Cool, warm, and heat-pain detection thresholds: Testing methods and inferences about anatomic distribution of receptors

We recently found that vibratory detection threshold is greatly influenced by the algorithm of testing. Here, we study the influence of stimulus characteristics and algorithm of testing and estimating threshold on cool (CDT), warm (WDT), and heat-pain (HPDT) detection thresholds. We show that continuously decreasing (for CDT) or increasing (for WDT) thermode temperature to the point at which cooling or warming is perceived and signaled by depressing a response key (“appearance” threshold) overestimates threshold with rapid rates of thermal change. The mean of the appearance and disappearance thresholds also does not perform well for insensitive sites and patients. Pyramidal (or flat-topped pyramidal) stimuli ranging in magnitude, in 25 steps, from near skin temperature to 9 °C for 10 seconds (for CDT), from near skin temperature to 45 °C for 10 seconds (for WDT), and from near skin temperature to 49 °C for 10 seconds (for HPDT) provide ideal stimuli for use in several algorithms of testing and estimating threshold. Near threshold, only the initial direction of thermal change from skin temperature is perceived, and not its return to baseline. Use of steps of stimulus intensity allows the subject or patient to take the needed time to decide whether the stimulus was felt or not (in 4, 2, and 1 stepping algorithms), or whether it occurred in stimulus interval 1 or 2 (in two-alternative forced-choice testing). Thermal thresholds were generally significantly lower with a large (10 cm2) than with a small (2.7 cm2) thermode. A topographic difference of CDT, WDT, and HPDT was demonstrated, with the face and volar arms having the lowest threshold and legs and feet having the highest threshold. In healthy subjects, warm threshold varied most among different sites, followed by uncomfortably hot, and last by heat-pain threshold. Particularly in older subjects, CDT could be determined on the dorsum of the foot whereas WDT sometimes could not, the first sensation experienced being heat-pain. A low density of warm receptors, especially in the foot and leg of old people, would explain these latter findings.