John O. Simon Joel S. Warm William N. Dember University of Cincinnati
Vigilance or sustained attention tasks require observers to detect transient signals over prolonged periods of time. A major characteristic of performance in such tasks is the event-rate effect, the finding that the accuracy of signal detections varies inversely with the rate of repetition of non-signal background events among which critical signals for detection appear ( Warm & Jerison, 1984).
Posner ( 1978) has suggested a mechanism involving the inhibition of psychological pathways, or sets of internal codes and their interconnections, which can be used to account for the event-rate effect. According to Posner, repeated stimulation leads to inhibition in a pathway; accumulated inhibition interferes with the activation of central processing mechanisms, thereby degrading performance efficiency. Since the accumulation of inhibition varies directly with the rate of stimulation, performance is poorer with fast than with slow event-rates.
Galinsky, Warm, Dember, Weiler, and Scerbo ( 1990) attempted to attenuate the build-up of pathway inhibition by shuttling stimulation between two sensory modalities (vision and audition) carrying common information. The results of their study indicated that sensory alternation did eliminate the event-rate effect.
One goal for the present study was to determine whether a similar effect could be achieved by alternating information pathways within a single sensory channel. Color channels (red-green, yellow-blue; Kandel, Schwartz, & Jessel, 1995) were chosen for that purpose. Using the study by Galinsky et al. as a guide, and the fact that duration is a common attribute of all sensory channels, the present test of Posner's pathway inhibition model also involved temporal discriminations, but with carrier-stimulus alternation across color channels. It was anticipated that cross-channel or heterogeneous-channel alternation (red-blue or yellow-green) would attenuate the build-up of inhibition within carrier channels and lead to a weakened event-rate effect in comparison to stimulation that was alternated within a color channel or homogeneous alternation (red-green or yellow-blue), or maintained on the same color channel throughout the vigil (no- alternation).
Ninety-six students, 48 men and 48 women, served as observers to fulfill a course requirement. They ranged in age from 18 years to 49 years with a mean age of 23 years. All of the students had normal color vision and normal or corrected-to-normal visual acuity. All observers participated in a 40-min vigil composed of four continuous 10-min periods of watch. In all conditions, they monitored the repetitive presentation of a 2mm x 9mm colored bar (red, green, blue, or yellow) which appeared in the center a video display terminal (VDT). Except for the bar, the VDT was uniformly black. Following Galinsky et al. ( 1990), neutral events were flashes of the bar lasting for 248 msec. Critical signals for detection were briefer (125 msec) flashes.
Observers in the no-alternation conditions monitored the same colored bar throughout the vigil. Each color, red, green, blue, or yellow, was assigned to equal numbers of male and female observers. In the alternating groups, bar colors were interchanged at 5 min intervals during each 10 min period of watch, consistent with the audio-visual alternation schedules employed by Galinsky et al. ( 1990). In the homogeneous (same channel) alternation conditions, equal numbers of observers (balanced for sex) experienced alternation in the orders red → green, green → red, blue → yellow, or yellow → blue. In the heterogeneous (cross-channel) alternation conditions, equal numbers of observers (balanced for sex) experienced