When imagining a new experience participants were explicitly told not to describe a remembered event or any part of one but rather to give free reign to their imaginations and create something new. We therefore tested whether a group of patients ( n = 5) with amnesia associated with bilateral hippocampal damage and a group of matched control subjects ( n = 10) could construct new imagined experiences in response to short verbal cues which outlined a range of simple commonplace scenarios (see Methods). For example, all patients can be asked to construct the same fictitious situations, and their performances can be compared and contrasted more directly than would be possible in a standard episodic memory recall paradigm. Constructions, then, have much in common with episodic memories but have the advantage of being easier to systematize and experimentally manipulate. Moreover, both episodic memory and construction involve the salient visualization of an experience within a rich spatial setting or context ( 24), and therefore differ markedly from “simple” visual imagery (e.g., for faces or single objects) ( 25), which is thought not to depend on the hippocampus ( 26). These include imagery ( 22), sense of presence ( 1), retrieval of semantic information and multimodal details ( 23), and narrative structure ( 22). In fact, episodic memory and imagining or constructing events share striking similarities in terms of the psychological processes engaged ( 19– 21). If patients with hippocampal damage are impaired at recollecting past events, we wondered, can they imagine new experiences? While there have been some suggestions that amnesic patients have difficulty envisioning themselves in the future ( 15– 18), surprisingly, the more general question of whether imagining new experiences depends on a functioning hippocampus has not been formally addressed to our knowledge. We therefore sought to further our understanding of the role of the hippocampus in episodic memory by adopting a different approach. This is not altogether surprising as studying memory for personal experiences is fraught with methodological issues ( 11– 13), not least of which is how to generalize across individuals when autobiographical memories are unique to each person ( 9, 14). Numerous studies have attempted to settle this debate by ascertaining the status of remote episodic memory in patients with hippocampal amnesia ( 10) but without resolution thus far. How exactly the hippocampus supports episodic memory ( 5– 7), or indeed whether its involvement is time-limited ( 5, 8) or permanent ( 7, 9) is uncertain, however. It has long been known that the hippocampus and related medial temporal lobe structures play a critical role in supporting episodic memory ( 2), and damage to even the hippocampus alone is sufficient to cause amnesia ( 3, 4). Recollection of these rich autobiographical or episodic memories has been likened to mentally traveling back in time and re-experiencing one's past ( 1). Given how closely imagined experiences match episodic memories, the absence of this function mediated by the hippocampus, may also fundamentally affect the ability to vividly re-experience the past.Įach of us has our own unique personal past, comprising a myriad of autobiographical experiences accrued over a lifetime. ![]() ![]() The hippocampus, therefore, may make a critical contribution to the creation of new experiences by providing the spatial context into which the disparate elements of an experience can be bound. The patients' imagined experiences lacked spatial coherence, consisting instead of fragmented images in the absence of a holistic representation of the environmental setting. Moreover, we identified a possible source for this deficit. Our results revealed that patients were markedly impaired relative to matched control subjects at imagining new experiences. We tested whether a group of amnesic patients with primary damage to the hippocampus bilaterally could construct new imagined experiences in response to short verbal cues that outlined a range of simple commonplace scenarios. Surprisingly, however, the question as to whether such patients can imagine new experiences has not been formally addressed to our knowledge. Amnesic patients have a well established deficit in remembering their past experiences.
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