How does our brain encode cool and warm?

Warm, cool, these sensations are an integral part of our daily lives. Our ability to detect the temperature of the objects is essential to living. For almost a century, scientists have been trying to determine where in the brain the ability to detect temperatures is located. A study published in Nature reports the discovery of a ‘thermal cortex’ located in a posterior region of the insular cortex. This cortex can detect cool or warm temperatures.

When our body is in motion, our brain processes the information from our sensory organs. This processing then builds a conscious perception of the world. Much of this process takes place in the outer, folded layer of the brain, called the cortex. The cortex has a major role! It is the house of high brain functions, such as voluntary movement and consciousness.

The scientists started from a previous observation: neurons in the primary somatosensory cortex are active when the skin comes into contact with cool temperatures. They therefore expected that warm temperatures would also be encoded in this region. A test was therefore carried out on mice. To do this, they exposed the front paws of the mice to slight changes in temperature. They then used imaging techniques to find out which part of the brain reacted to these temperature changes. To their surprise, the scientists found that the primary somatosensory cortex did not respond to warming, but on closer inspection, the posterior insular cortex instead responded not only to cooling but also to warming. Using a two-photon microscope, the response of individual neurons in the posterior insular cortex was studied. The scientists discovered that there are cool-specific neurons, warm-specific neurons and neurons that respond to both warm and cool. It should be noted that ‘warm’ and ‘cool’ neurons react in very different ways. Indeed, “warm” neurons are sensitive to absolute temperature, while “cool” neurons are activated following a variation in temperature. In addition, the responses of the “cool” neurons are faster than the responses of the “warm” neurons. This indicates that there are potentially separate pathways for the perception of cool and warmth. 

To prove conclusively that the insular cortex is involved in temperature perception, the scientists trained mice to respond to cool or warm temperatures. Using optogenetics they were able to temporarily deactivate the posterior insular cortex while delivering a thermal stimulus. In the end, the mice did not feel the thermal stimulus. However, when the scientists stopped deactivating the posterior insular cortex, they felt the stimulus again.

This discovery and the possibility of optically accessing the cortical representation of sensory processing in the insula opens up new avenues of research. First, to study the neural mechanisms of thermal perception, but also to extend similar research to other sensory systems represented in adjacent parts of the insular cortex (in particular the gustatory and visceral systems).

Reference

The cellular coding of temperature in the mammalian cortex.
M. Vestergaard, M. Carta, G. Güney & J. F. A. Poulet
Nature. 2023 Feb 8.
DOI: 10.1038/s41586-023-05705-5 
Online ahead of print.