Relationship between mood change, odour and its physiological effects in humans while inhaling the fragrances of essential oils as well as linalool and its enantiomers

Abstract

Humans can detect and discriminate a vast number of odours. The number perceived as distinguishable is estimated to be more than ten thousand. Humans are capable of distinguishing even slight alterations in the structure of an odorous molecule. A pair of enantiomers of an odorant, which possess the same molecular structures except for the chiral position, can trigger profoundly different odour perceptions. How precisely can humans and their olfactory system detect and discriminate such a great variety of odours and such subtle differences in the molecular structures? In a series of studies, we have attempted to examine the relationship between mood change, odour and its physiological effects, by focusing on the possible verbal and non-verbal changes in humans induced by smelling the fragrances of essential oils as well as linalool and its enantiometric isomers. In this article, we provide an overview of our recent verbal and non-verbal studies. We then discuss how our findings may contribute to the assessment of psychophysiological responses of essential oils as well as how our research can contribute to the study of human chemoreception science, by shedding light on the sophistication of the olfactory system in its ability to detect and discriminate odors.

Figure 1

Sensory spectra of peppermint and spearmint essential oils and linalool as a function of behavioural task. Redrawn from Sugawara et al. [15]. A sensory test was conducted twice before and after the task assigned to the subjects, in which aroma perception was evaluated by 13 impression descriptors consisting of contrasting pairs of adjectives. The pre-post task difference in the score of each of the impression descriptors is plotted on the ordinate as a bar graph. The statistical significance evaluated by t-test of each descriptor was marked with a single asterisk (*) if the pre-post impression difference was regarded significant with p < 0.05, ± if regarded significant with p = 0.05–0.1, and unmarked if p ≥ 0.1. The number of subjects was (a) 20, (b) 18, (c) 20, (d) 23, (e) 18 and (f) 22.

Figure 2

Sensory spectra of enantiomers of linalool [(RS)-(±)-, (R)-(−)- and (S)-(+)-forms] as a function of behavioural task. (a) (RS)-(±)-linalool, (b) (R)-(−)-linalool and (c) (S)-(+)-linalool when the subjects undertaking mental arithmetic; and (d) (RS)-(±)-linalool, (e) (R)-(−)-linalool and (f) (S)-(+)-linalool while undertaking the auditory task. Concentration of enantiomer was every 20 mg/mL (in diethyl phthalate), which was loaded and moistened in an inhalator (300 mL vol.) by applying 200 μL of each solution. Number of subjects was (a) 18, (b) 23, (c) 26, (d) 21, (e) 24 and (f) 23.

Figure 3

A duplication of sensory profiling conducted for cypress and lemon each versus physical task. In each column, a duplication of sensory profiling was conducted using two sets of mutually different panels, where no participant overlapped as panelist. The number of subjects was (a) 12 for Panel A, (b) 12 for Panel B and (c) 24 for Panel A+B as with cypress versus physical task; (d) 21 for Panel C, (e) 23 for Panel D and (f) 44 for Panel C+D as with lemon versus physical task. The total significance scores were 2.5 for (a), 0.5 for (b), 3.5 for (c), 6.5 for (d), 6.0 for (e) and 8.0 for (f).

Figure 4

A duplication of sensory profiling conducted for cinnamon each versus mental arithmetic and the auditory task. All other conditions were identical with those in Figure 3. The number of subjects was (a) 18 for Panel E, (b) 18 for Panel F and (c) 36 for Panel E + F as with cinnamon versus mental arithmetic; (d) 21 for Panel G, (e) 20 for Panel H and (f) 41 for Panel G + H as with cinnamon versus the auditory task. The total significance scores were calculated as 0.0 for (a), 2.5 for (b), 2.0 for (c), 5.5 for (d), 3.5 for (e) and 8.5 for (f).

Figure 5

The observed skin temperature changes following inhalation of peppermint in association with the auditory task. Redrawn from Sugawara et al. [15]. The numbers assigned to the graph represent the sensor spots on the left hand: 1, the tip of the thumb; 2, the tip of the first finger; 3, the tip of the second finger; 4, the tip of the third finger; 5, the tip of the fourth finger, and 6, the palm. The number of subjects was 20.

Figure 6

Summary of the verbal and non-verbal responses following inhalation of peppermint and spearmint essential oils and linalool in terms of the sensory evaluation spectrum and skin temperature changes as a function of behavioural task. Redrawn from Sugawara et al. [16]. Regarding sensory profiling study, the number of subjects was identical to that shown in Figure 1. As with skin temperature measurement study, the number of subjects was 18 for peppermint, 17 for spearmint and 20 for linalool in association with mental arithmetic; and 20 for peppermint, 18 for spearmint and 20 for linalool in relation to the auditory task.

Figure 7

Skin temperature changes following inhalation of lemon as a function of behavioural task. The net intensity change in skin temperature (see text) was calculated between pre- and post-task inhalations with respect to presentation of the odourless blank and the target fragrance in each trial. In line segment graph, these are connected by a solid line for each subject in each experimental run and the cases showing upward skin temperature changes after the task are plotted on the left, while those with a downward tendency are represented in the middle panels. The summarized mean values of net intensity changes obtained from pre and post task inhalations are depicted respectively as bar graph on the right. (a) The number of subjects was 18 as for lemon/mental arithmetic; and (b) was 20 as with lemon/auditory task.

Figure 8

Skin temperature changes following inhalation of ylang ylang as a function of behavioural task. The circumstances are identical to those shown in Figure 5. (a) The number of subjects was 18 as for ylang ylang/mental arithmetic; and (b) was 20 as with ylang ylang/auditory task.

Figure 9

Summary of the verbal and non-verbal responses following inhalation of lemon and ylang ylang essential oils in terms of the sensory evaluation spectrum and skin temperature changes as a function of behavioural task. The number of subjects as for sensory profiling study was 43 for lemon/mental arithmetic, 41 for lemon/auditory task, 19 for ylang ylang/mental arithmetic and 24 for ylang ylang/auditory task. In regards to the number of subjects concerning skin temperature study, see Figure 7 and Figure 8.

Figure 10

Essential oils showing “harmonization” in terms of the sensory spectra and skin temperature changes as a function of behavioural task. The conception of “harmonization” was introduced by Hongratanaworakit and Buchbauer as for ylang ylang [17]. From the view of this conception, we made attempts to search such an essential oil on the basis of our obtained task-dependent sensory spectra and fingertip skin temperature changes. With the ylang-ylang’s sensory spectra and net intensity fingertip skin temperature changes as a reference, two essential oils (peppermint and spearmint) and one monoterpenoid (linalool) were chosen.