Hot or cold? How the brain deciphers thermal sensations

When we touch something hot or cold, the temperature is consciously sensed. Previous studies have shown that the cortex, the outermost layer of the brain, is responsible for thermal sensations. However, how the cortex determines whether something is hot or cold is not well understood. Thermal sensitivity is often subjective and individualistic; what is a comfortable temperature for someone might be too hot or too cold for someone else.

In a new study, Professor Kei Nagashima from the Body Temperature and Fluid Laboratory, Faculty of Human Sciences, Waseda University, Japan, and Dr. Hironori Watanabe from Waseda University, in collaboration with others, used electroencephalography (EEG) to map the brain’s response to hot and cold temperatures and clarify the activity patterns. In this study, 20 participants were exposed to different temperature treatments on their right index and middle fingers. The temperature treatments were given in pulses for 15 seconds with a 10-second intermittent temperature of 32 ˚C. Brain neural activity in response to the two different temperatures, 40 ˚C and 24 ˚C, was recorded using a wearable EEG device. The study was published in Volume 564 of Neuroscience on 09 January 2025.

The recordings were analyzed to reveal region- and time-specific patterns of brain activity. Clustered brain activity was found in ten different areas within the cortex. Interestingly, both hot and cold temperatures invoked brain activity in the same ten areas. However, the EEG differed in response to the two types of thermal sensation. “Differences in these activity patterns will allow temperature differences to be distinguished, leading to different behaviors,” says Nagashima.

Different types of activity patterns in the same brain regions could be the underlying mechanism for distinguishing between hot and cold temperatures. It was also observed that most of the brain activity was concentrated in the right hemisphere, indicating that it has a greater role to play in thermal sensations than the left hemisphere.

The insights obtained in this study could be applied to developing more objective methods to evaluate thermal comfort. Nagashima states, “Thermal comfort is used as a standard for creating an optimal indoor environment (air conditioning) by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, but it is still based on subjective reporting. We thought that it was essential to evaluate it objectively and scientifically.” Health risks due to the ambiguity of subjective evaluations of thermal comfort could be circumvented with a better understanding of the brain’s response.

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Reference

Authors: Hironori Watanabe^a,b,c, Satoshi Shibuya^d, Yuta Masuda^e, Taisuke Sugi^c, Kiyoshi Saito^a,f, and Kei Nagashima^a,c

DOI: 10.1016/j.neuroscience.2024.11.045

Affiliations:  ^aInstitute for Energy and Environmental System, Sustainable Energy and Environmental Society Open Innovation Research Organization, Waseda University, Japan

 ^bAdvanced Research Center for Human Sciences, Waseda University, Japan

^cBody Temperature and Fluid Laboratory, Faculty of Human Sciences, Waseda University, Japan

^dDepartment of Integrative Physiology, Kyorin University School of Medicine, Japan

^eLaboratory of Animal Science, Graduate School of Life and Environmental Sciences, Kyoto Prefectural  University, Japan

^fDepartment of Applied Mechanics and Aerospace Engineering, School of Fundamental Science and Engineering, Waseda University, Japan

About Waseda University

Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University has produced many changemakers in its history, including nine prime ministers and many leaders in business, science and technology, literature, sports, and film. Waseda has strong collaborations with overseas research institutions and is committed to advancing cutting-edge research and developing leaders who can contribute to the resolution of complex, global social issues. The University has set a target of achieving a zero-carbon campus by 2032, in line with the Sustainable Development Goals (SDGs) adopted by the United Nations in 2015. To learn more about Waseda University, visit https://www.waseda.jp/top/en  

About Professor Kei Nagashima

Kei Nagashima is a Professor at the Faculty of Human Sciences at Waseda University, Japan. He received his medical degree in 1986 and Ph.D. in 1995 from the Kyoto Prefectural University of Medicine. His primary area of research is physiology with a focus on thermal regulation in humans and animals. He has over 100 publications and over 3,000 citations to his credit.