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. 2022 May 30;19(11):6672.
doi: 10.3390/ijerph19116672.

Effect of Indoor Forest Bathing on Reducing Feelings of Fatigue Using Cerebral Activity as an Indicator

Chie Imamura  1 , Kiyomi Sakakibara  1 , Kyosuke Arai  2 , Hideki Ohira  3 , Yuhei Yamaguchi  2 , Hitoshi Yamada  2
Affiliations

Effect of Indoor Forest Bathing on Reducing Feelings of Fatigue Using Cerebral Activity as an Indicator

Chie Imamura et al. Int J Environ Res Public Health. .

Abstract

We created an indoor forest bathing environment in a sunlight-type environmentally controlled chamber and both physiological and psychological measurements were conducted for the evaluation of mental fatigue reduction. At first, a working memory load experiment was performed among 10 participants in a space without plants to identify an indicator correlating with feelings of fatigue, using the cerebral activity of the prefrontal cortex. Then, the indicator was used to evaluate whether a 20-min exposure to an indoor forest bathing environment reduced the level of the feeling of fatigue. The working memory load experiment demonstrated that, when mental fatigue increased, the amount of oxygenated hemoglobin (oxy-Hb) in the right prefrontal cortex and the right-left difference in oxy-Hb (ΔRL oxy-Hb) in the prefrontal cortex increased. These were proposed as indicators of mental fatigue. In the indoor forest bathing experiment, staying in an indoor green space showed that the subjective values of feeling of fatigue decreased and ΔRL oxy-Hb decreased. Since these results demonstrated an opposite effect to the increase in ΔRL oxy-Hb related to the feeling of fatigue, it was inferred that the decrease in ΔRL oxy-Hb reflected the fatigue reduction in the indoor forest bathing environment.

Keywords: biophilic; brain activity; indoor forest bathing; near-infrared spectroscopy; prefrontal cortex activity; reduction of feeling of fatigue.

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Conflict of interest statement

The authors declare no conflict of interest. Author CI and KS were employed by Toyota Central R&D Labs., Inc., and KA, YY, and HY were employed by Toyota Motor Corporation. The remaining author declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
Study protocol. TRS, near-infrared time-resolved spectroscopy; HRV, heart rate variability; PVT, psychomotor vigilance task.
Figure 2
Figure 2
Experimental procedure. (a) Scene during staying spaces. (b) State at the time of measurement. (c) Study protocol. TRS, time-resolved spectroscopy; VR, visual reality; HRV, heart rate variability; GT, test space; Room Control, space without natural green features; Room A/B/C, space with natural green features.
Figure 3
Figure 3
Change in subjective feeling and performance before and after the fatigue-inducing task. Data are expressed as mean ± standard errors (n = 10). * p < 0.05, † p < 0.1. PVT, psychomotor vigilance task; VAS, visual analog scale; RAS, Roken Arousal Scale.
Figure 4
Figure 4
Correlation analysis of the visual analog scale (VAS) of the feeling of fatigue and near-infrared time-resolved spectroscopy (TRS) data. (a) Correlation coefficient and (b) scatterplot. † p < 0.1; * p < 0.05. NS, not significant; Hb, hemoglobin; oxy-Hb, oxygenated hemoglobin; deoxy-Hb, deoxygenated hemoglobin; tHb, total hemoglobin; StO2, tissue oxygen saturation; L, left prefrontal cortex; R, right prefrontal cortex; R/L ratio, right-left ratio; ΔRL, right-left difference.
Figure 5
Figure 5
Change in the visual analog scale value of the feeling of fatigue in the prefrontal cortex during the experiment (rest 1, VR, GT, rest 2). Data are expressed as mean ± standard errors (n = 7). VR, visual reality; GT, test space; Room Control, space without natural green features; Room A/B/C, space with natural green features.
Figure 6
Figure 6
Correlation analysis of feeling of fatigue and time-resolved near-infrared time-resolved spectroscopy data after staying in the experimental spaces. ΔRL oxy-Hb, right-left difference in oxygenated hemoglobin.
Figure 7
Figure 7
Change in the right-left difference in oxygenated hemoglobin (ΔRL oxy-Hb) in the prefrontal cortex during the experiment (rest 1, GT, rest 2), every 5 s. The vertical axis shows the difference value from 0 to 1-min means of rest 1. Data are expressed as means (n = 7). VR, visual reality; GT, test space; Room Control, space without natural green features; Room A/B/C, space with natural green features.
Figure 8
Figure 8
Change in the left-right difference value of the oxygenated hemoglobin (oxy-Hb) in the prefrontal cortex during the experiment (rest 1, GT, rest 2), every 5 min. The vertical axis shows the difference value from rest 1. (a) Comparison of rest 1 and the initial 5 min in GT; (b) comparison of Room Control and three indoor green spaces, and (c) comparison of the last 5 min of GT and rest 2. Data are expressed as mean ± standard errors (n = 7). ** p < 0.01, * p < 0.05, † p < 0.1. VR, visual reality; GT, test space; Room Control, space without natural green features; Room A/B/C, space with natural green features.
Figure 9
Figure 9
The mean values of the slope for oxygenated hemoglobin in 1-min periods immediately after the start of rest 1, GT, and rest 2 sessions. (a) Left prefrontal cortex and (b) right prefrontal cortex. Data are expressed as mean ± standard errors (n = 7). GT, test space; Room Control, space without natural green features; Room A/B/C, space with natural green features.
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