Participants
A total of 26 participants (13 males and 13 females) took part in our research. Table 1 summarises the characteristics of the participants. A priori power analysis and a review of the literature[16] indicated that our sample size provided sufficient power (0.8) to detect the effects of interests. Inclusion criteria for participants comprised being between 18 and 65 years old, having a BMI in the range of 18.5 to 24.9, and being able to provide informed content. Exclusion criteria consisted of having a history of psychiatric or neurological conditions, a cardiac illness, health or sensory conditions that might result in skin alterations (e.g., psoriasis), and suffering from claustrophobia. The study was approved both by the ethical committee of the University of Trento and by the Azienda Sanitaria of the province of Bozen, and was conducted in accordance with the Declaration of Helsinki (Fortaleza, 2013). Moreover, all participants gave their informed consent prior to start the experiment.
We administered an online questionnaire to assess participants' eligibility and gather information about relevant psychological aspects. The questionnaire included the following measures:
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Experienced Temperature Sensitivity and Regulation Survey (ETSRS[17]): This survey was used to determine participants' usual temperature preferences and classify them into three groups: those who prefer cold temperatures, those who prefer warm temperatures, and those without specific preferences.
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Empathy Quotient (EQ[18]): The EQ was used to assess participants' level of empathy. Empathy has been shown to affect activity in the somatosensory cortex, which is relevant to the processing of thermal stimuli[19].
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Body Perception Questionnaire (BPQ[20]): The BPQ was used to evaluate participants' level of awareness of their own body's internal states.
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Global Physical Activity Questionnaire (GPAQ[21]): We employed the GPAQ to gather information about participants' typical level of daily physical activity. This was relevant as our task involved periods of movement lasting 15 minutes.
Throughout the experiment, participants wore standardised clothing to ensure consistent skin coverage and equal thermal insulation between their bodies and the environment. Both male and female participants wore long jeans, short-sleeved t-shirts, and closed shoes. To monitor the participants’ skin temperature, we placed four thermal sensors (SHT31 Smart Gadget from Sensirion company) in specific locations on their bodies: the left part of the chest, the right upper arm, the right anterior thigh and the right anterior calf. We based this decision on a previous work of Liu and colleagues[22], who have shown that using four sensors in these sites reliably assesses the mean skin temperature of the body. Additionally, we measured participants’ core temperature at the beginning, middle, and end of the experiment using an infrared laser thermometer aimed at their foreheads (results reported in Supplementary Table S2).
Apparatus
We used four different climate chambers connected through an air-lock (Fig. 1), used as a waiting room before the test and between the different experimental blocks. The temperature in the airlock was monitored throughout the entire test, with a mean value of 22.8°C (see Supplementary Figure S2). The four chambers (3 m x 3 m x 3 m each) were kept at a constant relative humidity equal to 45% while the temperatures were oscillating between 23°C and 25°C as shown in Fig. 2. The temperature range was chosen in the comfort range to prevent some possible confounds that could emerge from an uncomfortable condition. We defined these temperatures as comfortable by looking at the ASHRAE standards (ANSI/ASHRAE, 2017) and by conducting a pilot. In order to evaluate the thermal stratification during the experiment, three PT100 temperature sensors (uncertainty of ± 0.16°C, k = 2) were placed at three different heights corresponding to the head, the arm and the calf of the participant. The effect of the thermal stratification is shown in Figure S1. We chose as temperature reference the average temperature measured at the head and arm level given the exposed participant’s skin due to the clothing. During the whole experimental campaign, the chambers’ temperature pattern was kept the same (Fig. 2), while each participant’s shift pattern between the chambers was randomised. Our aim was to prevent participants from developing a cognitive schema for detecting the temperature pattern and relying on it to make judgments about temperature differences between the chambers. To achieve this, we designed the temperature fluctuations to follow a complex pattern that would be difficult for participants to discern, thereby forcing them to rely solely on their sensory experience to make temperature judgments. This approach minimised the risk of bias in participant responses and ensured that the data collected was an accurate reflection of their perceptual experiences.
Procedure
Throughout the experiment, participants transitioned between different climate chambers and compared the temperature of the target chamber (the second they moved in) to that of the reference chamber (the first one they entered). We maintained stable communication with the participants using walkie-talkies, and we monitored the entire process using five video cameras. Participants were allowed to spend 5 seconds in each chamber (both reference and target) to perceive and assess the temperature difference. Upon leaving the target chamber, participants provided their response in the airlock area. Each experimental block consisted of 24 temperature comparisons and lasted approximately 15 minutes. There were a total of 5 experimental blocks, with 5-minute breaks between them to allow participants to rest. Overall, the entire procedure took 100 minutes to complete, involving a total of 120 temperature comparisons.
Analysis
Figure 3 reports the example of one experimental block with the temperature variations inside the four climate chambers and the 24 comparisons the participant made together with the answers. For each comparison, we measured the temperature difference between the target chamber (red triangle), i.e. the second chamber participant entered, and the reference chamber (black triangle), i.e. the first chamber participant entered. Then, we compared each temperature difference with the participant’s answer, where 1 means “warmer” while 0 means “colder”. We grouped the data obtaining a final dataset of 3120 observations in 3 variables (subject, temperature difference and answer).
Next, we fitted two generalised linear mixed-effects models[23] using the function glmer inside RStudio (version 2022.02.0 + 443). Given that we computed repetitively measures on single participants, we decided to analyse our data using generalised linear mixed-effects models that better take into account such source of variability[24]. The first model we computed contained the number of colder and warmer answers and the differences in temperature as fixed effects and the subjects ID as random effect (glmm0). The second model added to the previous one the difference in temperature as random effect (glmm1) to see whether this aspect of our experiment brought variability in the data. Then we compared the two models through means of an ANOVA and looked at which model had the smallest AIC criterion. Finally, we computed the marginal and conditional R2 of the best model to analyse the percentage of variance explained respectively by the fixed effects only and by the fixed effects plus the random ones.
Then, we extracted the average Point of Subjective Equality (PSE) and the Just Noticeable Difference (JND) using the MixedPsy R package[25]. Besides these data, we were also interested in the absolute thresholds, i.e. the minimum value needed to correctly identify the difference in the temperature 100% of the time. To obtain this data, for each participant we looked at the positive difference in temperature above which all answers were “warmer” (positive absolute threshold) and the negative difference in temperature below which all answers were “colder” (negative absolute threshold). Finally, we obtained the general JND and absolute thresholds by averaging these data for all the participants.