The experiment was conducted in a chamber with an artificial climate maintained at 24 °C and 50% relative humidity. In total, 21 Japanese female university students (mean age, 22.0 ± 2.0 years) participated. During the study period, we excluded smokers, people currently receiving treatment for diseases, and menstruating women. This study was conducted in compliance with the Ethical Guidelines for Medical and Health Research Involving Human Subjects of Japan under the Declaration of Helsinki. The protocol was approved by the Ethics Committee of the Center for Environment, Health and Field Sciences, Chiba University, Japan (Project Identification Code Number: 32), and was registered in the University Hospital Medical Information Network of Japan (Unique ID issued by UMIN: UMIN000029089). All participants gave written informed consent in accordance with the Declaration of Helsinki.
Sugi grown in Chiba Prefecture, Japan, was used. Four laminae without vertical joints (300 × 75 × 15 mm) were bonded together along their widths. To prevent bending, a second bonding was made using plywood (300 × 300 × 28 mm). The total thickness of the resulting product was 43 mm. The surface was uncoated and finished via: (1) brushing using a stainless steel wire brush (hereafter referred to as “brushed sugi”); or (2) sanding using an abrasive-band machine (hereafter referred to as “sanded sugi”). In our previous studies [5,6,7], we showed that physiological relaxation effects could be acquired by contact between the palm of the right hand and hinoki or white oak wood with an uneven surface due to a brushed finish compared with contact with building materials such as stainless steel, tiles, or wood with a thick coating. However, the wood typically used for desks, furniture, and floors is usually sanded to a smooth finish. In this study, therefore, we used sugi, which is a typical softwood in Japan, with two types of finish: the brushed finish used in previous studies that confirmed the induction of physiological relaxation effects and a more typical sanded finish that has not yet been studied for its physiological effects. Marble (300 × 300 × 15 mm) was used as a comparator material because it is a building material that has properties substantially different from wood. The marble was bonded to plywood of the same size as that bonded to sugi. All materials were maintained at room temperature. The thermal conductivity and arithmetic average roughness results of the various sample types were as follows: brushed sugi, 0.121 W/(mK) and 101.56 μm, respectively; sanded sugi 0.102 W/(mK) and 6.03 μm, respectively; and marble 0.151 W/(mK) and 0.66 μm, respectively. The conditions for the physical measurement followed those as previously described . Figure 1 presents the procedure of the entire experiment. The materials were presented in a counterbalanced order to eliminate any effects due to the order.
TRS was employed to assess prefrontal cortex activity. Changes of the oxy-Hb concentrations in the left and right prefrontal cortices as measured using TRS (TRS-20, Hamamatsu Photonics K.K., Shizuoka, Japan) reflect prefrontal activity [10,11,12]. In addition, post-measurement values were calculated in reference to pre-measurement values.
HRV was employed to assess autonomic nervous activity. R–R intervals were measured using a portable electrocardiograph (Activtracer AC-301A, GMS, Tokyo, Japan) [13, 14]. The power levels of the high-frequency (HF, 0.15–0.40 Hz) and low-frequency (LF, 0.04–0.15 Hz) components of HRV were calculated using the maximum entropy method (MemCalc/Win, GMS, Tokyo, Japan) [15, 16]. The HF component reflects parasympathetic nervous activity, and the LF/(LF + HF) ratio reflects sympathetic nervous activity . To normalize HRV parameters across participants, natural logarithmic-transformed values were used in the analysis . The HRV power spectra were also used to estimate the participant’s respiratory frequency . Respiratory changes influence HRV data, with heart rate generally increasing during inspiration and decreasing during expiration [20, 21]; thus, the respiratory rate can be estimated from the dominant frequency of the HF component, found by detecting the maximal power of the HF component. Participants with at least one case where the respiratory frequency could not be estimated because the power spectrum did not show a clear peak in the HF band (0.15–0.40 Hz) were excluded from HRV analysis. As a result, 3 participants were excluded from analysis and mean respiratory frequency of the remaining 18 participants was not significantly different between the conditions.
The modified semantic differential (SD) method  and the short version of the Profile of Mood States Second Edition (POMS 2) [23,24,25] were used to evaluate the psychological effects of touching the materials. In modified SD methods, seven pairs of adjectives were assessed: “comfortable–uncomfortable”; “relaxed–awakening”; “natural–artificial”; “warm–cold”; “uneven–flat”; “dry–moist”; and “soft–hard”.
Statistical Package for Social Sciences software (v21.0, IBM Corp., Armonk, NY, USA) was used for all statistical analyses. Paired t tests with Holm correction  were used to compare physiological responses, and Wilcoxon signed-rank tests with Holm correction were applied to analyze the differences in psychological indices. In all analyses, p < 0.05 indicated statistical significance. One-sided tests were used for both comparisons because our hypothesis was that humans would be more relaxed after touching the wood than after touching the other material (marble).