Abstract: Prospective memory (PM) refers to the task of remembering to perform an intended action at some point in the future. PM is divided into event-based prospective memory (EBPM) and time-based prospective memory (TBPM). However, there are some kinds of complicated PM in real life, one of which is that intended action is based on target event (i.e., PM target) with the time of PM targets occurrence known. This kind of PM is called mixed time-based and event-based prospective memory (Mixed PM). There is no research on this type of mixed PM. Whether its processing mechanisms can be explained by existing theories of EBPM and TBPM is worth exploring. The current study aimed to examine the cognitive mechanisms of this mixed PM with two experiments.
Experiment One manipulated the knowability of execution time of PM and examined whether the interference effect existed during PM task. Three conditions were tested. In the baseline condition subjects had no PM task and merely performed 1-back task. In the EBPM condition, subjects were told to perform 1-back task and EBPM task embedded, but when the PM targets would occur was unknown. In the mixed PM condition, the same 1-back task as in the baseline and a mixed PM task were required and when the PM target would occur was known. The only difference between the two PM conditions was whether the time of PM targets occurrence was known. Results show that preparatory attentional process was engaged in PM persistently in the EBPM condition. But in the mixed condition, preparatory attentional process was selectively engaged in the target time windows, and the allocation of attentional resources was dynamic in the process of PM task.
Experiment Two examined the cognitive process of the mixed PM in which PM targets occurred ahead of time. It was found that no subject allocated attentional resources to PM target before the first PM appeared in advance. Otherwise, some subjects fulfill the PM task successfully without monitoring. Then some subjects began to change their strategies of resources allocation and preparatory attentional process was engaged in the rest part of PM task. The other subjects still didn’t allocate attentional resources to monitor PM target until the second PM target appeared. Results also suggested that the way to allocate attentional resources to PM target could be selective and dynamic in the mixed PM task.
We come to the conclusion that processing mechanisms of the mixed PM were different from those of EBPM and TBPM. To make the multiprocess theory account for the cognitive process of the mixed PM, the current research forms a supplement to the multiprocess theory that multiple processes support PM execution in a dynamic mode. This research also demonstrates that preparatory attentional process is not necessary for PM execution but its involvement improves PM performance.

Key words: prospective memory, mixed time-based and event-based prospective memory, prospective interference effect, processing mechanisms