Patients with hippocampal amnesia cannot imagine new experiences

Abstract

Amnesic patients have a well established deficit in remembering their past experiences. Surprisingly, however, the question as to whether such patients can imagine new experiences has not been formally addressed to our knowledge. 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. Our results revealed that patients were markedly impaired relative to matched control subjects at imagining new experiences. Moreover, we identified a possible source for this deficit. The patients' imagined experiences lacked spatial coherence, consisting instead of fragmented images in the absence of a holistic representation of the environmental setting. 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. 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.episodic ͉ hippocampus ͉ imagination ͉ memory ͉ construction E ach of us has our own unique personal past, comprising a myriad of autobiographical experiences accrued over a lifetime. Recollection of these rich autobiographical or episodic memories has been likened to mentally traveling back in time and re-experiencing one's past (1). 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). 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. 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. 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).We therefore sought to further our understanding of the role of the hippocampus in episodic memory by adopting a different approach. 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. In fact, episodic memory and imagining or constructing events share striking similarities in terms of the psychological processes engaged (19-21). These include i...

The mental time line: An analogue of the mental number line in the mapping of life events

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Impaired ability to imagine future pleasant events is associated with apathy in schizophrenia

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Evidencing a place for the hippocampus within the core scene processing network

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