Abstract
The effects of four experimental conditions on performance in a vigilance task were studied using eight subjects in a repeated measurements design. The four conditions were: visual detection alone; auditory detection alone; combined (redundant) visual and auditory detection; and alternating, in 30-min periods, auditory and visual detection. Subjects were required to detect 0.1-sec aperiodic interruptions in either or both a visually displayed horizontal line or a 1000 c/s tone. The intensities of both signals were adjusted to near-threshold levels against constant noise backgrounds. Data were collected on the number of signals detected, the number of false responses, and response times during the 2-hr testing session. It was found that: (1) sensory alternation was an effective means of maintaining alertness; (2) redundant, dual sense monitoring resulted in significantly fewer false responses; (3) there were no differences between the four conditions with respect to median response times; and (4) response times did increase with the length of the watch over the four conditions.

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