Adaptation to warming but not cooling at slow rates of stimulus change in thermal threshold measurements

The relationship between thermal detection threshold and rate of temperature change of the thermal stimulus when slow (< 1 degrees Cs-1) rates of change are employed was investigated. Using both the reaction time ( RT) inclusive Method of Limits and RT exclusive Method of Levels healthy volunteers had warming ( WDT) and cooling detection thresholds ( CDT) measured at four different rates of temperature change ( 0.3, 0.5, 0.7 and 1.0 degrees Cs-1) from the thenar and/ or mental regions using a contact thermode. With the Method of Limits, CDT increased linearly with rate of temperature change suggesting increments were due to RT artefacts. This was further supported by threshold assessment with the Method of Levels which showed CDT were unaffected by the rate of change in the RT exclusive method ( P > 0.1). In contrast, WDT did not increase linearly with rate of stimulus temperature change when the Method of Limits was used and threshold assessment with the Method of Levels showed WDT assessed using a 0.3 degrees Cs-1 ramp rate were significantly higher than those measured with a 1 degrees Cs-1 rate of change ( P < 0.05). This study indicates that adaptation to a warming stimulus can occur at faster rates of stimulus change than previously anticipated and identifies differences in warming and cooling pathways in sensitivity to adaptation.