Elsevier

Neuropsychologia

Volume 49, Issue 8, July 2011, Pages 2246-2257
Neuropsychologia

Functional neuroimaging studies of prospective memory: What have we learnt so far?

https://doi.org/10.1016/j.neuropsychologia.2011.02.014Get rights and content

Abstract

The complexity of the behaviour described by the term “prospective memory” meant that it was not at all clear, when the earliest studies were conducted, that this would prove a fruitful area for neuroimaging study. However, a consistent relation rapidly emerged between activation in rostral prefrontal cortex (approximating Brodmann Area 10) and performance of prospective memory paradigms. This consistency has greatly increased the accumulation of findings, since each study has offered perspectives on the previous ones. Considerable help too has come from broad agreement between functional neuroimaging findings and those from other methods (e.g. human lesion studies, electrophysiology). The result has been a quite startling degree of advance given the relatively few studies that have been conducted. These findings are summarised, along with those from other brain regions, and new directions suggested. Key points are that there is a medial–lateral dissociation within rostral PFC. Some (but not all) regions of medial rostral PFC are typically more active during performance of the ongoing task only, and lateral aspects are relatively more active during conditions involving delayed intentions. Some of these rostral PFC activations seem remarkably insensitive to the form of stimulus material presented, the nature of the ongoing task, the specifics of the intention, how easy or hard the PM cue is to detect, or the intended action is to recall. However there are other regions within rostral PFC where haemodynamic changes vary with alterations in these, and other, aspects of prospective memory paradigms. It is concluded that rostral PFC most likely plays a super-ordinate role during many stages of creating, maintaining and enacting delayed intentions, which in some cases may be linked to recent evidence showing that this brain region is involved in the control of stimulus-oriented vs. stimulus-independent attending. Other key brain regions activated during prospective memory paradigms appear to be the parietal lobe, especially Brodmann Area (BA) 40 and precuneus (BA 7), and the anterior cingulate (BA 32). These regions are often co-activated with lateral rostral PFC across a wide range of tasks, not just those involving prospective memory.

Introduction

Prospective memory is a very new field of enquiry, but one in which there has been a startling increase of interest over the last few years. As Sellen, Louie, Harris, and Wilkins (1997) point out, a review approximately 20 years ago (Kvavilashvili, 1992) cited only twenty-four experimental studies of prospective memory. Yet there are now several entire books devoted to the topic (e.g. Brandimonte et al., 1996, Glicksohn and Myslobodsky, 2006, Kliegel et al., 2007, McDaniel and Einstein, 2007), and substantial international conferences occupied exclusively with research into prospective memory (the first International Conference on Prospective Memory was held in Hertfordshire, UK, in July, 2000). This explosion of interest has been accompanied by a change in the way that “prospective memory” has been viewed.

The earliest studies, which were largely behavioural only, tended to be concerned with the practical aspects of remembering intentions, often relating to everyday life situations, rather than experimental paradigms (e.g. Wilkins & Baddeley, 1978). Accordingly, the emphasis was very much on the study of situational and behavioural characteristics affecting delayed intentions. However as the field became more experimental, and with the help of a few key papers (e.g. Kvavilashvili, 1987) the notion grew that there might be cognitive processes or constructs unique to this form of memory (in the sense of not also being shared with memory for past events). From the viewpoint of a scientist trying to discover principles of behaviour, or of brain–behaviour relations, this would be fortuitous, making discovery simpler. However from a biological viewpoint, at the extreme at least, such an arrangement would likely be exceptionally inefficient: prospective memory is a highly complex and multi-componential behaviour so it is unlikely to have representations and brain structures dedicated entirely to it. Such a principle, carried across all cognitive functions would likely be far too costly, so it is always more likely that at both an information processing and functional level that processes and structures operate across psychological domains, or at least across our narrow definitions of subject fields. In this way it was clear to many from the outset that it would be implausible that the study of prospective memory would grow entirely independently from other fields of conceptual relevance (for review see Ellis, 1996).

And so, in large part has it transpired. However, there has been a particularly fortuitous finding for cognitive neuroscientists studying prospective memory: a consistent relation between activation in rostral PFC during prospective memory paradigms. This has given an invaluable inroad into a topic that at one time had little or no precedent within cognitive neuroscience. Moreover, as more information about this brain region from the study of other functions appears, the field is being encouraged to consider prospective behaviour more broadly, with the potential to encompass not just prospective memory, but many other forms of “prospection”.

Section snippets

Characteristics of typical prospective memory paradigms

There are a very wide variety of situations in everyday life that require prospective memory. However, for experimental purposes in cognitive neuroscience the essential features of these situations have been simplified. The typical features of prospective memory paradigms are summarised by Burgess, Scott, and Frith (2003). These are:

  • (a)

    There is an intention, or multiple intentions to carry out a mental or physical act. A variant might be that the intention is to withhold an act that is performed

Findings in brain regions outside rostral PFC

Finding 6 concerns activations outside rostral PFC that are also commonly found during PM tasks. A large proportion of them seem to be (bearing in mind the current temporal limits of our technology) co-activations with rostral PFC. In particular, there is a strikingly consistent finding in current functional neuroimaging studies of activation within BA 7 & 40 (precuneus; parietal lobe) in prospective memory studies. Activation within one, or both of, these regions is almost as common as for

Findings awaiting replication

Findings 7–14 (Table 1) relate either to single findings within the field, or where there is substantial apparent difference in findings amongst studies, but where each finding seems nevertheless secure. It is only on these grounds that the recommendation is made that the findings should be replicated: this is not at all a comment on the quality of the individual studies. In general, these demonstrations also might perhaps emphasise how it is that at least some regions of rostral PFC seem to

Conclusion: from prospective memory to future thinking

In recent years, investigators and theorists from various different disciplines (e.g. economics, cognitive neuroscience, psychology, anthropology) have shown an interest in an emerging field. This concerns itself with how people envisage future scenarios, and the effect that doing so has upon behaviour both in the present and the future. In general, it seems possible that the involvement of rostral PFC in prospective memory and also the many other forms of behaviour which have to do with

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