Abstract
In a system in which the operator must accomplish two or more tasks at the same time, an example of a method for preventing a serious accident would be sounding an alarm and giving a visual cue simultaneously. However, errors can come from inside or outside a system that might cause an alarm to fail. If this occurs, the operator may come to doubt or distrust the safety system, and its effectiveness is ruined. This can be very stressful for the operator's mental workload. In this study, we examined the influence of the absence of an alarm sound following some use of the system at 100% accuracy during performance of a cognitive task. We also examined the influence of the absence of the alarm as measured by EEG, HRV, and other tests as an index of mental workload. We used the difference in warning presentation rate as a way of measuring the influence on performance. The performance was the worst and the sympathetic nerve activity was highest in the 50% warning presentation rate condition, and the performance was better for where warning was not presented (0%) or at all of the presentation without fail (100%), and the relation between the warning presentation rate and the task performance showed the tendency to a reverse-u-shaped. It was suggested by this result that the absence of a warning sound brings about a decrease in a subject's performance and a rise in the operator's mental workload.
