Extrapolation of Thermal Sensation: Warm-Cold Stimulus Pair Elicits a Sense of Warmth Outside the Stimulus

Touching a warm stimulator with the base of the finger and a cold stimulator with the middle of the finger causes a sense of warmth at the fingertip outside the cold stimulator, revealing Extrapolation of Thermal Sensation (ETS). Although the ETS shares similarities with the Thermal Grill Illusion (TGI) regarding spatial thermal integration, the spatial distributions of the sensations are different. The TGI is limited to the inside boundaries that envelope physical stimuli, whereas the ETS crosses the boundaries. Although TGI is reproduced accompanied by overestimation of the cold stimulus, which is influenced by the spinal segmental distance between warm and cold stimuli, it remains to be seen whether ETS carries out the same. The study investigated the ETS and TGI using simultaneous warm, cold, and neutral stimulation of the fingers or lower leg. To show the difference between the ETS and TGI, we manipulated the segmental distance between warm and cold stimuli and observed the resulting perceived temperatures of the neutral and cold stimulators. The perceived temperatures of the ETS and TGI varied in units of segmental distance. However, the ETS was not reproduced where the TGI was. Thus, we conclude that the mechanism of the ETS is different from that of the TGI. The experimental results suggest that a non-uniform intersegmental connection contributes to the lower reproducibility of ETS on the lower leg.