To be clear, biological activities thus far discussed are not specific to essential oils. Flavonoids, for example, also show great structural diversity and a range of biological actions such as anti-inflammatory, antimicrobial, antioxidant, anxiolytic, neuroprotective etc., and can act on multiple targets as well. Is there anything special about essential oils, after all?
Well, yes. The small size of essential oil constituents enables them to diffuse into the air, and we can smell them. By definition, we cannot smell any plant products other than the volatiles.
Our ability to consciously or subconsciously perceive odours opens up entirely new territory for understanding how essential oils work. In contrast to pharmacological mechanisms, psychological effects are not substance specific: they don’t depend on the plasma concentration of the constituents, their binding affinity to target proteins, and other pharmacological parameters. They depend on the context of odour presentation: our memories and the current state of mind. Thus, when it comes to smell, it becomes more difficult to establish general relations in a sense “X oil has Y effect”, as psychological effects are very subjective.
HOW DOES THE PSYCHOLOGICAL MECHANISM WORK?
Psychological effects emerge from the cognitive processes initiated in the olfactory system and mediated through neuronal excitability – the ability of neurons to generate and conduct electrical signals.
Once an odorant binds to olfactory receptors, the chemical signal is transduced into a neural signal in the form of action potentials, tiny electrical currents that propagate along neurons. These are further encoded into large-scale synchronous patterns of neuronal oscillations that are communicated across the different parts of the brain and integrated into a holistic interpretation of the odorant. It’s at this stage that we consciously experience odours and assign meaning to them. The conscious experience initiated by odorants feeds back on the neurophysiological processes to modulate our mood, cognition, physiological arousal and behaviour.
Olfaction is pretty unique compared to other sensory modalities. One distinct neurological feature of olfaction is a rather direct connection of the olfactory tract with brain structures specifically involved in emotion, memory and associative learning. Such a targeted connection could help explain some peculiar properties of olfaction, such as its strong association with memories and emotions. On the other hand, some other neurological features make smell perception strongly susceptible to modulation with higher cognitive processes, such as beliefs and expectations.
Perhaps contrary to popular belief, limbic system does not seem to play the key role for the psychological effects of smells in humans. In fact, there is no universal agreement as to which structures of the brain belong to the limbic system, and whether it comprises an integrative functional unit. Although limbic structures (most notably, amygdala and hippocampus) play an important role in the early stages of olfactory processing, recent research suggests that piriform and orbitofrontal cortex are major associative structures for the conscious experience of smells, their emotional and behavioural relevance, and integration with other sensory modalities (Seubert et al. 2017).
In an influential article, Joseph Stephan Jellinek (1997) described three different psychological mechanisms how smell perception could affect mood, cognition, physiology and behaviour, which remain widely referenced in the research:
1. Via the hedonic valence of smell perception. Odour valence – its pleasantness or unpleasantness – is the most salient perceptual property of smell. Odours with distinct hedonic value, either positive or negative, can have an immediate effect on our mood and physiological arousal, as well as can indirectly influence our behaviour. For example, pleasant smells can help reduce emotional tension (or simply cheer you up), amplify social interaction and increase work efficiency, and unpleasant smells can lower tolerance for frustration (Baron 1990, 1997, Baron and Bronfen 1994, Knasko 1992, Rotton 1983, Weber and Heuberger 2008).
However, a variety of factors affect the value of perceived odour: familiarity with an odour, differences in culture, genetics and individual experience, metabolic and emotional status, and beliefs about the odour.
2. By invoking context-dependent memories through associative learning and recall (in Jellinek’s terminology this is known as the semantic mechanism; however, this term may cause confusion with other effects). Here, odours serve as contextual cues for the retrieval of associations and memories of events that were associated with them in the past, and they can affect our mood and behaviour consciously as well as at the subconscious level.
Odour-evoked memories were shown to be more emotional and evocative than memories evoked by other sensory cues, and this is especially the case in autobiographical memories (a.k.a. the Proust effect). Therapeutically, odour-evoked memories can be exploited for enhancing the mood and feelings of love and support (such as in comfort smelling), to tame cravings, and for enhancing nostalgia and associated feelings of social connectedness, optimism and self-esteem (Herz 2016).
3. Through expectancy of suggested effects (the placebo mechanism in Jellinek’s terminology). Here, effects are caused or modified according to prior beliefs and expectation regarding the odour. If you believe or are led to believe you’re smelling a ripe banana, you’ll evaluate the smell of the same odorant (isoamyl acetate in this case) as significantly more pleasant than when you think it’s a paint thinner (Djordjevic et al. 2008).
Something similar happens when you believe an oil you’re smelling will enhance your cognitive performance (Moss et al. 2006) or is harmful to your health (Dalton 1999). Your cognitive performance can actually increase, and you can viscerally start feeling bad, but this happens because you tricked your own mind into believing what you were told. Placebo effect (and related concepts of autosuggestion and emotional self-regulation) is a very powerful mechanism that should not be underestimated (Beauregard 2007), and it can be your friend or enemy.
How can we make sense of the different mechanisms discussed? Clearly, the mode of action will depend on the mode of application, but we can expect at least some interaction between different mechanisms – and here’s where it gets interesting.
For example, psychotherapeutic effects of essential oils can be mediated by both pharmacological and psychological mechanism. Linalool can work on the central nervous system through the pharmacological mechanism – by acting on specific ion channels and receptors – but it can also act through pleasantness, associations and expectations associated with its smell perception.
In a therapeutic context, smelling lavender can induce a calming effect through the expectancy of the calming effect itself, if such an experience has been previously associated with the smell of lavender. The calming effect can then be further consolidated by the inhibitory polypharmacological action of linalool on glutamate receptors and voltage-gated sodium channels, if sufficiently bioavailable. However, if you find the smell of lavender repulsive, or it reminds you of some bad experience in your life, it may rather act oppositely and make you frustrated.
In many cases, the mechanism behind an observed effect is not clear and further research is needed. But it’s the interactions between different modes of action that add a little bit of art to the science behind how essential oils work, making aromatherapy such a fascinating, yet often misunderstood discipline. We’ll go deeper into this subject in the next post!