Distinct monitoring strategies underlie costs and performance in prospective memory

Prospective memory (PM) describes the ability to remember to perform goal-relevant actions at an appropriate time in the future amid concurrent demands. A key contributor to PM performance is thought to be the effortful monitoring of the environment for PM-related cues, a process whose existence is typically inferred from a behavioral interference measure of reaction times. This measure, referred to as "PM costs," is an informative but indirect proxy for monitoring, and it may not be sufficient to understand PM behaviors in all situations. In this study, we asked participants to perform a visual search task with arrows that varied in difficulty while concurrently performing a delayed-recognition PM task with pictures of faces and scenes. To gain a precise measurement of monitoring behavior, we used eye-tracking to record fixations to all task-relevant stimuli and related these fixation measures to both PM costs and PM accuracy. We found that PM costs reflected dissociable monitoring strategies: higher costs were associated with early and frequent monitoring while lower costs were associated with delayed and infrequent monitoring. Moreover, the link between fixations and PM costs varied with cognitive load, and the inclusion of fixation data yielded better predictions of PM accuracy than using PM costs alone. This study demonstrates the benefit of eye-tracking to disentangle the nature of PM costs and more precisely describe strategies involved in prospective remembering.