpose of the electrode were made to subjects in the conditions where the instructions were focused on the cold pressor or to subjects in the control condition. Following the attachment of electrodes subjects were asked to sit and relax for 5 min. This time was used to calibrate the polygraph and to take the baseline readings of the physiological measure. Subjects then listened to an audiocassette with the manipulation instructions about the procedures they should follow. ImmediateIy after the taped instructions terminated. the physiological measure was taken for 60 sec. Subjects were then asked to complete the MAACL. They were exposed to the cold pressor for 2 min. During the cold pressor, subjects were asked to report their pain after 15 SK, 30 set, 60 set, 90 set, and 120 set from the beginning. Immediately after making the final pain report subjects were asked to remove their hand from the water. Subjects then completed the McGill Pain Questionnaire, the State-Trait Anxiety Inventory (Trait scale), and the manipulation checks.
Results Munipulation
check
Previous research had indicated that subjects with higher levels of trait anxiety were more susceptible to situational anxiety and may be more likely to report higher levels of pain than those with lower levels of trait anxiety. To check for a non-equivalent distribution of the level of trait anxiety across the 5 conditions an analysis of variance was conducted on the STAI-Trait scores. A significant main effect of condition was found, E’ (4, 94) = 3.19, P = 0.01. The means ranged from 35.5
to 41.8 with the subjects rccciving information about ;I potential shock reporting the lowest means ancl the subjects receiving information about the cold prossor having the highest means. The STAI-Trait scores were atso positively correlated with the MPQ scores (I’ = 0.33. P < 0.01) and the pain ratings (rx ranged from I). 13. P = 0.10. to 0.20, P = 0.02). Because there was an unequal distribution of trait anxiety across conditions and because trait anxiety was correlated with our primary dependent measures, it was decided that including trait anxiety as an independent variable in ail further analyses would help to increase the statistical precision of the experiment. That is, WC could rcducr: the error variance due to individual differcnccs. thcreb) increasing statistical power. and also examine the potential interactions between trait anxiety and our manipulations [26]. The independent variable wa.s created by dividing the subject sample in half at the median score (35) on the STAI-Trait. A 2 (level of anxiety) >c 2 (source of anxiety) j< 2 (Icvel of trait anxiety) analysis of covariance was conducted on the post-instruction skin conductance scores using the baseline skin conductance scores as the covariate. Equipment difficulties reduced the total number of subjects with complete data for this analysis to 6X. A significant main effect for level of anxiety was found, F (I, 59) L 4.87, P = 0.03. Subjects in the high anxiety conditions had higher adjusted mean skin conductance scores (M = 12.5) than subjects in low anxiety conditions (M = 10.2). The adjusted mean skin conductance level of the control group was X.X (N = 15). A significant interaction between source of’ anxiety and trait anxiety was also found, F t 1, 3) = 4.77. f’ -= 0.03. Because specific predictions were not made for intcractions of trait anxiety and source of anxiety, post hoc analyses for this and the following manipulation checks should be interpreted in light of the total number of possible comparisons among means. Using the Tukey correction, the critical value for F’, with alpha set at 0.05, is 6.99. (Please notc that although our interprctations have been adjusted to reflect this more stringent criterion for calling a difference significant the I’ values reported are not corrected.) Thus, for subjects who were given instructions about a potential shock. there wah a trend for subjects with high trait anxiety to have higher adjusted mean skin conductance \corcs (M 1 1.4) than subjects with Iow trait anxiety (M ==0.41, b ( I, 32) = 4.09, 1’ = 0.05. No difference was found bctween subjects with high trait anxiety (adjusted M = 10.3) and subjects with low trait anxiety (adjusted M = 12.4) when they received instructions focusing on the cold pressor, F (1. 30) = l.S.5, I’ = 0.22. No other cffects were significant. A 2 (level of anxiety) X 2 (source of anxiety) X 2 (level of trait anxiety) analysis of variance was conducted on the MAACL. anxiety scale scorcx. Kegard-
47
less of the level of anxiety, subjects who were given information about the possibility of receiving an electric shock reported higher scores on the MAACL (M = 12.2, SD. = 4.0) than those who were given information about the cold pressor task (M = 9.2, S.D. = 2.31, F (1, 73) = 5.48, P = 0.02. No other effects were significant. The mean of the control condition was 9.3, S.D. = 3.9. A 2 (level of anxiety) X 2 (source of anxiety) X 2 (level of trait anxiety) analysis of variance was conducted on each of the manipulation check ratings. The first question was about the degree to which subjects were anxious about the cold pressor. A significant interaction between level of anxiety and source of anxiety was found, F (1, 73) = 7.95, P = 0.01. Table I depicts the 2-way interaction between level of anxiety and source of anxiety. It can be seen that in the high anxiety conditions subjects given information about the cold pressor reported higher anxiety about the cold pressor than subjects given information about the shock, F (1, 37) = 6.84, P < 0.01. The two low anxiety conditions did not differ from each other in their reported levels of anxiety about the cold pressor, F (1, 38) = 1.60, P = 0.21. There was also a significant 3-way interaction among level of anxiety, source of anxiety, and trait anxiety, F (1, 73) = 9.33, P < 0.01. Post hoc analyses revealed that the 2-way interaction for low trait anxious subjects was significant, F (1, 35) = 17.38, P < 0.001, and was similar in pattern to the overall 2-way interaction. Subjects with high trait anxiety were equally anxious about the cold pressor regardless of the condition they were in, all Fs < 1.0. The second question asked subjects to rate the degree to which they were anxious about the possibility of getting an electrical shock. Significant main effects were found for level of anxiety, F (1, 73) = 8.46, P < 0.01, and source of anxiety, F (1, 73) = 9.72,P < 0.01. The level of anxiety by source of anxiety interaction
TABLE
1
MEAN RATINGS (and S.D.s) OF ANXIETY ABOUT THE COLD PRESSOR AND THE STOCK FOR EACH CONDITION Level of anxiety
P
Source of anxiety Cold
Shock
Exi;rent of anxiety abour the cold pressor Low N High N Control N
57.4 (30.0) 20 69.3 (23.5) 20 56.6 (20.0) IX
Extent of anxiety about clcctrical shock 19.2 (26.6) Low 20.9 (28.4) High 21.4 (29.3) Control
68.4 (24.2) 20 51.7 (26.5) 21
27.6 (22.Y) 54.9 (39.1)
0.21 0.01
0.29
0.01
was also significant, F (1, 73) = 5.10, P = 0.02. Table I also shows that subjects in the high anxiety conditions were much more anxious about receiving a shock when they were told about the possibility of shock than when they were told about the cold pressor, F (1,39) = 10.03, P < 0.01. Subjects in the two low anxiety conditions did not differ from each other in terms of how much they were anxious about the shock, F (1, 38) = 1.15, P = 0.29. There was also a significant 3-way interaction among level of anxiety, source of anxiety, and trait anxiety, F (1, 73) = 9.93, P < 0.01. Post hoc analyses to examine the 3-way interaction consisted of conducting a 2 (level of anxiety) X 2 (source of anxiety) analysis of variance at each level of trait anxiety. Among low trait anxious subjects, those who received information about the shock tended to report the most anxiety about the shock, F (1, 35) = 5.38, P = 0.02. Among subjects who reported high trait anxiety, the main effect for level of anxiety was significant, F (1, 38) = 11.02, P < 0.01. The effect for source of anxiety was not, F (1, 381= 4.29, P < 0.05. For high trait anxious subjects, the 2-way interaction was also significant, F (1, 38) = 16.25, P < 0.001, and mirrored the overall 2-way interaction except that the response to anxiety provoking information about the shock was exaggerated. In summary, these data demonstrate the efficacy of the manipulation. Subjects in the two high anxiety conditions were more anxious than subjects in the two low anxiety conditions as evidenced by increased skin conductance levels. Although the self-reports of anxiety on the MAACL did not differentiate between the high and low anxiety conditions, subjects in the two shock conditions reported higher levels of anxiety than subjects in the two cold pressor conditions. This finding is important for testing our primary hypothesis. Because we were predicting that anxiety about an irrelevant event would lead to a decrease in pain perception, it is good to know that subjects who were in the irrelevant condition were anxious. The ratings on the manipulation check questions indicated that all subjects were somewhat anxious about the cold pressor, but those who were supposed to be more anxious about the cold pressor reported being the most anxious. Subjects who were supposed to become anxious about the potential shock were anxious about the shock and subjects in the other conditions were relatively unconcerned about the possibility of getting shocked. Pain measures
Three subjects from 3 different conditions were not able to continue through the 2 min of exposure to the cold pressor. All of these subjects rated their pain level as being 100 before they removed their hands. It was assumed that the remaining unreported ratings would have been at the maximum level of pain and a score of 100 was assigned to each rating that was missed.
It will be recalled that we had hypothesized that subjects who were anxious about the cold pressor would report the most pain while, in comparison. subjects who were anxious about the shock would report significantly less pain. Therefore, we expected significant interactions between the level of anxiety and focus of anxiety and were especially interested in the direct comparison of the two high anxiety conditions. In addition, we were interested in testing whether high anxiety about an irrelevant event diminished pain in comparison to the no instruction control condition. The analyses reflect our interest in these planned comparisons. For the sake of brevity and clarity only those effects in the following analyses that achieved a significance level of 0.05 or less will be reported. A 2 (level of anxiety) x 2 (source of anxiety) x 2 (level of trait anxiety) x 5 (time of rating) mixed design analysis of variance was conducted on the pain ratings during the cold pressor trial. The time of rating was treated as a within-subjects variable. The mean pain ratings for each condition at each rating period are graphed in Fig. I. A significant main effect of time was found, F (4. 296) = 235.59, P < 0.001. The longer subjects left their hands in the water the higher their ratings of pain became. None of the other main effects were significant, all Fs < 1. A significant interaction
100
90
.5
I? z
60
Hi-Anx-Cold 50
E 2 E a
HcAnx-Shock 40
Low-An&old 30
TAB1.E MEAN
II PAIN
RATINGS
OF ANXIETY
AND
‘Vofc: numbs ratings
AND
in parentheses
averaged
over
MPQ
SOURCE
SCORES
FOR
EACII
i_I:VEI
OF ANXIETY are htandard
5 measurement
devtatlons.
periods.
MPQ
Rating
= pain
= McGill
Pain
Oue\twnnairc. ,\nwict>
Rating
LWd
Source
I .OH
Cold
53.2 (2
Shock
57.4 (20.8)
High
ix
MPO
1.O)
27.3
( I 1.7)
xl.8
(9.7)
20 ‘0
(‘old
5Y.l (17.2)
32.0 (IO.71
71I
Shock
40.0 (22.7)
22.5
‘I
Control
53.1 (19.0)
27.7 (1 1.3)
(9.7,
I ‘J
_
between level of anxiety and source of anxiety was found, F (1, 73) = 4.08, P = 0.04. Table 11 illustrates this interaction by reporting the overall means of the pain ratings collapsed across the 5 rating periods. Further analyses showed that subjects in the high anxietyabout-shock condition reported less pain than subjects in the high anxiety-about-cold condition, F (1, 37) = 4.09, P = 0.05. The two low anxiety conditions did not differ from each other, F < 1.0. Though the mean pain rating of the high anxiety-about-shock condition was lower than the control condition, the difference was not statistically significant, F < I .O. A 2 (level of anxiety) X 2 (source of anxiety) X 2 (level of trait anxiety) analysis of variance was conducted on the total score of the MPQ. The means and standard deviations for the MPQ scores are depicted in Table Il. A significant interaction between level of anxiety and source of anxiety was found, F (1, 74) = 5.09, P = 0.02. The results of further analyses showed a significant difference between the two high anxiety conditions, F (1, 39) = 8.74, P < 0.1. Subjects who were anxious about the cold pressor task reported more pain than those who were anxious about the potential shock. No significant difference was found between the low anxiety conditions, F < 1. An ANOVA was also conducted to test for a difference between the high anxiety-about-the shock condition and the control condition. Though the means were again in the predicted direction. the difference did not reach statistical significance, F (1, 38) = 2.43, P = 0.12.
Low-Anx-Shk
-
First
Second
Third
Rating Fig. I. Mean
pain ratings
Fourth
Control
Fifth
Period
for each condition
at each rating
period
Discussion Much of the literature on the relationship between anxiety and pain has indicated that these constructs arc positively correlated. There are, however, theoretical and empirical accounts which question the veridicality of a simple unidirectional positive association [5,21,25,
49
311. When examining the literature, it appears that evidence which supports a positive relationship between pain and anxiety has been obtained in contexts in which anxiety is relevant to the experienced pain. The present study was conducted to test a hypothesis forwarded by Weisenberg et al. [42]. This hypothesis could be stated in two forms. The strong form would predict that irrelevant anxiety should reduce pain. The weak form would simply predict that anxiety will not increase pain perception, even though, according to some theorists, it should. Prior to discussing the results, we should caution that the data derived from healthy young adults in the laboratory may not readily generalize to clinically induced pain or to other populations. The results of this study demonstrate that anxiety has a differential impact on pain perception depending on whether or not anxiety is relevant to the pain. It was found that subjects who either received anxiety provoking instructions about a potential electric shock (irrelevant anxiety) or about the cold pressor task (relevant anxiety) were equally anxious and significantly more anxious than subjects in the low anxiety conditions. In spite of high levels of anxiety, subjects in the irrelevant anxiety condition reported less pain when exposed to a cold pressor than those who received anxiety provoking instructions relevant to the cold pressor. The mean pain reports of subjects who were exposed to the irrelevant anxiety condition were lower than those of subjects in either of the low anxiety conditions and a no instruction control condition, though the differences did not attain statistical significance. Thus, we are unable to conclude that irrelevant anxiety reduces the experience of pain in comparison to low anxiety. The weak version of our hypothesis, however, was confirmed. We can say with certainty that increased anxiety is not necessarily associated with increased pain. Previous attempts to test this hypothesis may not have been as successful because of the difficulty in manipulating clearly different levels and sources of anxiety. Weisenberg et al. [42] used a learning task in which false negative feedback was presented to the subjects while they were exposed to a painful stimulus. The authors reported that this task may not have generated a level of anxiety equivalent to the fear of shock induction. Nor do we know if the feedback lights for the learning task sufficiently captured the attention of the subjects while they were experiencing the shock. Likewise, Cornwall and Donderi [S] did not utilize any measures to confirm whether or not subjects were at all concerned about the stressful interview that was to follow the pain tolerance trial. These studies point to some of the inherent challenges in investigating the relationship between anxiety and pain perception. Because the presentation of a painful stimulus may itself be anxiety provoking, it is
not enough to show that experimental conditions do not differ in levels of anxiety during the induction of pain. It is also necessary to demonstrate that subjects are aroused by different concerns and, if possible, to have some independent means for verifying that subjects in relevant and irrelevant conditions are differentially attending to the pain and the irrelevant task. We found that it was difficult to get subjects to maintain their focus of attention on the alternative stimulus when confronting a continuous pain like cold pressor pain. The cold pressor task was painful to the point where it was difficult for many of the subjects to think about anything other than the sensations in their hand. During the debriefing several subjects reported that they forgot all about the potential electric shock when they were confronted with the cold pressor task. Another test of this hypothesis may benefit from an anxiety induction which maintains the subject’s attention by making continued threats or anxiety inducing demands over a specified period of time and by using a pain stimulus that is intermittent, thereby allowing the subject to continue to focus attention on the irrelevant events. Further research should be directed toward understanding why irrelevant anxiety does not increase pain. Basing their ideas primarily on animal research, Belles and Fanselow [5] have proposed that fear and pain are associated with different and incompatible motivational systems. They postulate the presence of a physiological mechanism whereby fear inhibits pain. Although this model may very well apply to humans, we believe that either the form of anxiety which is represented in this study is a different construct from fear or otherwise there must be some cognitive mechanism which directs the influence of arousal. Otherwise, how else could we explain the differential pain reports of subjects who are equally aroused. This logic leads us to consider other possible cognitive interpretations. The subjects in the two high anxiety conditions were anxious. If skin conductance and the MAACL were to represent separate and additive components of anxiety, it might even be argued that subjects in the high anxiety about shock condition were more anxious than subjects in the high anxiety about the cold pressor condition. Yet, subjects in the shock condition reported significantly less pain than those in the cold pressor condition. The difference in pain reports between these conditions cannot be accounted for solely on the basis of their relative levels of anxiety. Therefore we must wonder about the potential mechanisms responsible for changing the perception of pain in the presence of anxiety. There may be several plausible explanations for these findings. Telling subjects that they would receive a shock to the arm may have served to distract them from the painful sensations in the hand [27]. To the
extent that subjects are aroused by a distracter they may be motivated to attend more closely to the distracter and stimuli related to it (i.e., though the difference was not statistically significant subjects who were highly anxious about the shock reported lower mean levels of pain than subjects who were less anxious about the shock). It could be argued that although subjects who were told about a potential shock were aroused, the arousal was not a functional part of the schema used to process stimulation from the cold prcsser. The painful stimuli would have been processed by a more objective schema and was therefore less distressing [21]. Finally, the data presented in this study could be explained by an attributional model [31]. If anxiety is attributed to the pain experience. then it may exacerbate this experience. When subjects attribute their arousal to a stimulus that is not related to the source of pain they will be less concerned about the pain and be more tolerant of it. These potential explanations arc not necessarily incompatible with each other. All of them are plausible and are ultimately testable. In summary we would like to return to the primary question posed by this investigation, “Can anxiety help us tolerate pain?” Based on the present data we are unable to conclude that increasing anxiety about an unrelated event was helpful. Subjects who were anxious about receiving an electric shock were not differcnt from less anxious subjects in their pain reports. From an applied perspective, if a patient were not anxious we should leave well enough alone. We certainly would not advocate making individuals anxious about irrelevant events in order to reduce their expericncc of pain. However, if WC were to ask. “Does anxiety necessarily increase pain?,” our answer. contrary to what many previous authors have stated, is definitely no. If an individual wcrc anxious about an upcoming surgery or about a medically unexplainable pain in the back, rather than reducing anxiety, an alternative approach to treatment might be to help that individual redirect the focus of his or her concerns or by making alternative attributions for the arousal. Regardless of any applied or causal speculations we might make, these data demonstrate that the relationship between anxiety and pain perception is not a unidirectional one.
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