Search asymmetry was used to test two theories of sustained attention lapses currently debated in the literature: the boredom-mindlessness theory and the resource depletion-mental fatigue theory. Participants performed feature present and a feature absent target detection tasks using either a sustained attention to response task (high Go low No-Go) or a traditionally formatted task (high No-Go low Go) response format. In addition to performance, functional near infrared spectroscopy was employed to measure lateral cerebral oxygenation levels and self-reports of tense arousal, energetic arousal, task related and unrelated thoughts occurring during the tasks were utilised. Detections were lower and reaction times longer in the feature absent search than the feature present search regardless of response format. Detections were lower, but reaction times shorter in the sustained attention to response task than the traditionally formatted task regardless of feature search. Greater right than left frontal hemisphere activation occurred in the sustained attention to response task than the traditionally formatted task. In addition, the sustained attention to response task was more fatiguing based on self-reports than the traditionally formatted task, but there were no differences in Task-Unrelated Thoughts across task conditions. Overall, the results of this study support a resource theory explanation of sustained attention lapses, not a mindlessness-boredom theory explanation. Moreover, the results suggest the sustained attention to response task places high response inhibition, not sustained attention, demands on participants.

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