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Thermal perception and thermal devices used on body parts other than hand or face Citation for published version (APA): Kappers, A., & Plaisier, M. (2019). Thermal perception and thermal devices used on body parts other than hand or face. IEEE Transactions on Haptics, 12(4), 386-399. [8747382]. https://doi.org/10.1109/TOH.2019.2925339 DOI: 10.1109/TOH.2019.2925339 Document status and date: Published: 01/10/2019 Document Version: Accepted manuscript including changes made at the peer-review stage Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim. Download date: 30. Sep. 2021 This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TOH.2019.2925339, IEEE Transactions on Haptics 1 Thermal Perception and Thermal Devices used on Body Parts other than Hand or Face Astrid M.L. Kappers and Myrthe A. Plaisier Abstract—Most fundamental research on thermal perception focuses on the fingers or the hand. Also most existing and proposed thermal devices are meant to be applied to hand or fingers. However, if the hands are needed for other tasks, application of thermal stimulation to other body regions should be considered. This paper surveys the literature on thermal perception and thermal devices relevant to such other body regions. It starts with a short description of the experimental methods used in the various studies, such as the methods of limits, the two-alternative forced choice method, and magnitude estimation. This is followed by thermal psychophysical studies on detection, adaptation, spatial summation and resolution. Next some striking thermal illusions are presented, such as a thermal grill and a seemingly continuously warming or cooling stimulus. Finally, the few studies on thermal communication and applications are summarized. These latter studies mainly focus on communicating emotions or playing computer games. The overall conclusion of this survey is that thermal devices should not focus on conveying complex messages, but especially in the areas of gaming or communication there seem to be interesting possibilities for further developments. Index Terms—Overview, thermal stimulation, psychophysics, devices, body F 1 INTRODUCTION HE majority of studies on thermal perception and ther- sensitivity variation over the human body surface. Finally, T mal devices have made use of stimulation on the hand, there are many different experimental methods to assess (e.g., [1]). This needs not be surprising as hands are easily psychophysical performance of individuals, but it is very accessible and flexible in reaching a stimulus. Moreover important to realize that different methods will lead to dif- hands and fingers are more sensitive to thermal stimulation ferent outcomes; a threshold measured via a two-alternative than most other body regions, although lips, cheek and forced choice method will not necessarily be the same as a forehead are even more sensitive [2]. However, if a thermal threshold obtained via the method of limits (see below for device is meant to be used as, for example, a feedback, an explanation of these methods). alerting or communication system, it might be desirable The studies discussed in this overview are all published to apply the stimulation on other body parts in order to in peer-reviewed scientific journals or conference proceed- keep the hands free for other tasks. In the special case of ings, so this excludes devices that might be on the market devices designed for use in daily life by individuals with without being published. All references of the included impaired vision, it is also undesirable to apply stimula- papers are checked for further relevant studies. In addition, tion to the face as this would involve some clearly visible papers that cite the included studies have also been checked apparatus connected to the head. The goal of the present for relevance. In several of the studies described below also paper is to give an extensive overview of studies on thermal thermal stimulation on hands or face has been studied in perception on other body parts than the hands or the face. addition to other body regions, but these results will be The studies included in this overview range from rigorous omitted from this overview. psychophysics to rather informal pilot studies of thermal As it is important to have some basic understanding devices. about the psychophysical methodology used in the various When comparing the results of various studies, many studies, this overview will start with a brief introduction issues have to be taken into account. Age and gender of of relevant terms and methods. Next, studies on thermal different populations of participants may directly influence detection, adaptation, summation and resolution will be the results [2]. In the present case of thermal stimulation, presented. As thermal perception is also susceptible to illu- the physical parameters of the stimulus such as contact sions, a separate section is devoted to illusions. Finally, more size, rate of change of temperature, and baseline temper- applied research on thermal communication and other ap- ature are all important and may have huge influences on plications will be presented. In the conclusion, the important the results. Also the body region stimulated is of major factors to take into account when designing applications importance, as Stevens and Choo [2] showed a 100-fold with thermal feedback are discussed. • A.M.L. Kappers and M.A. Plaisier are both with group Dynamics 2 METHODS AND TERMS USED IN THE STUDIES and Control of Eindhoven University of Technology, The Netherlands. A.M.L. Kappers is also with the groups Control Systems Technology and Perceptual performance of humans is often expressed in Human Technology Interaction of the same university. terms of thresholds. A threshold indicates the amount or E-mail: [email protected] intensity of a stimulus needed for the stimulus to be con- Manuscript received ??; revised ??. sciously perceived. Often the threshold is more precisely 1939-1412 (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TOH.2019.2925339, IEEE Transactions on Haptics 2 a difference threshold, which is the minimum change in in- and 100, are fixed by the experimenter. There exist many tensity needed to perceive the stimulus as different from variations of this method, all with their own advantages a reference stimulus. For thermal perception the threshold and disadvantages. indicates the minimal difference in temperature (either heat- Finally, some of the studies make use of scoring on a ing or cooling) from a baseline temperature that is noticed Likert scale. Participants have to score a property in more by a participant. The threshold is also termed just-noticeable psychological terms like dislike–like, not comfortable–very difference or JND. comfortable, etc. Such scales are typically used in question- Thresholds can be measured in different ways and the naires after an experiment. outcome will depend on the method. A method often used There are many books and papers that explain the above- for thermal thresholds is the method of limits (MoL). Starting mentioned terms and methods in much more detail. A from a baseline temperature (usually close to skin tempera- selection can be found here: [4], [5], [6], [7], [8]. Thermal ture), the temperature of the stimulus increases or decreases physiology and thermal receptors lie outside the scope of in small steps until the participant indicates that s/he no- this survey, but readers interested in these topics can find a ticed a change in temperature. Usually, this procedure is selection of papers here: [9], [10], [11]. repeated several times and the average end temperature is defined as the threshold. In this way, separate warm thresh- 3 PSYCHOPHYSICS olds (WT) and cold thresholds (CT) can be determined. In some studies, the stimulation reverses sign when a threshold is 3.1 Thermal Detection - Basic Psychophysics reached and stimulation continues to the opposite threshold. There are quite a number of studies that investigated how Thus, one goes back and forth between the warm threshold well participants can detect a change in temperature (see and the cold threshold.

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