The independent influences of heat strain and dehydration upon cognition
Purpose Many researchers have addressed the potential efects of hyperthermia and dehydration on cognition, often revealing contradictory outcomes. A possible reason for this inconsistency is that experiments may have been inadequately designed for such efects. In this study, the impact of hyperthermia, dehydration and their combination on cognition were evaluated in eight young males, after accounting for a range of experimental limitations. Methods Passive heating and thermal clamping at two mean body temperatures (36.5, 38.5°C) were performed under three hydration states (euhydrated, 3 and 5% dehydrated) to assess their efects on difculty-matched working memory and visual perception tasks, and on a difculty manipulated perceptual task. Data were analysed according to signal detection theory to isolate changes in response sensitivity, bias and speed. Results Neither moderate hyperthermia (P=0.141) nor dehydration (P>0.604) modifed response sensitivity, nor did they signifcantly interact (P>0.698). Therefore, the ability to distinguish correct from incorrect responses was unafected. Nevertheless, hyperthermia, but not dehydration (P=0.301), reduced the response bias (−0.08 versus 2.2 [normothermia]; P=0.010) and reaction time (mean reduction 49 ms; P<0.001), eliciting more liberal and faster responses (P=0.010). Response bias was reduced for the memory relative to the perceptual task (P=0.037), and this efect was enhanced during hyperthermia (P=0.031). Conclusions These observations imply that, once potentially confounding infuences were controlled, moderate hyperthermia, signifcant dehydration and their combined efects had insufcient impact to impair cognition within the memory and perceptual domains tested. Nonetheless, moderate hyperthermia elicited more liberal and rapid responses.