Auditory Vigilance in Repeated Sessions

In spite of the widespread interest in problems of vigilance and the relatively large number of vigilance studies, few attempts have been made to study the effects of practice. Although Buckner, et al. (1960) found that performance on a combined auditory discrimination vigilance task improves with repetition, no practice effects were noted for a similar task of visual monitoring. The primary purpose of the Buckner study, however, was to measure individual differences in monicoring ability. In another study, specifically designed to measure the effects of repeated and prolonged sessions, Webb and Wherry (1960) found no change in the detection of auditory signals. Their sample, however, consisted of only three Ss and the signal duration used was quite long (3 sec.). Not only have vigilance investigators consistently found that individuals differ widely in their monicoring ability, but also at least one investigator, Adams (1956), reported that detection probability over time is a direct function of signal duration. Since the Webb and Wherry results may well have been due to either one or both of these factors, it was felt desirable to repeat the Webb and Wherry study using a larger sample of Ss and a signal of shorter duration. METHOD Ss were 20 Army enlisted men undergoing military training at Fort Knox, Kentucky. The average age was 23 yr., and all were free of hearing defects as determined by Army medical standards. The signal was a brief interruption (.03 sec. as measured by a Hunter Klockounter) of a continuous white noise source presented to S over PDR-8 earphones. To prevent S from learning a particular schedule, two different aperiodic signal schedules were used. In the two schedules, the intersignal intervals ranged from 45 to 600 and from 32 to 360 sec. In both schedules, the intersignal intervals were randomized by means of a table of random numbers. Both schedules, however, had the same mean, over-all intersignal interval of 2.5 min. White noise was provided by a Grayson-Stadler noise generator (Model 455B). The sound level was 44 db above the average threshold for 10 staff military Ss. The signals, interruptions in the white noise, were made by a Gerbrands variable interval programmer using punched-paper tape and a simple 24-v d.c. relay timing circuit. Ss' signal detections were recorded on a model AW voltage type Esterline-Angus operations recorder. Each S monitored the noise from an individual isolated room for 30 min. in the morning (0800-0930) and 90 min. in the afternoon (1300-1430) for five successive