displays are monitored, suggesting that searching among more displays results in decreased performance. This effect receives partial support from the present study. Performance was found to be significantly higher in the small as compared to large display size condition, but the decline in performance over time was comparable across display size. Observers also adopted a more conservative search strategy over time. A change in response criteria often accompanies a change in perceptual sensitivity in vigilance studies because observers become more familiar with the actual probability of the occurrence of a critical signal and are less willing to respond when they are unsure ( Davies & Parasuraman, 1982).
Although these results agree with predictions made from vigilance research, an interaction between feature type and display size was also found. This interaction appears to support the predictions of Treisman and her colleagues ( Treisman & Gelade, 1980; Treisman & Souther, 1985). Mean A' scores in the feature presence conditions were similar in the two display sizes, suggesting the use of parallel processing. In the feature absence conditions, however, mean A′ scores were significantly higher in the small display size condition, indicating the use of a more deliberate type of processing that is negatively impacted by the number of distractors on the display.
A three-way interaction between feature type, display size, and monitoring period was not found. Instead, performance declined over time in a similar manner in every condition. These results suggest that Treisman's predictions do not generalize to vigilance situations, but only hold up under alerted conditions. Another possibility is that the absence of a significant interaction may have been due to the differences in exposure times of the large and small display size conditions. By using longer exposure times in the large display size condition, differences in performance over time may have been eliminated. This possibility is currently being investigated in a study in which the initial levels of difficulty (as indicated by mean A′ scores) have been equated.
The current study also examined subjective mental workload ratings before and after the vigil. In accordance with other vigilance studies (e.g., Scerbo et al., 1992), workload scores were found to be higher after the vigil. Further, there was some evidence that the magnitude of this increase was depended upon signal type. Although workload ratings increased somewhat in the feature presence conditions, the postvigil ratings were nearly twice as high in the feature absence conditions. These results suggest that although observers in both feature conditions found the vigilance task to be demanding, those searching for the absence of a line experienced higher levels of mental workload.
Results from the present study suggest that monitoring for the presence or absence of features in automated systems can have dramatically different effects on both performance and mental workload. When designing automated systems, care should be taken to ensure that critical information is represented by the presence of features instead of their absence. Operators who must look for the absence of features run a greater risk of missing critical information. Moreover, they may perceive that activity as more demanding.
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