Tracking changes in touch desire and touch avoidance before and after the COVID-19 outbreak.

Touch is essential for survival, social bonding, and overall health. However, the COVID-19 pandemic calls for an abrupt withdrawal from physical contact, and the prolonged lockdown has left many people in solitude without touch for months. This unprecedented dissociation from touch has cast a shadow on people's mental and physical well-being. Here we approached the issue by examining COVID-19's impact on people's touch attitudes. We analyzed people's desire and avoidance for animate and inanimate targets based on large-scale Japanese Twitter posts over an 8-year span. We analyzed the impact of the COVID-19 outbreak with the difference-in-differences estimation method, which can estimate the impact while accounting for other changes over time such as seasonality or long-term effects. As a result, we found that people's desire for touching the human body and pet animals increased significantly after the COVID-19 outbreak and remained high afterward. In contrast, the avoidance of touching everyday objects (e.g., doorknobs and money) increased immediately after the outbreak but gradually returned to the pre-COVID-19 levels. Our findings manifest the impact of COVID-19 on human touch behavior. Most importantly, they highlight the sign of "skin hunger," a public health crisis due to social distancing, and call attention to the trend that people are becoming less aware of infection control as COVID-19 persists.

The Effect of Temperature on Tactile Softness Perception.

We are adept at discriminating object properties such as softness and temperature using touch. Previous studies have investigated the nature of each object property, but the interactions between these properties are not fully understood. Tactile softness perception relies on multiple sensory cues such as the size of the contact area, indentation depth, and force exerted. In addition to these cues, the temperature of the stimulus may contribute to tactile softness perception by changing the sensitivity to changes in stimulus compliance. To test this hypothesis, we conducted two psychophysical experiments in which the subjects estimated the magnitude of perceived softness after touching deformable objects. We varied the compliance and temperature of the stimuli. The linear functions of compliance fit to the magnitude estimates under cold conditions (9-15°C) were steeper than the functions fit to the magnitude estimates under room temperature (21-25°C). These results indicate that temperature can sharpen our tactile softness perception of deformable surfaces by increasing the sensitivity to differences in compliance.

Thermal-Tactile Integration in Object Temperature Perception.

The brain consistently faces a challenge of whether and how to combine the available information sources to estimate the properties of an object explored by hand. While object perception is an inference process involving multisensory inputs, thermal referral (TR) is an illusion demonstrating how the interaction between thermal and tactile systems can lead to deviations from physical reality-when observers touch three stimulators simultaneously with the middle three fingers of one hand but only the outer two stimulators are heated (or cooled), thermal uniformity is perceived across three fingers. Here, we used TR of warmth to examine the thermal-tactile interaction in object temperature perception. We show that TR is consistent with precision-weighted averaging of thermal sensation across tactile locations. Furthermore, we show that prolonged contact with TR stimulation results in adaptation to the local variations of veridical temperatures instead of the thermal uniformity perceived across three fingers. Our results illuminate the flexibility of processing that underlies thermal-tactile interactions and serve as a basis for thermal display design.

Material recognition based on thermal cues: Mechanisms and applications.

Some materials feel colder to the touch than others, and we can use this difference in perceived coldness for material recognition. This review focuses on the mechanisms underlying material recognition based on thermal cues. It provides an overview of the physical, perceptual, and cognitive processes involved in material recognition. It also describes engineering domains in which material recognition based on thermal cues have been applied. This includes haptic interfaces that seek to reproduce the sensations associated with contact in virtual environments and tactile sensors aim for automatic material recognition. The review concludes by considering the contributions of this line of research in both science and engineering.

Perceptual-Semantic Congruency Facilitates Semantic Discrimination of Thermal Qualities.

The ability to sense temperature is vital to our life. It signals the environmental condition, reflects the physiological conditions of our own body, and generates feelings of pleasantness or unpleasantness. Moreover, recent studies have demonstrated implicit associations between physical temperature and social/emotional concepts, suggesting the processing of temperature may even influence cognition. In this work, we examined the effect of physical warmth and coldness on semantic cognition. Participants performed speeded target categorization for thermal descriptors in the form of semantic words or illustrative figures representing the thermal qualities "warm" or "cold" while physical thermal stimulation was presented. We compared the average reaction time (RT) for the congruent and incongruent conditions managed by response key assignments. In the congruent condition, the response key for the symbol associated with warmth (coldness) was assigned to the hand with warm (cold) thermal stimulation, and in the incongruent condition the key assignment was reversed. Our results demonstrate that the average RT in the congruent condition was faster than in the incongruent one for both forms of thermal descriptors, suggesting that the experience of physical temperature facilitates the internal processing of the meaning of thermal quality.

Physical-Perceptual Correspondence for Dynamic Thermal Stimulation.

Thermal displays have been applied in various haptic applications, from material simulation to interpersonal communication; however, there is insufficient knowledge about the temporal processing in human thermal sense to provide a　knowledge basis for thermal display design. In this study, we investigated the physical-perceptual correspondence for dynamic thermal stimulation to shed a light on the temporal processing of human thermal sense. In the experiments, participants reported subjective timings of the temperature onset and temperature peak of continuous temperature changes applied to the thenar eminence. We found that the physical-perceptual correspondence was not consistent for warm and cold stimulations. For warm stimulation, the subjective experience always came after the corresponding physical event. On the other hand, for cold stimulation, while the subjective onset always lagged the physical onset, the subjective temperature peak preceded the physical temperature peak. We analyzed these results in the framework of linear systems theory. The results suggest that the senses of warmth and cold have distinct temporal filtering properties, with the sense of cold being more transient than the sense of warmth. These findings advance our knowledge regarding temporal processing in human thermal sense and serve as a basis for thermal display design.

The pleasant heat? Evidence for thermal-emotional implicit associations occurring with semantic and physical thermal stimulation.

The association between thermal and emotional experiences in interpersonal relations is intuitively apparent and has been confirmed by previous studies. However, research has not yet elucidated whether such an association is grounded in mental processes occurring at an intrapersonal (internal) level. In two experiments we examined whether the thermal-emotional associations can be observed at an intrapersonal level. We looked at the speed and accuracy of stimuli categorization. Experiment 1 examined the implicit semantic association between temperature (warm versus cold) and emotional valence (positive versus negative). Experiment 2 examined the association between experience of physical temperature and emotional valence. In both experiments warm-positive/cold-negative associations were demonstrated. These results suggest a conceptual and perceptual mapping in the mental representation of emotion and temperature, which occurs at an intrapersonal level, and which might serve as the ground to the interpersonal thermal-emotional interactions.

Combining colour and temperature: A blue object is more likely to be judged as warm than a red object.

It is commonly believed that reddish colour induces warm feelings while bluish colour induces cold feelings. We, however, demonstrate an opposite effect when the temperature information is acquired by direct touch. Experiment 1 found that a red object, relative to a blue object, raises the lowest temperature required for an object to feel warm, indicating that a blue object is more likely to be judged as warm than a red object of the same physical temperature. Experiment 2 showed that hand colour also affects temperature judgment, with the direction of the effect opposite to object colours. This study provides the first demonstration that colour can modulate temperature judgments when the temperature information is acquired by direct touch. The effects apparently oppose the common conception of red-hot/blue-cold association. We interpret this phenomenon in terms of "Anti-Bayesian" integration, which suggests that the brain integrates direct temperature input with prior expectations about temperature relationship between object and hand in a way that emphasizes the contrast between the two.

Colour-temperature correspondences: when reactions to thermal stimuli are influenced by colour.

In our daily lives, information concerning temperature is often provided by means of colour cues, with red typically being associated with warm/hot, and blue with cold. While such correspondences have been known about for many years, they have primarily been studied using subjective report measures. Here we examined this correspondence using two more objective response measures. First, we used the Implicit Association Test (IAT), a test designed to assess the strength of automatic associations between different concepts in a given individual. Second, we used a priming task that involved speeded target discrimination in order to assess whether priming colour or thermal information could invoke the crossmodal association. The results of the IAT confirmed that the association exists at the level of response selection, thus indicating that a participant's responses to colour or thermal stimuli are influenced by the colour-temperature correspondence. The results of the priming experiment revealed that priming a colour affected thermal discrimination reaction times (RTs), but thermal cues did not influence colour discrimination responses. These results may therefore provide important clues as to the level of processing at which such colour-temperature correspondences are represented.

Mechanisms underlying referral of thermal sensations to sites of tactile stimulation.

When three stimulators are simultaneously touched with the middle three fingers of one hand but only the outer two stimulators are cooled or heated, the central (neutral) stimulator is also perceived to be cold or warm. This phenomenon is known as thermal referral and it shares phenomenological similarities with filling-in, in which the discontinuity in the signals of interest can be compensated perceptually on the basis of the spatially adjacent context. Although the mechanisms underlying filling-in have been well substantiated, those underlying thermal referral are still poorly understood. In the present study, we examined the intensity perception of the sensation resulting from thermal referral with human participants. We found that the sensation was uniform among the three fingers, but its apparent intensity was always lower than the physical intensity applied to the outer two fingers. These results indicate that the thermal uniformity perceived under thermal referral is not created by the brain's interpolating the thermal changes applied to the outer two fingers, as one would expect for those induced by typical filling-in. Instead, the thermal changes applied to the outer two fingers are summated and redistributed to all the fingers in contact. Our findings suggest that thermal referral is mediated by two separate processes. One determines the apparent intensity from the physical intensity and the areal extent of the thermal stimulation; the other determines the localization of the resulting sensation from the apparent sites of tactile stimulation.

Somatotopic or spatiotopic? Frame of reference for localizing thermal sensations under thermo-tactile interactions.

The thermal sense is diffuse and incapable of providing precise spatial information. From a phenomenon known as thermal referral, we know that touch influences the localization of cold or warmth, leading to our perceiving illusory thermal sensations at a thermally neutral site. This study investigated the frame of reference for localizing thermal sensations under thermal referral in order to shed light on how thermal and tactile modalities coordinate to process localization information. One thermally neutral tactile stimulator and two cold (warm) stimulators were presented to different sets of three fingers of both hands. The location of the neutral tactile stimulator varied, and the strength of the referral was estimated from participants' performance in localizing the neutral tactile stimulator. By manipulating the somatotopic and spatiotopic distances between the stimulated sites, we found that the somatotopic distance--more specifically, the distance between and among the sites being defined in cortical topography--determines the strength of thermal referral. Our findings suggest that localization of thermal sensations under thermo-tactile interactions is processed with respect to the somatotopic frame of reference and that this cross-modal processing resides in early cortical areas whose organization conserves topographic information.

Modeling the thermal responses of the skin surface during hand-object interactions.

The objective of this research is to analyze and model the decreases in skin temperature when the hand makes contact with an object at room temperature so that thermal feedback can be incorporated into haptic displays. A thermal model is proposed that predicts the thermal responses of the skin and object surface as well as the heat flux exchanged during hand-object interactions. The model was evaluated by comparing the theoretical predictions of temperature changes to those experimentally measured using an infrared thermal measurement system. The thermal measurement system was designed to overcome the limitations imposed by contact thermal sensors, and was able to measure skin temperature during contact, together with the contact area and contact force. The experimental results indicated that over the pressure range of 0.73-10.98 kPa, changes in skin temperature were well localized to the contact area and were affected by contact pressure. The pressure in turn influenced both thermal contact resistance and blood flow. Over the range of contact forces typically used in manual exploration, blood perfusion and metabolic heat generation do not appear to have a significant effect on the skin's thermal responses. The theoretical predictions and the measured data were consistent in characterizing the time course and amplitude of the skin temperature change during contact with differences typically being less than 1 degrees C between the two for pressures greater than 4 kPa. These findings indicate that the proposed thermal model is able to characterize and predict the skin temperature responses during hand-object interactions and could be used in a thermal display that simulates the properties of different materials.

Contribution of thermal cues to material discrimination and localization.

The objective of these two experiments was to determine the role of thermal cues in material discrimination and localization, using materials that spanned a range of thermal properties. In the first experiment, the subjects were required to select the cooler of two materials presented to the index fingers. In the second, the finger that was in contact with a material that was different from that presented to the other two fingers on the same hand had to be identified. The results indicated that the subjects were able to discriminate between materials, using thermal cues, when the differences in their thermal properties were large. The changes in skin temperature when the fingers were touching the materials were, however, smaller than those predicted by the theoretical model. The ability to localize the thermal changes when three fingers on the same hand were stimulated was poor and depended on both the thermal properties of the target and the distractor materials.