Arousing Stresses Increase Vigilance

Abstract

The optimum environment for vigilance tasks is rather more arousing than the optimum for tasks which are intrinsically more interesting. Moderate heat reduces vigilance, but heat intense enough to raise the body temperature probably increases vigilance. There may also be an increase in vigilance in mild heat which is a little uncomfortable, and an initial increase in vigilance on first entering the heat. Vigilance declines rapidly with heat exhaustion.

Continuous unvarying noise increases vigilance. Performance deteriorates only when the noise masks the auditory feedback from the man’s controls which he uses in quiet, or when the noise masks the inner speech which he uses to assist his short-term memory. Intermittent or variable noise also increases vigilance, unless the task is susceptible to distraction.

Vertical vibration at 5 Hz increases vigilance. This is probably because the vibration of the shoulders at this frequency can be attenuated by increasing the tension of the trunk muscles. The need to tense the trunk muscles provides man with an alerting mechanism.

Vigilance is high when a person in perceptual isolation is asked to perform a vigilance task. Physical exercise also probably increases vigilance, unless it is too exhausting.

Unfortunately many of the potentially most useful experiments on arousing stresses use designs in which each person performs a number of conditions one after the other. The differences between the conditions are then confounded by uncontrolled transfer between conditions. The reliable interactions reported when two or more stresses are combined are also confounded by uncontrolled transfer.