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Article

Physiological Effects of Visual Stimulation Using Knotty and Clear Wood Images among Young Women

by
Harumi Ikei
1,2,†,
Masashi Nakamura
3,† and
Yoshifumi Miyazaki
1,*
1
Center for Environment, Health and Field Sciences, Chiba University, Chiba 277-0882, Japan
2
Department of Wood Engineering, Forestry and Forest Products Research Institute, Ibaraki 305-8687, Japan
3
Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan
*
Author to whom correspondence should be addressed.
†
Co-first authors who contributed equally to this work.
Sustainability 2020, 12(23), 9898; https://doi.org/10.3390/su12239898
Submission received: 16 October 2020 / Revised: 21 November 2020 / Accepted: 23 November 2020 / Published: 26 November 2020
(This article belongs to the Special Issue Sustainable Nature Therapy: Accumulation of Physiological Data on the Wellbeing Effect of Nature)
Browse Figures
Versions Notes

Abstract

:
Wood is a sustainable and natural material used in interior design for living environment. Knots are prominent features on wood surfaces, and they affect a user’s building preference and impression. Data on the effects of wood knots on human physiological responses are limited. Hence, further studies should be conducted. This study examined the effects of interior wall images comprising knotty or clear wood on physiological responses. Computer graphics were used to prepare wall images of knotty or clear lumber. A gray image was set as the control. In total, 28 adult Japanese female university students were included in this study. They observed two types of wood interior wall images for 90 s. The control was also set for 90 s. The oxyhemoglobin level in the prefrontal cortex measured by near-infrared time-resolved spectroscopy (TRS) and the activities of parasympathetic and sympathetic nerves assessed using the heart rate variability (HRV) were utilized as physiological indexes. TRS sensors, which emit and receive near-infrared light, were attached to frontal pole (Fp) 1 and Fp2, based on the international 10–20 method. R-R interval was measured using HRV sensors attached based on the three-point guidance method, and frequency data were analyzed to assess high frequency (HF), which reflects parasympathetic nervous system activity, and the ratio of high and low frequencies (LF/HF), which reflects sympathetic nervous system activity. The knotty wood sedated the right prefrontal cortex activity compared with the control and enhanced parasympathetic nerve activity compared with before stimulation. Clear wood sedated the left prefrontal cortex activity compared with the control and suppressed sympathetic nerve activity compared with before stimulation. Subjective evaluations revealed that compared with gray wall images, both knotty and clear wood images significantly promoted comfort, relaxation, and natural feeling and improved overall mood states. In addition, clear wood image had a more positive subjective effect than knotty image. Wall images comprising knotty or clear wood, when used as a visual stimulus, have a physiological relaxation effect among adult women in their 20s.

1. Introduction

With respect to promoting comfort in an indoor environment, the importance of removing elements with a negative impact on humans has been emphasized. In relation to this, heat, air quality, sound, and lighting have been the targets [1,2,3]. In the past few years, interest in achieving positive effects, including quality of life (QOL) improvement in the field of indoor environment, has been increasing [4,5,6]. The incorporation of natural elements into the living environment has a positive effect on the health and well-being of occupants [4]. Workspaces that incorporate natural elements, including houseplants, can change the attitudes and behaviors of office workers, thereby improving their productivity and overall well-being [5]. Indoor exposure to nature, such as gardening and gardening work, is effective in improving cognitive, psychological well-being, social outcomes, and life satisfaction among elderly adults requiring care [6]. Moreover, research focusing on well-being is now conducted [7].
Wood is a sustainable and natural material used in indoor environment. The efficient use of wood from forests is one of the key tenets of the 2030 Agenda for Sustainable Development (2030 Agenda), which was adopted as a common goal for the international community in the United Nations Summit in September 2015. It is utilized as a structural component as well as an interior finishing material that is visible and tangible in different buildings, such as dwellings, offices, and public facilities. Moreover, it brings a positive impression, such as comfort and relaxation, among residents and users. When woods are used as interiors, the appearance is a critical factor that influences a user’s preference and impression of the buildings [8]. In particular, knots are a prominent feature that appears on the surface of wooden material. This notion has been examined via subjective evaluations. For example, surveys that use questionnaires to validate whether knots induce the gaze among humans [9,10] and whether it has a negative influence on preference and impression [11,12,13,14,15] have been conducted. However, the number of studies evaluating the physiological response of humans to wood knots is extremely small [16]; hence, more studies should be conducted.
Hence, the purpose of this study was to validate the physiological and psychological impact of visual stimuli using full-scale wall images made up of knotty and clear wood. In terms of physiological responses, brain activity (oxyhemoglobin level in the prefrontal cortex evaluated with near-infrared time-resolved spectroscopy (TRS)) and activity of autonomic nerve (the activity of parasympathetic/sympathetic nerves analyzed by heart rate variability (HRV)) were simultaneously measured.

2. Literature Review

2.1. Effects of Wood-Derived Stimuli on Human Physiological Responses

In Japan, studies carried out in the last few years have concentrated on the impacts of wood on the human physiological aspects and evidence using objective indicators, for example, the brain and the autonomic nerve, endocrine, and immune activities. As evidence, in a literature review on the effects of wood on human physiological responses conducted in 2015 [17], 35 of the peer-reviewed articles discussed, excluding review articles, were all reported by Japanese research teams.
Hence, our research team developed a simultaneous measurement system for brain activity and autonomic nerve activity. Then, a series of studies were conducted on adult female university students. The results showed that the use of Japanese cypress (Chamaecyparis obtusa) as an olfactory stimulus [18,19,20,21] and wood as a tactile stimulus for the palm [22,23,24,25] and soles [26,27] induced a physiological relaxing effect, such as sedated prefrontal cortex activity, enhanced parasympathetic nerve activity, which increases as one relaxes, and suppressed sympathetic nerve activity, which elevates during stress.

2.2. Effects of Nature-Derived Visual Stimuli Such as Wood on Human Physiological Responses

The number of studies on the effects of wood when used as a visual stimulus on physiological responses is extremely limited. Sakuragawa et al. [16] investigated the physiological effect of viewing knotty wood by continuously measuring blood pressure. In this study, 14 male students viewed a 2.67 × 3.60 m2 wall composed of Japanese cypress for 90 s. A subjective evaluation on impression was performed using the semantic differential method. Based on the scores, the participants were divided into two groups: those who liked knotty wood and those who did not. Results showed that the blood pressure of participants who liked knotty wood walls significantly decreased, and that of participants who disliked knotty wood walls did not change. Tsunetsugu et al. [28,29,30] reported differences in the physiological effects of visual stimulation in rooms with different proportions of wood. Living rooms in Japan typically use wood in about 30% of the surface area. In this study, 15 young men entered rooms with wood interiors at varying proportions and looked at the interior for 90 s. Visual stimulation in rooms with 30% wood content decreased diastolic blood pressure and pulse rate. Hence, visual stimulation in this type of room induced a physiological relaxation effect. In contrast, visual stimulation in rooms comprising 40% or 45% wood increased pulse rate. Thus, changes in the percentage of wood caused differential physiological changes, and visual stimulation in a room comprising 30% wood, a standard type of living room commercially available in Japan, induces a physiological relaxation effect. Recently, Nakamura et al. [31] examined the effects of visual stimulation using two full-size square mural images arranged vertically and horizontally as well as knot-free clear wood on brain and autonomic nervous system activities among young women in their twenties. Results showed that visual stimulation using mural images arranged vertically and horizontally, compared with gray wall images, significantly decreased left and right prefrontal oxygenated hemoglobin concentrations.
In relation to the effects of wood on human physiological responses, several studies have reported the effects of naturally occurring stimuli in indoor environment, including house plants, on the human brain and autonomic nervous system activities. Based on a systematic review of laboratory experiments on the effects of nature-derived visual stimuli on physiological responses [32], we extracted six studies [33,34,35,36,37,38] that simultaneously measured TRS or near-infrared spectroscopy (NIRS) and HRV. The results are summarized in Table 1. Regarding prefrontal cortex activity, three studies [34,35,36] showed significant differences in right prefrontal cortex activity. One study [33] revealed a change only in left prefrontal cortex activity, and two studies [37,38] did not observe any significant differences. In terms of autonomic nervous system activity, only three studies [34,35,38] showed changes (including a significant tendency) in the sympathetic nervous system activity. Two studies [33,37] revealed significant differences in both parasympathetic and sympathetic nervous system activities, and one study [36] did not reveal significant differences in both activities. As described above, the current data regarding the effect of nature-derived visual stimulation on cerebral activity and autonomic nerve activity were not consistent.

3. Materials and Methods

Adult women in their 20 s were included with consideration of continuity with our previous research [18,19,20,21,22,23,24,25,26,27,31] on the effect of wood-derived stimuli on the physiological responses in humans. In addition, visual stimulation, study protocol, physiological measurements, psychological measurements, and statistical analysis in the study of Nakamura et al. [31] were applied in the current investigation. Their research revealed the physiological effects via a vision when using images of a full-scale wall made up of horizontally and vertically arranged wooden elements on prefrontal cortex activity.

3.1. Participants

In total, 28 female university students took part in this study (Table 2). The inclusion criteria included healthy Japanese women in their 20 s (range: 20–29 years). The exclusion criteria were habitual smokers, those undergoing treatment for any illness, and those who joined the experiment during their menstrual period. Before the measurement, we explained the purpose of the experiment, how the assessment will be conducted, and the measurement index that will be used. Then, a consent form signed by all individual participants was sought and received. The ethics review committee of Center for Environment, Health and Field Sciences, Chiba University (approval No. 35) approved the study and consequently recorded in the University hospital Medical Information Network (UMIN) Center in Japan (ID: UMIN000034012).

3.2. Visual Stimulation

The base material was Japanese cedar (Cryptomeria japonica). Based on the knot diameter standardized in the Japan Agriculture Standards for fixture lumber, the knotty wood is classified into small knot type with knots of under 20 mm in diameter scattered at intervals of approximately 1 m, and the clear wood is classified into the clear type. A digital still camera was used to take high-definition color photographs of individual boards, while the lumber images were retouched with the aim of amending and standardizing their color and contrast. Fifteen board images each for the knotty wood and clear wood with fairly regular grain patterns were used as wall components. Knotty wood and clear wood wall images (2160 × 3840 pixels) were created by placing 15 lumber images randomly (Figure 1). A gray wall image with similar brightness to the wooden wall images was prepared and utilized as a control.
Participants observed three full-size images using a high-vision liquid-crystal display with a size of 1053 × 1872 mm and a resolution of 3840 × 2160 pixels. The chair in which the participant was seated was 1 m 10 cm away from the large display. Further, the display was set to dim to prevent participant eye strain. All images had almost similar luminance and illuminance (Table 3).

3.3. Study Protocol

Figure 2 and Figure 3 depict the experimental protocol and visual stimulations, respectively. Measurements were performed in an artificial climate chamber with soundproofing that was retained at 24.0 °C and 50% RH. After being briefed about the experiment in the anteroom, the participant entered the artificial climate chamber. For measurement, the sensor was attached to the participants’ forehead and chest. Then, the measurement procedure was explained while the participants were sitting on a chair. Next, a trial measurement was performed using a dummy image (red-brown brick wall). The procedure was as follows: after the light in the room was dimmed, the participant observed a gray image for 60 s while sitting (rest). Then, the screen was switched, and the interior wall image composed of knotty wood (Figure 3a) or clear wood (Figure 3b) was displayed for 90 s. In the control condition, the screen was not switched, and the participant continued to look at the gray image (Figure 3c). The knotty wood, clear wood, and gray images were presented in a counterbalanced manner with the aim of getting rid of any order effects, for example, adaptation and fatigue as a result of the order of visual stimulation. The participants’ physiological response was continuously evaluated during the rest period and presentation of visual stimulus. After the visual stimulation for 90 s, the display was first switched to a dark background, followed by switching on the room lights, and finally, a subjective evaluation was carried out.

3.4. Physiological Parameters

3.4.1. Near-Infrared Time-Resolved Spectroscopy

To evaluate the participants’ brain activities during visual stimulation, the oxyhemoglobin levels in the bilateral cortices were assessed based on near-infrared time-resolved spectroscopy (TRS) (TRS-20; Hamamatsu Photonics K.K., Shizuoka, Japan; [39,40,41]), a type of near-infrared spectroscopy (NIRS). This measurement approach is premised because increased local cerebral activity increases blood flow, resulting in significant perfusion. Hence, local blood flow exceeds oxygen consumption, resulting in a high oxyhemoglobin level [42]. The regional cerebral blood flow associated with focal neural activity in the brain correlates with hemoglobin levels (oxyhemoglobin and total hemoglobin) using NIRS [43,44,45]. The prefrontal cortex is linked to higher-order judgments, for example, solving problems and making decisions. The oxyhemoglobin level in the bilateral prefrontal cortices was calculated for 60 s during the rest in which the participants looked at the gray image (pre-measurement) as well as during the 90-s visual stimulation when study participants viewed a wooden interior wall image or gray image (post-measurement). The difference between each data point and the mean of the 30-s resting period was determined. Further, the overall mean over the 90 s of visual stimulation was also assessed.

3.4.2. Heart Rate Variability and Respiratory Frequency

HRV was used to assess the activity of the autonomic nerve. The R-R interval was determined using a portable electrocardiograph (Activtracer AC-301A; GMS, Tokyo, Japan; [46,47]. The power levels in the low frequency (LF) component of 0.04–0.15 Hz and the high frequency (HF) component of 0.15–0.40 Hz were calculated based on the maximum entropy method (MemCalc/Win; GMS, Tokyo, Japan; [48,49]). The HF represented the activity of parasympathetic nerves, while the LF/HF ratio represented the activity of sympathetic nerves [46,50]. The HF and LE/HF data were normalized using natural logarithms [51]. For ln(HF) and ln(LF/HF), the average value of visual stimulation (90 s) was determined and used as the difference from the average value within the last 30 s of the rest period.
Respiratory frequency was assessed based on the power spectrum of HRV because changes in breathing affected HRV [52]. Generally, the heart rate increases with inhalation and decreases with exhalation. Hence, the respiratory frequency could be evaluated based on the main frequency of the HF component [53,54].

3.5. Psychological Parameters

The modified version of the semantic differential method [55] and the Profile of Mood States Second Edition (POMS2) [56,57,58,59] were adopted to assess the psychological effects of visual stimulation using the images. The semantic differential method is used for the subjective assessment of mental state and is rated on a 13-grade scale [55]. In this study, four adjective pairs were used to assess the degree of comfort, relaxation, nature, and homogeneity. The POMS2 can simultaneously evaluate seven moods, which are as follows: tension–anxiety (T–A), depression–dejection (D–D), anger–hostility (A–H), fatigue–inertia (F–I), confusion–bewilderment (C–B), vigor–activity (V–A), and friendliness (F) [56,57,58,59]. Further, the total mood disturbance (TMD) score was computed. In the current experiment, the short form of 35 questions (POMS2 in Japanese) was used so as not to burden the participants.

3.6. Statistical Analysis

The Statistical Package for the Social Sciences (version 21.0, IBM Corp., Armonk, NY, USA) was utilized. A p-value < 0.05 was considered statistically significant. Paired t-tests were utilized to analyze the physiological parameter for the (1) comparison between images (knotty wood versus gray image, clear wood versus gray image, and knotty wood versus clear wood) and (2) comparison before and after the visual stimulation (before versus after stimulation) for each image. The Wilcoxon signed-rank test was used to analyze the psychological parameters among the three images. The Holm method was used to adjust the family-wise error rate while comparing three images [60].

4. Results

4.1. Physiological Effects

4.1.1. TRS

Figure 4 indicates every second of the continuous oxyhemoglobin level in the bilateral prefrontal cortex during the visual stimulation of the knotty wood, clear wood, and gray images. During the visual stimulation of clear wood, oxyhemoglobin level in the left prefrontal cortex was lower than that before the visual stimulation and returned to the baseline after 72 s (Figure 4a, beige line). Moreover, the oxyhemoglobin level in the left prefrontal cortex while viewing the knotty wood transitioned around the baseline (Figure 4a, black line). The oxyhemoglobin level in the left prefrontal cortex while observing the gray image was temporarily below the baseline at 7–10 s and 36–40 s. However, the level remained higher than the initial level (Figure 4a, line of gray). In the right prefrontal cortex, the oxyhemoglobin level while observing the knotty wood gradually decreased and returned to baseline after 85 s (Figure 4b, black line). The oxyhemoglobin level in the right prefrontal cortex while viewing the clear wood remained around the baseline and gradually increased after 64 s (Figure 4b, line of beige). The oxyhemoglobin level in the left prefrontal cortex while observing the gray image increased after 7 s and remained higher than at first (Figure 4b, line of gray). Notably, no statistical significance was observed among the three visual stimuli in the mean oxyhemoglobin level (left prefrontal cortex: knotty wood  = 44.40 ± 1.11 µM [mean  ±  SE], clear wood = 44.96 ± 1.05 µM, and gray image =  44.37 ± 1.13 µM; right prefrontal cortex: knotty wood =  44.31 ± 1.34 µM, clear wood = 44.21 ± 1.28 µM, and gray image =  44.48 ± 1.26 µM, p > 0.05) during the 30 s of the rest (gray part in Figure 4). The unit M represented molar concentration (mol/dm3).
The overall mean oxyhemoglobin level (computed as the measurement after the stimulation less than before the stimulation) in bilateral prefrontal cortices during the 90-s visual stimulation period is shown in Figure 5. The oxyhemoglobin level in the left prefrontal cortex while observing the knotty wood, clear wood, and gray images were −0.04 ± 0.15, −0.19 ± 0.11, and 0.30 ± 0.11 µM, respectively. The oxyhemoglobin level was significantly lower during the visual stimulation of the clear wood image than with the gray image (Figure 5a, p < 0.05). The oxyhemoglobin level during the visual stimulation of the gray image was significantly higher than that during the pre-measurement value (Figure 5a, p < 0.05). The oxyhemoglobin level in the right prefrontal cortex was −0.21 ± 0.11, 0.00 ± 0.10, and 0.32 ± 0.11 µM while observing knotty wood, clear wood, and gray images, respectively. The oxyhemoglobin level was significantly lower during the visual stimulation of the knotty wood image than the gray image (Figure 5b, p < 0.05). The oxyhemoglobin level, while observing the gray image was significantly higher than the pre-measurement value (Figure 5b, p < 0.05).

4.1.2. HRV

One participant who presented with a large difference in respiratory rate while viewing the images was excluded as variations in these values could affect HRV data. No statistically significant difference was observed between the three images and pre and post-measurements in terms of the mean value of the estimated respiratory rate in the remaining 27 participants (knotty wood: pre-measurement = 17.1 ± 0.7 times/min, post-measurement = 16.9 ± 0.7 times/min; clear wood: pre-measurement = 16.4 ± 0.7 times/min, post-measurement = 17.0 ± 0.5 times/min; gray image: pre-measurement = 16.7 ± 0.6 times/min, post-measurement = 16.8 ± 0.6 times/min, p > 0.05).
Figure 6 shows the before and after stimulation of ln(HF) values reflecting parasympathetic nerve activity assessed using HRV in 27 participants. The ln(HF) was 5.20 ± 0.21 lnms2 before measurement and 5.60 ± 0.14 lnms2 after measurement in the knotty wood, 5.33 ± 0.15 lnms2 before measurement and 5.42 ± 0.16 lnms2 after measurement in the clear wood, and 5.40 ± 0.13 lnms2 before measurement and 5.21 ± 0.13 lnms2 after measurement in the gray image. In contrast, the average baseline ln(HF) within 30 s before viewing was not significantly different among the three images (p  >  0.05). The ln(HF) value while observing the knotty wood image was significantly higher than the pre-measurement (Figure 6, p  <  0.05). No statistically significant difference was observed between the three images concerning ln(HF) for 90 s after measurement (p > 0.05).
Figure 7 shows the average values of ln(LF/HF) reflecting the sympathetic nerve activity before and after measurement in 27 participants. The ln(LF/HF) values were 0.18 ± 0.20 before measurement and 0.03 ± 0.18 after measurement in the knotty wood, 0.35 ± 0.25 before measurement and −0.06 ± 0.17 after measurement in the clear wood, and 0.17 ± 0.19 before measurement and 0.50 ± 0.20 after measurement in the gray image. Notably, the average baseline ln(LF/HF) within 30 s before the viewing was in no way significantly different among the three images (p  >  0.05). The ln(LF/HF) was observed to be statistically lower than the pre-measurement value while observing the clear wood image (Figure 7, p  <  0.05). The difference between the three images in ln(LF/HF) for 90 s after measurement (p > 0.05) was not significant.

4.2. Psychological Effects

Figure 8 illustrates the participants’ findings from the subjective evaluation after observing each image obtained by the modified version of the semantic differential method. The participants felt comfortable (slightly to moderately) after observing knotty and clear wood images and indifferent to slightly uncomfortable after observing the gray image. Thus, viewing wooden interior wall images could significantly induce comfort when contrasted with viewing the gray image (Figure 8a, p < 0.05). In terms of feeling relaxed, subjective reporting by participants revealed that they felt slightly to moderately relaxed while observing knotty and clear wood images and indifferent to slightly awakened while observing the gray image. Thus, wooden interior wall images could significantly induce relaxation in contrast to the gray image (Figure 8b, p < 0.05). Regarding feeling natural, knotty and clear woods were construed from slightly to moderately natural, and they could significantly induce a natural feeling compared with the gray image. This feeling was interpreted as moderate to very artificial (Figure 8c, p < 0.05). In terms of homogeneous feeling, the participants felt a bit different after observing the knotty wood, indifferent to slightly ordered after observing clear wood, and moderately to extremely ordered after viewing the gray image. Thus, the knotty wood image was perceived as significantly more varied than the clear wood or the gray image (Figure 8d, p < 0.05). Moreover, for the homogeneous feeling, a significant difference was observed between the clear wood and gray images (Figure 8d, p < 0.05).
In the POMS2, negative mood subscales (C–B, D–D, F–I, and T–A) were significantly lower for the images of the wooden walls than for the gray image (Figure 9, p < 0.05). In contrast, the positive mood subscales (V–A and F) were significantly higher for the wooden wall images than for the control one (Figure 9, p < 0.05). In the A–H subscales, significant differences were found only between the clear wood and gray images (Figure 9, p < 0.05). The TMD score was significantly lower for the wooden wall images than the gray image (Figure 9, p < 0.05). The C–B score was significantly lower in the clear wood than the knotty wood (Figure 9, p < 0.05).

5. Discussion

This is the first case study that investigated the effects of visual stimulation using knotty and clear wooden images on physiological responses among adult women in their twenties. We investigated the physiological impacts of visual stimulation with wooden wall images composed of knotty and clear wood. As a measurement indicator, we used the left/ right prefrontal cortex activities by measuring the oxyhemoglobin level by TRS. Moreover, the parasympathetic nerve activity reflects the relaxation state, and the sympathetic nerve activity, which reflects the stress state, was evaluated by measuring HRV. In addition, as supporting evidence on the physiological response, we examined the psychological effects of wooden wall images via subjective assessments.
When participants observed the wooden interior wall image composed of knotty wood for 90 s, the oxyhemoglobin level in the right prefrontal cortex was significantly decreased (versus control image). Moreover, it significantly increased the parasympathetic nerve activity (versus pre-stimulation). By contrast, while viewing the clear wood image, the oxyhemoglobin level in the left prefrontal cortex and the activity of sympathetic nerve (versus pre-stimulation) significantly decreased (versus control). Subjective evaluations revealed that compared with gray wall images, both knotty and clear wood images significantly promoted comfort, relaxation, and natural feeling and improved overall mood states. In addition, the clear wood image had a more positive subjective effect than the knotty image.

5.1. Physiological Effects

Nakamura et al. reported the research on the effect of wood-derived visual stimulation on prefrontal cortex activity [31]. Results indicated a significant reduction in the oxyhemoglobin level in bilateral prefrontal cortex activity of adult female university students by visual stimulation of horizontally and vertically displayed full-scale wall images composed of Japanese cedar wood for 90 s on a large display. Furthermore, Song et al. [49] found that oxyhemoglobin level in the right prefrontal cortex was significantly reduced among adult female university students after visual stimulation of a forest image using a large display as opposed to a city image. However, these two reports showed a lack of significant difference in the indices of autonomic nerve activity (HF- and LF/HF-based HRV).
Ikei et al. assessed the effect of nature-derived visual stimulation on parasympathetic and sympathetic nerve activity via a series of studies on flower and foliage plants. Results showed that observing fresh rose flowers in a vase enhanced the parasympathetic nerve activity in male office workers [61], and observing three pots of dracaena foliage plants enhanced the activity of parasympathetic nerves and reduced the activity of sympathetic nerves in male and female high school students [62]. In addition, Igarashi et al. [63] found that observing fresh flowers of pansies planted in planters suppressed sympathetic nerve activity as opposed to observing artificial flowers among high school students. However, prefrontal cortex activity was not evaluated in these three studies.
In the current study, the oxyhemoglobin level in the right prefrontal cortex was found to be significantly decreased while viewing images composed of knotty wood. The oxyhemoglobin level in the left prefrontal cortex significantly reduced while viewing the clear wood image (both versus control image). A significant increase in ln(HF), which represented the activity of parasympathetic nerves, was only observed while viewing images composed of knotty wood, and a significant decrease in ln(LF/HF), which represented the activity of sympathetic nerves, was noted while viewing only images composed of clear wood (versus pre-stimulation). This study showed the results of cerebral and autonomic nerve activity. That is, it can be interpreted that the human body was relaxed after wood-derived visual stimulation. However, whether the relationship between the changes in the activity of the left and right prefrontal cortex, the activity of parasympathetic nerves, and activity of sympathetic nerves caused after viewing images composed of knotty wood or clear wood was not elucidated. Currently, research about the influence of nature-derived stimuli, for example, wood and wooden materials, on human physiological responses is conducted by a Japanese team [17,64]. However, it is still in its early stages. Hence, a continuous investigation must be conducted, and more scientific data should be collected.

5.2. Psychological Effects

The findings of the subjective evaluation agreed with those for physiological responses. In the impression assessment using the modified semantic differential approach, the interior wall images composed of knotty or clear wood significantly enhanced comfortable, relaxed, and natural feeling compared to the gray image. In the mood evaluation using the POMS2, scores of negative mood subscales and TMD scores were significantly lower, and scores of positive mood state subscales were significantly higher in the wooden interior wall images than for the gray image. In previous studies, the preference of knotty wood was influenced by the balance of harmony and activity that people perceive when looking at the timber surface [12], the homogeneity of knots that influences people’s preference [13,14], and the evenness of the wooden material surface that significantly influences a person’s impression [15]. All knotty lumber used in this study had small knots, with diameters specified in the Japan Agriculture Standards for lumber. In addition, the size and arrangement of the knots were similar among the lumber. Therefore, the knotty wood was considered similarly favorable to the clear wood in terms of the participants’ impression and mood in this study.

5.3. Generalization

Novel and important points from this study are as follows: we focused on visual stimulation using interior wall images composed of knotty or clear wood which had not been physiologically verified, and the evaluation including the simultaneous measurement of the activity of the prefrontal cortex and the activity of parasympathetic/sympathetic nerves to elucidate the possible physiological relaxing effects of observing wooden interior wall images.
Yoshifumi Miyazaki, a physiological anthropologist, proposed the Back to Nature theory [65,66] for the physiological relaxing effect of natural stimuli, including wood. This theory is similar to the Biophilia hypothesis [67], which was introduced by Edward O. Wilson in his book published in 1984. That is, humans have an inherent preference for parts of the natural world before they acquired knowledge. Miyazaki’s argument is as follows: human evolution started about 7 million years back, and most evolutionary processes, including those observed to date, occur in a natural environment. Therefore, human has a physiological function that corresponds to the natural environment. If we consider the industrial revolution as the beginning of urbanization and artificialization, less than 0.01% of the total time spent to date has been spent in an artificial environment. Thus, modern humans cannot adapt to an artificial environment, and they experience a high level of stress on a daily basis. In this condition, exposure to the natural environment and nature-derived stimuli can promote relaxation and can help humans get closer to their natural and proper state. The successful incorporation of wood in our daily lives can help improve the QOL of modern people living in a stressful society.
Furthermore, wood is a natural material that can be seen in daily life, and it can be useful in reducing challenges within the modern urbanized and artificial society to attain sustainability. The Sustainable Development Goals (SDGs) are included in the 2030 Agenda, which is based on the idea that the world as a whole should address these issues [68]. Wood produced in sustainably managed forests (SDGs Goal 15) is directly correlated with SDGs Goal 12: Responsible consumption and production, which seeks to ensure sustainable production and consumption patterns. In addition to this, wood and various wood-based materials would hold out their carbon storage function when used in buildings. Compared with other building materials, wood is beneficial because less energy is required during manufacturing and processing. The present study showed that visual stimulation with solid cedar wood, with or without knots, has a physiological relaxing effect on humans. Thus, in “sustainable nature therapy,” as defined in the special issue of Sustainability [69], proactive use of wood is recommended.

5.4. Limitations and Future Research

Visual stimulation using interior wall images composed of knotty or clear wood had physiological relaxing effects. However, the physiological reaction of prefrontal cortex activity and parasympathetic/sympathetic nerve activity slightly differed based on the presence or absence of knots. Results showed that the surface properties of the wooden material used for interior decoration in different places have different effects on human physiological responses. In future studies, the allowable range of knots that have positive physiological and psychological effects can be validated by conducting a detailed examination of the proportion or size of the knots occupying the surface.
In the current study, to minimize visual inhomogeneity in genuine wood, the lumber images were tuned well via image processing, and clear or knotty wooden wall images were used. To present these images as full-size visual stimuli, a large high-vision liquid-crystal display (LCD) was used. Thus, participants were exposed to the LCD, which was used as light source. As shown in Table 2, the correlated color temperatures (CCTs) were fairly different between wooden wall and gray images. The light with a high CCT is rich in blue-band light (short wavelength in the visible light). Several studies (e.g., Vandewalle et al. [70,71]) showed that such blue light affecting brain activities, particularly non-visual effects evoked by it, has been well investigated. In the current study, when the gray image was displayed, the LCD light might have caused wavelength-dependent modulation in brain activities. However, this impact was extremely challenging to validate. A close examination of the spectroscopic quality to the visual stimuli using the LCD will be a future task.
A gray image was selected as a control in this study because it provides a neutral impression when viewed. Further studies that use a monotonous brown image hue-matched to wood can help elucidate the efficacy of visual stimulation using wood on human physiological responses.
Young women in their twenties were included in this study. Previous studies showed that there were differences in human physiological responses based on factors such as gender and age. To generalize the findings obtained from this study, larger and more diverse samples should be included. Such samples could focus on the contrasting effects among young men and women in their twenties, and those in various age groups should be assessed. In the future, new scientific data that can contribute to the acquisition of active comfort [72] in the building research field can be obtained.
Visual stimuli were presented by projecting images on a large display in this study. To obtain more scientific data, future research should be conducted to investigate the effects of visual stimulation using actual materials on physiological responses. Then, the QOL of the general public can be improved by investigating the effect of the wooden interior space on human physiological responses in actual places, such as dwellings, offices, and public buildings. The results of this study and future related studies may provide useful data on the design and construction of wood interiors in general, including not only walls but also floors.

6. Conclusions

This study found that knotty wood sedated right prefrontal cortex activity compared to control and enhanced parasympathetic nerve activity compared to before stimulation, and clear wood sedated left prefrontal cortex activity compared to control and suppressed sympathetic nerve activity compared to before stimulation. Based on the impression evaluation, subjective comfort, relaxation, and natural feelings were significantly enhanced, and the mood evaluation revealed reduced negative mood and increased positive mood after viewing the wooden interior wall images compared with the gray image. These results were in agreement with the noticeable changes in the activities of prefrontal cortex and parasympathetic/sympathetic nerves. Taken together, viewing interior wall images composed of knotty or clear wood induces relaxation physiologically and psychologically among adult women in their twenties.

Author Contributions

Conceptualization, H.I. and Y.M.; methodology, H.I. and Y.M.; formal analysis, H.I.; investigation, H.I. and Y.M.; resources, H.I., M.N. and Y.M.; data curation, H.I.; writing—original draft preparation, H.I.; writing—review and editing, H.I., M.N. and Y.M.; visualization, H.I.; project administration, Y.M.; funding acquisition, H.I. and Y.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Tokyo log wholesalers’ association.

Acknowledgments

The current manuscript arose from commissioned research “physiological effects of visual and tactile stimulation by solid wood” by the Tokyo log wholesalers’ association. We would like to thank Hiromitsu Kobayashi of the Ishikawa Prefectural Nursing University for his contribution to provide us the knowledge and analysis software necessary for calculating the estimated respiratory frequency.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Images used in the visual experiment. (a) Interior wall image composed of knotty lumber. (b) Interior wall image composed of clear lumber. (c) Control: the brightness of the gray image was adjusted similarly to that of wooden interior wall images.
Figure 1. Images used in the visual experiment. (a) Interior wall image composed of knotty lumber. (b) Interior wall image composed of clear lumber. (c) Control: the brightness of the gray image was adjusted similarly to that of wooden interior wall images.
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Figure 2. Experimental procedure. TRS (near-infrared time-resolved spectroscopy), HRV (heart rate variability). * To remove the viewing order effect, a counterbalance was included between the three images.
Figure 2. Experimental procedure. TRS (near-infrared time-resolved spectroscopy), HRV (heart rate variability). * To remove the viewing order effect, a counterbalance was included between the three images.
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Figure 3. Images during the visual stimulus. (a) Interior wall image composed of knotty lumber. (b) Interior wall image composed of clear lumber. (c) Control: gray image with a brightness similar to that of the wooden interior wall images.
Figure 3. Images during the visual stimulus. (a) Interior wall image composed of knotty lumber. (b) Interior wall image composed of clear lumber. (c) Control: gray image with a brightness similar to that of the wooden interior wall images.
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Figure 4. Oxyhemoglobin level in the prefrontal cortex during the visual stimulation using wooden interior wall images and the gray image every second of the 90-s period. N = 28, Mean ± SE.
Figure 4. Oxyhemoglobin level in the prefrontal cortex during the visual stimulation using wooden interior wall images and the gray image every second of the 90-s period. N = 28, Mean ± SE.
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Figure 5. Overall mean oxyhemoglobin level in the prefrontal cortex. (a) Oxyhemoglobin level in the left prefrontal cortex, (b) oxyhemoglobin level in the right prefrontal cortex; n = 28, mean ± standard error (mean ± SE), * p < 0.05 (comparison between stimuli) as calculated based on the paired t-test with the Holm method, ‡ p < 0.05 (comparison of pre- and post-measurements) obtained from the paired t-test.
Figure 5. Overall mean oxyhemoglobin level in the prefrontal cortex. (a) Oxyhemoglobin level in the left prefrontal cortex, (b) oxyhemoglobin level in the right prefrontal cortex; n = 28, mean ± standard error (mean ± SE), * p < 0.05 (comparison between stimuli) as calculated based on the paired t-test with the Holm method, ‡ p < 0.05 (comparison of pre- and post-measurements) obtained from the paired t-test.
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Figure 6. Pre- and post-measurement values of the natural logarithm of the high frequency (HF) reflecting parasympathetic nerve activities evoked by wooden interior wall images and the gray image. N = 27, mean ± SE, ‡ p < 0.05 (comparison of before and after stimulation) obtained from the paired t-test.
Figure 6. Pre- and post-measurement values of the natural logarithm of the high frequency (HF) reflecting parasympathetic nerve activities evoked by wooden interior wall images and the gray image. N = 27, mean ± SE, ‡ p < 0.05 (comparison of before and after stimulation) obtained from the paired t-test.
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Figure 7. Pre-stimulus and post-stimulus values of the natural logarithm of the low frequency/high frequency (LF/HF) reflecting sympathetic nerve activities evoked by wooden interior wall images and the gray image. N = 27, mean ± SE, ‡ p < 0.05 (comparison of before and after stimulation) obtained from the paired t-test.
Figure 7. Pre-stimulus and post-stimulus values of the natural logarithm of the low frequency/high frequency (LF/HF) reflecting sympathetic nerve activities evoked by wooden interior wall images and the gray image. N = 27, mean ± SE, ‡ p < 0.05 (comparison of before and after stimulation) obtained from the paired t-test.
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Figure 8. Subjective evaluations of the knotty, clear wood, and gray images after visual stimulation using the modified version of the semantic differential method questionnaire. (a) Comfortable, (b) relaxed, (c) natural, and (d) homogeneous feeling; n = 28, mean ± SE, * p < 0.05 obtained from the Wilcoxon signed-rank test with the Holm method.
Figure 8. Subjective evaluations of the knotty, clear wood, and gray images after visual stimulation using the modified version of the semantic differential method questionnaire. (a) Comfortable, (b) relaxed, (c) natural, and (d) homogeneous feeling; n = 28, mean ± SE, * p < 0.05 obtained from the Wilcoxon signed-rank test with the Holm method.
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Figure 9. Subjective feelings measured using the Profile of Mood States 2 questionnaire after viewing wooden interior wall images and the gray image. A–H, anger–hostility; C–B, confusion–bewilderment; D–D, depression–dejection; F–I, fatigue–inertia; T–A, tension–anxiety; V–A, vigor–activity; F, friendless; TMD, total mood disturbance. N = 28, mean ± SE, * p < 0.05 obtained from the Wilcoxon signed-rank test with the Holm method.
Figure 9. Subjective feelings measured using the Profile of Mood States 2 questionnaire after viewing wooden interior wall images and the gray image. A–H, anger–hostility; C–B, confusion–bewilderment; D–D, depression–dejection; F–I, fatigue–inertia; T–A, tension–anxiety; V–A, vigor–activity; F, friendless; TMD, total mood disturbance. N = 28, mean ± SE, * p < 0.05 obtained from the Wilcoxon signed-rank test with the Holm method.
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Table
Table 1. Indoor study on the physiological effects of nature via visual stimuli on brain activity (measured with TRS or NIRS) and autonomic nerve activity (assessed using HRV).
Table 1. Indoor study on the physiological effects of nature via visual stimuli on brain activity (measured with TRS or NIRS) and autonomic nerve activity (assessed using HRV).
AuthorsStimulation/ControlStimulation TimeStimulation MethodSummaryParticipants
Brain Activity
(Measured Using TRS or NIRS)		Autonomic Nerve System Activity (Assessed Using HRV)
Left Prefrontal ActivityRight Prefrontal ActivityParasympathetic Nerve ActivitySympathetic Nerve Activity
Ochiai et al.
[33]		Bonsai/without bonsai60 sReal* Decrease−* Increase* DecreaseAdult male patients (spinal cord injury) N = 24
Igarashi et al.
[34]		3D image (water lily)/
2D image (water lily)		90 s3D projector−* Decrease−* DecreaseMale university students N = 19
Song et al.
[35]		Rose flowers/without rose flowers3 minReal−* Decrease−(*) Marginal decrease (p < 0.06)Female university students N = 15
Song et al.
[36]		Forest image/city image90 sDisplay−* Decrease−−Female university students N = 17
Song et al.
[37]		Bonsai/without bonsai60 sReal−−* Increase* DecreaseElderly patients (undergoing rehabilitation) N = 14 (4 men, 10 women)
Park et al.
[38]		Pot with foliage plant/pot without foliage plant3 minReal−−−* DecreaseMale university students N = 24
Modified from Ref. [32]. *: significant difference (vs. control, p < 0.05), (*) marginal difference (vs. control, p < 0.10), −: no significant difference. HRV, heart rate variability; NIRS, near-infrared spectroscopy; TRS, near-infrared time-resolved spectroscopy.
Table
Table 2. Participant information (N = 28).
Table 2. Participant information (N = 28).
ParametersMean Value ± Standard Deviation
Age (years)22.3 ± 2.1
Height (cm)158.8 ± 6.0
Weight (kg)51.9 ± 5.9
Eyesight scores aLeft: 1.0 ± 0.3, right: 1.0 ± 0.2
a The eyesight score includes corrected eyesight (n = 6, naked eyes; n = 8, with eyeglasses; and n = 14, with contact lenses).
Table
Table 3. Physical property values of each image at display projection.
Table 3. Physical property values of each image at display projection.
Knotty WoodClear WoodGray Image
La (cd/m2)11.2 ± 1.311.7 ± 1.211.5 ± 0.3
Evb (lx)7.57.37.6
Tcp (K)385038396330
L: luminance on display, Ev: illuminance at the participant’s eye level. a The average value and standard deviation (mean ± SD) of knotty wood image were calculated using six different points apart from knots with a color meter (CS-100A; Konica Minolta, Tokyo, Japan). This device could measure luminance from a distance similar to photography. In the clear and gray images, six luminance values were measured at the same locations as the knotty wall image, and data were presented as mean ± SD. b It was measured thrice at the participant’s eye level by an illuminance spectrophotometer (CL-500A; Konica Minolta, Tokyo Japan). Then, the average was computed. It was evident that Ev values variation was extremely low (from 7.29 to 7.56 lx).
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Ikei, H.; Nakamura, M.; Miyazaki, Y. Physiological Effects of Visual Stimulation Using Knotty and Clear Wood Images among Young Women. Sustainability 2020, 12, 9898. https://doi.org/10.3390/su12239898

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Ikei H, Nakamura M, Miyazaki Y. Physiological Effects of Visual Stimulation Using Knotty and Clear Wood Images among Young Women. Sustainability. 2020; 12(23):9898. https://doi.org/10.3390/su12239898

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Ikei, Harumi, Masashi Nakamura, and Yoshifumi Miyazaki. 2020. "Physiological Effects of Visual Stimulation Using Knotty and Clear Wood Images among Young Women" Sustainability 12, no. 23: 9898. https://doi.org/10.3390/su12239898

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