https://orcid.org/0000-0003-3969-150X
ABSTRACT
This study investigated the effects of essential oil odors from Japanese citrus fruits, iyokan (Citrus iyo) and yuzu (Citrus junos), on human psychology and both the autonomic and central nervous systems. The inhalation of both essential oils significantly increased miosis rate and fingertip temperature and could induce parasympathetic dominance by suppressing sympathetic nerve activity. Oxyhemoglobin concentration in the prefrontal cortex increased after the inhalation of yuzu essential oil and decreased after the inhalation of iyokan essential oil. Subjectively, the inhalation of both essential oils reduced the feelings of fatigue and improved the feelings of refreshment, suggesting that the effect of autonomic nervous activity might involve in these psychological changes directly. Moreover, we observed that task performance improved after inhaling yuzu essential oil, which may be due to the increase in oxyhemoglobin concentration in the prefrontal cortex.
Odors of iyokan and yuzu essential oils induce autonomic sedation while showed different effects on central nervous system.
Abbreviations
- NIRS:
near-infrared spectroscopy
- oxy-Hb
oxyhemoglobin
- SE
standard error
- VAS
visual analog scale
The essential oils of citrus fruits contain a large number of aromatic components, and various physiological responses caused by inhalation of the odor of essential oils have been reported. Most of them are reports related to essential oils from Western citrus fruits (eg orange, grapefruits, lemon, and lime). For example, sweet orange essential oil showed antianxiety effects in animal and human studies (Lehrmer et al. 2005; Faturi et al. 2010; Goes et al. 2012), inhalation of its odor affected subjective alertness and significantly increased heart rate in humans (Hongratanaworakit and Buchbauer 2005), and orange essential oil enhanced spatial working memory by improving subjective alertness and calmness (Hawiset et al. 2016). In rats and mice, olfactory stimulation with the odor of grapefruit oil increases sympathetic nervous activity and decreases gastric vagal nervous activity (Shen et al. 2005; Nagai et al. 2014), and limonene, the active component of grapefruits oil, affects autonomic neurotransmission and blood pressure through central histaminergic nerves and the suprachiasmatic nucleus (Tanida et al. 2005). However, most previous studies interpreted physiological effects using only either the autonomic or central nervous system.
Japanese citrus fruits (eg yuzu, iyokan, mikan, kabosu, and sudachi) are widely used in Japan. Essential oils from these Japanese citrus fruits are used as flavors and fragrances for food and cosmetics. In addition, iyokan and mikan are mainly eaten raw, whereas yuzu, kabosu, and sudachi are often added to seasonings and used as ingredients in juices, sauces, and dressings. Although Japanese citrus fruits are familiar to Japanese people, only a few studies have focused on the effects of inhalation of their odor on psychology and physiology.
Although the autonomic and central nervous systems do not always respond in the same way, to date, most psychological and physiological effects, including mood/emotion/stress states, central nervous activity, and autonomic nervous activity, have only partially been interpreted and discussed. Therefore, it is difficult to interpret the overall effects of odor inhalation. To evaluate the integrative effects of odor inhalation on psychology and physiology using multiple methods, it is necessary to reveal the functionality and propose the availability of the odors.
Recently, we measured the mood state, autonomic nervous activity, and central nervous activity after inhalation of the odor in heated foods, and reported the integrative physiological effects of the odor (Ohata et al. 2020). In that report, we evaluated autonomic nervous activity by measuring the pupillary light reflex and fingertip temperature instead of conventional heart rate and blood pressure measurement, and showed that these 2 kinds of measurements were appropriate experimental techniques for evaluating autonomic nervous activity changes caused by food odor stimulation.
The aim of the present study was to investigate the effects of 2 kinds of Japanese citrus fruits on human mood and physiology using integrative physiological methods. We used iyokan, which is a typical edible citrus fruit frequently consumed by Japanese people, and yuzu, which is a fruit most frequently used in blended spices. Autonomic nervous activity was evaluated based on pupil diameter and peripheral skin temperature, and central nervous activity was assessed by measuring oxyhemoglobin (oxy-Hb) levels in the frontal cortex of the brain using near-infrared spectroscopy (NIRS). The mood and emotion changes before and after inhalation of each odor of the essential oils were examined using a visual analog scale and multiple mood scales.
Materials and methods
Participants
The objective, types of essential oils, and measurement/evaluation methods used in this study were explained to 32 healthy participants enrolled in the study (16 men and 16 women, aged 20-24 years). All participants provided consent for inclusion in the study. The inclusion criteria of the participants were as follows: no dysosmia, no smoking habits, no habit of taking or applying drugs for disease treatment, and no scent of citrus fruits (yokan and yuzu). This study was ethically reviewed and approved for human studies (ethics code: TKB187C1, Institutional Review Board of Chiyoda Paramedical Care Clinic; UMIN-CTR ID: UMIN000032773, https://upload.umin.ac.jp/cgi-open-bin/ctr/ctr_view.cgi?recptno=R000037389).
Preparation and presentation of essential oil samples
Essential oils were prepared from the peel of iyokan (Citrus iyo) and yuzu (Citrus junos) using a steam distillation method. The essential oils of the test samples used in this study were provided by Ogawa & Co., Ltd. (Ibaraki, Japan). A preliminary experiment was conducted to decide the application doses of iyokan and yuzu essential oils to cotton (1 cm × 1 cm). Each essential oil at several amounts from 0.1 to 7.0 µL was presented to the 32 participants, and 1.0 µL of iyokan essential oil and 0.5 µL of yuzu essential oil were determined as all participants perceived them as preferred odor quality. Cotton was soaked in 1.0 µL of iyokan essential oils and 0.5 µL of yuzu essential oil and placed on the bottom of an amber vial (opening diameter: 35 mm; height 73 mm; volume: 50 mL). The lid of the vial was closed and left to stand for 2 h at 25 °C, and the headspace of the vial was filled with odor. Immediately after removing the lid, the odor was presented to the participant's nose. The distance between the nose and the opening of the vial was approximately 10 mm. The participants were seated comfortably on an armchair during the experiment, and they inhaled the vapor from the sample for 2 min with uncontrolled breathing. The participants were presented with control (clean air) and odor samples in a blind manner. The autonomic nervous activity, central nervous activity, and affective mood and emotion were measured and evaluated.
Study procedure
All measurements were performed in a sensory room with controlled ambient temperature and humidity (25 °C, 60 ± 2%). The participants were seated comfortably on an armchair for 5 min for acclimation to the experimental environment before each experiment. Measurements 1-3 (1: pupillary light reflex and fingertip temperature measurement; 2: NIRS measurement; 3: subjective mood and emotion assessment) were all performed on different days. Each measurement was performed on different days in order to eliminate any interactions between odors. The time zone to be measured was matched for each participant. Participants were divided into 2 groups randomly. One group first performed all tests with iyokan essential oil. The other group first performed all tests with yuzu essential oil. After that, a crossover trial was performed. All subjects were evaluated for the 2 essential oils. The details of each method are described in the following subsections.
Evaluation of autonomic nervous activity by pupillary light reflex and fingertip temperature measurements
Detailed procedures for the measurement of autonomic nerve activity have been previously described (Ohata et al. 2020). Briefly, participants breathed natural air for 2 min as a control, followed by measurement of the miosis rate, which is the ratio of the pupil diameter before and after light stimulation, measured using an Iriscorder Dual C10641 (Hamamatsu Photonics, Shizuoka, Japan). The miosis rate was measured for the inhalation of each essential oil odor and compared.
After the pupillary light reflex measurement, the fingertip temperature was measured using a thermometer (LT8A, Gram Corporation, Saitama, Japan). The participants breathed natural air for 2 min as a control, and the fingertip temperature was measured as a control. After resting for 5 min, the participants inhaled the odor of each essential oil for 2 min, and the fingertip temperature was measured again.
Evaluation of the central nervous activity by NIRS
Changes in the concentration of oxy-Hb in the frontal cortex of the brain were assessed using NIRS (Spectratech OEG-16, Spectratech Inc., Tokyo, Japan). Detailed procedures for measurement using NIRS have been previously described (Ohata et al. 2020). Briefly, participants settled in a relaxed position. The probe holder was attached to the participants’ heads. The 16 channels and the source and center of the probe holder were located at the frontopolar central position based on the International 10/20 System. The oxy-Hb concentration (m m/mm) was recorded for a total of 6 min: 2 min before odor inhalation (breathing natural air; control), 2 min during odor inhalation, and 2 min after odor inhalation. The oxy-Hb concentrations were recorded during the latter half of each 2 min section (control, odor inhalation, and post odor inhalation), and the mean changes in oxy-Hb concentration in each channel was calculated during the latter half of the odor inhalation and post odor inhalation sections compared to the control. The oxy-Hb concentration was calculated using the modified Beer–Lambert law.
Evaluation of subjective mood and emotion
Visual analog scale (VAS) (Hongratanaworakit and Buchbauer 2004) and multiple mood scale (Terasaki et al. 1992) questionnaires were used as psychological indicators to examine mood and emotion changes before and after inhalation of each odor of essential oils. The VAS consists of 7 sets of mood states: overall fatigue, spontaneous stress, boredom, clear head, concentration, ambition, and refreshment. The multiple mood scale consists of 8 sets of mood states: depression (anxiety), hostility, boredom, liveliness, well-being, friendliness, concentration, and startlement. After breathing natural air for 2 min (control), the participants underwent VAS and multiple mood scale examinations. The participants then inhaled the odor of each essential oil for 2 min and the VAS and multiple mood scale examinations were repeated. The panelists indicated the rate of each mood state by marking on a 100 mm visual line scale (maximum score of 100; “extremely,” minimum of 0; “not at all”) in VAS and by checking on a scale from 0 (not at all) to 6 (extremely) in multiple mood scale, for the control and each odor of the essential oils. The total score for each state during each examination was calculated.
Statistical analysis
All data are shown as mean ± standard error (SE) and were processed statistically using Student’s t-test after confirming the normal distribution in SPSS (IBM SPSS software, version 20, Chicago, IL, USA) (Dayawansa et al. 2003; Yada et al. 2007). Statistical significance was set at P < .05, but set at P < .00 001 in the case of NIRS study.
Results
Effects of the odor of each citrus essential oil on the autonomic nervous system
The miosis rates after inhalation of clean air (control) and the odors of each citrus essential oil are shown in Figure 1. The miosis rate significantly increased to 0.368 ± 0.011 after iyokan odor inhalation (P < .01) compared to 0.337 ± 0.009 for the control, and increased significantly to 0.359 ± 0.012 after yuzu odor inhalation (P < .01) compared to 0.323 ± 0.012 for the control.
Effect of the odor of iyokan and yuzu essential oils on miosis rate. Data are presented as mean ± standard error and analyzed by Student's t-test; n = 32; **P < .01.
The changes in fingertip temperature after odor inhalation are shown in Figure 2. The fingertip temperature increased significantly to 32.39 ± 0.32 °C after iyokan odor inhalation (P < .01) compared to 31.66 ± 0.31 °C for the control, and increased significantly to 32.82 ± 0.23 °C after yuzu odor inhalation (P < .01) compared to 32.03 ± 0.24 °C for the control.
Effect of the odor of iyokan and yuzu essential oils on fingertip temperature. Data are presented as mean ± standard error and analyzed by Student's t-test; n = 32; **P < .01.
Taken together, both these citrus odors predominantly induce parasympathetic nervous activity.
Effects of the odor of each citrus essential oil on the central nervous system
For the 2 kinds of essential oils, Table 1 shows the mean changes in oxy-Hb concentration in each channel measured during the latter half of odor inhalation and post odor inhalation sections compared to the control. Figure 3 shows a typical example of an optical topogram of the mean oxy-Hb concentrations in the prefrontal cortex after 80 s from the start of the experiment (Control), from the start of odor exposure (Odor inhalation), and from the end of odor exposure (Post odor inhalation), respectively.
Illustration of changes in oxyhemoglobin (oxy-Hb) concentration captured by near-infrared spectroscopy. Mean (n = 32) of oxy-Hb concentration of the control (breathing natural air; before odor inhalation), during odor inhalation, and post odor inhalation of (a) iyokan and (b) yuzu essential oils. Red color represents an increase and blue color represents a decrease in oxy-Hb concentration.
Changes in oxyhemoglobin (oxy-Hb) concentration (m m/mm) during and post odor inhalation compared to before odor inhalation of the essential oils. Changes were calculated during latter half of odor inhalation section and post odor inhalation section compared to the control (before odor inhalation) in each participant (n = 32).
| Channel No.a | Iyokan | Yuzu | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Odor inhalation | Postodor inhalation | Odor inhalation | Postodor inhalation | |||||||||
| ⊿Meanb | SEc | Pd | ⊿Mean | SE | P | ⊿Mean | SE | P | ⊿Mean | SE | P | |
| 1 | −0.1470 | 0.0105 | 5.29E-36 | −0.2136 | 0.0102 | 2.52E-29 | 0.1360 | 0.0075 | 3.02E-53 | 0.1307 | 0.0118 | 3.33E-16 |
| 2 | −0.0300 | 0.0056 | 1.42E-07 | −0.0679 | 0.0076 | 1.09E-12 | 0.0593 | 0.0071 | 1.63E-15 | −0.0049 | 0.0082 | 0.5530 |
| 3 | −0.1542 | 0.0086 | 4.25E-52 | −0.2163 | 0.0067 | 1.88E-39 | 0.0731 | 0.0062 | 4.20E-27 | 0.0163 | 0.0073 | 0.0298 |
| 4 | 0.0258 | 0.0055 | 4.63E-06 | 0.0105 | 0.0080 | 0.1979 | 0.1391 | 0.0104 | 1.21E-33 | 0.0762 | 0.0129 | 1.63E-07 |
| 5 | −0.0009 | 0.0061 | 0.8812 | −0.0457 | 0.0113 | 0.0001 | 0.1046 | 0.0090 | 7.01E-27 | 0.0064 | 0.0119 | 0.5921 |
| 6 | −0.0266 | 0.0066 | 7.42E-05 | −0.0425 | 0.0074 | 3.22E-07 | 0.0960 | 0.0076 | 1.78E-30 | 0.0749 | 0.0086 | 2.93E-12 |
| 7 | 0.0336 | 0.0127 | 0.0084 | −0.0153 | 0.0216 | 0.4809 | 0.0137 | 0.0109 | 0.2113 | −0.0584 | 0.0143 | 0.0001 |
| 8 | −0.0230 | 0.0097 | 0.0183 | −0.0869 | 0.0158 | 7.82E-07 | 0.0333 | 0.0143 | 0.0210 | 0.0029 | 0.0209 | 0.8898 |
| 9 | −0.0100 | 0.0072 | 0.1705 | −0.0154 | 0.0090 | 0.0928 | 0.0251 | 0.0087 | 0.0041 | −0.0427 | 0.0088 | 8.96E-06 |
| 10 | −0.0671 | 0.0114 | 8.00E-09 | −0.1086 | 0.0175 | 5.95E-08 | 0.0131 | 0.0126 | 0.2991 | −0.0396 | 0.0196 | 0.0480 |
| 11 | −0.1006 | 0.0106 | 2.95E-19 | −0.1393 | 0.0151 | 4.61E-13 | 0.0120 | 0.0121 | 0.3229 | −0.0121 | 0.0174 | 0.4881 |
| 12 | −0.0121 | 0.0083 | 0.1435 | −0.0202 | 0.0124 | 0.1076 | 0.1587 | 0.0108 | 5.50E-39 | 0.1656 | 0.0110 | 5.90E-22 |
| 13 | 0.0034 | 0.0271 | 0.8997 | 0.0115 | 0.0418 | 0.7850 | 0.0877 | 0.0226 | 0.0001 | 0.0484 | 0.0328 | 0.1451 |
| 14 | −0.0673 | 0.0078 | 1.51E-16 | −0.1463 | 0.0081 | 6.23E-26 | −0.0283 | 0.0095 | 0.0030 | −0.0777 | 0.0139 | 5.72E-07 |
| 15 | 0.0116 | 0.0130 | 0.3743 | −0.0026 | 0.0236 | 0.9108 | 0.1359 | 0.0088 | 7.93E-42 | 0.1244 | 0.0096 | 4.58E-19 |
| 16 | 0.0123 | 0.0114 | 0.2789 | −0.0747 | 0.0192 | 0.0002 | −0.0123 | 0.0095 | 0.1968 | −0.0514 | 0.0141 | 0.0005 |
| Channel No.a | Iyokan | Yuzu | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Odor inhalation | Postodor inhalation | Odor inhalation | Postodor inhalation | |||||||||
| ⊿Meanb | SEc | Pd | ⊿Mean | SE | P | ⊿Mean | SE | P | ⊿Mean | SE | P | |
| 1 | −0.1470 | 0.0105 | 5.29E-36 | −0.2136 | 0.0102 | 2.52E-29 | 0.1360 | 0.0075 | 3.02E-53 | 0.1307 | 0.0118 | 3.33E-16 |
| 2 | −0.0300 | 0.0056 | 1.42E-07 | −0.0679 | 0.0076 | 1.09E-12 | 0.0593 | 0.0071 | 1.63E-15 | −0.0049 | 0.0082 | 0.5530 |
| 3 | −0.1542 | 0.0086 | 4.25E-52 | −0.2163 | 0.0067 | 1.88E-39 | 0.0731 | 0.0062 | 4.20E-27 | 0.0163 | 0.0073 | 0.0298 |
| 4 | 0.0258 | 0.0055 | 4.63E-06 | 0.0105 | 0.0080 | 0.1979 | 0.1391 | 0.0104 | 1.21E-33 | 0.0762 | 0.0129 | 1.63E-07 |
| 5 | −0.0009 | 0.0061 | 0.8812 | −0.0457 | 0.0113 | 0.0001 | 0.1046 | 0.0090 | 7.01E-27 | 0.0064 | 0.0119 | 0.5921 |
| 6 | −0.0266 | 0.0066 | 7.42E-05 | −0.0425 | 0.0074 | 3.22E-07 | 0.0960 | 0.0076 | 1.78E-30 | 0.0749 | 0.0086 | 2.93E-12 |
| 7 | 0.0336 | 0.0127 | 0.0084 | −0.0153 | 0.0216 | 0.4809 | 0.0137 | 0.0109 | 0.2113 | −0.0584 | 0.0143 | 0.0001 |
| 8 | −0.0230 | 0.0097 | 0.0183 | −0.0869 | 0.0158 | 7.82E-07 | 0.0333 | 0.0143 | 0.0210 | 0.0029 | 0.0209 | 0.8898 |
| 9 | −0.0100 | 0.0072 | 0.1705 | −0.0154 | 0.0090 | 0.0928 | 0.0251 | 0.0087 | 0.0041 | −0.0427 | 0.0088 | 8.96E-06 |
| 10 | −0.0671 | 0.0114 | 8.00E-09 | −0.1086 | 0.0175 | 5.95E-08 | 0.0131 | 0.0126 | 0.2991 | −0.0396 | 0.0196 | 0.0480 |
| 11 | −0.1006 | 0.0106 | 2.95E-19 | −0.1393 | 0.0151 | 4.61E-13 | 0.0120 | 0.0121 | 0.3229 | −0.0121 | 0.0174 | 0.4881 |
| 12 | −0.0121 | 0.0083 | 0.1435 | −0.0202 | 0.0124 | 0.1076 | 0.1587 | 0.0108 | 5.50E-39 | 0.1656 | 0.0110 | 5.90E-22 |
| 13 | 0.0034 | 0.0271 | 0.8997 | 0.0115 | 0.0418 | 0.7850 | 0.0877 | 0.0226 | 0.0001 | 0.0484 | 0.0328 | 0.1451 |
| 14 | −0.0673 | 0.0078 | 1.51E-16 | −0.1463 | 0.0081 | 6.23E-26 | −0.0283 | 0.0095 | 0.0030 | −0.0777 | 0.0139 | 5.72E-07 |
| 15 | 0.0116 | 0.0130 | 0.3743 | −0.0026 | 0.0236 | 0.9108 | 0.1359 | 0.0088 | 7.93E-42 | 0.1244 | 0.0096 | 4.58E-19 |
| 16 | 0.0123 | 0.0114 | 0.2789 | −0.0747 | 0.0192 | 0.0002 | −0.0123 | 0.0095 | 0.1968 | −0.0514 | 0.0141 | 0.0005 |
Channel number of the measurement probe holder in near-infrared spectroscopy.
Differences in oxy-Hb concentrations during or post odor inhalation compared to the control
Standard error.
Significance level.
Changes in oxyhemoglobin (oxy-Hb) concentration (m m/mm) during and post odor inhalation compared to before odor inhalation of the essential oils. Changes were calculated during latter half of odor inhalation section and post odor inhalation section compared to the control (before odor inhalation) in each participant (n = 32).
| Channel No.a | Iyokan | Yuzu | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Odor inhalation | Postodor inhalation | Odor inhalation | Postodor inhalation | |||||||||
| ⊿Meanb | SEc | Pd | ⊿Mean | SE | P | ⊿Mean | SE | P | ⊿Mean | SE | P | |
| 1 | −0.1470 | 0.0105 | 5.29E-36 | −0.2136 | 0.0102 | 2.52E-29 | 0.1360 | 0.0075 | 3.02E-53 | 0.1307 | 0.0118 | 3.33E-16 |
| 2 | −0.0300 | 0.0056 | 1.42E-07 | −0.0679 | 0.0076 | 1.09E-12 | 0.0593 | 0.0071 | 1.63E-15 | −0.0049 | 0.0082 | 0.5530 |
| 3 | −0.1542 | 0.0086 | 4.25E-52 | −0.2163 | 0.0067 | 1.88E-39 | 0.0731 | 0.0062 | 4.20E-27 | 0.0163 | 0.0073 | 0.0298 |
| 4 | 0.0258 | 0.0055 | 4.63E-06 | 0.0105 | 0.0080 | 0.1979 | 0.1391 | 0.0104 | 1.21E-33 | 0.0762 | 0.0129 | 1.63E-07 |
| 5 | −0.0009 | 0.0061 | 0.8812 | −0.0457 | 0.0113 | 0.0001 | 0.1046 | 0.0090 | 7.01E-27 | 0.0064 | 0.0119 | 0.5921 |
| 6 | −0.0266 | 0.0066 | 7.42E-05 | −0.0425 | 0.0074 | 3.22E-07 | 0.0960 | 0.0076 | 1.78E-30 | 0.0749 | 0.0086 | 2.93E-12 |
| 7 | 0.0336 | 0.0127 | 0.0084 | −0.0153 | 0.0216 | 0.4809 | 0.0137 | 0.0109 | 0.2113 | −0.0584 | 0.0143 | 0.0001 |
| 8 | −0.0230 | 0.0097 | 0.0183 | −0.0869 | 0.0158 | 7.82E-07 | 0.0333 | 0.0143 | 0.0210 | 0.0029 | 0.0209 | 0.8898 |
| 9 | −0.0100 | 0.0072 | 0.1705 | −0.0154 | 0.0090 | 0.0928 | 0.0251 | 0.0087 | 0.0041 | −0.0427 | 0.0088 | 8.96E-06 |
| 10 | −0.0671 | 0.0114 | 8.00E-09 | −0.1086 | 0.0175 | 5.95E-08 | 0.0131 | 0.0126 | 0.2991 | −0.0396 | 0.0196 | 0.0480 |
| 11 | −0.1006 | 0.0106 | 2.95E-19 | −0.1393 | 0.0151 | 4.61E-13 | 0.0120 | 0.0121 | 0.3229 | −0.0121 | 0.0174 | 0.4881 |
| 12 | −0.0121 | 0.0083 | 0.1435 | −0.0202 | 0.0124 | 0.1076 | 0.1587 | 0.0108 | 5.50E-39 | 0.1656 | 0.0110 | 5.90E-22 |
| 13 | 0.0034 | 0.0271 | 0.8997 | 0.0115 | 0.0418 | 0.7850 | 0.0877 | 0.0226 | 0.0001 | 0.0484 | 0.0328 | 0.1451 |
| 14 | −0.0673 | 0.0078 | 1.51E-16 | −0.1463 | 0.0081 | 6.23E-26 | −0.0283 | 0.0095 | 0.0030 | −0.0777 | 0.0139 | 5.72E-07 |
| 15 | 0.0116 | 0.0130 | 0.3743 | −0.0026 | 0.0236 | 0.9108 | 0.1359 | 0.0088 | 7.93E-42 | 0.1244 | 0.0096 | 4.58E-19 |
| 16 | 0.0123 | 0.0114 | 0.2789 | −0.0747 | 0.0192 | 0.0002 | −0.0123 | 0.0095 | 0.1968 | −0.0514 | 0.0141 | 0.0005 |
| Channel No.a | Iyokan | Yuzu | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Odor inhalation | Postodor inhalation | Odor inhalation | Postodor inhalation | |||||||||
| ⊿Meanb | SEc | Pd | ⊿Mean | SE | P | ⊿Mean | SE | P | ⊿Mean | SE | P | |
| 1 | −0.1470 | 0.0105 | 5.29E-36 | −0.2136 | 0.0102 | 2.52E-29 | 0.1360 | 0.0075 | 3.02E-53 | 0.1307 | 0.0118 | 3.33E-16 |
| 2 | −0.0300 | 0.0056 | 1.42E-07 | −0.0679 | 0.0076 | 1.09E-12 | 0.0593 | 0.0071 | 1.63E-15 | −0.0049 | 0.0082 | 0.5530 |
| 3 | −0.1542 | 0.0086 | 4.25E-52 | −0.2163 | 0.0067 | 1.88E-39 | 0.0731 | 0.0062 | 4.20E-27 | 0.0163 | 0.0073 | 0.0298 |
| 4 | 0.0258 | 0.0055 | 4.63E-06 | 0.0105 | 0.0080 | 0.1979 | 0.1391 | 0.0104 | 1.21E-33 | 0.0762 | 0.0129 | 1.63E-07 |
| 5 | −0.0009 | 0.0061 | 0.8812 | −0.0457 | 0.0113 | 0.0001 | 0.1046 | 0.0090 | 7.01E-27 | 0.0064 | 0.0119 | 0.5921 |
| 6 | −0.0266 | 0.0066 | 7.42E-05 | −0.0425 | 0.0074 | 3.22E-07 | 0.0960 | 0.0076 | 1.78E-30 | 0.0749 | 0.0086 | 2.93E-12 |
| 7 | 0.0336 | 0.0127 | 0.0084 | −0.0153 | 0.0216 | 0.4809 | 0.0137 | 0.0109 | 0.2113 | −0.0584 | 0.0143 | 0.0001 |
| 8 | −0.0230 | 0.0097 | 0.0183 | −0.0869 | 0.0158 | 7.82E-07 | 0.0333 | 0.0143 | 0.0210 | 0.0029 | 0.0209 | 0.8898 |
| 9 | −0.0100 | 0.0072 | 0.1705 | −0.0154 | 0.0090 | 0.0928 | 0.0251 | 0.0087 | 0.0041 | −0.0427 | 0.0088 | 8.96E-06 |
| 10 | −0.0671 | 0.0114 | 8.00E-09 | −0.1086 | 0.0175 | 5.95E-08 | 0.0131 | 0.0126 | 0.2991 | −0.0396 | 0.0196 | 0.0480 |
| 11 | −0.1006 | 0.0106 | 2.95E-19 | −0.1393 | 0.0151 | 4.61E-13 | 0.0120 | 0.0121 | 0.3229 | −0.0121 | 0.0174 | 0.4881 |
| 12 | −0.0121 | 0.0083 | 0.1435 | −0.0202 | 0.0124 | 0.1076 | 0.1587 | 0.0108 | 5.50E-39 | 0.1656 | 0.0110 | 5.90E-22 |
| 13 | 0.0034 | 0.0271 | 0.8997 | 0.0115 | 0.0418 | 0.7850 | 0.0877 | 0.0226 | 0.0001 | 0.0484 | 0.0328 | 0.1451 |
| 14 | −0.0673 | 0.0078 | 1.51E-16 | −0.1463 | 0.0081 | 6.23E-26 | −0.0283 | 0.0095 | 0.0030 | −0.0777 | 0.0139 | 5.72E-07 |
| 15 | 0.0116 | 0.0130 | 0.3743 | −0.0026 | 0.0236 | 0.9108 | 0.1359 | 0.0088 | 7.93E-42 | 0.1244 | 0.0096 | 4.58E-19 |
| 16 | 0.0123 | 0.0114 | 0.2789 | −0.0747 | 0.0192 | 0.0002 | −0.0123 | 0.0095 | 0.1968 | −0.0514 | 0.0141 | 0.0005 |
Channel number of the measurement probe holder in near-infrared spectroscopy.
Differences in oxy-Hb concentrations during or post odor inhalation compared to the control
Standard error.
Significance level.
The oxy-Hb concentration significantly decreased in channels 1, 2, 3, 6, 10, 11, and 14 (P < .00 001) during inhalation of the iyokan essential oil odor, and significantly decreased in channels 1, 2, 3, 6, 8, 10, 11, and 14 (P < .00 001) after inhalation of the odor. In contrast, yuzu essential oil induced a significant increase in the oxy-Hb concentration during odor inhalation in channels 1, 2, 3, 4, 5, 6, 12, and 15 (P < .00 001), and after odor inhalation in channels 1, 4, 6, 9, 12, 14, and 15 (P < .00 001).
Changes in subjective mood and emotion after inhalation of the odors of each citrus essential oil
Figure 4 shows the changes in 3 negative moods (overall fatigue, spontaneous stress, and boredom) and 4 positive moods (clear head, concentration, ambition, and refreshment) by VAS after inhalation of each essential oil. For iyokan (Figure 4a), the scores of all positive moods increased significantly (clear head: P < .01; concentration: P < .05; ambition: P < .01; refreshment: P < .00 001), whereas those of negative moods tended to decrease, especially “overall fatigue” (P < .01) and “boredom” (P < .05), which were significantly relieved. The “clear head” (P < .00 001), “concentration” (P < .00 001), and “refreshment” (P < .00 001) among the positive moods for yuzu (Figure 4b) significantly increased, and “overall fatigue” (P < .01) and “spontaneous stress” (P < .01) scored significantly lower after inhalation.
Visual analog scale scores for breathing natural air (control) and each odor. Mean (n = 32) of the scores for (a) iyokan and (b) yuzu essential oils. Significant P-values vs control, compared by Student's t-test; n = 32; *P < .05; **P < .01; ††P < .00 001.
Figure 5 shows the changes in emotions by multiple mood scales after inhalation of each essential oil (iyokan: Figure 5a; yuzu: Figure 5b). “Fatigue” for both these essential oils (iyokan: P < .05; yuzu: P < .00 001) showed a significant decrease as seen in the VAS results. “Depression/anxiety” scores were also significantly reduced for all essential oils (iyokan: P < .00 001; yuzu: P < .00 001). “Hostility” scored significantly lower for yuzu (P < .05) but not iyokan. On the contrary, the score of “active pleasure,” which consists of concrete emotions such as “liveliness,” “energy,” “vigor,” “cheerfulness,” and “breeziness” (Terasaki et al. 1992) increased significantly after inhalation of the odors of both essential oils (iyokan: P < .01; yuzu: P < .00 001). The “concentration” score for iyokan increased significantly (P < .05) but not yuzu.
Multiple mood scale scores for breathing natural air (control) and each odor. Mean (n = 32) of the scores for (a) iyokan and (b) yuzu essential oils. Significant P-values vs control, compared by Student's t-test; n = 32; *P < .05; **P < .01; ††P < .00 001.
Discussion
To date, human physiological responses to the inhalation of Japanese citrus essential oil have never been evaluated using integrated physiological methods. In this present study, we successfully evaluated the effects of yuzu and iyokan odor on blood flow in the prefrontal cortex, the response of the autonomic nervous system, and psychological fluctuations each in humans. The results show that the odor from the 2 Japanese citrus fruits induced different effects on the peripheral nervous system and cerebrum.
Recently, we reported that pupillary light reflex and peripheral skin temperature measurements are appropriate experimental parameters for evaluating autonomic nervous activity changes caused by odor stimulation (Ohata et al. 2020). Pupillary size is regulated by 2 antagonistic smooth muscle systems: the sympathetic nervous system, which induces the mydriasis by activating the dilator muscle, and the parasympathetic nervous system, which induces the miosis by shrinking the sphincter muscle (Wilhelm et al. 2001). As the sphincter muscle is regulating by the parasympathetic nervous system or the dilator muscle is regulating by the sympathetic nerve system, the miosis rate is increased. In the present study, the miosis rate was significantly increased by inhalation of each of the essential oils of iyokan and yuzu, indicating that odors of both of these Japanese citruses affect the autonomic nervous systems dominating the parasympathetic system.
Next, the fingertip temperature was measured to assess the sympathetic nervous activity without voluntary activation of the parasympathetic nervous system. Cutaneous blood flow is controlled by vascular smooth muscles, which are dominated by sympathetic vasoconstrictor fibers (Kistler et al. 1998) but are devoid of innervation of the parasympathetic vasodilator fiber. Suppressing the activity of sympathetic vasoconstrictor fibers leads to dilation of blood vessels and increases blood flow and skin temperature. In our study, fingertip temperature increased significantly following the presentation of the odor of iyokan and yuzu, indicating that sympathetic nervous activity was suppressed. From the results of miosis rate and fingertip temperature, it was considered that inhaling the odor of the Japanese citrus essential oils, iyokan and yuzu, induced parasympathetic dominance by suppressing sympathetic nervous activity, as well as induce sedative effects in humans. To date, the effect of the inhalation of iyokan essential oil on autonomic nerve activity has not been reported, and it was clarified for the first time in the present study. Reportedly, yuzu essential oil significantly reduces heart rate and increases the high-frequency component of heart rate variability, which reflects parasympathetic nerve activity (Matsumoto et al. 2016). The results of our study support these findings.
To investigate central nerve activity, changes in oxy-Hb concentration in the prefrontal cortex were measured using NIRS. Brain activity leads to an increase in oxygen consumption, accompanied by an increase in cerebral blood flow (Aoyama et al. 2010; Scholkmann et al. 2014). Oxy-Hb concentration in the prefrontal cortex was significantly decreased after the inhalation of iyokan essential oil and was significantly increased after the inhalation of yuzu essential oil. Compared with before inhalation of iyokan essential oil odor, a significant decrease in oxy-Hb concentration was observed in a wide area of the prefrontal cortex during inhalation of odor, indicating that the cerebrum became sedated. Then, after inhalation of the odor, the oxy-Hb concentration remain decreased, and the effect of odor inhalation have continued. In the case of yuzu essential oil, the cerebrum was significantly activated in a wide area of the frontal lobe during inhalation of odor compared with that before inhalation. The change in oxy-Hb concentration after odor inhalation was still significantly higher than that before odor inhalation, indicating that cerebral activation continued even after inhalation of the odor. Although there are a few reports on the effects of olfactory stimulation of Japanese citrus essential oils on the central nervous system, this present study demonstrated that the odor of iyokan essential oil induces sedation in the cerebrum due to a decrease in the concentration of oxy-Hb in the prefrontal cortex, and the odor of yuzu essential oil activates the cerebrum due to an increase in the concentration of oxy-Hb in the prefrontal cortex (Table 1 and Figure 3). In particular, the effects of odor inhalation of both essential oils continue even after inhalation is completed. It has been reported that the odor of rose and orange essential oil significantly decreased oxy-Hb concentration in the right prefrontal cortex (Igarashi et al. 2014), and the authors concluded that these odors induce physiological and psychological relaxation. Therefore, the decreased oxy-Hb concentration after iyokan odor stimulation in the present study suggests that the effect was similar to the olfactory stimulation of the aforementioned odor stimuli.
As the olfactory system has close nerve fiber communication with the limbic system, mood changes are brought about quickly by sniffing odor components (Helwany and Neuroanatomy 2020). Therefore, in this study, subjective changes in mood and emotion were assessed using the VAS and multiple mood scale scores. The results indicate that inhalation of the odors of the 2 citrus essential oils reduces negative moods, such as anxiety and fatigue. Subjective mood and emotional tendencies were deemed to be similar. Since the sedative effect on the autonomic nervous system was similar after inhalation of both iyokan and yuzu, the subjective mood and emotional changes in this study were considered to be due to the effect of autonomic nervous activity. As described above, Matsumoto et al. (2016) reported that inhalation of yuzu essential oil significantly decreased heart rate and increased high frequency power of heart rate variability reflecting parasympathetic nerve activity, which affected the reduction of negative moods such as “tension/anxiety” and “fatigue.” Hence, this study suggests the presence of an interaction between these psychological changes and underlying physiological mechanisms of the autonomic nervous system. As the prefrontal cortex controls cognition and emotion (Salzman and Fusi 2010), cognitive and emotional changes were considered to be caused by the inhalation of the odor of Japanese citrus essential oils. However, the increase in oxy-Hb concentration in the prefrontal cortex by inhalation of yuzu odor is contradictory to the present iyokan data and a previous report (Igarashi et al. 2014), and does not seem to directly affect mood and emotion. These results suggest that odor components that affect the central nervous system are likely to be different between iyokan and yuzu. There have been several studies on the characteristic odor components of iyokan and yuzu. It has been shown that there are high amounts of limonene and linalool in iyokan (Maekawa et al. 1967; Uchida et al. 1983; Takei 1985). On the other hand, in addition to the high amounts of limonene and linalool, specific odorants such as (6Z,8E)-6,8,10-undecatrien-3-one and (6Z,8E)-6,8,10-undecatrien-4-ol have been found in yuzu peel oil (Miyazawa et al. 2009; Tomiyama et al. 2012). Limonene is a major odorant of citrus essential oil and has been reported to have an activating effect on the sympathetic nervous system (Tanida et al. 2005). Further studies are required to confirm which odor chemicals in yuzu essential oil induce the increase in the oxy-Hb concentration in the prefrontal cortex.
Increasing oxy-Hb concentration in the prefrontal cortex observed during and after inhalation of yuzu essential oil suggests that the odor inhalation may have positive effects on task performance. Therefore, a count-up task was conducted as a preliminary experiment (data not shown). Eighteen subjects participated in a crossover study and touched a total of 40 numbers of digits and hiragana displayed on a computer screen in order. It was confirmed that the required time for the task was significantly reduced by the inhalation of yuzu essential oil: 1.69 ± 0.34 s, P < .01 vs control 1.88 ± 0.37 s. On the other hand, the difference in the required time between iyokan oil and the control was not statistically significant. In other words, activation of the central nervous system might improve performance of a task. It has been reported that an increase in the oxy-Hb concentration in the prefrontal cortex, which it means an increase in blood flow, affects improvement of task performance and working memory (Berman et al. 1995; Burbaud et al. 2000). In the future, the effect of inhalation of Japanese citrus essential oil, especially yuzu essential oil, on task performance and executive function of brain such as working memory should be scrutinized.
Conclusions
The results of this study showed that the odor of iyokan and yuzu induced different effects on human brain and body using integrated physiological methods. Olfactory stimulation by iyokan induced sedation of the autonomic and central nervous systems while olfactory stimulation by yuzu induced sedation of the autonomic nervous system and arousal of central nervous system. Subjectively, inhalation of both essential oils reduced the feelings of fatigue and improved the feelings of refreshment, suggesting that the effect of autonomic nervous activity might involve in these psychological changes directly. Moreover, we observed that task performance improved after inhaling yuzu essential oil, which may due to the increase in oxy-Hb concentration in the prefrontal cortex
Data availability
The data that support the findings of this study are available from the corresponding author, Motoko Ohata, upon reasonable request.
Author contribution
M.O. contributed to the investigation, conceptualization of the manuscript, writing the original draft, and project administration. L.Z. contributed to the investigation and writing of the original draft. S.A. contributed to the investigation. S.K., K.O., and Y.Y. contributed to manuscript review and supervision of the study. All authors read and approved the final manuscript.
Funding
This work was funded by the Japan Society for the Promotion of Science KAKENHI Grant Number 21K02137.
Disclosure statement
The authors declare no conflicts of interest associated with this article.
