A Sensory Processing Hierarchy for Thermal Touch: Thermal Adaptation Occurs Prior to Thermal-Tactile Integration

The brain consistently faces a challenge of whether and how to combine the available information sources to estimate the properties of an object explored by hand. Thermal referral (TR) is a phenomenon that demonstrates how thermal and tactile modalities coordinate to resolve inconsistencies in spatial and thermal information. When the middle three fingers of one hand are thermally stimulated, but only the outer two fingers are heated (or cooled), thermal uniformity is perceived across three fingers. This illusory experience of thermal uniformity in TR compensates for the discontinuity in the thermal sensation across the sites in contact. The neural loci of TR is unclear. While TR reflects the diffuse nature of the thermoceptive system, its similarities to perceptual filling-in and its facilitative role in object perception also suggest that TR might involve inference processes associated with object perception. To clarify the positioning of this thermo-tactile interaction in the sensory processing hierarchy, we used perceptual adaptation and Bayesian decision modelling techniques. Our results indicate that TR adaptation takes place at a peripheral stage where information about temperature inputs are still preserved for each finger, and that the thermal-tactile interaction occurs after this stage. We also show that the temperature integration across three fingers in TR is consistent with precision weighted averaging effect - Bayesian cue combination. Altogether, our findings suggest that for the sensory processing hierarchy of thermal touch, thermal adaptation occurs prior to thermo-tactile integration, which combines thermal and tactile information to give a unified percept to facilitate object recognition.\n\nSignificance StatementThermal touch refers to the perception of temperature of objects in contact with the skin and is key to object recognition based on thermal cues. While object perception is an inference process involving multisensory inputs, thermal referral (TR) is an illusion demonstrating how the brains interpretation of object temperature can deviate from physical reality. Here we used TR to explore the processing hierarchy of thermal touch. We show that adaptation of thermal perception occurs prior to integration of thermal information across tactile locations. Further, we show that TR results from simple averaging of thermal sensation across locations. Our results illuminate the flexibility of the processing that underlies thermal-tactile interactions and facilitates object exploration and identification in our complicated natural environment.