Cooling the Thermal Grill Illusion through Self-Touch

Abstract

Acute peripheral pain is reduced by multisensory interactions at the spinal level [1]. Central pain is reduced by reorganization of cortical body representations [2, 3]. We show here that acute pain can also be reduced by multisensory integration through self-touch, which provides proprioceptive, thermal, and tactile input forming a coherent body representation [4, 5]. We combined self-touch with the thermal grill illusion (TGI) [6]. In the traditional TGI, participants press their fingers on two warm objects surrounding one cool object. The warm surround unmasks pain pathways, which paradoxically causes the cool object to feel painfully hot. Here, we warmed the index and ring fingers of each hand while cooling the middle fingers. Immediately after, these three fingers of the right hand were touched against the same three fingers on the left hand. This self-touch caused a dramatic 64% reduction in perceived heat. We show that this paradoxical release from paradoxical heat cannot be explained by low-level touch-temperature interactions alone. To reduce pain, we often clutch a painful hand with the other hand. We show here that self-touch not only gates pain signals reaching the brain [7-9] but also, via multisensory integration, increases coherence of cognitive body representations to which pain afferents project [10].