Electrophysiological Correlates of Strategic Monitoring in Event-Based and Time-Based Prospective Memory

Giorgia Cona,Vincenza Tarantino,Patrizia Silvia Bisiacchi,Giorgio Arcara,Sam Gilbert

doi:10.1371/journal.pone.0031659

Abstract

Prospective memory (PM) is the ability to remember to accomplish an action when a particular event occurs (i.e., event-based PM), or at a specific time (i.e., time-based PM) while performing an ongoing activity. Strategic Monitoring is one of the basic cognitive functions supporting PM tasks, and involves two mechanisms: a retrieval mode, which consists of maintaining active the intention in memory; and target checking, engaged for verifying the presence of the PM cue in the environment. The present study is aimed at providing the first evidence of event-related potentials (ERPs) associated with time-based PM, and at examining differences and commonalities in the ERPs related to Strategic Monitoring mechanisms between event- and time-based PM tasks.The addition of an event-based or a time-based PM task to an ongoing activity led to a similar sustained positive modulation of the ERPs in the ongoing trials, mainly expressed over prefrontal and frontal regions. This modulation might index the retrieval mode mechanism, similarly engaged in the two PM tasks. On the other hand, two further ERP modulations were shown specifically in an event-based PM task. An increased positivity was shown at 400–600 ms post-stimulus over occipital and parietal regions, and might be related to target checking. Moreover, an early modulation at 130–180 ms post-stimulus seems to reflect the recruitment of attentional resources for being ready to respond to the event-based PM cue. This latter modulation suggests the existence of a third mechanism specific for the event-based PM; that is, the “readiness mode”.

Highlights

In everyday life, individuals are often required to retrieve intentions from memory for correctly fulfilling a task at the appropriate time
The present study provides the first evidence of commonalities and differences in the electrophysiological correlates of Strategic Monitoring between time-based and event-based prospective memory tasks
Modulations were selectively found in the event-based prospective memory (PM) task: an increased positivity over occipital and parietal regions occurring between 400–600 ms post-stimulus, and an early modulation, occurring between 130–180 ms post-stimulus

Summary

Introduction

Individuals are often required to retrieve intentions from memory for correctly fulfilling a task at the appropriate time. The retrieval mode, conceptualised as the process of maintaining the intention continuously active in memory [6,10] would be common between event-based and time-based PM tasks; for this reason, it should be reflected in a similar pattern of sustained ERP modulations in the two PM tasks. We expected this ERP activity to be more expressed over frontal regions, in line with the majority of previous neuroimaging findings [e.g. 25,43–45]. Since the ERPs were time-locked to the onset of the ongoing stimuli, possible modulations expressed on the ERPs in the event-based, but not in time-based task, might be associated with this additional process of verifying whether ongoing stimuli contain the PM cue, which is a process required to fulfil event-based intentions

Materials and Methods

Results

Discussion