III. Introduction

Working with scents is an art that dates back to before recorded history, with different combinations of resins, oils and fragrant plants being used in one form or another by the majority of civilizations– as part of ceremonies, celebrations and religious rituals, as well as for medical use. The therapeutic benefits of scents have been known for some time and, as a result, the term aromatherapy has become established in the fields of medicine and psychotherapy. Coined in 1936 by René-Maurice Gattefossé, the term is defined as the topical (local) or oral administration – or therapeutic inhalation – of essential oils to promote health, hygiene and psychological well-being.

All essential oils are derived from plants, which themselves are capable of synthesizing two different types of oils: plant oils and essential oils. While plant oils consist of fatty acids and glycerin, essential oils are volatile organic compounds that are responsible for a plant's scent and flavor. Generally speaking, essential oils allow plants to communicate, and are used either as chemical attractants that are targeted at insects, or as chemical messengers that are targeted at other plants of the same type. The term essential is used to indicate the very nature – or essence – of the plant itself.

Even in the times of Hippocrates (460-340 BC), aromatic plants were being used for medicinal purposes (see Dierbach 2013). The use of distilled oils can be traced back to the 10th Century AD and the development of the extraction process that is now known as distillation (Forbes 1970). It is estimated that of a total of 350,000 plant species that are believed to exist on our planet, approximately 17,500 are aromatic plants. Of these, approximately 400 are commercially processed for their aromatic raw materials. While essential oils are generally thought to have only limited toxic potential (Tisserand &Young 2014), some plants contain toxic aromatic compounds with highly specific therapeutic benefits. Some of the most effective antimicrobial substances we know are essential oils (extracted from e.g. eucalyptus and pepper weed). These can be used to kill viruses, bacteria, fungi, and tiny organisms such as lice (Tisserand & Young 2014).

Today, we might consider scents a luxury, simply because our awareness and conscious perception are more focused on our sense of hearing and our sight. As was mentioned above, however, the olfactory system plays a far more prominent role in the normal functioning of the human brain than we may think. The mammalian sense of smell represents an important part of neurosensory perception, and allows both us and other mammals to "probe" our chemical environment. Scents can trigger feelings of sympathy and antipathy, influence our mood and emotions, control our social interactions and sexual behavior, trigger changes in our hormone levels, and act as chemical messengers.

In terms of its medical significance, however, aromatherapy continues to be seen as belonging predominantly to folklore. According to conventional medicine, there is a lack of (sound) scientific evidence to support claims that aromatherapy is effective in preventing and/or treating disease, and of course alternative medicine practitioners are likely to disagree with this assessment. Indeed, there are a number of interesting studies that effectively challenge this position. However, we do not wish to engage in the controversy that exists between conventional and alternative medicine. This is because there is no need for aromatherapy to engage in this controversy.

Let us briefly explain our reasoning. A disease, a condition, or "simply" the state of feeling unwell is all usually more than just the sum of individual symptoms. In order to make a person feel ill, the illness has to be consciously experienced by the affected person (after all, an unconscious person does not feel pain). In other words, signals from the body have to be interpreted in order for a person to be able to experience suffering. Thure von Uexküll, the founder of integrative medicine, recognized this and because of this severely criticized the prevailing medical doctrine, which was based on a purely mechanist view of human biology. As the founder of modern psychosomatic medicine, von Uexküll saw healing processes as consisting of an exchange of information (for a summary of these issues, please refer to Hontschik et al. 2013). According to this school of thought, the exchange of information and its interpretation form the building blocks of any type of healing process, and includes both within-system communication of biological systems (e.g. cell-to-cell communication) and between-system communication (e.g. the human sensory apparatus and physiological processes). This is also a good point at which to remind ourselves of the meaning of the term information. The term originally stems from the Latin word informare, which means "to fashion" or "to give shape (and order) to". Therefore, (new) order can only be created when information flows between (sub)systems.

In short, it is conscious awareness (the interpretation of symptoms) that creates suffering. This phenomenon explains, for instance, why a person in shock does not initially experience any pain from their injuries: stress hormones ensure that pain signals are blocked and cannot interact with the brain’s pain centers. However, as soon as the injured person becomes aware of a "deviation from the norm", i.e. a symptom, processes that are responsible for interpreting this information are triggered, producing affective reactions. Affective responses that enter our conscious awareness are then experienced or interpreted as emotions, thus prompting our brain (or more specifically, the self-referential processing centers in the prefrontal cortex; see Schneider 2012) to search for and evaluate potential explanations for why the body produced specific physiological signals. The brain does this with a remarkable degree of "creativity" in order to produce explanations that agree with both the context and previous experience. Vice versa, emotions can produce highly specific responses within the autonomic nervous system, which in return trigger highly specific physiological responses.

What we are getting at is this: if feelings are both the cause and the effect of physiological changes within the body, then scents as mediators of these emotions are capable of producing distinct physiological and psychological changes. While the question of how scents manage to influence our emotions, actions, and behavioral responses remains largely unanswered, research efforts are increasing and new knowledge is becoming available. We know that, when scent molecules enter the body via the nose, they can either elicit a direct response in the brain, via olfactory receptors in the nose, or they can reach the lungs, where they are absorbed into the blood stream and transported to various organs and tissues within the body.

Furthermore, there is a direct link between scents and behavior. Scents can influence the autonomic nervous system, neurophysiological processes, and the neuroendocrine system (hormones, neurotransmitters, and neuromodulators). Scents can also influence the physiology of our skin, an organ that acts as an important interface between “outside” and “inside”(Angelucci et al. 2014; Kiecolt-Glaser et al. 2008; Strous & Schoenfeld 2006; Tanida et al. 2008).

The nose is not the only part of our bodies to contain olfactory receptors, and it is not the only part of our bodies to treat scent signals as information. Research studies have shown that olfactory receptors can be found in the liver, heart and prostate, as well as in the skin and even the gastrointestinal tract, with the latter of course providing a possible explanation for the fact that herbs and spices can have such a pronounced effect on a person's digestion. One of the reasons why our organs can detect scents is that this enables them to distinguish between harmful and beneficial chemical substances.

For instance, our gastrointestinal tract needs to be able to eliminate toxic substances and pathogens as quickly as possible, and a network of neurons allows it to respond through reflex actions such as diarrhea and vomiting (Brand 2006; Blackshaw et al. 2007; Sternini et al. 2008). Once again, it is our evolutionary history that provides an explanation: living creatures were able to detect smells before they were ever able to see or hear which suggests that – rather than (hedonistic) extravagance – the ability to detect smells was used as a means of communication and a strategy for survival. This is also likely to be one of the reasons why the large variety of olfactory receptor neurons that are found in the nose are organized into such a complex network, and why this network of neurons contains such an enormous variety of receptor proteins (GPCR: G protein-coupled receptors) which, in humans, are encoded by a total of over 350 genes (Borin & Boulpaep 2011; Feldmesser et al. 2006; Fleischer et al. 2009).

We know today that when organisms coexist in groups, their growth and development are largely mediated via the use of signs or symbols (Greek: semeion). Thure von Uexküll was – as previously mentioned – one of the researchers who called attention to these issues, and it was he who coined the term biosemiotics. According to biosemiotics, molecules are used as signs; they act as signals and messengers, as well as carriers of information or as information storage, and occur in solid, liquid or gaseous form (Witzany 2006). This sign-based communication is possible because the molecules that combine to make up different scents are themselves capable of interacting with the receptors that modulate the release of neurotransmitters (Sergeeva et al. 2010), thus effectively becoming themselves like messengers in signal transmission!