Using Imagination to Understand the Neural Basis of Episodic Memory

Abstract

Functional MRI (fMRI) studies investigating the neural basis of episodic memory recall, and the related task of thinking about plausible personal future events, have revealed a consistent network of associated brain regions. Surprisingly little, however, is understood about the contributions individual brain areas make to the overall recollective experience. To examine this, we used a novel fMRI paradigm in which subjects had to imagine fictitious experiences. In contrast to future thinking, this results in experiences that are not explicitly temporal in nature or as reliant on self-processing. By using previously imagined fictitious experiences as a comparison for episodic memories, we identified the neural basis of a key process engaged in common, namely scene construction, involving the generation, maintenance and visualization of complex spatial contexts. This was associated with activations in a distributed network, including hippocampus, parahippocampal gyrus, and retrosplenial cortex. Importantly, we disambiguated these common effects from episodic memory-specific responses in anterior medial prefrontal cortex, posterior cingulate cortex and precuneus. These latter regions may support self-schema and familiarity processes, and contribute to the brain's ability to distinguish real from imaginary memories. We conclude that scene construction constitutes a common process underlying episodic memory and imagination of fictitious experiences, and suggest it may partially account for the similar brain networks implicated in navigation, episodic future thinking, and the default mode. We suggest that additional brain regions are co-opted into this core network in a task-specific manner to support functions such as episodic memory that may have additional requirements.

The construction system of the brain

“…Sugiura et al 2005; figure 5). Moreover, there was no overlap between this simple object network (Sugiura et al 2005) and that of complex scene construction (Hassabis et al 2007a), suggesting that they represent dissociable cognitive processes with distinct neural bases. Nevertheless, complex scenes and experiences are clearly constructed out of simpler elements.…”

“…We have demonstrated that scene construction is a dissociable set of processes supporting the episodic memory system (Hassabis et al 2007a), both past and future, but what are some of the other processes that together with the construction system underpin the special properties of episodic memory? We addressed this question using our fMRI imagination paradigm by contrasting the recall of real memories to the recall of previously created imaginary memories matched for difficulty, age, detail and vividness, thus partialling out the effects of the common construction network (Hassabis et al 2007a).…”

“…Crucially, tasks involving imagined scenarios can be designed to de-emphasize key features allowing insights to be gained into the neural substrates of these features when compared with episodic memories (Hassabis et al 2007a). For example, participants can be asked to construct fictitious experiences in their imagination that are atemporal (i.e.…”

“…Our fMRI study also included tasks requiring vivid visualization of acontextual single objects as a baseline task (Hassabis et al 2007a). Recalling previously seen or previously imagined objects, or imagining objects for the first time in the scanner resulted in activation of brain areas associated with supporting object representations and manipulations, namely lateral occipital complex and intraparietal sulcus (e.g.…”

“…From functional magnetic resonance imaging (fMRI ) studies, we also know that a distributed and highly consistent network of brain regions supports memory for past experiences. This comprises dorsal and ventromedial prefrontal cortex (PFC), lateral PFC, the hippocampus, parahippocampal gyrus, lateral temporal cortices, temporoparietal junction, thalamus, retrosplenial cortex (RSC), posterior cingulate cortex (PCC), precuneus and cerebellum (Maguire 2001a;Maguire & Frith 2003;Svoboda et al 2006;Cabeza & St Jacques 2007;Hassabis et al 2007a; figure 1). Despite over a decade of activating this network, however, surprisingly little is understood about the contributions individual brain areas make to the overall recollective experience .…”