The independent roles of temperature and thermal perception in the control of human thermoregulatory behavior

The present study independently evaluated temperature and thermal perception as controllers of thermoregulatory behavior in humans. This was accomplished using a self-paced exercise and heat stress model in which twelve physically active male subjects exercised at a constant subjective rating of perceived exertion (16, ‘hard - very hard’) while their face was thermally and non-thermally cooled, heated, or left alone (control trial). Thermal cooling and heating were achieved via forced convection, while non-thermal cooling and heating were accomplished via the topical application of menthol and capsaicin solutions. Evidence for thermoregulatory behavior was defined in terms of self-selected exercise intensity, and thus exercise work output. The results indicate that, in the absence of changes in temperature, non-thermal cooling and warming elicited thermal sensory and discomfort sensations similar to those observed during thermal cooling and warming. Furthermore, the perception of effort was maintained throughout exercise in all trials, while the initial and final exercise intensities were also similar. Thermal and non-thermal cooling resulted in the highest work output, while thermal warming the lowest. Non-thermal warming and control trials were similar. Heart rate, mean skin and core (rectal) temperatures, and whole body and local (neck) sweat rates were similar between all trials. These data indicate that changes in temperature are not a requirement for the initiation of thermoregulatory behavior in humans. Rather, thermal sensation and thermal discomfort are capable behavioral controllers.     

Cold thermoregulatory changes induced by sleep deprivation in men

The aim of this study was to evaluate the thermoregulatory changes induced by 27-h of sleep deprivation (SD) in men at rest both in a comfortable ambient temperature and in cold air. A group of 12 male subjects were placed in a comfortable ambient temperature (dry bulb temperature,T _db = 25° C, relative humidity, rh = 40%–50% , clothing insulation = 1 clo) for 1 h and then they were submitted to a standard cold air test in a climatic chamber for 2h (T _db=1° C, rh = 40%–50%, wind speed = 0.8 m·s^–1, nude), before and after 27 h of sleep deprivation. Thermoregulatory changes (rectal temperature,T _re; mean skin temperature, _sk; metabolic heat production ) were monitored continuously. At comfortable ambient temperature, no significant change was observed after SD forT _re, _sk and . During the cold test,T _re did not change but _sk and were higher after SD (P<0.05). Increased (+ 6%,P < 0.05) was related to earlier and higher shivering, with a possible increase in the sensitivity of the thermoregulatory system as shown by the shorter time to onset of continous shivering (d): 8.66 (SEM 1.33) min versus 28.20 (SEM 1.33) min (P < 0.001) and by a higher _sk observed at d: 27.60 (SEM 1.40)° C versus 21.40 (SEM 0.60)° C (P < 0.001). These results were associated with higher cold sensations and shivering following SD. They also suggested that SD modified thermoregulatory responses at a central level especially in a cold environment.     

Effect of voluntary dehydration on thermoregulatory responses to heat in men and women

Summary The effects of dehydration prior to heat exposure on sweating and body temperature were tested in 8 men and 8 women, dehydration being 1.3 and 1.0% of body weight, respectively. The subjects were exposed to 40° C for 60 min. Compared with controls (C), in the dehydrated men (D) there was a longer delay in the onset of sweating (C, 7.8, D, 11.6 min,p<0.05), a lower total sweat loss (C, 153, D, 127 g · m^-2 · h^–1,p< 0.001), and a greater increase in Tre (C, 0.31, D, 0.43° C,p<0.002). In women, dehydration did not influence the control time course of sweating significantly, nor were these significant body temperature increases during heat exposure. Delay in the onset of sweating in women (C, 18.1, D, 18.7 min) was generally longer than in men (C, 7.8, D, 11.6 min), [F(1, 14)=7.41,p<0.05]. A significant correlation was found between the inertia time of sweating and Tre in both control and dehydration conditions in the men (r=0.81,p<0.01). The rectal temperature increases in men were also related to the inertia time of electrical skin resistance (r=0.83,p<0.01). It is concluded that dehydration affects sweating and body temperature in men more severely than in women.This work was supported in part by the Centre National de la Recherche Scientifique and the Polish Academy of Sciences within the Project 10.4     

Direct measurement of the thermal responses of nude resting men in dry environments

Summary Two nude resting men were exposed for two-hour periods to each of 25 dry environments, with air temperatures ranging between 12.8° C and 49.1° C and wind speeds between 0.67 m/sec and 4.94 m/sec. The mean radiant temperature of the surroundings was kept equal to the air temperature. Rates of radiant and convective heat exchange were measured directly, separately and continuously. The men had reached a thermal steady state after 105 min in the warm environments, but not in the cold environments. Graphs are presented to show the effect of ambient temperature and wind speed on the radiation and convection rates attained after 105 min, as well as on metabolic rate, sweat evaporation rate, rectal temperature and mean skin temperature. These graphs revealed some important aspects of the behaviour of man's thermal control system. In particular the physiological conductance increased with increasing ambient temperature and then saturated at an ambient temperature near 35° C. This saturation resulted in a constant difference between rectal temperature and mean skin temperature irrespective of the environmental conditions.Published with the permission of the Chamber of Mines of South Africa.     

Mild hypohydration induced by exercise in the heat attenuates autonomic thermoregulatory responses to the heat, but not thermal pleasantness in humans

Hypohydration caused by exercise in the heat attenuates autonomic thermoregulation such as sweating and skin blood flow in humans. In contrast, it remains unknown if behavioral thermoregulation is modulated during hypohydration. We assume that thermal unpleasantness could drive the behavioral response, and would also be modulated during hypohydration. Nine healthy young men participated in the present study. Body and skin temperatures were monitored. Ratings of thermal sensation and pleasantness were conducted. After ∼ 45 min rest at 27 °C, they performed 50-min cycling exercise, which was at the level of 40% of heart rate range at 35 °C (hypohydration trial) or at the level of 10% of heart rate range at 23 °C (control trial), respectively. Subjects returned to the rest at 27 °C, and the ambient temperature was then changed from 22 to 38 °C. Body weight decreased by 0.9 ± 0.1% immediately after exercise in the hypohydration trial and 0.3 ± 0.1% in the control trial. In the cold, no significant difference in thermal sensation or pleasantness was observed between trials. There was no significant difference in thermal pleasantness between trials in the heat, although thermal sensation in the heat (32.5-36 °C) was significantly lower in the hypohydration trial than in the control trial. In addition, laser Doppler flow of the skin and sweat rate were attenuated in the heat in the hypohydration trial. These results may indicate that mild hypohydration after exercise in the heat has no influence on behavioral responses to the heat.     

Reduced responsiveness of locus coeruleus neurons to cutaneous thermal stimuli in capsaicin-treated rats

Recent electrophysiological experiments have shown that brain norepinephrine (NE) neurons in the locus coeruleus (LC) are activated by cutaneous thermal stimuli of both non-noxious and noxious character. In the present study the LC neuronal response to thermal stimuli was used to evaluate cutaneous thermal sensitivity in capsaicin-treated rats, a treatment that is described to cause impaired thermoregulation. Capsaicin treatment, of neonates as well as of adult rats, caused a reduced responsiveness of brain LC neurons to thermal stimuli. The results suggest that a reduction in peripheral thermal afferent transmission may be one mechanism underlying the capsaicin-induced thermoregulatory dysfunction.     

Influence of the menstrual cycle and oral contraceptives on thermoregulatory responses to exercise in young women

Summary Thermoregulatory responses to exercise in relation to the phase of the menstrual cycle were studied in ten women taking oral contraceptives (P) and in ten women not taking oral contraceptives (NP). Each subject was tested for maximal aerobic capacity ( ) and for 50% exercise in the follicular (F) and luteal (L) phases of the menstrual cycle. Since the oral contraceptives would have prevented ovulation a quasi-follicular phase (q-F) and a quasi-luteal phase (q-L) of the menstrual cycle were assumed for P subjects. Exercise was performed on a cycle ergometer at an ambient temperature of 24° C and relative air humidity of 50%. Rectal (T _re), mean skin ( ), mean body ( ) temperatures and heart rate (f _c) were measured. Sweat rate was estimated by the continuous measurement of relative humidity of air in a ventilated capsule placed on the chest, converted to absolute pressure (PH₂O_chest). Gain for sweating was calculated as a ratio of increase inPH₂O_chest to the appropriate increase inT _re for the whole period of sweating (G) and for unsteady-state (G_u) separately. The did not differ either between the groups of subjects or between the phases of the menstrual cycle. In P, rectal temperature threshold for sweating (T _{re, td}) was 37.85° C in q-L and 37.60° C in q-F (P < 0.01) and corresponded to a significant difference fromT _re at rest. TheT _re, andf _c increased similarly during exercise in q-F and q-L. No menstrual phase-related differences were observed either in the dynamics of sweating or in G. In NP,T _{re, td} was shorter in L than in F (37.70 vs 37.47° C,P<0.02) with a significantly greater value fromT _re at rest. The dynamics and G for sweating were also greater in L than in F. The G_u was 36.8 versus 16.6 kPa · ° C^–1 (P<0.01) while G was 6.4 versus 3.8 kPa · ° C^–1 (P<0.05), respectively. TheT _re, andf _c increased significantly more in phase F than in phase L. It was concluded that in these women performing moderate exercise, there was a greater temperature threshold and larger gains for sweating in phase L than in phase F. Intake of oral contraceptives reduced the differences in the gains for sweating making the thermoregulatory responses to exercise more uniform.     

Neuronal responses in the rat's thalamus to scrotal heating

Summary Twenty-one single neurons from the rat's thalamus were recorded as they responded to scrotal skin thermal stimulation. Eighteen of them were warm und three inverse-warm cells. After the injection of Lidocain (Xylocain) into the scrotal skin, the response of each neuron was abolished. Eight neurons could be followed until the drug effect had disappeared; their responsiveness was fully restored. External application of Xylocain had the same effect as intracutaneous injection, apart from longer time intervals before the effect became apparent. From this, conclusions are drawn on the origin of the thalamic responses, and speculations are made on the functional role of warm and inverse-warm cells.This project was supported by the Deutsche Forschungs-gemeinschaft, SFB 114 (BIONACH)     

Evoked cerebral responses to noxious thermal stimuli in humans

Summary Brief pulses of Laser emitted radiant heat were used to induce cutaneous painful sensations in human volunteers. Accurate timing of the stimuli permitted recording of scalp averaged evoked potentials. A late negative-positive component of the EP which correlated in amplitude with the subjective sensation was abserved in four subjects. The latency of this component (130–160 msec) correlated with stimulus intensity.This study was supported by a David Rose Grant.     

Interindividual differences in the thermoregulatory response to cool exposure in sleeping neonates

The responses of the thermoregulatory effectors vary greatly among neonates. Therefore, we assume that a small decrease in air temperature from thermoneutrality induces various thermoregulatory responses within neonates that represent an energy cost due to the cold defence processes. To determine the importance of this variability in nursing, 26 neonates were explored at thermoneutrality and in a cool environment (−1.5 °C from thermoneutrality) similar to that which occurs currently in clinical procedure. Oxygen consumption (V˙O₂), oesophageal and skin temperatures, as well as sleep parameters were recorded continuously in both conditions. Analysis of all of the data from all of the neonates revealed that the cool exposure induced thermal and sleep disturbances, but V˙O₂ did not increase and was not negatively correlated to body temperature (as might be expected). Analyses of individual data showed large variability in body temperature regulation: the neonates could be assigned to one of three groups according to the direction of the individual slopes of V˙O₂ versus oesophageal or skin temperature. The groups also differed according to the sleep changes recorded in the cool condition. The results show that the definition of thermoneutrality should be revised by incorporating non only changes in the body temperature, but also the sleep disturbances (increased wakefulness and active sleep, decreased quiet sleep), which are criteria that are more sensitive to mild cool exposure. Thermoneutrality should be defined for each individual, since the results stress that the variability does not help to predict a general pattern of thermoregulatory responses in cool-exposed neonates. Accepted: 22 November 1999     

Effects of lesions in central thermosensitive areas on thermoregulatory responses in rat

Behavioral and autonomic responses elicited by ambient heat were tested before and after lesioning five thermosensitive areas previously localized in the rat brain. Lesions in the PO/AH area, where local warming elicits prone body extension, markedly reduced or abolished extension but had no effect on locomotion, grooming, or tail vasodilation. Lesions in the posterior hypothalamus, where local warming induces grooming, and lesions in the septum, medial midbrain, and dorsomedial medulla, where local warming evokes locomotion, had no significant effects on thermoregulatory responses except for reduced body extension following midbrain lesions, which may have interrupted the efferent path from PO/AH thermosensors. It was concluded that extension is predominantly dependent on PO/AH thermosensors and/or integrative interneurons, while locomotion, grooming, and vasodilation are controlled by input from peripheral and central sensors. The present and earlier data from the rat are less consistent with a relatively localized general integrating mechanism producing a unitary output signal to thermoregulatory effectors than with a spatially distributed network of specific paths from sensors to separate response mechanisms, where summation of inputs may occur.     

Responses to acoustic stimuli of units in the auditory cortex of awake squirrel monkeys

Summary The responses of single units in the superior temporal gyrus of awake squirrel monkeys to tone pips, noise, clicks, and frequency-modulated sounds were recorded with extracellular microelectrodes. A majority of the units responded to acoustic stimulation, pure tone pips being the most effective in terms of the percentage of units responding (71%). No simple catalogue of response types could be elicited. Units varied in terms of the combinations of stimuli to which they were responsive, the frequency range over which pure tones were effective, and the temporal pattern of discharge to different frequencies and different types of stimuli. A substantial majority of units gave precise timed responses to the onset or offset of the stimuli, while a few introduced long delays between the stimulus and the response. With regard to the area studied, acoustic units could be found between stereotaxic coordinates A3 and A10, both on the lateral surface of the hemisphere and in the superior temporal plane, as well as in the caudal insular region. No precise tonotopic organization could be discerned.Peter Winter died in a skiing accident in March, 1972.     

Skin derivative control of thermal environment in a closed incubator

Defining a thermoneutral environment remains difficult because thermoneutrality depends on both physical and physiological factors. A servocontrolled skin temperature derivative (SCS) heating device has been designed to control the thermal environment in closed incubators without the necessity of setting an air or skin reference temperature. The thermal environment obtained with the SCS program is controlled only by the neonate's skin temperature changes. For each neonate, the program allows the attainment of a specific individual thermal equilibrium (T_eq). Although the mean value of the thermal equilibrium level measured on 29 neonates does not differ significantly from the neutral air temperature defined from the charts of other researchers, individual values of T_eq differed greatly among neonates of similar birthweight and postnatal age. When compared with on/off heating programs, the SCS system permits greater quiet sleep occurrence and seems to provide an optimal thermal environment. The results suggest that the skin temperature derivative heating program takes into account both the ambient and physiological factors affecting body temperature regulation of each neonate.     

Sensitivity to mechanical stimuli and the role of general sensory and perceptual processes in heartbeat detection

Sensitivity to heartbeat sensations is commonly assessed using tasks that require individuals to judge the simultaneity of heartbeats and tones. In two experiments, we investigated the suitability of this paradigm for examining cardioception. In the first experiment, participants judged the simultaneity of near–threshold vibrations and suprathreshold tones. Precision in judging vibration–tone simultaneity was directly related to the detectability of the mechanical stimuli, thereby supporting use of the simultaneity paradigm to assess heartbeat detection. In the second experiment, we examined the influences of sensitivity to mechanical stimuli and the ability to make intermodality simultaneity judgments on the precision of heartbeat detection. We measured participants' vibrotactile thresholds, precision in judging light–tone simultaneity, and precision in judging heartbeat–tone simultaneity. The ability to judge the simultaneity of lights and tones accounted for 24.3% of the variance in precision of heartbeat detection, and mechanical sensitivity accounted for a further 8.5%.     

Thermoregulatory and subjective responses of clothed men in the cold during continuous and intermittent exercise

Summary Thermoregulatory and thermal subjective responses were studied in ten male, clothed subjects during continuous (C) and intermittent (I) exercise at the same average level of oxygen consumption. The subjects performed both I and C twice, dressed in two different three-layer cold-protective clothing ensembles of two thermal insulation levels [total clothing insulation = 2.59 clo (L) and 3.20 clo (H)]. Experiments were carried out at an ambient temperature of –10°C. Rectal temperatures increased similarly in both types of exercise. Mean skin temperature (T _sk) was lower in 1 compared to C with both levels of clothing insulation. Over the last 0.5 h of the experimentT _sk was approximately 1.3°C lower in 1 than in C for clothing L. The skin evaporation rate was higher in clothing H than L but did not differ between I and C. Subjective ratings for thermal sensations of the whole body (BTS) and hands were close to neutral in I and around slightly warm in C. The BTS was lower in I than in C and was lower in L compared to H. It was concluded that, at equal average energy expenditure, thermal responses to intermittent and continuous exercise in the Gold differ in clothed subjects, principally as a result of different patterns of heat exchange.     

Visual and vergence eye movement-related responses of pursuit neurons in the caudal frontal eye fields to motion-in-depth stimuli

The caudal parts of the frontal eye fields (FEF) contain smooth-pursuit related neurons. Previous studies show that most FEF pursuit neurons carry visual signals in relation to frontal spot motion and discharge before the initiation of smooth-pursuit. It has also been demonstrated that most FEF pursuit neurons discharge during vergence tracking. Accurate vergence tracking requires information about target motion-in-depth. To further understand the role of the FEF in vergence tracking and to determine whether FEF pursuit neurons carry visual information about target motion-in-depth, we examined visual and vergence eye movement-related responses of FEF pursuit neurons to sinusoidal spot motion-in-depth. During vergence tracking, most FEF pursuit neurons exhibited both vergence eye position and velocity sensitivity. Phase shifts (re target velocity) of most neurons remained virtually constant up to 1.5 Hz. About half of FEF pursuit neurons exhibited visual responses to spot motion-in-depth. The preferred directions for visual responses of most neurons were similar to those during vergence tracking. Visual responses of most of these neurons exhibited sensitivity to the velocity of spot motion-in-depth. Phase shifts of most of the responding neurons remained virtually constant up to 2.0 Hz. Neurons that exhibited visual responses in-depth were mostly separate from neurons that showed visual responses in the frontal plane. To further examine whether FEF pursuit neurons could participate in initiation of vergence tracking, we examined latencies of neuronal responses with respect to vergence eye movements induced by step target motion-in-depth. About half of FEF pursuit neurons discharged before the onset of vergence eye movements with lead times longer than 20 ms. These results together with previous observations suggest that the caudal FEF carries visual signals appropriate to be converted into motor commands for pursuit in depth and frontal plane.     

Temporal and thermal variations in site‐specific thermoregulatory sudomotor thresholds: Precursor versus discharged sweat production

Temporal and thermal differences between the initiation of precursor, eccrine sweat and its surface discharge were investigated during passive heating. Sudomotor activity was evaluated using electrodermal (precursor) and ventilated sweat capsule measurements (dorsal fingers, dorsal hand, forehead, forearm). Passive heating significantly elevated auditory canal (0.5^oC) and mean body temperatures (0.9^oC). At each site, the precursor sudomotor thresholds occurred at a lower mean body temperature (P < .05), with an average elevation of 0.35^oC (SD 0.04). However, discharged thresholds were delayed until this temperature had risen 0.53^oC (SD 0.04), producing significant phase delays across sites (mean: 4.1 min [SD 0.5]; P < .05). It is concluded that precise sudomotor threshold determinations require methods that respond to sweat accumulating within the secretory coil, and not discharged secretions, reinforcing the importance of electrodermal techniques.     

Effects of body morphology and mass on thermal responses to cold water: revisited

Summary Seven male volunteers were divided into two groups based on body morphology and mass. The large body mass (LM) group (n = 4) was 21.2 kg heavier and 0.32 m2 · kg ^–1 smaller in surface area-to-mass ratio (P< 0.05) than the small body mass (SM) group (n = 3) Both groups were similar in total body fat and regional skinfold thicknesses. All subjects were immersed to the first thoracic vertebrae for 120 min in stirred water at 18°C. Overall, tissue insulation was similar (P>0.05) between the SM and LM groups across time. Perceptual ratings of thermal sensation increased (P<0.05) across time. Therefore, as cold water immersion progressed, tissue insulation was similar and thermal sensation increased (P<0.05) in males matched for subcutaneous fat and total fat but differing in body mass and morphology.     

Abnormal responses of retinal ganglion cells in Siamese cats to moving stimuli

Summary The responses of X and Y-Type retinal ganglion cells in Siamese cats to moving slits were compared to those in common cats, in order to assess the center/surround interactions in Siamese cat receptive fields. Responses in the latter animal were quantitatively lower than those in common cats, the RF centers were larger, and the encounter rate for Y-type optic tract fibers was significantly lower than in common cats. Whereas response enhancement of common cat units was predictable following masking of the surround, such an effect could only be observed in Siamese cats when the contrast between the stimulus and background was increased considerably. These data suggest anomalous center/surround interactions in Siamese cats, probably due to the presence of weaker surround influences in that animal.This investigation was supported by NIH grants EY01444 to Y.M.C. and EY00701 to D.I.H.     

Day-night difference in thermoregulatory responses to olfactory stimulation

Previously, we observed that olfactory stimulation with scent of grapefruit oil (SGFO) or scent of lavender oil (SLVO) affected, elevated or lowered brown adipose tissue temperature (BAT-T) in conscious mice, respectively. In the present study, to test the day-night difference in the actions of olfactory stimulations, we examined the responses of BAT-T and body temperature (BT) measured as the abdominal temperature to SGFO or SLVO during day-time at 14:00 and night-time at 2:00 in conscious rats. In the light period, BAT-T and BT were suppressed after SLVO and elevated after SGFO whereas in the dark period, these parameters remained unchanged with olfactory stimulations. Bilateral lesions of the hypothalamic suprachiasmatic nucleus (SCN) eliminated the effects of olfactory stimulations with SGFO and SVLO on BAT-T and BT. Moreover, sympathetic nerve activity innervating brown adipose tissue (BAT-SNA) changes after SGFO or SLVO were abolished in SCN-lesioned rats. Thus, we concluded that there is day-night difference in the effects of SGFO or SLVO on BAT-T and BT, and that the SCN might be involved in these effects.