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ORIGINAL ARTICLES |
From the Psychology Academic Group, School of Health and Social Sciences, Middlesex University, London, England.
Address correspondence and reprint requests to G. Neil Martin, MA, PhD, FRSA, Psychology Academic Group, School of Health and Social Sciences, Middlesex University, Queensway, Enfield, London EN3 4SA, England, UK. E-mail: n.martin{at}mdx.ac.uk
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ABSTRACT |
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Method: Sixty healthy men and women experienced experimentally induced pain (cold-pressor test) during exposure to an ambient pleasant odor (lemon), an ambient unpleasant odor (machine oil), or no odor. Participants reported the degree of pain they experienced at 5-minute intervals for 15 minutes.
Results: Individuals exposed to both odors reported significantly greater pain than did those in the control condition at 5 minutes. At 15 minutes, individuals exposed to the unpleasant odor experienced greater pain than did the control group.
Conclusion: The results suggest that exposure to odors judged to be pleasant and unpleasant may not lead to pain relief. Rather, their perception is associated with greater pain than is no odor. The implications for attentional theories of pain are discussed, as are avenues for future research.
Key Words: odor • pain • aromatherapy • smell • cold-pressor test • attention
Abbreviations: CAM = complementary or alternative medicine.
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INTRODUCTION |
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Aromatherapy is one of the complementary, or alternative, medicines (CAM). It refers to the administration of odor (usually an essential oil) to alleviate the symptoms of mental or physical ill health (3) and is one of the three most commonly recognized and used forms of CAM (4,5). Most aromatherapy as practiced, however, can also involve interventions such as massage and/or exposure to relaxing music (6). This introduces an obvious confound when attempting to disentangle the potentially beneficial effects of ambient odor and massage on well-being. One review (3) has also noted that, up to 1995, studies reporting aromatherapeutic effects recruited individual participants who were often given unvalidated measures by individuals not trained in psychology or medicine and who were exposed to extraneous variables as well as odor. Furthermore, when data were reported, control groups were rarely employed, studies were not single or double blind, no statistical analysis beyond the calculation of means was presented, alternative odors were not administered, the rationale for the odors chosen was unclear, and alternative explanations of the data were not broached. This context contrasts significantly with the large body of psychological evidence demonstrating the significant positive (and negative) effects of odor on mood and cognition (7–10).
More recently, some experimental studies have noted some reduction in self-reported pain during exposure to pleasant odor. However, they do not reach a consistent conclusion. For example, one study of the effects of lavender and rosemary inhalation on contact heat, pressure, and ischemic pain perception in 13 men and 13 women reported no direct analgesic effect of odor inhalation (11); an experiment with a repeated-measures design in which participants endured hot water during exposure to self-selected pleasant and unpleasant odors (and no odor) found an ameliorating effect of odor only in women (12). A study of anxiety in dental waiting rooms also found a significant reduction in self-reported anxiety during exposure to orange oil odor but only in women (13). Well-controlled studies of the effect of ambient odor on the behavior of ill patients continue to find no statistically significant effect of odor on the alleviation of ill-health symptoms, such as mood, physical symptoms, and quality-of-life measures in 42 to 46 hospice patients with cancer (14,15) and agitation in seven patients with dementia (16). In an paradigmatically different experiment using a highly stylized, counterbalanced, repeated-measures design where participants rated the intensity and unpleasantness of pain during exposure to pleasant (unspecified) or unpleasant (pyridine) odor delivered via a Y-shaped glass apparatus inserted into the nostrils, one study (17) found that the reported intensity of pain was unaffected by exposure to different odors but the reported unpleasantness of the pain was.
To examine whether odor can affect or modulate the experience of aversive experiences, therefore, the current study tested the strongest form of the aromatherapeutic hypothesis: the suggestion that exposure to a pleasant odor can alleviate pain. We required men and women to endure experimentally induced pain for a maximum of 15 minutes in the presence of either a pleasant or unpleasant odor or no odor. If pleasant odor is effective in alleviating pain, participants should report a lower degree of pain than those in the control and unpleasant conditions.
Additionally, the study allowed the examination of two current conceptions of the role of attention and distraction in the perception of pain. The first approach, the distraction hypothesis, argues that any perceived sensory, environmental stimulus is sufficient to reduce experienced pain because the stimulus is drawing attention away from the pain and the source of pain, thus reducing the cognitive resources available to focus on the pain (18,19). The second approach, the emotional distractor hypothesis, posits a more specific relationship and argues that in order for a stimulus to distract a person from his or her pain, it must first be perceived as pleasant. An unpleasant stimulus detected during the experience of pain will lead to an increase in the perception of pain because two negative emotional experiences are being paired and the pain is compounded (20,21).
We hypothesized that if the distraction hypothesis is correct, exposure to any odor will lead to a reduction in the perception of pain. If the emotional distractor hypothesis is correct, then exposure to the pleasant odor will lead to a reduction in perceived pain, whereas exposure to unpleasant odor will lead to an increase in perceived pain when compared with the other two conditions. To our knowledge, this is the first study to have tested these hypotheses in this explicit way in a context with a high degree of ecologic validity.
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METHODS |
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Procedure
Participants were recruited via posters displayed in a university psychology department, and the experiment took place over 3 weeks. On the day of the experiment, participants were informed that they would be exposed to a version of the cold-pressor test in order to determine the effect of environmental variables on pain perception. Although informed that the experiment was interested in the effect of environment on pain perception, participants’ attention was not explicitly drawn to the odor. Testing took place in a comfortable, minimally decorated room specifically built for olfactory research.
Participants were seated at a desk and were asked to place their nondominant hand and forearm in a bucket of uncirculated water and ice (approximately 3–4°C) for up to 15 minutes. A researcher was present in the adjacent room and could monitor participants at all times. Although exposure times in variants of the cold-pressor test vary in published literature, with some stipulating immersion for 2 to 3 minutes, another a total of 9 minutes (2), and still others an exposure period determined by the participant, the current design was intended to explore the effect of odor on sustained discomfort (although not unbearable discomfort; hence, the presence of the experimenter in the next room and the constant supervision of the participant). Because the water was uncirculated, participants reported feeling slight numbness but not overbearing pain. None reported unbearable pain.
Participants were informed that the researcher would ask them to rate the degree of pain they felt at various intervals. Measurements of pain were taken at 5-minute intervals: immediately after the arm had been placed in the water and then at 5-minute intervals, up to 15 minutes, when the experiment ended. At these intervals, participants placed a mark along an 11-cm line that corresponded to how they felt at that moment (1 = no pain at all; 11 = unbearable pain). All participants reported various degrees of pain during the experiment, all undertook the experiment for 15 minutes, and none expressed a wish to abandon the study (see Ethical Approval section below). Before and after the cold-pressor test, participants rated the room on various dimensions (relaxing, pleasant-smelling, warmth, comfort) on an 11-cm line scale.
Odors were supplied by Aroma Co. and were diffused using an AromaCube. In the pleasant condition, lemon odor was diffused 20 minutes before the experiment began. In the unpleasant odor condition, the same procedure was adopted with machine oil. In the no-odor condition, the room was not scented. The odors were chosen based on a pilot study in which 10 participants rated a range of odors using 11-cm line scales (as above). Those odors, rated the most and the least pleasant, respectively, were selected for the experiment.
All participants gave informed consent, were informed that they could withdraw from the experiment at any point, for whatever reason, and were told to abandon the experiment if at any point they were unable to continue. Ethical approval was obtained from the departmental ethics committee.
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RESULTS |
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Table 1 provides participants’ mean pain ratings by condition and time; Figure 1 illustrates changes in pain perception over time.
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The time x odor interaction was attributable to differences between the control and the lemon and machine oil conditions at 5 minutes and no odor and machine oil at 15 minutes. Participants exposed to lemon and machine oil odors experienced significantly more pain than did the control group at 5 minutes. At 15 minutes, exposure to machine oil was associated with greater pain than was exposure to no odor.
Participants rated the room along various psychometric measures before and after the experiment. Participants rated the room as significantly less relaxing in the machine oil condition (F (2,57) = 32.47, p < .05) and as most pleasant (F (2,57) = 41.12, p < .05), warm (F (2,57) = 5.66, p < .05), and comfortable (F (2,57) = 10.79, p < .05) in the lemon condition before the beginning of the experiment.
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DISCUSSION |
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In terms of current theories of pain and attention, the result is intriguing because all distractors, pleasant and unpleasant, were associated with increases in self-reported pain in the early stages of pain perception. One explanation for this finding might be that, although the pleasant odor was regarded positively, it may have been overly alerting. Lemon scent is pleasant but refreshing, as other studies have reported, and exposure to the odor has been associated with increased vigilance during a basic driving exercise (22). It is possible, therefore, that exposure to the odor of lemon heightened participants’ vigilance and this increased sensory awareness made people more alert to their pain.
Interestingly, the participants’ responses differed even at 0 minutes (the first point at which pain was experienced and recorded), with participants in the machine oil test reporting more pain. Together with the increased pain experienced during exposure to machine oil at 5 and 15 minutes, the data suggest partial support for the emotional distractor hypothesis (which suggests that an aversive stimulus might increase or intensify, rather than reduce, the experience of pain) and is consistent with studies showing that aversive distractors are associated with increases in pain perception (20). It is also interesting to note that odor did not exert effects on pain consistently (i.e., there was no main effect of odor), although this may be attributable to the length of the experiment and the phenomenon of olfactory habituation.
The current study extends previous findings by showing that the number of aversive distractors that can enhance pain perception can include odor. Importantly, however, it has demonstrated that exposure to a pleasant and an unpleasant ambient odor can increase pain perception, possibly by drawing attention to the experience of pain in its early stages. However, the current study employed one odor described as pleasant and another described as unpleasant (these were the only two psychometric properties examined and for which hypotheses were constructed). It is possible that the pleasant odor used here, lemon, is ineffective in reducing pain for various sensory or psychological reasons (although the hypotheses tested were not qualified by specifying a type of pleasant—or unpleasant—emotional stimulus). Future research might usefully compare whether a pleasant-relaxing and a pleasant-alerting odor, for example, have differential effects on pain perception; this would help determine whether a) any pleasant scent can increase pain perception or whether the odor must be characterized by some property that enhances vigilance; and b) whether an odor judged to be "relaxing" does alleviate the experience of pain, possibly by reducing environmental vigilance.
It is also worth noting that the current study used the cold-pressor test as the pain-induction measure; this leaves open the possibility that other types of pain generated by different means might be affected differently by odor. In addition, although the group tested, in the context of the current hypothesis-testing strategy, was an effective opportunity sample, the fact that the study recruited young, healthy undergraduate adults may limit interpretation of the findings to this group.
Finally, it may be useful in future to record pain intensity ratings (as well as degrees of pain) under each odor condition, as reported elsewhere (17). Although the present study was concerned only with measuring the level of experienced pain, rather than qualities of pain, further fractionation of the pain experience and the factors that affect it might be informative.
The authors thank Vicky Chatzidimitraki, Ghenti Etan, Michelle Joseph, Walla Keshk, and Konstantinos Konstantinidis for assistance with data collection, Konstantina Bezioula for assistance with data analysis, and three anonymous reviewers for very helpful and constructive comments on the original version of this paper.
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NOTES |
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Received for publication September 23, 2005; revision received April 12, 2006.
DOI:10.1097/01.psy.0000227753.35200.3e
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