Deconstructing episodic memory with construction

It has recently been observed that the brain network supporting recall of episodic memories shares much in common with other cognitive functions such as episodic future thinking, navigation and theory of mind. It has been speculated that ‘self-projection’ is the key common process. However, in this Opinion article, we note that other functions (e.g. imagining fictitious experiences) not explicitly connected to either the self or a subjective sense of time, activate a similar brain network. Hence, we argue that the process of ‘scene construction’ is better able to account for the commonalities in the brain areas engaged by an extended range of disparate functions. In light of this, we re-evaluate our understanding of episodic memory, the processes underpinning it and other related cognitive functions.

Introduction

Episodic memory 1, 2, the memory for our everyday personal experiences, is an essential ingredient in shaping how we perceive ourselves [3]. Tulving 2, 4 seminally defined three key properties of episodic memory recall: a subjective sense of time (mental time travel), connection to the self, and autonoetic consciousness – a special kind of consciousness that accompanies the act of remembering, enabling one to be aware of the self in subjective time [2]. Others have since identified visual imagery [5], narrative structure [5], retrieval of semantic information [6], and feelings of familiarity [7] as also being important aspects of recollection. Episodic memory recall is, therefore, a highly complex cognitive function that can be conceptually divided into several distinct component processes 3, 6, 8, 9 and is accompanied by a rich recollective experience (Box 1) 2, 4. Although numerous functional magnetic resonance imaging (fMRI) studies investigating the neural basis of episodic memory recall 10, 11, 12 have revealed a consistent and distributed network of associated brain regions, surprisingly little is understood about the contributions individual areas make to the overall recollective experience 10, 11, 12.

In a stimulating Opinion article published in the February 2007 issue of Trends in Cognitive Sciences, Buckner and Carroll [13] make the astute observation that there is extensive overlap in the brain network activated during fMRI studies of remembering the past 10, 11, 12, and that engaged during other activities as diverse as thinking about the future 14, 15, 16, navigation [17], theory of mind (perspective taking) [18], and the ‘default network’ [19] (and related to the ‘default network’, perhaps also ‘mind wandering’ [20]). This presents the intriguing possibility that these disparate cognitive functions, hitherto treated as distinct, might share common underlying processes. Buckner and Carroll [13] suggest that self-projection might be a crucial common process. They define self-projection as ‘the ability to shift perspective from the immediate present to alternative perspectives...requiring a shift in perception from the immediate environment to the alternative, imagined future environment...referenced to oneself’. Thus, their proposal closely connects to Tulving's original ideas of mental time travel of the self [4]. Self-projection is undoubtedly important, perhaps even uniquely so for episodic memory recall and thinking about the future [2]. However, recent convergent neuropsychological [21], electroencephalographic (EEG) [22] and fMRI (D. Hassabis, D. Kumaran and E.A. Maguire, unpublished) findings suggest that at least one further important cognitive function, namely that of richly imagining fictitious experiences [21], is also reliant on the same brain network, but is not explicitly connected to either the self or a subjective sense of time.

Our Opinion article differs from that of Buckner and Carroll [13] in several ways. First we put forward the case for mental scene construction as a well-defined and key component process in supporting recollective experiences. Second, we argue that rather than self-projection [13], the process of scene construction is better able to account for the commonalities in the brain networks activated by all the disparate cognitive functions noted above (and possibly others in addition – see Box 2). Third, we link the process of scene construction with existing theories that view the recollection of complex episodic memories as a (re)constructive process 3, 4, 5, 8, 23, 24, 25.