Person:

Schacter, Daniel

Recent neuroimaging work has demonstrated that the hippocampus is engaged when imagining the future, in some cases more than when remembering the past. It is possible that this hippocampal activation reflects recombining details into coherent scenarios and/or the encoding of these scenarios into memory for later use. However, inconsistent findings have emerged from recent studies of future simulation in patients with memory loss and hippocampal damage. Thus, it remains an open question as to whether the hippocampus is necessary for future simulation. In this review, we consider the findings from patient studies and the neuroimaging literature with respect to a new framework that highlights three component processes of simulation: accessing episodic details, recombining details, and encoding simulations. We attempt to reconcile these discrepancies between neuroimaging and patient studies by suggesting that different component processes of future simulation may be differentially affected by hippocampal damage.

Imagining or simulating future events has been shown to activate the anterior right hippocampus (RHC) more than remembering past events does. One fundamental difference between simulation and memory is that imagining future scenarios requires a more extensive constructive process than remembering past experiences does. Indeed, studies in which this constructive element is reduced or eliminated by “pre-imagining” events in a prior session do not report differential RHC activity during simulation. In this fMRI study, we examined the effects of repeatedly simulating an event on neural activity. During scanning, participants imagined 60 future events; each event was simulated three times. Activation in the RHC showed a significant linear decrease across repetitions, as did other neural regions typically associated with simulation. Importantly, such decreases in activation could not be explained by non-specific linear time-dependent effects, with no reductions in activity evident for the control task across similar time intervals. Moreover, the anterior RHC exhibited significant functional connectivity with the whole-brain network during the first, but not second and third simulations of future events. There was also evidence of a linear increase in activity across repetitions in right ventral precuneus, right posterior cingulate and left anterior prefrontal cortex, which may reflect source recognition and retrieval of internally generated contextual details. Overall, our findings demonstrate that repeatedly imagining future events has a decremental effect on activation of the hippocampus and many other regions engaged by the initial construction of the simulation, possibly reflecting the decreasing novelty of simulations across repetitions, and therefore is an important consideration in the design of future studies examining simulation.

Recent research has demonstrated commonalities between remembering past events and imagining future events. Behavioral studies have revealed that remembering the past and imagining the future depend on shared cognitive processes, whereas neuropsychological and neuroimaging studies have shown that many of the same brain regions are involved in both remembering the past and imagining the future. Here, we review recent cognitive and neuroimaging studies that examine remembering the past and imagining the future in elderly adults. These studies document significant changes in elderly adults’ capacities to imagine future events that are correlated with their memory deficits; most strikingly, older adults tend to remember the past and imagine the future with less episodic detail than younger adults. These findings are in line with the constructive episodic simulation hypothesis [Schacter and Addis: Phil Trans R Soc B 2007;362:773-786], which holds that that past and future events draw on similar information and rely on similar underlying processes, and that episodic memory supports the construction of future events by extracting and recombining stored information into a simulation of a novel event. At the same time, however, recent data indicate that non-episodic factors also contribute to age-related changes in remembering the past and imagining the future. We conclude by considering a number questions and challenges concerning the interpretation of age-related changes in remembering and imagining, as well as functional implications of this research for everyday concerns of older adults.