Thermal referral: evidence for a thermoceptive uniformity illusion without touch

When warm thermal stimulators are placed on the ring and index fingers of one hand, and a neutraltemperature
stimulator on the middle finger, all three fingers feel warm. This illusion is known as
thermal referral (TR). On one interpretation, the heterogenous thermal signals are overridden by
homogenous tactile signals. This cross-modal thermo-tactile interaction could reflect a process of object
recognition, based on the prior that many objects are thermally homogenous. Interestingly, the illusion
was reported to disappear when the middle digit was lifted off the thermal stimulator, suggesting that
tactile stimulation is necessary. However, no study has investigated whether purely thermal stimulation
might induce TR, without any tactile object to which temperature can be attributed. We used radiant
thermal stimulation to deliver purely thermal stimuli, which either were or were not accompanied by
simultaneous touch. We found identical TR effects in both the original thermo-tactile condition, and in
a purely thermoceptive condition where no tactile object was present. Control experiments ruled out
explanations based on poor spatial discrimination of warm signals. Our purely thermoceptive results
suggest that TR could reflect low-level organization of the thermoceptive pathway, rather than a
cognitive intermodal modulation based on tactile object perception.