Citrus Study Documents and Perplexes
A new study from the Institute of Experimental Psychology at Heinrich Heine University in Düsseldorf, Germany, reports:
Aromatherapy claims that citrus essential oils exert mood lifting effects. Controlled studies, however, have yielded inconsistent results. Notably, studies so far did not control for odor pleasantness, although pleasantness is a critical determinant of emotional responses to odor.1
In a nutshell, this study will trigger a negative mood in study participants, so the researchers can discover whether citrus odor itself elevates mood or if it is the pleasantness of the citrus odor perceived by the participants. If you are thinking, “Huh?” you are not alone!
Before we explain how the study was set up, we must note that the German researchers quoted three studies that documented citrus essential oils alleviated stress and exerted anxiolytic (calming anxiety) effects in rodents. Remember this: the rodents did not have the capability of stating whether it was citrus odor or just the pleasantness of the scent that made them less stressed.
Ninety-seven volunteers joined this study. All had to be healthy and free of neurological or psychiatric conditions. A cover story had to be developed to set up what is called a “hopelessness induction.”
Inducing Hopelessness by Deception
All in the name of science, a situation is set up that cannot be solved and will certainly cause frustration and “hopelessness.” Only participants who work in the social domain (e.g., social worker) or studying a subject related to the social sciences (e.g., psychology, educational science) were recruited. Because of technical problems and disbelief in the “cover story,” 19 participants were excluded. The study explains the cover story:
Participants were asked to take part in a study investigating the effects of right brain hemisphere activation on odor habituation. They were informed that they would be working on a computer-based emotional intelligence test, which leads to activation of the right brain hemisphere, while inhaling an odor. It was stated that the emotional intelligence test would usually be applied to test professional aptitude in the social domain (e.g., physiotherapy, social work, or psychotherapy). Participants were told that it was crucial to do their best at the task in order to determine whether they possessed a skill that is important for their profession.2
I am sure that the participants who did not figure out that “something was rotten in Denmark” with the “cover story” were not terribly upset at the conclusion of the testing. The study tells us that at “the end of the experiment, participants were debriefed and informed of the true nature of the study. Happily, those who had to endure the frustration and stress of the computer-based emotional intelligence test were nicely compensated by either course credit or 15 euros.
After the questioning, 19 departed and the 78 remaining participants were divided into three groups.
These participant groups tested three different “odors.” The first tested the citrus essential oil constituent D-(+)-limonene (hereafter limonene) diluted 1:1 in diethyl phthalate. The study participants in this group numbered 27 with 23 of them female.
Limonene: By Ben Mills and Jynto – Derivative of File:(R)-(−)-carvone-from-xtal-3D-balls-A.png and File:Diketene-from-xtal-3D-balls.png., Public Domain, https://commons.wikimedia.org/w/index.php?curid=8803903
The second group tested the odor of the fragrance chemical vanillin. This chemical has such an intense vanilla odor that it can be added to lavender or peppermint essential oil in the tiny amount of 0.01 percent to give a candy-like aroma. (This chemical adulterates without being detected unless a 60-meter GC column is used to test for adulteration.) In the study this chemical was also diluted 1:10 in diethyl phthalate. The vanillin group consisted of 26 participants, 22 of whom were female.
Vanillin: By Sbrools – CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2067518.
The third “odor” is diethyl phthalate, which an agency of the CDC explains, “Diethyl phthalate is a colorless liquid that has a bitter, disagreeable taste. This synthetic substance is commonly used to make plastics more flexible.”3 This last group of 25 had 21 females. (The gender of the participants is discussed later.)
The Helplessness Induction Procedure
At last we get to “the cover story.” Somewhere in a tiny retirement home for former lab rats, the little rodents are cheering that HUMANS get the short end of the stick at last! The poor study participants are about to get slammed with a not very fun attempt to “Name that face!”
Using the Karolinska Directed Emotional Faces System, a series of faces with different emotions on them will be shown, and a task developed. In this case, 175 faces were presented in a facial expression assessment task. Below are examples of these faces:
Of the faces presented to the study participants, 92 percent were neutral: 45.8 percent showed surprise, and 8 percent were negative (fear: 5.7 percent, sadness: 0.4 percent, anger: 1.5 percent, disgust: 0.8 percent).4
Now the testing. Each participant was tested before the “cover story” to show their ability to discriminate between the three odors using emotion ratings. The odors of each group were constantly being presented through an oxygen mask. Then they did the faces test and were tested again. Odor quality was rated regarding intensity, pleasantness, unpleasantness, and familiarity. The odors were presented continuously during the actual experiment.
Now the participants are tricked into the emotion of “helplessness.” While faces are being flashed on a screen at 3 second intervals, each participant had to classify the expressions as positive or negative. Remember, the deck is stacked against the participants: 92 percent of the faces are neutral! Decisions had to be made by mouse click, and no photos could be skipped. The study notes, “This was an unsolvable task due to the mostly neutral facial expressions of the stimuli presented.”5 Imagine having to choose between negative or positive emotional faces when over 90 percent were neutral!
Even worse, the participants “received false feedback regarding their performance over time after every 6th decision.”6 Feedback would then list “below average,” progressively worsening until the score indicated a “quite poor performance.” Helplessness? How about anger and fury developing?
The helplessness induction was successful! No matter the odor, the participants indicated they were in a more negative mood following this test. They were more submissive, angrier, and less happy.
The study determined. “Limonene was perceived as more intense than diethyl phthalate and vanillin. . . . Odors did not differ regarding pleasantness. All odors were rated as more unpleasant after compared to before the helplessness induction.”7
Regarding the over-representation of females in the study, the researchers claim, “gender does not modulate the effects of pleasant and unpleasant odors on mood, rendering a similar gender bias within the current results unlikely. Further, as women were equally distributed among the odor groups, possible odor effects could not have been confounded by gender.”8 (We remain unconvinced.)
We are also not sure what to make of the study results. At one point, the study states, “Odors had no effect on mood.”8 Yet it also reports, “On the other hand, the effectiveness of the helplessness induction varied between individuals in accordance with their ratings of the odors’ pleasantness. In detail, the more pleasant the odors were rated, the less successful (in terms of a smaller decrease in happiness) the helplessness induction was.”9
The study concludes:
The current study indicates that odor pleasantness and not limonene itself has a mood enhancing effect. Odor effects in humans are provoked by the individual perception of a particular odor, and not by the intrinsic properties of the odor. Thus, the study highlights the necessity to evaluate the odor judgments of the participants in aromatherapy research.10
We wonder if this is a little like the chicken and the egg question. We are not so sure the science shows it is mere pleasantness that has a mood enhancing effect rather than an actual effect of limonene. This study is available to download,11 and we invite our readers to review it and decide for themselves.
While the study authors suggest that “the effects of aromatherapy in humans may primarily be attributed to psychological effects,”12 we remain certain there are physical effects from aromatic components.
Frankincense Compound Elicits Emotion Regulation
A 2008 study13 of incensole acetate, a component of frankincense found in both Boswellia carterii and Boswellia sacra, reports that this constituent triggers an ion channel in the brain that regulates emotion. The study by researchers from the Hebrew University, Indiana University, and Johns Hopkins School of Medicine, states:
Here we show that incensole acetate (IA), a Boswellia resin constituent is a potent TRPV3 agonist that causes anxiolytic-like and anti-depressant-like behavioral effects in wild-type (WT) mice with concomitant changes in c-Fos activation in the brain. . . . Furthermore, the biochemical and pharmacological effects of IA may provide a biological basis for deeply rooted cultural and religious traditions.14
Here, a component of frankincense resin (that is also found in the essential oil of frankincense) causes a physical and emotional effect in the brain. The study researchers report, “Taken together, our data support our original contention, namely that Boswellia resin may affect sensation and emotional states.”16
We appreciate the German research studied today while reserving judgment on its conclusions.
- Hoenen M, et al. Fancy Citrus, Feel Good: Positive Judgment of Citrus Odor, but Not the Odor Itself, Is Associated with Elevated Mood During Experienced Helplessness. Front Psychol. 2015 Feb 2;7:74. [Epub ahead of print]
- Hoenen M, op cit.
- Moussaieff A, et al. Incensole acetate, an incense component, elicits psychoactivity by activating TRPV3 channels in the brain. FASEB J. 2008 Aug;22(8):3024-34.