Effects of EOs might be because of emotional effects of smell pleasantness rather than inherent pharmacological properties. Hedonic processing of odours has been shown to be an emotional rather than an analytical task (Vernet-Maury, Alaoui- Ismaili, Dittmar, Delhomme, & Chanel, 1999), which is affected by sex and the handedness of the individual, (Dijksterhuis, Moller, Bredie, Rasmussen, & Martens, 2002), the time of day and year, mental state (Goel & Grasso, 2004), ethnic origins (Schiffman, Suggs, & Sattely-Miller, 1995) and age (Fitzgerald et al., 2007). For example, in women and men at mid-life, pleasant odours (perfumes) significantly improved mood (Schiffman, Sattely-Miller, Suggs, & Graham, 1995). In general, pleasant odours have been shown to positively affect the emotions, and unpleasant odours negatively affect emotion (Knasko, 1992). For example, smelling lavender, which was viewed as a pleasant odour, increased happiness; while, camphor, which was neither pleasant nor unpleasant, produced less specific emotions, inducing both happiness, surprise or sadness; whereas, butyric acid, an unpleasant odour, elicited anger and disgust (Vernet-Maury, et al., 1999).
Even though the hedonic processing of odours is mainly emotional, the effects of odours are not limited only to emotion and odours can also play a large part in association and conditioned behaviours. Furthermore, odours can have direct effects on cognition and behaviour, even when not consciously perceived. For example, the smell of cleaning fluid caused participants to keep their direct environment cleaner than usual during an eating task (Holland, Hendriks, & Aarts, 2005).
In addition to the influence of odour hedonics on psychological and behavioural factors, odour pleasantness can influence physiological parameters too. Pleasant, versus unpleasant, odours have been shown to influence physiological measures in opposite directions. The inhalation of odours, which participants had identified as having an unpleasant smell, were found to increase heart-rate (Bensafi et al., 2002). However, smells that were pleasing to participants increased alpha EEG waves, which are related to relaxation (Yagyu, 1994).
Furthermore, although lavender is pleasant smelling at low concentrations it is often reported to be an unpleasant odour at higher concentrations. Low concentrations of
inhaled lavender (1/1000 in propylene glycol) decreased serum cortisol and increased FRSA, an index of stress that decreases in response to physical fatigue and increases when mood is positive (Atsumi & Tonosaki, 2007). However, very high concentrations of lavender, which participants found unpleasant, failed to have any effect.
Differences in the physiological response to odours are not limited to effects of pleasant versus unpleasant odours. Different pleasant smelling odours have been shown to elicit different effects on brain waves. Torii et al., (1988) found that lavender odour reduced a pattern of brain waves called contingent negative variation (CNV). Increased CNV is thought to be related to cortical arousal and increased stress (Nagai et al., 2004). In comparison, jasmine, regarded as a pleasant but stimulating odour, increased CNV in a similar manner to the stimulant caffeine in the same study. Congruent with these findings, electronically diffused lavender odour (for ten minutes in four sessions over two weeks) increased alpha and mean EEG activity (Ceballos, Matthews, Catledge, & Geisler, 2000). In another study, inhaled lavender (6 drops of lavender in 10ml of warm water for 10 minutes) decreased sympathetic arousal in young female adults (Saeki & Shiohara, 2001). Systolic blood pressure, galvanic skin conductance, and the low frequency component of heart-rate variation were reduced; while parasympathetic activity increased, as indicated by an increase in the high frequency component of heart-rate variation. In contrast, citronella and rosemary odours increased sympathetic nervous system activity. However, none of the participants or experimenters was blinded to the odours and these effects are just as likely to be due to odour hedonics as they are to pharmacological properties of the oils, and so they do not provide unequivocal evidence for specific pharmacologically anxiolytic effects caused by lavender EO.
In one study where odour pleasantness was taken into account, lavender was compared with rosemary (either oil was applied to the wrist, 3 drops in water, or a water only control) and participants were subjected to a timed crossword study (Burnett et al., 2004). When odour pleasantness was controlled, lavender and rosemary had differing effects and lavender did not affect physiological measures (heart-rate and body temperature). However, lavender did have positive effects on psychological self-report measures producing higher positive affect, increased vigour, activity and less fatigue and inertia, than the no-odour control. However, lack of
blinding to the odour in this study means that expectation effects were not controlled. This result is in contrast with Heuberger’s study where the odour, albeit only linalool and not the whole oil, was removed and only physiological, and not psychological, measures were affected. There are other differences which make the two studies difficult to compare directly; in one study peanut oil was used as the carrier and in the other water, also the part of the body where the oil was administered differed. Thus, the bio-availability and absorption rates of the diluted oils might have differed.
Contrary to the Heuberger (2004) study, however, are the results of a study conducted in 5-day-old neonates of depressed mothers where there was found to be no difference between rosemary and lavender oil odour on measures related to wellbeing and negative affect. Both odours (3 drops of 10% in grape seed oil on a dental swab suspended above each baby’s head) shifted brain activity (frontal EEG activity) to the left, particularly in babies who had predominant right frontal activity (Fernandez et al., 2004; Sanders, et al. 2002). Increased left frontal EEG activity is an indication of increased positive affect. It was not possible to ascertain whether the babies had any preference for either of the odours because they were too young to be asked! In addition, no information was given on whether the depressed mothers routinely used EOs to help lift their depressed mood state or which odour the mother preferred if any. Therefore, because there was no difference in response between the two odours, one could argue for non-specific effects rather than specific pharmacological effects.
1.4.2.2.3 Does the type of anxiogenic stimuli have an effect on lavenders effects to relieve