Odour Perception and Physiological Response

Olfactory perception is composed of three basic tasks: intensity estimation, qual- itative description, and hedonic tone [68]. One of the most significant features of olfaction is the high sensitivity and thus low detection threshold. Certain odours in air can be detected, but not identified (“I smell something”) at very low concentra- tions such as 4×10−15g/L and are identified at 2×10−13g/L [128]. These numbers represent an estimate due to high diversity of population but in general may differ by a maximum of 50-fold [52, 162].

On the other hand, qualitative description of an odour still remains a difficult task, mostly because humans have the ability to discriminate up to 10,000 dif- ferent odorants (compared to vision where we have only four different kinds of receptors) [88, 162]. Various schemes have been proposed in the past in an at- tempt to classify odours into smaller groups. The first such attempt was made by Amoore, who proposed 7 primary odours because of their high frequency of occurrence amongst 600 organic compounds: camphor, musk, floral, peppermint, ether, pungent and putrid [6, 84]. Rimmel proposed a more general classification system which included 18 categories, while a more abstract system was proposed by Zwaardemaker that included only 9 main categories, each of which was further divided into two or more categories (see [119]). Henning’s odour prism represents an attempt to identify primary odours (equivalent to red, green and blue colours) [72]. Each corner of the prism represents a primary quality: fragrant, putrid, ethereal, spicy, burnt and resinous (see Figure 3.3).

Henning claimed that odours would either be fully captured by a primary odour de- scriptor or fall on the surface or edges of the prism if in between categories. This as- sumption caused a huge number of researchers to run various experiments, conclud- ing that many odours could not be located on the surface of the prism [59, 69, 109]. For example, in MacDonald’s study, geraniol was reported to have three principal qualities (flowery, fruity and resinous) but according to the prism, it should also have a spicy quality; therefore, participants’ judgments were not in accordance with Henning’s solution [109, 171]. Due to the lack of success of these classification theo-

Figure 3.3: Henning’s smell prism [142].

ries, current approaches employ odour profiling techniques, in which a large number of verbal descriptors are used to describe individual odours adequately, if at all possible [68, 162]. Needless to say, the use of verbal descriptors assumes partici- pants have the same olfactory experience and that they will use the same words in the same way to describe a particular odour [199]. Furthermore, odours that are nameable are almost by definition highly familiar [170]. Individual differences in reception are very much influenced by individual differences in cognition, culture and experience [201]. This would seem to suggest that there are no primary odour qualities (for a similar conclusion see [35, 171]).

The third task of olfactory perception, the hedonic tone, represents a qualitative property according to which odours are further divided into two groups: pleas- ant (positive hedonic value) and unpleasant (negative hedonic value). The hedonic quality is highly subjective and is influenced by our cultural background and emo- tions [156,197]. Whether a person likes an odour or not depends largely on the asso- ciation made through life. In addition, some odours are considered pleasant at low concentrations and not at high, meaning that the hedonic tone is non-monotonically dependent on the exposure level [67]. Differences in odour perception across cul-

tures may be significant, since cultures differ in their use of various flavours, specific odours and in their use of odorants in different contexts (eg., cleaning detergents, perfumes, and so on) [120, 171].

Koelega and K¨oster reported that there is a significant difference between sexes in the hedonic perception of smell and also in sensitivity to an odour [93]. This was also shown by [101]. However, various researchers have reported that men are more likely to guess, compared to women who will not report a smell being present unless absolutely sure [93, 101, 114, 187]. Such responses may significantly affect experi- mental results and future research directions. Furthermore, scientists have shown that changes in the ability to perceive odours, which are known to occur in elderly people, are greater in men than in women [42]. This was also shown by [47, 95]. Psychophysiological studies exposing subjects to pleasant and unpleasant odours suggest that the hedonic factor is of significant influence, and that odours are pro- cessed differently depending on the hedonic tone. For example, perception of un- pleasant odours produces “automatic emotional arousal as indicated by the measures of skin conductance and heart rate” [4,5,155]. There is evidence that pleasant ambi- ent odour has a positive effect on relieving stress and improving mental relaxation. For example, in the area of aromatherapy, essential oils are being used allegedly to help people relax and release the stress they accrue doing their everyday jobs. It may be beneficial to introduce olfactory stimulation as a treatment. For example, the experiment conducted by Lehrner et al. suggests that an ambient odour of orange in a dental office could reduce anxiety and improve mood in female patients [100]. Numerous studies indicate that unpleasant odours produce a much higher degree of activation and leave a deeper/longer lasting impression than pleasant odours [95]. Recognized odours may affect our mood and they can have a psychological impact on the olfactory system. People associate odours with past experiences and odour perception is influenced by learning, and relies much more on experience than do other senses [31, 138]. Like and dislike of a particular odour may change depending on odour concentration or intensity [141]. On the other hand, our olfactory experi- ence and knowledge progressively improves, especially through adolescence and early adulthood [195, 196]. Barfield and Danas reported that peak performance in olfac- tory identification occurs between the third and fourth decade of life [10]. For vision

and audition, major developmental changes such as object discrimination, hearing threshold, colour and depth perception are nearly complete by the age of one [171]. Even though olfactory sensitivity (which depends on the duration of stimulation and concentration [26]) gradually but steadily declines with age [42,101,186], it has been shown that pleasant fragrances have positive effects across all age groups. However, even though there is strong evidence that a pleasant smell can improve our mood and our sense of well-being, researchers also found that just by telling participants that a pleasant or unpleasant odour was being used in the study, which they might not be able to smell, altered their reports of mood and well-being [60]. In an inter- esting research done by Stockhorst and colleagues, insulin was injected into healthy male volunteers once a day for four days and their blood glucose was measured (it fell). At the same time, they were exposed to a smell. On the fifth day they were just given the smell, and their blood glucose again fell [173].

Even though we may learn to discriminate different odours with age and experience (eg., wine tasters), we are still not able to separate an odorant from a mixture of more components [67]. We may never be able to do so due to the nature of our olfactory information processing [78, 171]. However, one of the important charac- teristics of olfaction is the ability of an odour to evoke an old memory as if it was a current moment. This phenomenon might be described as an implicit memory effect on perception [169].