Heat exposure-induced changes in motor outflow component of reaction time

Passive heat acclimation does not modulate processing speed and executive functions during cognitive tasks performed at fixed levels of thermal strain

PURPOSE

This study evaluated if passive controlled hyperthermia heat acclimation (HA) modulates cognitive performance during passive heat stress.

METHODS

Eight healthy adults (25 ± 4 years) underwent 7 consecutive days of hot water immersion (core temperature ≥38.6°C) and a 7-day time-control period. On days 1 and 7 of HA, participants performed a digital Stroop test at baseline, when core temperature reached 38.6°C, and after 60 minutes at a core temperature ≥38.6°C to evaluate reaction time during tasks targeting processing speed (reading and counting) and executive functions (inhibition and switching). On days 1 and 7 of the time-control intervention, participants performed the Stroop test with equivalent amounts of time separating each task as for HA.

RESULTS

During day 1 of HA, reaction time was quicker during the reading (-44 ms [-71, -17], P<0.01) and counting (-39 ms [-76, -2], P=0.04) tasks when rectal temperature reached 38.6°C, but after a further 60 minutes of heat exposure, reaction time only remained quicker during the reading task (-56 ms [-83, -29], P<0.01). Changes in reaction time during heat exposure were unaffected by subsequent HA (interaction, all P≥0.09).

CONCLUSION

Seven days of HA does not modulate processing speed and executive functions during passive heat exposure. Novelty: - Whether heat acclimation (HA) to improve cognitive performance during heat exposure remains understudied. - We tested the hypothesis that HA modulates reaction time during cognitive tasks performed at matched levels of thermal strain. - Despite classical signs of HA, reaction time during heat exposure was unaffected by HA.

Hyperthermia impairs short-term memory and peripheral motor drive transmission

The aims of this study were to determine (i) the effect of passive hyperthermia on motor drive and cognitive function, and (ii) whether head cooling can limit the hyperthermia-induced alterations. Sixteen subjects were randomly exposed for 2 h to three different conditions: control (Con, 20 degrees C), hot (Hot, 50 degrees C) and hot head cool (HHC--where cold packs were applied to the head under Hot conditions). Three cognitive tests measuring attention and two measuring memory were performed. Neuromuscular testing included electrically evoked muscle action potentials (M-waves) and reflex waves (H-reflex) at rest and during brief (4-5 s) and sustained (120 s) maximal voluntary contractions (MVC) of the plantar flexors. All the tests were performed in the environmental room. During brief MVC, torque was significantly lower in both Hot and HHC as compared to Con (P < 0.05). The decrease in muscle activation was significant in Hot (P < 0.05) but not in HBC (P = 0.07). This was accompanied by peripheral failures in the transmission of the neural drive at both spinal (significant decrements in H-reflexes and V-waves, P < 0.05) and neuromuscular junction (significant decrements in M-waves, P < 0.05) levels. During sustained MVC, muscle activation was further depressed (P < 0.05) without any concomitant failures in M-waves, suggesting neural activation adjustments occurring probably at the supraspinal level. Cerebral perturbations were confirmed by significant decrements in both memory tests in Hot as compared with Con (P < 0.05) but not in simple tests (attention tests) that were not affected by hyperthermia. The decrement in memory capacity suggested the existence of frontal lobe activity impairments. Thus, HHC preserved memory capacity but not the visual memory.

Neurobehavioral test performance in the third National Health and Nutrition Examination Survey

The third National Health and Nutrition Examination Survey (NHANES III) contained three computerized neurobehavioral tests from the Neurobehavioral Evaluation System (NES): simple reaction time, symbol-digit substitution and serial digit learning. The neurobehavioral data that were collected came from a nationally representative sample of adults 20-59 years old. Performance on the tests was related to sex, age, education level, family income and race-ethnicity. Performance decreased as age increased, and increased as education level and family income increased. Differences in performance between sexes, levels of education and racial-ethnic groups tended to decrease as family income increased. The relationship between age and performance on the symbol-digit substitution test varied by education level and by racial-ethnic group. The relationship between age and performance on the serial digit learning test varied by racial-ethnic group. Questionnaire variables that were related to performance on one or more of the tests included the reported amount of last night's sleep, energy level, computer or video game familiarity, alcoholic beverages within the last 3 h and effort. Persons who took the tests in English or Spanish performed differently on the symbol-digit substitution and serial digit learning tests. Performance on all the tests decreased as test room temperature increased.

Temperature-induced changes in neuromuscular function: central and peripheral mechanisms

Three series of experimental tests were conducted on subjects under both elevated and depressed thermal conditions. Tripartite series consisted of whole-body immersion excepting the head, whole-body immersion excepting the head and response limb, and immersion of the discrete-response limb. Measures of physiological and behavioural responses were made at sequential .4 degrees C changes during whole-body immersions and approximately 5 degrees C changes of water temperature during the immersion of a limb only. Results suggested that velocity of nerve conduction decreased with thermal depression. Premotor, motor, simple, and choice reaction times varied differentially as a function of the hot and cold conditions. Implications of these differential effects on neuromuscular function are examined with respect to person-machine performance in artificially induced or naturally occurring extremes of ambient temperature.