In this section, studies aimed at elucidating the effects of certain elements of the forest environment within the laboratory setting will be surveyed by reviewing the reports of previous studies. Prior to the “Therapeutic Effects of Forests” project, there had been little field research on “Shinrin-yoku.” It is clear, nonetheless, that people have always recognized forests as familiar places and have somehow felt that the forest atmosphere benefitted human health. The number of studies targeting “Shinrin-yoku” gradually increased in the late 1990s following the launch of the “Shinrin-yoku plan” in 1982. Further, improvement in the technology of physiological measurements provided the impetus for an increase in the number of studies.
Forest environments affect humans via the five senses, providing stimulation of various senses, such as vision (scenery), olfaction (smell of wood), audition (sound of running streams or the rustle of leaves), tactile sensation (feel of the surfaces of trees and leaves). Sensory information inputs via the five senses are processed in the corresponding sensory areas of the brain and are further transmitted through interaction among the various sensory inputs. These signals subsequently reach the areas of the brain that control emotions and physiological functions, where they effect physiological changes. The objectives of the laboratory experiments that were designed to elucidate the physiological effects of each sensory input were: (1) to obtain backup data for the field studies or to undertake a detailed analysis of the results of these studies and (2) to assess the effects of elements of nature when subjects are introduced to an artificial environment (e.g., scenery shown in the form of a poster or smell introduced in the form of aroma therapy). Examples of laboratory-based studies that have investigated the therapeutic effects of wood and forest are summarized below.
Suda et al.  investigated the physiological effects by viewing scenery of “Sakura” and “Shinrin-yoku” on 70-inch high-resolution displays. Sakura consisted of a photograph of a cherry tree in full bloom, and Shinrin-yoku was a photograph of people taking a walk in the forest of Vincennes, Paris. In the subjective evaluation, the photograph of Shinrin-yoku was considered to be significantly more “comfortable” and “soothing” than the gray screen used as the control; in contrast, Sakura was evaluated as relatively “awakening.” Subjects viewing Shinrin-yoku had a significantly decreased blood pressure and prefrontal activity, whereas those viewing Sakura had a significantly increased pulse rate and blood pressure. Further, viewing Sakura significantly increased activity in the prefrontal area. Although in certain cases there was a considerable correlation between the physiological responses and subjective evaluations, in other cases no such correlation was observed. In this particular study, however, there was a considerable correlation between the two. This study revealed that people experience elation on viewing cherry blossoms. It would be interesting to learn whether or not this physiological response is unique to Japanese.
Lee and Watanuki  investigated the differences between the cardiovascular responses of Types A and B subjects to visual stimulation of displeasure-evoking images and nature-video clips. Significant differences were observed in the heart rate and cardiac output between Types A and B persons when they viewed displeasure-evoking images, i.e., Type A individuals showed a smaller decrease in heart rate and a significant increase in the cardiac output. However, in the “post-stress session” where the subjects watched the nature-video clips, no significant main effects of the Type A/B tendency or interactions between Type A/B tendency and type of visual stimulus were observed for any physiological recoveries. The authors assumed that nature-video clips may be effective in normalizing the stress-induced cardiovascular responses in Type A individuals as there were no differences between Types A and B subjects with respect to the physiological recovery responses.
Tsunetsugu et al. [19, 20] investigated the visual effects of wooden room interiors by evaluating physiological responses in mock-up living rooms. The objective of the study was to determine whether the wood ratio (i.e., the ratio of the area covered with wooden material to the whole area of the ceiling, walls, and floor) affected physiological responses. In this study, the authors attempt to focus attention solely on the visual effects; thus, they used veneer on the floor and walls of the rooms, and they ventilated the rooms appropriately. Subjects in a room with a 30% wood ratio, which is the standard type of living room commercially available in Japan  had a significantly decreased pulse rate and diastolic blood pressure, indicating the extent of the relaxing effect of this type of interior. A 45% wood ratio room tended to be evaluated as the most comfortable; further, when this wood ratio was utilized, the highest scores in the “vigorous” feeling in the mood test were generated. Pulse rate significantly increased in the 45% wood ratio room, which was considered to be in agreement with the results of the mood evaluation. A 90% wood ratio room initially caused a significant and large decrease in both systolic and diastolic blood pressure; however, this wood ratio subsequently led to a rapid decrease in prefrontal activity and an increase in pulse rate. In the subjective evaluation, all of the wooden interiors were rated as “comfortable”; however, physiological measurements revealed that an excessive amount of wood caused an awakened response. Consequently, the researchers hypothesized that there is an appropriate wood quantity (wood to room surface area ratio) in the presence of which people experienced physiological relaxation.
Phytoncides [derived from “phyto” (referring to plant) and “cide” (meaning “to kill”)] are defined as volatile or nonvolatile substances produced by all types of plants that have an influence on other organisms . The word phytoncide has become so popular that there is a persistent misunderstanding that the effects of “Shinrin-yoku” are solely attributable to phytoncide activity. However, phytoncide is a general name for a range of substances. These chemicals tend to differ from forest to forest. The atmospheric concentrations of phytoncides in forests are very low and vary depending on many factors, such as season, climate, and forest composition. This leads to the reasonable assumption that the effect of “Shinrin-yoku” is complex and comprises all of the elements associated with the senses, including phytoncides.
Phytoncides are olfaction-related elements of the forest environment. Smell has commonly been considered to be associated with instinct, emotion, and preference, and to have a greater influence on physiological change than stimuli for other senses .
Trees have scents that are species-specific. Japanese cedar (Cryptomeria japonica) is a representative tree species in Japan, and its smell is one of the most familiar to the Japanese population since cedar wood is frequently employed in the construction of houses. Miyazaki et al.  revealed that the smell of Japanese cedar chips significantly decreased systolic blood pressure, particularly at 40–60 s following the commencement of inhalation. The prefrontal area also exhibited a rapid decrease in activity with the inhalation of the smell of cedar chips, which became significant at 70–90 s following the first inhalation. The subjects evaluated the smell as “preferable.” The smell of Japanese cedar wood improved subjective comfort and caused a relaxed physiological state.
There are large individual differences in odor preferences. In the experiment just described, although Japanese cedar was evaluated as “preferable” on average, there were some subjects who disliked the smell. However, contrary to expectation, there was no rise in the blood pressure of those subjects with an aversion to the smell. The authors assumed that human physiology has adapted to the natural environment and that the smell of natural matter does not lead to stress despite the fact that it is perceived as being unpleasant.
Hiba (Thujopsis dolabrata) is also a commonly used species in Japan that possesses a characteristic smell. Itai et al.  examined the effects of aromatherapy on the mood by exposing subjects to hiba oil and demonstrated that the smell of hiba oil significantly mitigated depression and anxiety in chronic hemodialysis patients. Hiruma et al.  investigated the effects of the odor of hiba on contingent negative variation (CNV) and mismatch negativity (MNN). The amplitude of the CNV components was significantly larger under the odor condition than under the no-odor condition, while the amplitude of the MNN showed no difference whether or not the odor existed. The authors interpreted the results to be attributable to the nervous system being roused by the odor of hiba without affecting the autonomic information process.
Miyazaki et al.  reported the effects of the smell of Taiwan cypress (Chamaecyparis taiwanensis) on subjects. The inhalation of Taiwan cypress essential oil decreased the maximal blood pressure. In a work efficiency test in which the subjects deleted a specific character from a random character string, the odor of Taiwan cypress tended to improve task performance. The inhalation of Taiwan cypress essential oil appeared to relax the subjects, possibly enabling them to concentrate on the task.
One of the major components of wood scent, α-pinene, is generally detected in the air of coniferous forests. Tsunetsugu et al.  investigated the effects of the concentration of α-pinene on human physiological responses. Dilutions were made to obtain 10, 100, and 500 μL of α-pinene in 30 L of air. The odor was emitted at a rate of 3 L/min from a position approximately 15 cm beneath the nose of the subjects. The 10 μL/30 L and 100 μL/30 L concentrations, which were rated as “slight odor” and experienced to be “slightly comfortable” in the subjective evaluation, caused decreased systolic blood pressure. However, the 500 μL/30 L concentration, which was rated as a “strong odor” and “slightly uncomfortable,” did not cause a decrease in systolic blood pressure although it did increase the pulse rate. The 500 μL/30 L concentration that caused the slightly uncomfortable experience was considered to induce dominant sympathetic nervous activity. It was also observed that the degree of increase in prefrontal activity was enhanced concomitant with the increase in the concentration of α-pinene. Collectively, these results indicate that a weak smell of α-pinene induces a relaxed physiological state, whereas a relatively strong smell of α-pinene induces a stress state. Although α-pinene is a natural product, human beings have likely never been exposed to relatively high concentrations of α-pinene in the natural environment during evolution. Thus, it caused a stress state that was experienced as “uncomfortable.”
Limonene is another common phytoncide of wood. It can also be found in citrus peel and has a lemony, grapefruit-like smell. Tsunetsugu et al.  demonstrated that limonene at a concentration of 10 μL/30 L emitted at a rate of 3 L/min from a position approximately 15 cm beneath the nose of subjects was evaluated as being “slightly comfortable” and “slightly soothing.” Blood pressure started to decrease after 20 s from the commencement of inhalation; this decrease reached a significant level at 33–44 s. These researchers concluded that the inhalation of limonene was experienced as comfortable and soothing and that it suppressed sympathetic nervous activity.
Dayawansa et al.  tested the effects of cedrol, which is a compound that occurs in cedar extract, on cardiovascular and respiratory functions and observed that cederol inhalation significantly decreased the heart rate, systolic blood pressure, diastolic pressure, and respiratory frequency. The authors assumed that cedrol inhalation reduced respiratory frequency, which in turn, decreased the blood pressure via an enhancement of the baroreceptor sensitivity that has been reported to be inversely related to sympathetic activity.
Despite the large number of studies, it remains unclear to date whether odor affects human physiology through the olfactory pathway or via the blood-borne route in the form of chemical compounds. Buchbauer et al.  reported that odorous compounds were observed in the serum of mice, though in low concentrations, after an inhalation of sedative odors. Dayawansa et al.  referred to both possibilities in their above-mentioned report that cedrol could act via the central olfactory and limbic systems, and/or via the blood-borne route or peripheral afferent fibers of the vagal nerve innervating the respiratory system. The most recent human study demonstrated that cedrol directly inhaled through the lower airway, but not introduced to the upper airway, causes a suppression of sympathetic outflow and an increase in parasympathetic outflow . The results of various studies on other scents, such as lavender, lemon, valerian, and others, showed considerable agreement with respect to the fact that the effectiveness of odors in prolonging pentobarbital sleep time [32, 33], lowering blood pressure , and suppressing renal sympathetic nerve activity  disappeared in anosmic mice or rats, thus indicating the mediation of the olfactory system in the whole process. The authors consider that odors affect human physiology mainly through the olfactory processing pathway; however, in the case of laboratory experiments, the possibility that odorous compounds act directly via the blood-borne pathway could not be eliminated since the concentration of inhaled compounds is relatively high. In the case of “Shinrin-yoku” performed in the field, where the odorous compounds (phytoncides) exist in very low atmospheric concentrations, it is likely that phytoncides exert their effects via the olfactory processing pathway, and not via the blood-borne route.
Among the elements of the forest environments, interior wood is considered to have the strongest association with tactile sensation. Sakuragawa et al.  attempted to clarify the effects of contact with wood used as an interior material. They compared the physiological responses shown by the subjects to a metal plate and an oak plate. The effect of the temperature of the materials was controlled by warming the metal plate and cooling the oak plate. The blood pressure of subjects was observed to increase when they touched the metal at room temperature; however, this increase was suppressed when the plate was warmed. The expectation was that the cold wood panel would cause an increase in blood pressure; however, it did not. In the subjective evaluation, the cold oak was felt to be uncomfortable but natural; this probably explains the suppression of a blood pressure increase.
Miyazaki et al.  reported on the effects of paint. They compared changes in systolic blood pressure caused by touching Japanese cedar panels that had not been painted, that had been painted with a thin layer (oil finish), and that had been painted with a thick layer (polyurethane paint). A metal plate was also prepared as a reference. In the case of no paint and thin paint, systolic blood pressure rose transiently and then rapidly fell back to baseline levels, whereas it remained high when the subjects touched the thick-painted panel and metal. These results indicate that cedar wood can be as much a stressor as metal when coated with a thick layer of paint. Wood is usually coated with paint for maintenance reasons; however, this tends to negate the comparative advantage of the material in terms of human-friendliness.
Mishima et al.  compared the changes in brain activity and systolic blood pressure caused while listening to the sound of a turbine (dental implement), representing an artificial sound, or to a stream, representing a natural sound. No sound was introduced in the control session, i.e., the subjects heard no sound. Systolic blood pressure was significantly increased with the sound of the turbine; however, it was not changed by the sound of a stream. Prefrontal activity estimated using the NIRS method was considerably reduced when the subject was listening to the turbine, while it showed a slight but significant decrease when he/she was listening to the stream. Brain activity is frequently enhanced when stress is perceived; however, in this case, the sound of a turbine was so unpleasant that the flight reaction may have been induced. In the “Null” session, systolic blood pressure and cerebral activity did not show significant changes. The study was conceived with a view to masking the alarming sound of a turbine with the gentle sound of a stream; however, further studies on individual differences in physiological responses in terms of age, dental experiences and other factors would be required in order to be able to implement this idea into practice.