Mixing Memory

Apparently so. Recent research has shown that pleasant smells can increase pain tolerance, and a recent paper by Prescott and Wilkie(1) suggests that it is specifically sweet smells that do so. I’ll just skip to the experiment, and spare you the background, because the experiment contains all you need to know.

They started with three types of smells: sweet and pleasant (caramel), unsweet and pleasant (after shave), and unpleasant (civet musk, which I hear smells awful). The inclusion of both sweet and unsweet pleasant smells allowed Prescott and Wilkie to distinguish between the analgesic effects of the pleasantness of the smell from that of the sweetness (apparently these two properties had been confounded in previous studies). The smells were placed on the inside of a mask, which participants were told was used to measure their breathing rate so that they would have no knowledge of the experiment’s purpose. Participants put on the mask, and then placed their hands into a vat of water at 5° Celsius (41° Fahrenheit) for up to 4 minutes. They did this in two trials, separated by 15 minutes, one trial with the smell and one without (they wore the mask for both). The key measure was how long participants kept their hand in the cold water. Prescott and Wilkie also asked them to rate how intense the pain was right after putting their hands in, thirty seconds later, and when they pulled their hands out of the water.

Here are the main results:

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The results are pretty impressive. Participants in the sweet and pleasant smell condition were able to keep their hands in the water more than twice as long, on average, than the participants in the other two conditions, when the smell was present. Participants in the unsweet and pleasant and unpleasant conditions held their hands in the water about equally long, on average, when the odor was present. When the odor wasn’t present, participants in all three conditions held their hands in the water about equally long, with no difference between the three no-odor conditions and the unsweet and pleasant and unpleasant conditions with the odor. Interestingly, the pain ratings did not differ across all three conditions. Thus, while participants in the sweet and pleasant condition were able to tolerate the pain longer, they did not experience subjectively less pain.

How do sweet smells increase pain tolerance? This experiment obviously doesn’t test any causal hypotheses, but Prescott and Wilkie suggest that because the sweetness of smells is largely learned, through associations with sweet tastes, the analgesic effects of sweet smells are likely the result of learned associations as well. Sweet tastes have been show to increase pain tolerance, and by association, sweet smells come to do so as well.


Prescott, J., & Wilkie, J. (2007). Pain tolerance selectively increased by sweet-smelling odor. Psychological Science, 18(4), 308-311.

Comments

  1. #1 Torbjörn Larsson, OM
    July 2, 2007

    Prescott and Wilkie suggest that because the sweetness of smells is largely learned, through associations with sweet tastes, the analgesic effects of sweet smells are likely the result of learned associations as well.

    I’m not sure this has anything to do with general pain response, but I am reminded of how some TRP pain receptors are also sensitive to other environmental stimuli like heat, cold and a variety of chemical compounds.

    “TRPA1 is capable of translating diverse signals of hostility into a singular sensation, i.e. pain. […] engineering TRPA1 could help to counter the effects of noxious compounds in airways” ( http://www.expasy.org/spotlight/back_issues/sptlt082.shtml )

    It seems for example TRPA1 is ubiquitous in sensory nerve endings ( http://lib.bioinfo.pl/auth:Julius,D ). So I guess at least local pain response (nose or mouth) indeed can be modified by smell or taste. And perhaps it could affect the general response.

    OT, it seems btw as TRP receptors “could prove to be an exceptional therapeutic target and may help to find ways of relieving those that suffer from chronic pain, or the secondary effects of chemotherapy and medication” (Ibid.) In any case, an interesting group (~ 30 variants) of receptors, most of which seems to have unknown function.

  2. #2 Jasper
    July 4, 2007

    How are subjects in one of three smell conditions with no odor present? I could just look it up, but I’m lazy, and you probably know the answer.

  3. #3 Chris
    July 6, 2007

    Jasper, I’m not sure what you’re asking. All subjects completed the task one time without any odor.

  4. #4 Matt
    July 8, 2007

    It would be interesting to see if sweet taste and sweet smell have additive or synergistic analgesic effects. If the authors are correct and it is a learned association, then the sweet taste effect would like over-ride the sweet smell effect.

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