Response to pain and stress: A multivariate analysis

Journal of Psychosomatic Research, Vol. 18, pp. 2S to 32. Pergamon Press, 1974. Printed in Great Britain

RESPONSE TO PAIN AND STRESS: ANALYSIS*

A MULTIVARIATE

PARK 0. DAVIDSON?and RICHARDW. J. NEUFELD~ (Received 6 August 1973)

problem of stress as a focal area of research appears to have commanded the attention of psychologists at an accelerated rate in recent years [l, 21. McGrath [3] in reviewing some of this research has noted that “ . . . our techniques for measuring stress and its effects are as yet crude, far less advanced, for example, than our ability to measureintelligence,or attitudes or perceptualskills.” Part of the problem, according to McGrath has been the failure to consider the “multidimensional” nature of stress. Interestingly, recent reviews of pain research [4, 51 have noted similar problems of measurement and Melzack and his associates [6-81 have increasingly stressed the need to consider pain as a multidimensional concept. Problems associated with measurement of multidimensional concepts have traditionally been attacked by mapping units of input (stimulus parameters) to units of effect (response parameters). Measurement studies dealing with psychological stress have primarily been concerned with mapping response parameters [9, lo]. The multivariate responses to stress can be allocated to three distinct levels. According to McGrath [3]:

THE

“One is the physiologicallevel, having to do with body functions and conditions. The second is the psychologicof level, having to do with cognitive, emotional and motivational functions and conditions. The third can be termed behavioral; it has to do with (overt) responses of the organism vis a vis environmental settings, including both interpersonal and task behaviors” (p. 65).

A very limited number of studies have attempted to measure the stimulus parameters of psychological stressors [I I]. Experimental studies of pain, on the other hand, have been almost exclusively concerned with measurement of stimulus parameters [12-141. These studies have typically examined a single stimulus dimension such as pain threshold although Wolff [ 151has recently reported a multidimensional study. The relatively fewer studies of response parameters of pain have usually only examined one aspect of the cognitive or “psychological” domain-that of verbal self-report. It is somewhat surprising (given that verbal self-report is the most frequently measured response to pain in both experimental and clinical studies) that no systematic scale of pain adjectives currently exists beyond the simple “mild-moderate-severe” form [16, 171. Melzack and Torgerson [8] have recently categorized a wide variety of pain adjectives and attempted to scale them on a common intensity dimension. Behavioral responses are seldom measured in experimental studies of pain. Zimbardo et al. [18] have used a simple learning task and Dinnerstein and Lowenthal[19] have demonstrated the disorganizing effect of pain on hand steadiness and reaction time. *This study was supported in part by NRC Grants APA 213 and A 8579. tuniversity of British Columbia. $Dept. of Psychology, University of Western Ontario, London 72, Canada.

26

PARK0. DAVIDSON and RICHARDW. J. NEUFELD

A number of studies have attempted to identify physiological response patterns to painful stimuli [14, 20-221. These experiments have examined autonomic responses such as skin resistance, blood pressure, heart rate, frontalis muscle potential, respiration rate, pulse volume etc. As Sternbach [4] notes in his review of these studies “a major difficulty with attempting to specify patterns of response that are unique to pain, is that very few studies exist in which pain stimuli were systematically compared with other activating stimuli” (p. 55). He could only find two such studies [23, 241 both of which used the “cold pressor” test and examined only physiological responses. The present study was designed to compare the effects of a pain stimulus and a psychological threat stimulus on a variety of measures chosen from across the three response domains-psychological, physiological and behavioral-in order to see whether response to pain can be differentiated from other aversive stimuli by any maximal combination of these response measures. METHOD Subjects The subjects (Ss) were 60 female undergraduate university students ranging in age between 18 and 21. All subjects were volunteers and were paid for their participation in the study. The Ss were randomly divided into three equal groups of 20 Ss each. The three groups were labelled; pain group, stress group and control group according to the stimulus used. Stimuli Pain group. The pain stimulus used was the pressure algometer [25]. It was chosen because previous research has shown it to be a safe and reliable pain stimulus [26, 271. Pain threshold and tolerance levels were established for each subject prior to the experiment using the procedure described by Davidson and McDougall (271. During the experimental session pressure was applied to the thumb on the distal phalanx and maintained within 0.2 kg of tolerance level. Stress group. The stressor stimuli consisted of a set of five slides of homicide victims, obtained from the police department of a large Canadian city. These stimuli were chosen on the basis of previous work by the present authors [l l] and others [28,29] establishing their highly stressful properties. The slides, projected on to a screen 7 ft in front of S, were unretouched, colored photographs of the victims at the scene of the crime or “on the slab” at the morgue. Blood and other evidence of trauma typified the scenes. Control group. The non-stressor stimuli used for this group consisted of a set of five slides of male undergraduates bare from the waist up, projected upside down. These stimuli are novel and unusual but show significant multivariate separation on response measures to stressful stimuli [30]. Measures and apparatus Behavioral. On the basis of previous research demonstrating that hand steadiness [31, 321 and digit span (33, 34) are affected by situational stress, measures of these two behavioral tasks were selected for the present study. Hand steadiness, which has also been shown to be affected by pain [19], was measured by a tracking task. The apparatus for the tracking task consisted of a large wooden box, approximately 2 x 2 x 4 ft, housing a black rotating drum with a white irregular line traced around it. The path was visible through a 3 x 6 in. plexi-glass-covered aperture on top of the box. A photoelectric cell in the stylus registered time-on-target through a relay connected to a Stoelting timer. The digit span measure was the digits-backwards subtest of the Wechsler Adult Intelligence Scale [351. Subjective. Since there is currently no subjective pain adjective scale, three adjective scales that have frequently been used in stress studies were selected. These were the Affect Adjective Check List (AACL) [36], the Anxiety Differential [37], and a resealed version [38] of the Subjective Stress Scale (SSS) [33,38]. PhysiologicaL Heart rate, skin conductance, respiration rate and frontalis muscle tension were measured using a Grass Model 7 polygraph. Two 7 Pl preamplifiers were used in recording skin conductance (SC) and respiration rate (RR). A 7 P3 preamplifier with an integrator circuit was used for muscle potential (MT) while a 7 P4 preamplifier and cardiotachometer monitored heart rate (HR). Skin conductance electrodes, covered with K-Y sterile lubricant, were placed on the palm and forearm

Response to pain and stress

27

of the left hand. Respiration was monitored with a Phipps-Byrd bellows and a Grass PT-5 pressure transducer. Frontalis muscle tension was recorded with surface electrodes placed midway between the hairline and each eyebrow. Heart rate was recorded using a photoelectric finger plethysmograph which was placed on the index finger and shielded with a dark cloth to prevent interference from ambient light. Procedure Each S, tested individually was seated in a comfortable chair with a small desk top. The nature of the experiment and details of the stimuli to be presented were discussed with the S and opportunity given for withdrawal from the study. All Ss remained to participate. The surface electrodes, respiration bellows and finger plethysmograph were then attached. The S was administered the Wechsler Adult Intelligence digits-backwards subtest and the tracking task was performed. The order of these 2 tasks was randomized independently for each S. A IO-set practice interval on the tracking task preceded a 20-see interval with proportion of time on target for the latter interval retained as the pre-test measure. Performance on the digits-backward subtest was also retained as the digit-span pre-test measure. Attention was then directed to a panel on the desk top in front of S which displayed the 7 adjectives of the SSS with a separate push-button adjacent to each adjective. S was requested to push the button adjacent to the adjective best describing how she felt after each stimulus presentation. The S then completed a booklet consisting of the other subjective measures-the Anxiety Differential and the Affect Adjective Check List (AACL)-with reference to her current state. The order of questionnaires in the booklet was independently randomized for each S as they were in the post-test booklet. The booklet was retained for calculating difference scores with the post-test questionnaires. A 5-min rest interval followed completion of the questionnaires permitting calibration of the polygraph and tabulation of the pre-test data. For the stress and control groups the stimulus presentation consisted of a set of 10 slides. first a set of 5 neutral “habituation” slides of household items (e.g. a matchbox, pen, cup) followed by a set of 5 “experimental” slides of either homicide victims or male undergraduates bare from the waist up and projected upside-down. Duration of each slide was 20 set with inter-slide intervals averaging 30 set and randomly varying between 27 and 33 sec. The order of slides within each subset of 5 was randomized for each S. For the pain group the stimulus consisted of placing the pressure algometer on the distal phalanx of alternate thumbs for 5 trials at threshold level (“habituation” trials) followed by 5 trials near tolerance level (“experimental” trials). Duration of each trial was 10 set with the same intertrial intervals used in the slide presentations to the other two groups. Physiological recording was continuous throughout the duration of stimulus presentation. Upon completion of stimuli presentation, S was again given the tracking task for 20 set and was also given the digitsbackwards test. The order of each of these was randomized independently for each S. The post-test completion of the questionnaires followed the behavioral tasks. The S was requested to answer with reference to her subjective experience during the experimental trial presentation. Scoring of measures Skin conductance. The maximum conductance (n7.J) reached during the duration of each of the 5 experimental trials was obtained. The average of these values was adjusted by subtracting the average maximum taken for the habituation trials.* The conductance was then scored as (@) x (0.10) to facilitate computations. Respirntion rate. The total number of complete cycles during each trial averaged over the experimental trials was adjusted by subtracting the average over the habituation trials.? Muscle tension. Muscle tension for each of the experimental trials was obtained by finding the area under the integrated record. The average score thus obtained was again adjusted for the average over the habituation trials. Heart rate. The relatively straightforward procedures for scoring other physiological responses did not extend to the cardiac response. At least two possible estimates of cardiac reaction were available as suggested by Graham and Clifton [39]-heart rate increase and heart rate decrease. Several researches [28, 40, 411 have reported an impressive decelerative component to the onset of aversive visual stimuli. Engel 1231and Shor [21] have reported heart rate increase to the presentation of painful *Responses to the first neutral slide and the iirst habituation pain trial, largely reflecting Ss orientation to the onset of stimuli, were not included in the scoring of physiological responses. tScores on respiration and muscle tension were corrected for time difference in presentation of the pain stimulus trials (score X 2).

28

PARK 0. DAVLDSON and RICHARDW. J. NEUFELD . stimuli. Therefore both acceleration and i!leceleration to the present stimuli were evaluated separately. The average minimum and maximum rates during each of the habituation trials were subtracted from the average of the minimum and maximum, respectively, reached during the experimental trials. Subjective stres scale. The scale values for the adjectives selected in response to the habituation trials were averaged and subtracted from the average scale value taken over the experimental trials. Anxiety differential, AACL and digit span The scores obtained on pre-test measures were again subtracted from the post-test measures. Digit span and tracking performance Post-test scores were subtracted from pre-test scores for both tasks. However, lack of reliability in the tracking task measure resulted in a decision to drop this variable from the analysis of results, Analytical methods The method of Multiple Discriminant Function Analysis [42] was employed to assess the number of configurations-the number of “discriminant dimensions”-among the selection of 9 measures separating the 3 groups of Ss. The discriminant functions provided weights for the respective measures such that between group variance on the discriminant scores (weighted sums) relative to within-group variance was maximized. Analogously, the sets of discriminant scores (one set per discriminant function), when translated into F ratios, produced maximum values given the obtained raw data. Results provided an estimate of the number of orthogonal modes of variation-orthogonal in the sense that the sets of discriminant scores are uncorrelated-among the groups on the selection of measures. Tests of significance of group separation on each discriminant function were available [43] with potentially two (the number of groups-l), one or zero significant functions for a 3-group analysis. Descriptions of the significant functions employed correlations between the original measures and the discriminant scores, as suggested by Veldman [43], and univariate analyses of the respective original variables. In the present study, if reactions to the stress, pain and control stimuli were all similar on the 9 measures, no significant functions would be obtained. If the stress and pain groups departed from the control group in the same manner, one function would be necessary to describe the separation. If the stress and pain groups differed from the control group, each in a distinct fashion, two significant dimensions would be required to describe variation among the groups. RESULTS The overall multivariate separation among the groups was highly significant (Wilk’s lambda = 0.282. F = 4.81. d.f. = 18.98. D <0%0001). indicating one or two sienificant discriminant functions. The f&t function, account&for 71.56 peicent of the total discrimi;ating variance of the battery of measures, was found to be highly significant (x” = 45.54, d.f. = 10, p
This study represents using multiple measures

a preliminary empirical demonstration that it is possible, and appropriate statistical analyses, to discriminate patterns

Response to pain and stress

29

x

-_d.‘l

Stress .

-03

-02

. Pain

-0.2

J-o.3 FIG. I.-Average

discriminant scores (centroids) in a two-dimensional

discriminant

space.

TABLE 1.-CORRELATIONS OF ORIGINAL MEASURES WITH DISCRIMINANT SCORES

Measure Digit span Anxiety diff. AACL sss HR (dec.) HR (inc.) SC RR EMG

11 0.465 0.481 0.017 0.579 0.237 -0.180 0.302 -0.536 -0.374

A, -0.269 0.200 -0225 -0.655 0.343 0.129 0.172 -0.310 -0.514

of response to pain from other types of stressors. These results are encouraging and somewhat surprising, in that theories of response to aversive stimuli such as Cannon’s [44] and Selye’s [45] predict essentially similar response patterns to any strong emotion, whether pain, fear, or threat. Further mitigating against the expectancy of finding differential response patterns for pain and other aversive stimuli is the principle of autonomic response stereotypy [46]. According to this principle one individual may respond maximally with GSR change, another with blood pressure changes, whether the stimulus is mental arithmetic or cold pressor. As Sternbach [4] notes, the distribution of response stereotypy among individuals greatly increases the variance of responses to a particular stimulus and hence increases the difficulty of detecting patterns of response specific to pain. The discriminant function, as used in this study, is particularly appropriate for overcoming this difficulty. The discriminant power of the SSS in identifying psychological stress confkms the results of an earlier study by the present authors [ll]. The significant increases in frontalis muscle tension and respiration rate for the pain group indicate that these

30

and RICHARD W.J. NEUFELD

PARK O.DAVID~~N

06

sss.

Digit

span .Anxtety

.

dlff

04

.

SC

-02

i

I

*Hi?

AACL IU

I

-10 -08 -06 -04 -02

(decl

/

I

/

,

02

34

25

03

,

IO 12

‘HR(I~c) --02

EMG

.

--0

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FIG. L-Correlations

of the original measures with discriminant

TABLE2.--GROUP MEANS ON THE F TESTS Measure

Digit span Anxiety diff. AACL sss HR (dec.) HR (inc.) SC Resp. R EMG

* p
Stress group O%O 16.65 3.80 2.40 1.09 -0.70 0.64 --iI. 0.04

Means Pain group -103 4.15 3.75 0.76 -1.43 -1.04 -3.77 0.66 0.08

ORIGINAL BETWEEN

(OFFE.RENCE)

MEASURES

scores.

AND

GROUPS

F ratio Control group -0.95 13.25 2.75 0.23 2.02 1.13 -0.12 -0.08 0.03

Overall 4.96* 4%’
Stress/ control 7.05*
Stress/ pain 7.s1* 9.ost
Pain/ control
t p
responses may be particularly useful for future studies of pain. On the other hand, more frequently used measures of skin conductance and heart rate appear, from the present study, to be less discriminative of pain. The failure to find a significant increase in heart rate for the pain group is at variance with Engel’s [23] results but consistent with Schachter’s [24]. It could be that other cardiac responses, particularly diastolic blood pressure, may be better discriminant responses for pain. The significant decrease in digit span performance following a pain experience (compared to no change following a psychological stressor) suggests the need to look at the effects of pain on other behavioral responses, This is a response domain that has been largely neglected in pain research. The significant differences between the pain group and the other two groups on the Anxiety Differential Test was an unexpected finding since the Anxiety Differential is thought to measure situational anxiety [37]. Both the control and stress groups

two the

Response to pain and stress

31

showed large increases in situational anxiety, whereas the pain group showed relatively little change. It was thought that this may be due to inflated pre-test scores in the pain group (in anticipation of the pain experience). Statistical evaluation of the pre-test scores for the three groups, however, showed no significant differences. This failure to show any increase in situational anxiety may be specific to the experimental pain situation in contrast to the typical experience of clinical pain. Clearly the results of the present study need to be qualified by future multivariate studies using different responses and different pain stimuli-both experimental and clinical. Before one can establish that there are discriminant multivariate responses unique to pain it is necessary to demonstrate that the pattern of responses is consistently similar across a variety of pain stimuli and consistently different from a variety of other aversive stimuli. Finally, it should be noted that the tentative findings from the present study may have implications for broadening our understanding of methods of treating pain. The increased muscle tension and respiration rate response for the pain group compared to the increased subjective stress response for the psychological stress group is consistent with the findings of Bobey and Davidson [47], that relaxation procedures are more effective than cognitive rehearal procedures in increasing pain tolerance while the opposite is the case for stress tolerance [lo]. REFERENCES 1. LAZARUSR. S. Psychological Stress and the Coping Process. McGraw-Hill, New York (1966). 2. APPLEY M. H. and TRUMBULLR. Psychological Stress. Appleton-Century-Crofts, New York (1967). 3. MCGRATHJ. E. Social and Psychological Factors in Stress. Holt, Rinehart & Winston, New York (1970). R. A. Pain: A PsychophysioIogical Analysis. Academic Press, New York (1968). 4. STERNBACH 5. MELZACK R. Pain. International Encyclopedia of the Social Sciences, Vol. II, pp. 357-363. Macmillan, New York (1968). (1961). 6. MELZACKR. The perception of pain. Scient. Am. 204,41-49 (1965). 7. MELZACKR. and WALL P. D. Pain mechanisms: A new theory. Science 150,971-979 8. MELZACKR. and TORGER~~NW. S. On the language of pain. Anesthes. 34,50-59 (1971). 9. MORDKOFFA. M. The relationship between psychological and physiological response to stress. Psychosom. Med. 26,135-150 (1964). 10. FOLKMS C. H., LAWSONK. D., OPTON E. M. JR. and LAZARUSR. S. Desensitization and the experimental reduction of threat. J. Abnorm. Psychol. 73, 100-113 (1968). 11. NEUFELDR. W. J. and DAMD~QNP. 0. Dimensions of sex differences in responding to stress. Paper presented at Canadian Psychol. Ass. Annual Meetitw (1973). 12. BE&&R H. K. The measurement of pain. Phurmucol. Revyd, 54-209 (1957). 13. CLARKW. C. and MEHL L. A sensory decision theory anaylsis of the effect of age and sex on d’, various response criteria, and 50% pain threshold. J. Abnorm. Psycho!. 78,202-212 (1971). 14. HARDYJ. D., WOLFFH. G. and G~~DELL H. Pain Sensations and Reactions. Williams & Wilkins, Baltimore (1952). 15. WOLFFB. B. Factor analysis of human pain responses: Pain endurance as a soecific =~ oain factor. ~~~---~J. Abnorm. Psychol. 78,292-298 (1971): _ K. D. The pain chart. Luncet 2,6 (1948). 16. KEELE 17. CLARKEP. R. F. and SPEARF. G. Reliability and sensitivity in the self-assessment of well being. Bull. Br. Psychof. Sot. 17, 55 (1964). M., DWORKINL. and FIRESTONEI. Control of pain 18. ZIMBARDOP. G., COHENA. R., WEI~ENBERG motivation by cognitive dissonance. Science 151,217-219 (1966). 19. DINNER~TEINA. J. and Lo ~ENTHAL M. Behavioral consequences of painful electric shock. J. appl. Physiof. 17, 333-337 (1962). 20. BARBERT. X. and HAHN K. W., JR. Physiological and subjective responses to pain-producing stimulation under hypnotically-suggested and waking-imagined “analgesia”. J. ubnorm. Sot. Psychol. 65, 411-418 (1962).

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