Hot or cold? How the brain deciphers thermal sensations

The brain discriminates between hot and cold sensations by inducing distinct temporal activity patterns in shared cortical regions, offering insights into how sensory information is encoded.

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.