Thermal Grill Illusion

As a child, I remember being amazed by the thermal grill perceptual illusion in a science museum. In this illusion, subjects feel an unpleasant burning sensation when simultaneously touching juxtaposed cold and warm bars of a grill. However, when touching only warm bars or only cold bars, the subject does not report a feeling of pain (just the warm or cool temperatures).

Thermography of the grill surface. Panel C results in the Thermal Grill Illusion (alternating warm and cold temperatures)

In an interesting fMRI study, researchers (Lindstedt et al.) examined this phenomenon of the thermal grill illusion. It was known that touching warm and cold temperatures at the same time caused an imbalance of firing from spinal neurons, but more neurophysiological evidence and spatio-temporal data on human brain function was needed. In the Lindstedt at al. study, participants placed their left leg on the cold and warm bars on the surface of the grill so their calf muscle was in contact with the stimulating surface. While their leg was being stimulated, fMRI recordings were taken from their brain to image the supraspinal activation from the thermal grill illusion. After statistically analyzing the results, the most notable finding was a strong activation of the contralateral thalamus as compared to the patterns of activation seen by its constituent temperatures.

Thermal Grill Illusion activation (right thalamus) vs. constituent cold or warm activation

The results of this study are really interesting as they can be compared to future fMRI studies analyzing neuropathic pain.