Figure 1. Experimental design.

The study comprised of three different testing days differing only by the conditioning modalities applied. On the first day, all individuals were familiarized with the stimulation devices (CHEPS Stimulator (Pathway, Medoc, RamatYishai, Israel) and customized Thermal Grill device (http://www.sms.hest.ethz.ch/; Engineering Department)) and the temperatures (i.e., neutral, warm, cool, and interlaced cool/warm) that would be used as conditioning modalities. Employing the thermal grill device, all bars were set to either 32 °C (neutral), 20 °C (cool), 40 °C (warm), or alternating 20/40 °C (i.e., the TGI). The individuals were also exposed to three contact heat stimulations delivered by the CHEPS thermode. A 30-minute break followed the familiarization phase upon the commencement of the first experimental session to avoid thermal carry-over effects. The experiment consisted of three measurement blocks of combined conditioning and contact heat stimulation separated by breaks of 5 minutes. For the first two blocks, neutral (32 °C) was chosen as the conditioning modality, while either warm (W), cool (C), or interlaced cool/warm (TGI) (i.e., pseudo-randomized order over the three days: C/TGI/W or W/TGI/C) was presented in the third block. Each block was initiated with a 30 s exposure to the conditioning modality (i.e., neutral, cool, warm, or interlaced cool/warm) by placing the left hand on the thermal grill. Subsequently, individuals were instructed to keep the left hand on the thermal grill device, to fix on a point on the ceiling with their eyes, and to remain relaxed and quiet while recording of the 15 contact heat evoked potential stimulations (CHEPS) applied on the contralateral right hand. All contact heat stimulations were made from a baseline temperature of 42 °C to a peak temperature of 52 °C. The nominal heating rate was 70 °C/s and the cooling rate was 40 °C/s.