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Abstract

Behavioural responses are the most efficient thermoregulatory responses, since relatively simple actions
can prevent the need to activate the more metabolically costly autonomic responses. Dependence on
behavioural responses may increase in environmental conditions where the prevailing nonthermal factors
(NTF) attenuate autonomic responses and alter thermal perception (Mekjavic et al., 2003; Mekjavic and
Eiken, 2006). Thermal (dis)comfort is considered to be the driving force for the initiation of behavioural
thermoregulatory responses (Weiss and Laties, 1961). Thus, alteration in the perception of thermal comfort
by an NTF might jeopardize normothermia by preventing the initiation of appropriate behavioural
responses (Mekjavic et al., 1994). This was confirmed in animal studies (Pertwee et al., 1986; Macdonald
et al., 1989; Pertwee et al., 1990), but data in humans are lacking. Furthermore, there is currently no
accepted method to evaluate behavioural thermoregulation in humans. The studies presented in this
dissertation address both the need for a new method of assessing behavioural thermoregulation in humans
(Study 1), and the need to evaluate how NTFs may influence this ability. Particular reference was given to
the effects of mild narcosis (Study 2) and prolonged bed rest (Study 3). Both of these NTFs represent
conditions that humans may encounter when performing in extreme environments (e.g., deep sea and
space).
Study 1: A new method for assessing thermal comfort and behavioural thermoregulation in humans
There being no accepted method for assessing thermoregulatory behaviour in humans, the aim of the first
study was to develop an experimental procedure for assessing behavioural thermoregulation. It is well
accepted that behavioural thermoregulatory responses are initiated once the surrounding environment is
perceived as thermally uncomfortable. The correlation between these two events, namely the change in
thermal perception and initiation of a behavioural response to counteract it, were assessed by the new
procedure. The experimental set-up comprised a water-perfused suit (WPS) with a manual control system
designed to allow both the experimenter and the subject to control the temperature of the water perfusing
the WPS (Twps). The zone of thermally comfortable Twps (TCZ) was evaluated in 12 subjects (6 males and 6
females) in two trials. In each trial, a control unit changed the Twps in a sinusoidal manner from 27°C to
42°C, at a rate of 1.2°C/min. In the first trial, the subjects could only report their perception of the
fluctuating Twps and in the second, they could change the direction of Twps when they perceived it to be
slightly uncomfortable. The subjects regulated the Twps within a preferred range for a total duration of one
hour. The results demonstrate that the subjects reproducibly identified the boundaries of their TCZ on three
separate occasions. The threshold Twps perceived as slightly uncomfortable was highly correlated with the
threshold Twps at which the subjects initiated a behavioural response to counteract it. It was concluded that
thermal preference could be reliably detected by evaluating the manner in which subjects behaviourally
regulate their surrounding temperature. Furthermore, gender-related differences were found when the subjects controlled the WPS temperature. Females preferred higher Twps than males for thermal comfort.
This suggested that the method could be used to determine the effect of nonthermal factors on behavioural
temperature regulation.
Study 2 investigated how inert gas narcosis, a nonthermal factor that humans might encounter in hyperbaric
environments, influences temperature perception, thermal comfort, and behavioural temperature regulation,
based on the methodology that was developed in study 1. Twelve subjects (the same as in study 1)
participated in two trials. During the trials, subjects wore a water-perfused suit (WPS). The temperature of
the WPS (Twps) fluctuated sinusoidally from 27°C to 42°C, at a heating and cooling rate of 1.2°C/min. In
the first trial, the subjects had no control over the Twps, they determined their thermal comfort zone (TCZ)
by providing a subjective response whenever they perceived the temperature changing from a comfortable
to an uncomfortable level and vice versa, and provided subjective ratings of temperature perception and
thermal comfort on 7-point and 4-point scales, respectively, at each 3°C change in Twps. As in study 1, in
the second trial, subjects could change the direction of Twps whenever it became uncomfortable by
depressing a button on a manual control. The protocols were conducted with subjects breathing either room
air (AIR), or a normoxic breathing mixture containing 30% N2O. Subjects perceived increasing Twps as
equally warm and the decreasing Twps as equally cold with AIR or N2O. However, equal changes in Twps
were perceived as significantly less discomforting (P<0.05) in the presence of N2O and the magnitude of
the TCZ significantly (P<0.01) increased. Thus, narcosis did not alter thermal sensation, but it significantly
changed the perception of comfort. These changes were not reflected in the behavioural response. Subjects
produced similar Twps damped-oscillation patterns in the AIR and N2O trials. It was concluded that the
narcosis-induced alteration in the peception of thermal comfort does not change either the preferred
temperature, or the ability to behaviourally maintain thermal comfort.
Study 3 evaluated the utility of the developed methodology in assessing changes in behavioural
temperature regulation due to overall deconditioning. The musculoskeletal and cardiovascular
deconditioning that is observed with microgravity or prolonged inactivity, was established by maintaining
subjects in a horizontal position for 21 days (bed rest experiment). Previous thermal studies using a bedrest
model suggested that changes in thermal sensitivity and comfort might contribute to the elevated core
temperature observed during spaceflights (Rimmer et al., 1999). Thus, in addition to assessing behavioural
thermoregulation, cutaneous cold and warm sensitivity were also evaluated. Healthy male subjects (n=10)
were accommodated in a hospital ward for the duration of the study, and were under 24-hr medical care.
All activities (eating, drinking, hygiene, etc.) were conducted in the horizontal position. On the 1st and 22nd
days of the bed rest cutaneous temperature sensitivity was tested by applying cold and warm stimuli of
different magnitudes to the volar region of the forearm via a Peltier element thermode. As in studies 1 and
2, behavioural thermoregulation was assessed by having the subjects regulate the temperature of the water
within a WPS they were wearing. In this study, however, the Twps varied from 27°C to 42°C at a faster rate
(2.1°C/min) and required that the subjects alter the direction of the change in Twps more frequently. The
magnitude of the oscillations towards the end of the trial was assumed to represent the upper and lower
boundaries of the TCZ. The results demonstrate that there were no significant differences in the TCZ or the
pattern of the regulated Twps after bedrest. Furthermore, a higher rate of thermal changes at the WPS did not
significantly change the preferred temperature or its behavioural control. In contrast, the cutaneous threshold for detecting cold stimulus decreased (p<0.05) from 1.6 (1.0) ºC on Day 1 to 1.0 (0.3) ºC on Day
22. No effect was observed on the ability to detect warm stimuli. It was concluded that although cold
sensitivity increased after bed rest, it was not of sufficient magnitude to cause any alteration in behavioural
thermoregulatory responses.
The contribution of the present study is the development of a new, reliable, experimental procedure for
evaluating behavioural thermoregulation in humans. As opposed to obtaining subjective scale ratings of
thermal comfort, with the new method behavioural responses can be evaluated directly with minimal
intervention of the observer. The results indicate that the proposed experimental protocol enabled subjects
to identify the thresholds of warm and cold discomfort in a reproducible manner and that a higher rate of
change in Twps did not alter this ability. The upper and lower peaks of the Twps pattern that are obtained
when subjects maintain thermal comfort provide a valid measure of the threshold temperature eliciting
warm and cold discomfort.
With respect to the influence of NTFs on behavioural thermoregulation, the results of the present
research do not support the hypothesis that altered perception of comfort attenuates behavioural
thermoregulation. Mild narcosis and prolonged bedrest did not significantly change thermoregulatory
behaviour despite their significant influence on thermal sensation and comfort.