The processing mechanism of mixed prospective memory: Changes in internal and external attention.

While time-based prospective memory (TBPM) includes only time cues, mixed prospective memory (MPM) is a special form of prospective memory including both time and event cues. Depending on the classification of the clarity of time cues, MPM can be divided into time-period MPM and time-point MPM. While the time cue of the latter is a definite time point, the time cue of the former is a vague time period. As such, MPM and TBPM may have different processing mechanisms due to the additional event cue. This study aimed to investigate whether there are differences in the processing mechanisms between TBPM and the two types of MPM. A total of 240 college students were recruited to participate in the experiment. They were randomly assigned to a TBPM group, time-point MPM group, time-period MPM group, and baseline group. We adopted the performance of ongoing tasks to reflect internal attention indirectly and the frequency of time checks to measure external attention. The results showed that in terms of prospective memory, time-point MPM had the best performance, followed by time-period MPM, while TBPM had the worst performance. In relation to ongoing tasks, the two types of MPM had a better performance than TBPM in some stages, although worse than the baseline. In addition, the two MPMs evoked a lower time monitoring frequency than TBPM under different monitoring conditions. These results suggested that, compared with TBPM, MPM reduced both internal and external attention consumption and achieved better prospective memory performance. Internal attention consumption displayed dynamic changes for both types of MPM, and the time-point MPM had higher internal attention effectiveness than the time-period MPM. These results support the Dynamic Multiprocess Theory and the Attention to Delayed Intention model.