Unique Influence of Stimulus Duration and Stimulation Site (glabrous Vs. Hairy Skin) on the Thermal Grill-induced Percept

Background: The application to the skin of spatially interlaced innocuous warm (40 °C) and cool (20 °C) thermodes (termed a thermal grill--TG) can produce an unusual thermal percept, but the mechanisms remain unclear.

Methods: We compared the percept quality and intensity over a 120-s period evoked by each of three configurations of a 6-bar thermal stimulator (6 TS): all 40 °C(WARM); all 20 °C(COOL); alternating bars 40/20 °C (TG) at two body sites (forearm and palm).

Results: Both unpleasantness and pain were significantly greater for the TG-induced (vs. either COOL- or WARM-induced) percept. Unpleasantness ratings were significantly higher than pain intensity ratings. Several emotional qualitative descriptors were unique to the TG-induced percept. TG palmar (vs. forearm) stimulation produced a more intense percept and was perceived as painful in more subjects. Temporal profiles of intensities of TG-induced percepts differed from those induced by the COOL or WARM thermodes alone. For both unpleasantness and pain, the site differences in the temporal profile were also unique for TG versus the COOL- or WARM-evoked percepts. Qualitative characteristics of the TG-induced percept varied over time and between subjects.

Conclusions: The TG percept intensity and temporal profile were different from those evoked by either of its component parts. The perceived quality is person-specific. These differences suggest that the classic 'TG illusion' results from complex central integration of several types of peripheral afferent inputs activated by the TG. Differing body site-related roles of thermosensory afferents in discrimination versus temperature homeostasis may explain site-related variations in the percept.