Contact with plants has been found to be associated with health benefits, including improvements in physical, cognitive, psychological, and social functioning (Simson and Straus, 1998). The people–plant relationship has been explored in terms of theoretical concepts such as learning, evolution, and overload/arousal (Kaplan and Kaplan, 1989). For example, the evolution theory maintains that because we evolved in environments consisting primarily of plants, we tend to have positive psychological and physiological responses to them (Frumkin, 2001; Ulrich, 1983). Increasingly, empirical studies demonstrate that people tangibly benefit from contact with plants and other nature elements using different kinds of measures in different populations (Kuroko and Fujii, 2002; Rodiek, 2002; Ulrich 1981, 1984). These studies show that passive or active involvement with nature such as gardening or viewing/experiencing a natural setting can have a positive impact on physical and mental health. These findings can be applied to the design of improved environments; they can also be used in therapeutic horticulture to create more effective healing experiences (Cooper Marcus and Barnes, 1999; Gonzalez et al., 2010).
Although there is empirical evidence of the restorative effects of nature and plants, a majority of studies have focused on either visual perception of nature (Chang and Chen, 2005; Parsons et al., 1998; Ulrich et al., 1991) or else active involvement with plants (Mcbey, 1985). However, therapeutic horticulture professionals have documented the importance of the non-visual sensory perceptions such as scent, sound, touch, and taste. Some have suggested that a sensory or memory garden should stimulate as many senses as possible by using natural elements such as sunlight, water, breezes, fragrance, and views of wildlife (Wagenfeld, 2009). Multisensory stimulation has been found to enhance the healing effects of nature, especially for specific users such as children, hospital patients, and persons with dementia (Chapman et al., 2005; Said and Abu Bakar, 2007).
In a nature setting, people may experience plant scents consciously or unconsciously. Gardens may be designed to take advantage of attractive floral scents, which may create feelings leading to specific kinds of therapeutic experience. Floral scents can often trigger memories of particular times, events, places, or feelings (Haas and McCartney, 1996). Studies of the brain have shown that the olfactory sense is closely related to the limbic system, which in turn is responsible for the function of instinct. The olfactory sense is also intimately related to mood and feelings and, unlike the other senses, is linked to long-term memory (Brennan et al., 1990; Engen, 1982; Herz et al., 2004; Onoda, 2000; Zatorre et al., 1992). The aspects of the olfactory sense relating to emotion and memory suggest the possible therapeutic benefits of floral scent. As a potential natural healing element, floral scent may provide substantial mental or emotional benefits such as relieving anxiety or depression and maintaining memory abilities for individuals with Alzheimer’s disease or other memory disorders (Brawley, 2004; Cohen-Mansfield and Werner, 1999).
By measuring cerebral and autonomic nervous system responses, studies have demonstrated the physiological responses resulting from fragrances; for example, lavender fragrance increased beta power in electroencephalographic activity, suggesting increased drowsiness and also increased parasympathetic nervous activity in the direction associated with relaxation. Rosemary fragrance, on the other hand, decreased alpha and beta power, suggesting increased alertness; it also stimulated sympathetic nervous activity associated with excitement (Diego et al., 1998; Saeki and Shiohara, 2001). However, a majority of these studies has used essential oils or fragrance components instead of actual plant materials. Few previous studies have examined the effect of the scent that is naturally diffused by the plant, which is generally how we experience scent in everyday life or in horticultural activities. Kweon et al. (2003) compared the brain responses between people who were tearing peppermint leaves compared with people who were tearing paper and found those handling peppermint showed more vigor and brain stimulation than those handling paper. Interestingly, by comparing brain responses with intuitive verbal responses to the same scent, Jo et al. (2007) found that familiar plant scents such as shiso (Perilla frutescens) and Japanese pepper tree leaves (Zanthoxylum piperitum), often used as foods or medicines, are typically thought of as stimulating, whereas they actually have a relaxing effect on the prefrontal area in the cerebrum involved in judgment and feeling function. These studies suggest that the natural scent diffused by plants can have positive physical and mental benefits that may be useful in horticultural therapy.
The present study measured the immediate effects of natural floral scents on the cerebrum and autonomic nervous system as well as on psychological evaluations of mood and stimuli using the scent diffused by the flower itself. The findings presented here may lead to better understanding of how fragrance affects people in indoor and outdoor gardens, therapeutic environments, or during horticultural activities.
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