We investigated the physiological and psychological responses to touching wood and other materials in young male and female participants to compare wood/non-wood, hardwood/softwood, and coated/uncoated wood. We compared the relationships between heat transfer, psychological responses, and physiological changes, as well as the potential differences between the male and female respondents. Through a preliminary investigation, a round-shaped bar was chosen as specimen to ensure that the entire hand was in contact with the material. Consequently, the participants were instructed to hold the specimens instead of only touching them, as in previous studies.
The overall trend in changes in physiological indices over time was similar among the six materials. Blood pressure and pulse rate exhibited an initial increase and a subsequent decrease, and then the blood pressure gradually recovered or increased whereas the pulse rate remained below the baseline. Increased blood pressure and decreased pulse rate induced by local or whole-body cooling have been well documented by various previous studies, including those related to the so-called hunting reaction [8]. One study reported that immersing a hand in water at 5 °C increased the systolic blood pressure by 10–15 mmHg, and it caused an initial increase and a subsequent decrease in heart rate within 2 min in male and female participants [9]. These results were supported by various studies that demonstrated time-dependent blood pressure and heart rate changes during hand immersion in water at 5 °C or 2 °C [10, 11]. As previously explained, we assume that the initial rise in blood pressure and pulse rate upon contact in this study could be attributed to a sympathetic activation triggered by the cold sensation owing to the temperature difference between the palm (approximately 37 °C) and the grab bars (approximately 25 °C) [11, 12]. The subsequent drop in the pulse rate may be mainly caused by baroreflex to compensate for the rise in blood pressure [13], although other factors such as a parallel activation of both sympathetic and parasympathetic nervous systems may also be responsible [12].
The simultaneous decrease in oxy- and deoxy-hemoglobin concentrations upon contact in this study can be because of the decrease in the blood volume in the measured area [14]. Cerebral blood flow is maintained via autoregulation, which is a process for maintaining adequate and stable blood flow while arterial pressure changes [15]. A recent study suggested that cerebral autoregulation is achieved up to 62% by three different physiological mechanisms, that is, sympathetic, cholinergic (parasympathetic), and myogenic components combined. Generally, neurogenic (sympathetic and parasympathetic) components cause vasoconstriction in counteracting a rise in arterial pressure. In addition, cerebral autoregulation has been reported to be effective for a slower change in blood pressure (> 30 s), whereas it is less so during faster changes [16, 17]. Based on the previous findings, the changes in cerebral blood flow in this study may be explained as follows: the fastest parasympathetic and the subsequent sympathetic reflexes counteracted the fast increase in blood pressure, thus resulting in vasoconstriction that was reflected in the decreased oxy- and deoxy-hemoglobin signals of the NIRS. The following increase in oxy-hemoglobin concentration can be attributed to autoregulation becoming effective in a delayed manner.
Although speculative, the above interpretations of the physiological mechanism underlying the time-course responses are partly supported by the correlation analysis. We demonstrated that the heat transfer and the subjective warmth exhibited a significant correlation with blood pressure, but not with the pulse rate and brain hemoglobin concentrations. On the other hand, the pulse rate and brain hemoglobin concentrations significantly correlated with blood pressure. These relations support the interpretation that emphasizes the effect of physiological reflex responses on changes of the blood pressure, pulse rate and brain blood flow. The relation to perceived comfort was not negligible in blood pressure and pulse rate but it was less dominant in the brain blood flow in the results in this study.
The smaller change in SBP during the first 30 s when contacting coated and uncoated Japanese cypress and uncoated Japanese oak indicates that physiological stress was smaller when touching these materials. The lower minimum SBP in these three materials may be attributed to the relatively smaller increase in SBP by the contact (the lower maximum SBP), which also indicates lower physiological stress. On the other hand, the lowest maximum PR in aluminum can be a result of the physiological reflex to a larger increase in SBP and DBP when touching this material. To summarize, the changes in blood pressure and pulse rate simultaneously suggest that touching the wood, particularly Japanese cypress or uncoated Japanese oak induces less physiological stress than touching aluminum.
In the subjective ratings, wooden materials were rated to be significantly warmer compared to the non-wood materials, that is, aluminum and polyethylene, regardless of the species or surface coating, which corresponds to a smaller amount of heat transfer in the wooden materials. Moreover, the subjective comfort perception was generally higher when touching the wooden materials than when touching aluminum, except that the difference did not reach statistical significance in the case of the uncoated Japanese oak (p = 0.074). Compared to aluminum, polyethylene was evaluated to be more comfortable and almost as comfortable as the wooden specimens. The correlation analysis suggests that although perceived comfort partly relates to the amount of heat transfer or perceived warmth, it might be influenced by other factors. Such factors may include naturalness, smoothness, and dryness of the surface as suggested by previous studies [18, 19].
In this study, the effect of species on the amount of heat transfer was significant. This can be attributed to the lower thermal conductivity of the Japanese cypress than that of the Japanese oak, as heat flux is known to correlate with thermal conductivity positively [20], which is related to the density of wood [21]. Conversely, the effect of surface coating on the heat transfer was not significant. This may be because the layer thickness of the coating employed in this study was thin enough to neglect compared to the diameter of the grab bars. Therefore, the thermal properties of the coated specimens can be considered to be equal to that of the wood. Although not evident in the heat transfer, the surface coating slightly influenced the subjective ratings of both species. The perceived warmth of the unpainted Japanese cypress was rated highest, and only the unpainted cypress showed significant differences between the hardwood specimens. The coated oak was evaluated to be significantly more comfortable compared to the non-wooden bars, whereas the uncoated oak was not. These perceptions may be affected by thermal or other properties arising from the presence or absence of surface coating, which are not reflected in the amount of heat transfer.
The difference between the female and the male participants was significant in the score of subjective comfort and the maximum pulse rate: female participants generally gave higher comfort scores to all the materials and exhibited lower maximum pulse rate. Although not statistically significant, the p-values were relatively small in some of the hemoglobin indices. Therefore, gender differences in both physiological and psychological responses need to be carefully investigated in further studies, considering the fact that gender differences have been reported in both thermal perception [22, 23] and thermoregulation [24].
This study demonstrated that even a mild local cold could cause similar, although smaller, physiological changes that were previously shown by more severe cold exposures. Part of the physiological mechanisms underlying the observed response was speculated based on the correlation analysis among the physiological indices. However, as the correlation coefficients we obtained were relatively small, further investigations are required to understand the interactions among different physiological functions entirely. There should be other factors that affect physiological changes, including individual differences at various levels. More precise investigations with other related physiological indices such as peripheral (finger) blood flow and temperature, cardiac output, or sympathetic and parasympathetic nervous activities are needed.
The number of participants in this study was relatively larger than previous studies that dealt with physiological responses to various sensory simulations originating from wood. However, it still may not be sufficient to fully discuss the influence of gender and other individual differences. Since the development of measurement devices is rapidly progressing, physiological experiments may become more accessible and easier to implement in the future. Further studies can attempt to accumulate physiological and psychological data with a group of larger sample sizes and scrutinize the influence of gender, age, and other factors more precisely.
