Episodic Prospection

The massive reuse model says imagining sensorily rich prospective episodes is fundamental to deliberation. One line of evidence in favor of this view is that the brain houses an elaborate system for constructing just these kinds of episodic prospections. This system has been intensively investigated over the last decade. Let us get better acquainted with its workings.

Much of our knowledge of how the brain constructs episodic representations of the future builds on an understanding of how it constructs episodic memories of the past (Schacter et al., 2012). Longterm memory is usually divided into procedural memory, (knowing how) and declarative memory (knowing that). The latter is then divided again. "Semantic memory" is memory for facts. It resembles pulling data from a file: Carson City is the capital of Nevada; 13 squared is 169. "Episodic memory" is memory for autobiographical personal experiences (Tulving, 2002). Episodic memory has a rich quasi-sensory phenomenology and contains information about the "w"-questions: What happened and who was involved? Where did it occur; what was the context? When did it occur and in what order did it unfold? Episodic memory fuses together information about what, where, and when into a single composite representation of the overall event.

Interestingly, the brain appears to maintain relatively separate stores of information for each component of episodic memory (Allen & Fortin, 2013). "What" information—information about persons or things that were involved—is stored in the perirhinal and entorhinal cortex, regions deep in the medial temporal lobe of the brain. "Where" information is stored in other regions, including the parahippocampal and post-rhinal cortex and in the medial temporal lobe. Less is known about how "when" information is stored: Very short time scales—seconds to minutes—have been tied to an internal pacemaker in the striatum, a set of structures underneath the cortex. Time information over much longer scales—weeks to months to years—is much harder to study and much less is known.

In episodic memory, these three streams of information—what, where, and when—are fused together to create a complex, composite representation of an overall episode (Allen & Fortin, 2013). Orchestrating this union is the hippocampus, a seahorse-shaped region deep in the medial temporal lobe (Davachi, 2006; Konkel & Cohen, 2009). The hippocampus plays a central role in relational memory. When items are presented with others, for example as pairs, the perirhinal and entorhinal regions—the "what" regions discussed earlier—are needed to recall the identity of the individual items. The hippocampus, especially its anterior regions, is required specifically to recall that the items co-occurred. More generally, the hippocampus stores all sorts of spatial, temporal, and other abstract relationships between items.

An emerging hypothesis says imaginative prospection uses the brain's vast storehouse of episodic memories as raw material; the elements in these memories are recombined to create the imagined scenario (Schacter, Addis, & Buckner, 2007). Initial evidence in favor of this account arises from the observation that states or conditions that impair episodic memory seem to have similar effects on episodic prospection. For example, individuals with damage to the medial temporal lobe experience a loss of episodic memories of past personal experiences and these individuals are reliably found to also be impaired in constructing rich and detailed episodic simulations of the future (Hassabis, Kumaran, Vann, & Maguire, 2007; Race, Keane, & Verfaille, 2011; Squire et al., 2010; Tulving, 1985). Developmental studies similarly find that it is only once children gain an ability to recall personal experiences from the past that they are able to construct episodic prospections about the future (Busby & Suddendorf, 2005).

Perhaps the most remarkable evidence comes from neuroimaging. During retrieval of episodic memory, distributed regions of the brain's default mode network are activated. The various regions contribute to a complex integrative process that has been called scene construction. During imaginal prospection, this very same set of regions is activated, suggesting that highly similar scene construction processes are engaged (Buckner & Carroll, 2007; Hassabis & Maguire, 2007; Schacter & Addis, 2007; Schacter et al., 2007; Spreng & Grady, 2010; Suddendorf & Corballis, 2007). There appears to be an important difference, however. Imagined scenes can't simply be replays or attempts at replaying what happened before. Rather, one must engage in additional activity to recombine elements from prior episodes. The persons, things, and places from episodic memories are the raw materials from which entirely new, never-experienced scenes are generated.

Donna Addis, Daniel Schacter, and their colleagues examined the neural substrates of this recombination process using fMRI. Before scanning, participants were asked to provide episodic memories of actual experiences that included details about a person, object, and place. During a subsequent fMRI scan, they were instructed to either (a) recall events that had actually occurred; or (b) imagine a novel scene in which the person, object, and place from separate prior episodes had to be recombined. Consistent with the idea that episodic memory and imagination draw on a common brain network, default network regions were activated in both conditions.

Several regions, especially the anterior hippocampus, were more vigorously activated during imagined scenarios, suggesting they are the substrates for the recombination process (see also Gaesser, Spreng, McLelland, Addis, & Schacter, 2013). Given the anterior hippocampus's role in relational memory noted earlier, it is possible that during imaginative prospection existing links between persons, places, and times are broken and new links are temporarily formed. Thus a person regularly located in one context is "unlinked" from it and instead placed in a different context at a different time. Through this unlinking process, the elements in existing memories could provide the "ingredients" to construct an unlimited array of novel scenarios.

Let us return to the massive reuse view. Notice that it requires that the brain be outfitted with a mechanism to generate detailed senso- rily rich prospective episodes that represent what would be the case were the person to undertake various candidate actions. Only then will affective systems have something concrete and detailed enough that they can target, thus delivering up emotional evaluations of these episodes. There is now abundant evidence that the brain does in fact have the machinery required by the massive reuse view.

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