The motivation to behaviorally thermoregulate during passive heat exposure in humans is dependent on the magnitude of increases in skin temperature

We tested the hypothesis that the motivation to behaviorally thermoregulate in humans is dependent on the magnitude of changes in mean skin temperature. Ten healthy subjects (22 ± 3 y, 5 females) underwent 60 min of seated rest in a 32±1 °C or 42±1 °C environment (20% relative humidity). Trials were completed in a counterbalanced order. The motivation to behaviorally thermoregulate was measured using an operant behavior task on a fixed ratio schedule, in which subjects received thermal reinforcement after clicking a button 100 times. The reinforcer was 30 s of cooling on the dorsal aspect of the neck. The motivation to behave was defined as the cumulative number of button clicks over time and behavioral thermoregulation was defined as the change in neck skin temperature. Mean skin temperature was higher throughout the 42 °C versus the 32 °C trial (at 60 min: 36.3±0.5 °C vs. 34.5±0.5 °C, P <.01) and core temperature became higher in this trial 40 min into heat exposure (at 60 min: 37.2±0.2 °C vs. 37.1±0.1 °C, P ≤.04), but did not differ from pre- heat exposure (P =.81). Neck skin temperature was lower in the 42 °C compared to the 32 °C trial starting at 30 min (33.7±0.8 °C vs. 35.3±0.5°C, P <.01), which was maintained thereafter (P ≤.04). Cumulative responding for thermal reinforcement was greater in the 42 °C trial compared to the 32 °C trial at 20 min (180±155 clicks vs. 0±0 clicks, P <.01), which persisted thereafter (P <.01). These data indicate that the motivation to behaviorally thermoregulate during passive heat exposure in humans is dependent on the magnitude of increases in skin temperature.