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

Abstract

Touching a warm stimulator with the base of the finger and a cold stimulator with the middle part of the finger causes a sense of warmth at the fingertip outside the cold stimulator, revealing Extrapolation of Thermal Sensation (ETS). Although ETS shares similarities with the Thermal Grill Illusion (TGI) regarding spatial thermal integration, the spatial distributions of sensations are different. The TGI is limited inside boundaries that envelop physical stimuli, whereas ETS crosses the boundaries. This difference indicates ETS, 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 present study investigated ETS and TGI by simultaneous warm, cold, and neutral stimulation on the fingers or the lower leg. To show the difference between ETS and TGI, we manipulated the segmental distance between warm and cold stimuli and observed the resulting perceived temperature of neutral and cold stimulators. The perceived temperatures of ETS as well as TGI varied in units of segmental distance, however, ETS was not reproduced where TGI was. Thus, we concluded ETS has a different mechanism from the TGI. The experimental results suggest a non-uniform intersegmental connection contributes to the lower reproducibility of ETS on the lower leg.