If you are amongst the brave individuals that have ever dared to winter-bathe, you may have noticed a strange experience when first dipping your foot into the cold water. Instead of the expected ice-cold, you might feel a warmth or prickling heat sensation on your skin, a bizarre feeling that may be amplified if you have just come out of the hot sauna. Within scientific literature, this experience is called paradoxical heat sensation; which specifically refers to the feeling of heat when your skin is actually being cooled (1). Although this phenomenon was discovered more than a century ago (2), its existence has long baffled scientists. Our new study, published in Communications Psychology, provides a solution to this century-old puzzle: that the illusion is driven by the contrast between warm and cold temperatures.

Whilst it is well known that our senses can be deceived, the vast majority of research on perceptual illusions is specific to the visual system. We drew from this rich background to generate our hypothesis about what drives paradoxical heat. In particular, we were inspired by the concept of contrast enhancement, a process in which the perceived difference between adjacent and typically opposing elements is exaggerated in perception (3). For example, when we view alternating dark and light bars, the boundaries between the bars appear to be brighter than they really are, a process that is driven by our visual system enhancing the perceived contrast between opposing bars. If you look closely at the example image below, each bar appears as a gradient from light to dark, even though they are all one solid colour. We expected that this logic could apply to our other senses, specifically thermosensation - the perception of temperature both on and inside our bodies. We hypothesised that temperature perception might be distorted by experiencing highly contrasting temperatures in quick succession - just like stepping into the cold sea after sweating in a sauna.

In the lab, we experimentally induce feelings of paradoxical heat in our participants by alternating warm and cold temperatures on a single spot of skin. To test our theory, we designed an experiment using this approach in over 200 young, healthy individuals. By carefully varying the contrast between warm and cold we could present participants with small (i.e., low contrast) to large differences (i.e., high contrast) in temperature, and then measure how frequently they felt paradoxical heat. Our results clearly showed that participants reported feeling paradoxical heat much more often when the temperature contrast was high compared to when it was low. We also quantified the relationship between paradoxical heat and thermal contrast with a mathematical formula, the Thermal Contrast Function, which calculates the standardised difference between warm and cold stimuli. We found that we could use this function to predict whether a participant felt paradoxical heat during the experiment with up to 70% accuracy - a massive improvement over models that did not account for thermal contrast (which were 5-10% accurate)!

Our research reveals that, much like in visual illusions, the amplification of the difference between opposing features (i.e. contrast enhancement) underlies illusory experiences of temperature. This means that the sensation of warmth in cold conditions may not be a flaw in our perception, but a sophisticated feature of how our brain processes sensory information. Ultimately, this insight challenges our understanding of sensory reality, suggesting that how and what we feel is not merely a passive reflection of the world, but highly dependent on the complex interaction between the nervous system and our environment. Going beyond the lab - individuals with nerve damage have been shown to be more vulnerable to misperceptions of temperature and pain. Our findings could one day help us to understand why nerve damage leads to sensations that feel incredibly real, yet lack a clear physical explanation. This discovery of why the every-day person can experience something as simple as a feeling of warmth when stepping into cold water could reshape how we approach the misperception of temperature and pain in critical and life-changing pain disorders.

Cited references

1. 1. Hämäläinen, H., Vartiainen, M., Karvanen, L. & Järvilehto, T. Paradoxical heat sensations during moderate cooling of the skin. Brain Res. 251, 77–81 (1982).
   2. Goldscheider, A. Über die Empfindung der Hitze. Z. Für Klin. Med. 75, 1–14 (1912).
   3. Hess, R. F., Dakin, S. C. & Field, D. J. The role of “contrast enhancement” in the detection and appearance of visual contours. Vision Res. 38, 783–787 (1998).