The influence of fragrances such as perfumes and room fresheners on the psychophysiological activities of humans has been known for a long time, and its significance is gradually increasing in the medicinal and cosmetic industries. A fragrance consists of volatile chemicals with a molecular weight of less than 300 Da that humans perceive through the olfactory system. In humans, about 300 active olfactory receptor genes are devoted to detecting thousands of different fragrance molecules through a large family of olfactory receptors of a diverse protein sequence. The sense of smell plays an important role in the physiological effects of mood, stress, and working capacity. Electrophysiological studies have revealed that various fragrances affected spontaneous brain activities and cognitive functions, which are measured by an electroencephalograph (EEG). The EEG is a good temporal measure of responses in the central nervous system and it provides information about the physiological state of the brain both in health and disease. The EEG power spectrum is classified into different frequency bands such as delta (0.5–4 Hz), theta (4–8 Hz), alpha (8–13 Hz), beta (13–30 Hz) and gamma (30–50 Hz), and each band is correlated with different features of brain states. A quantitative EEG uses computer software to provide the topographic mapping of the brain activity in frontal, temporal, parietal and occipital brain regions. It is well known that decreases of alpha and beta activities and increases of delta and theta activities are associated with brain pathology and general cognitive decline. In the last few decades, many scientific studies were conducted to investigate the effect of inhalation of aroma on human brain functions. The studies have suggested a significant role for olfactory stimulation in the alteration of cognition, mood, and social behavior. This review aims to evaluate the available literature regarding the influence of fragrances on the psychophysiological activities of humans with special reference to EEG changes.
The aroma components from natural products have been used for mental, spiritual and physical healing since the beginning of recorded history. In aromatherapy, fragrance substances (aroma/odor/scent) from various natural sources have been used for the treatment of various disorders. The aromatherapy treatment is a natural way of healing a person’s mind, body and soul. Many ancient civilizations, including Egypt, China and India, have used aromatherapy as a popular complementary and alternative therapy for more than thousands of years . In traditional medicine as well as in aromatherapy and herbal medicine, essential oils and fragrance compounds have been used for the treatments of various psychological and physical disorders such as headaches, pain, insomnia, eczema, stress-induced anxiety, depression and digestive problems [2,3]. In recent years, various studies have revealed that olfactory stimulation through fragrance inhalation exerts various psychophysiological effects on human beings. There are various methods available to administer the fragrances in small quantities, including inhalation, massage or simple applications on the skin surface and, sometimes, they can be taken internally [4,5].
In our daily life, several fragrances appear and a sense of smell plays an important role in the physiological effects of mood, stress, and working capacity. Fragrance is a volatile chemical component with a molecular weight of <300 Da that humans perceive via the olfactory system. In the olfactory process, the fragrant molecules in the air attach to the cilia of olfactory receptors in the olfactory epithelium, located in the nasal cavity. Then the guanine nucleotide binding protein (G-protein) coupled receptors (GPCR) are activated and electrical signals are generated. Subsequently, the electrical signals are transmitted to the brain by olfactory sensory neurons via olfactory bulb and higher olfactory cortex [5,6]. Consequently, these electrical signals modulate the brain functions including memory, thoughts, and emotions. Many studies describe that the inhalation of fragrances highly affect the brain function since the fragrance compounds are able to cross the blood-brain barrier and interact with receptors in the central nervous system [7,8]. Furthermore, many studies have suggested that the olfactory stimulation of fragrances produces immediate changes in physiological parameters such as blood pressure, muscle tension, pupil dilation, skin temperature, pulse rate and brain activity [5,9,10]. Hence, the studies in relation to the role of fragrances in the brain functions of healthy and diseased subjects have significantly increased in the past decades.
There are numerous techniques that have been developed to examine the brain function. The emotional and behavior alterations by fragrance inhalation have been assessed by different electrophysiological methods such as electroencephalograph (EEG), contingent negative variation, near infrared spectroscopy, and functional magnetic resonance imaging [5,11,12]. Among them, EEG is the best temporal measure of responses in the central nervous system and is susceptible to alteration during exposure to fragrance. Furthermore, the perfect classification of electrical activity for a particular state of human brain supports the diagnoses of neurological diseases. Previous studies reported that the odors affected spontaneous brain activities and cognitive functions were estimated by EEG [13,14,15,16,17]. The EEG power spectra were estimated by using Fast Fourier Transform that allows the quantitative analysis of electrical signals in the total as well as in single frequency bands. The EEG spectrum is a complex signal resulting from postsynaptic potentials of cortical pyramidal cells and these signals can be recorded by the metal electrodes placed on the surface of scalp . Based on the above knowledge, we present an overview of scientific experimentation in regards to psychophysiological effects of fragrances with special reference to EEG studies.
2. Fragrance Components
The fragrances are mainly volatile organic compounds with characteristic, usually pleasant odors. They have been used for thousands of years to deliver a variety of benefits, especially for the physical and psychological well-being of humans. In the 1920s, Rene-Maurice Gattefosse, a French chemist, coined the term aromatherapy, referring specifically to the use of natural fragrance essential oils to treat injury and disease . Nowadays, a variety of consumer products such as candles, perfumes and other personal care products, room fresheners, detergents, etc., are commercially available with aromatherapy benefits. The aromatic properties of these products play a major role in the psychophysiological functions of human beings . The fragrance materials are exposed to consumers ranging from skin contact to inhalation. The fragrance components have some specific molecular properties in order to provide sensory properties. It has a sufficiently high vapor pressure, low polarity, some ability to dissolve in fat and surface activity. Fragrance materials vary from highly complex mixtures to single chemicals. The fragrance molecules are mainly limited to the molecular weight of 200 to 300 Da but within that range, there are essentially a vast number of fragrant components and their molecular structures are highly varied. The natural fragrance materials are mainly obtained from plants, resins, animal secretion and their metabolites .
Among the various natural fragrant components, essential oils are the main therapeutic agents, which are said to be a highly concentrated volatile and complex mixture of aromatic components obtained from different organs of the plant. There are about 17,500 aromatic plant species from different angiospermic families producing essential oils, particularly Lamiaceae, Rutaceae, Myrtaceae, Zingiberaceae and Asteraceae. The essential oils contain approximately 20–60 different components at various concentrations. They are characterized by two or three major components at relatively higher levels (20%–70%) with several other minor components (trace amounts) [20,21]. In general, these major components are responsible for the biological potentials of the essential oils. The components of essential oils are classified into two major groups (terpenes and aromatic compounds) based on their biosynthetic origin. The terpenes are the largest group of natural fragrances. The classification of terpenes is mainly based on the number of isoprene units present in their structure. Depending on the number of C5 units, the terpenes are classified into hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15) and diterpenes (C20). Based on the functional groups, the terpenes and other aromatic compounds have been classified into hydrocarbons, alcohols, aldehydes, ketones, phenols, esters, ethers, etc. [19,22,23]. Thousands of different terpene and aromatic structures occur in perfume ingredients, both natural and synthetic.
In the terpenes, monoterpenes are the most abundant molecules of the essential oils (about 90%) with a great variety of structures. Geraniol/nerol, linalool, citronellol, citronellal and citral are the most important terpenes and are widely used in the perfume industries [6,21]. In aromatherapy, the medicinal and aromatic plants including bergamot, caraway, eucalyptus, geranium, juniper, lavender, lemon, lemongrass, mint, orange, peppermint, pine, rosemary, sage, tea tree, thyme and ylang-ylang have been used to cure a variety of physical and psychological disorders. These plants contain various bioactive monoterpene and sesquiterpene components along with other aromatic components. Table 1 shows the names of some important essential oil-bearing plants with their major bioactive components [1,21,22]. Some of the fragrance components from animal origin such as macrocyclic ketones and esters as well as aromatic nitro compounds and polycyclic aromatics (group of musk fragrances) are also widely used in the perfume industries. The natural fragrances from plants and animals were predominantly used until the end of the 19th century. At present, synthetic fragrances are increasingly applied due to the constant and reproducible quality over natural fragrances