PROGRESS These issues have diverted attention away from the relationship between future event Remembering the past to imagine simulation and memory processes in humans. During the past year, however, the growing the future: the prospective brain number of papers published on this topic have changed this situation dramatically. Daniel L. Schacter, Donna Rose Addis and Randy L. Buckner Insights from memory impairments Abstract | A rapidly growing number of recent studies show that imagining Early indications of a link between the the future depends on much of the same neural machinery that is needed processing of past and future events were for remembering the past. These findings have led to the concept of the prospective provided by observations of patients with memory impairments. In a seminal descrip- brain; an idea that a crucial function of the brain is to use stored information tion of patients with Korsakoff’s amnesia, to imagine, simulate and predict possible future events. We suggest that processes marked deficiencies in personal planning such as memory can be productively re-conceptualized in light of this idea. were noted18. The amnesic patient K.C., who showed a total loss of episodic memory after a head injury, reported a ‘blank’ when asked For more than a century, memory research which has traditionally been defined as a about his personal future or past8. (For has focused on the past. Psychologists have memory system that supports remembering related observations, see REF. 19.) analysed the cognitive processes that allow personal experiences, allows individuals to Expanding on these observations, the individuals to retain past experiences, and engage in “mental time travel” into both the ability of five amnesic patients with bilateral neuroscientists have identified the brain past and the future7,8. Tulving also claimed hippocampal damage to imagine novel expe- structures, such as the hippocampus, that that the capacity for mental time travel is riences was examined systematically20. The support this ability. A function of memory uniquely human9. patients were asked to generate everyday that has been largely overlooked until Perhaps as a result of this claim, much imaginary experiences and were specifi- recently is its role in allowing individuals to research has focused on whether non-human cally instructed not to provide a memory imagine possible future events. In this arti- animals are capable of mental time travel of a past event, but to construct something cle, we consider emerging evidence which (for reviews, see REFS 10–12), using ingenious new. Participants described their imaginary indicates that memory — especially episodic demonstrations to question the claim for scenarios, which were scored based on memory — is crucially involved in our abil- human uniqueness. For example, there is their content, their spatial coherence and ity to imagine non-existent events and simu- compelling evidence that food-caching their subjective qualities. The imaginary late future happenings. Indeed, brain regions scrub jays can retrieve detailed information constructions produced by four of the five that have traditionally been associated with about what food they have cached, as well patients were greatly reduced in richness and memory appear to be similarly engaged as when and where they cached it10,13. content compared with those of controls. when people imagine future experiences. Furthermore, recent work indicates that The impairment was especially pronounced We believe that such observations might jays can cache food in a manner that reflects for the measure of spatial coherence, indicat- have far-reaching implications for concep- some form of planning for the future14 that ing that the constructions of the amnesic tions of memory and its functions. is not simply a reflection of current patients tended to consist of isolated frag- motivational needs15. ments of information, rather than connected Memory for the future: background Debates about mental time travel in scenes. It is important to note that this In 1985, D. H. Ingvar published a paper non-human animals might never be set- study did not specifically require patients to with the seemingly paradoxical title tled definitively, given that animals lack construct scenes pertaining to future events, “Memory for the future”. According to the linguistic capacity to describe mental suggesting a more general deficit in the Ingvar, “concepts about the future, like contents. At the same time, research in the patients’ ability to construct novel scenes. memories of past events, can be remem- child-development literature has investi- The ability to remember the past and bered, often in great detail” (REF. 1). Ingvar gated the development of mental time travel imagine the future is also affected in summarized evidence which indicated that in children, and found that both episodic psychiatric disorders. A decade ago, a link regions within the prefrontal cortex have a remembering and future thinking emerge was reported between deficits in accounts crucial role in the planning, foresight and relatively late in development, between of past and future events in patients with programming of complex action sequences approximately three and five years of age. suicidal depression21. In response to word — examples of “memory for the future” However, similar issues regarding the ability cues, depressed patients showed less specific (REFS 2–6). At approximately the same time, of young children to communicate their retrieval of past events, and less specific E. Tulving argued that episodic memory, mental contents have arisen9,16,17. imagining of future autobiographical events. NatURE REViewS | NEUROSCIENCE VOLUme 8 | SeptemBER 2007 | 657 © 2007 Nature Publishing Group PROGRESS A similar cueing procedure was recently effect occurred for future events: imagined future, and the differences between the two used to study past and future events in happenings also contained sparse episodic temporal directions of episodic thought. schizophrenic patients22. Schizophrenics information (TABLE 1). In the first study, participants were recalled fewer specific past events and The ability of older adults to generate epi- instructed to talk freely about either the near imagined fewer specific future events than sode-specific details of both past and future or distant past or future26 while a positron did control subjects, but the schizophrenic events was correlated with their ability to emission tomography scan was carried out. patients’ deficits were even greater for future integrate information and form relationships The scans showed evidence of shared activ- than for past events. Interestingly, this reduc- between items (relational memory). This ity during descriptions of past and future tion in past and future specificity was corre- suggests that the simulation of future episodes events in a set of regions that included the lated with the extent of the patients’ positive draws on relational processes that flexibly prefrontal cortex and parts of the medial symptoms (delusions and hallucinations), recombine details from past events into novel temporal lobe (namely the hippocampus and but not with their negative symptoms (such scenarios. Combined with other recent data the parahippocampal gyrus). as blunted affect). Other evidence correlated that indicate similar temporal distributions for More recent studies have taken advan- positive symptoms in schizophrenics with past and future events in older adults25, there tage of the temporal resolution of functional problems remembering contextual details, is now an empirical basis for proposing that MRI (fMRI) (BOX 1). In one study, partici- suggesting that the patients’ impairments aging has parallel effects on both imagining pants were instructed to remember specific on the past and future tasks reflect impair- the future and remembering the past. past events, imagine specific future events ments in accessing contextual details for the or imagine specific events that involved purpose of constructing specific simulations Insights from neuroimaging a familiar individual (specifically, Bill of their personal future or past. Several recent neuroimaging studies have Clinton) in response to event cues27. Again, A similar pattern was also recently directly contrasted situations where young there was striking overlap in the activity reported in healthy older adults, when they adults either recall from their own personal associated with past and future events in were compared with college students23. pasts or imagine future events. These prefrontal and medial temporal regions, as When asked to generate past and future studies have provided insights into three well as in a posterior midline region at or events, the older adults generated fewer central issues: whether common brain sys- near the precuneus. These regions were not episode-specific details relating to the past tems are used while remembering the past activated to the same extent when imagin- events than the younger adults, which repli- and imagining the future, which specific ing events that involved Bill Clinton, which cated earlier results24. Importantly, the same brain system(s) are used for imagining the demonstrates the existence of a neural Table 1 | Examples of past and future events generated by older and younger adults Age Event Cue Event description group type word Non-episodic information Episodic information Young Past Tree “I went hiking in Muir Woods in California … with my boyfriend then and his room-mates … we went through all these different ecosystems … and you would see different kinds of plants, so like we would see orchids … and I said, “Wow it’s so beautiful and it’s like wild” … that part was like a jungle: wet, very lush and green. At the end of that trail, was like the beach … so we had bread and cheese and it was very fun and good… …because I love cheese.” Young Future Oven None “I’m going to bake my first loaf of bread. It’s going to be probably Friday afternoon before Yom Kippur … I’ll get the recipe from an old cookbook… The room’s going to be hot even though we have the windows open, because we’re going to turn on the oven in the middle of the summer.