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Resilience and Survival in Extreme Environments 155

Psychological Stress

Psychological resilience to stress contributes substantially to the ability
(or inability) to withstand environmental stress, and thus presents one of
the most accessible avenues for modification of response to environmen-

tal demands. For example, in some conditions, resilience can be improved
by training human subjects to achieve optimal physiological responses
to chronic or acute psychological stress (Crews & Landers, 1987). Acute
psychological stress has been studied extensively in sport parachutists and
in military trainees who perform dangerous jumping or sliding maneu-
vers. Experienced parachutists show an initial prejump surge of autonomi-
cally mediated heart rate followed by a decline to normal levels when they
finally jump. On the contrary, novice jumpers experience a continuous rise

in heart rate right up to the moment of the jump (Fenz & Epstein, 1967;
Ursin, Levine & Baade, 1978; Wittels, Rosenmayr, Bischof, Hartter & Haber,

1994). Th ese different responses appeared to reflect individual diff erences in
perception of the stressful event, even with increasing experience, and were
also associated with jump performance (Fenz & Jones, 1972). Th e important
point of these studies is that training by experience seems to modify the
autonomic responses of individuals who will become successful parachut-

ists, and that this modified control of autonomic responses is important for

expert performance. Personal spiritual beliefs, self-confidence, and other
coping skills contribute to the perception and understanding of psycho-
logical stress, and these changes are internalized through brain physiology.
Only recently have we begun to unravel some of the mechanisms involved
in these eff ects, such as the role of endocannabinoids, their relationship to
nitric oxide and vascular relaxation, and their release by relaxation strate-
gies (produced by repeated mental or physical actions during avoidance of
distracting thought) (Stefano et al., 2003).
Chronic stress affects the hypothalamic-pituitary-adrenal (HPA) axis

and sustained adrenergic activation produces some maladaptive conse-
quences such as increased susceptibility to infectious disease, impaired short-
term memory, and loss of muscle mass and strength (Sapolsky, 1996, 2005).
Th ese effects obviously impair health and performance. However, both the

response to chronic stress and its outcome can be mediated by psychological
resilience, as discussed in other chapters within this volume.
Psychological performance has been studied in relation to environmental
stressors such as dehydration, heat, cold, altitude, and workload. One of the
more reliable observed effects of environmental stress on cognition is a result-

ing reduction in short-term memory. Neurobiological mechanisms for this

effect appear to be mediated in the hippocampus, possibly through the release
of stress hormones. Imaging studies of the hippocampus in individuals suff er-
ing from major depression and posttraumatic stress disorder (PTSD) suggest

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