ART AND ORGANISM
Memory and Imagination
Science 19 January 2007: Vol. 315. no. 5810, p. 312. DOI: 10.1126/science.315.5810.312. http://www.sciencemag.org/cgi/content/short/315/5810/313
News of the Week NEUROBIOLOGY:
A Surprising Connection Between Memory and Imagination
People with amnesia struggle to remember their past. They may also struggle to envision their future, according to a new study. Researchers have found that people with amnesia caused by damage to the hippocampus, a brain region intimately tied to memory, have difficulty envisioning commonplace scenarios they might reasonably expect to encounter in the future.
The findings challenge long-held views about the function of the hippocampus and the nature of memory, says Lynn Nadel, a cognitive neuroscientist at the University of Arizona in Tucson. “The claim here is that the same system we use to remember the past we also use to construct possible futures,” says Nadel.
In the new study, published online this week by the Proceedings of the National Academy of Sciences, cognitive neuroscientist Eleanor Maguire of University College London and colleagues examined five amnesic patients. All of them had severe memory deficits caused by damage to the hippocampus; they had great difficulty forming new memories and recalling events that happened after their injuries. Ten healthy individuals who matched the patients’ ages and education levels participated in the study as controls.
Maguire’s team asked each subject to imagine and describe several ordinary experiences, such as meeting a friend or visiting a beach, a pub, or a market. The healthy subjects provided rich descriptions, remarking for example on the curve of a beach, the sound of waves hitting the shore, and the feel of burning hot sand. The amnesic patients were able to follow the researchers’ instructions, but their descriptions were far less vivid. They described fewer objects, fewer sensory details such as sounds and smells, and fewer thoughts or emotions that might be evoked in the imagined scenario. The patients’ responses on a questionnaire indicated that what they saw in their mind’s eye were fragmented collections of images rather than coherent scenes.
The work suggests that the hippocampus may have a broader role in cognition than many researchers have thought, says Morris Moscovitch, a cognitive neuroscientist at the University of Toronto in Canada. The textbook view is that the main function of the hippocampus is to encode new memories, creating an initial memory trace that is eventually filed away to the cortex for long-term storage. In this view, the hippocampus is not needed to maintain or retrieve memories once they’ve been stored in the cortex. If this view were correct, Maguire says, the patients in her study, who did not have substantial damage to the cortex, should have been able to construct imaginary experiences by drawing on memories formed before their injuries. But their inability to integrate those memories into a coherent imagined scene suggests that the hippocampus does more than simply record current events.
Nadel, Moscovitch, and others have argued in recent years that the traditional view of the hippocampus’s role is too narrow. Work from Moscovitch and colleagues, for example, suggests that the hippocampus binds together elements of remembered scenes to create vivid and coherent memories. Maguire’s findings point to a similar role in constructing imagined scenes, Moscovitch says. “In order to have vivid constructions of the past, the future, or of imaginary events, you always need the hippocampus,” he says.
The idea that thinking about the past has much in common with thinking about the future has ancient roots, says Yadin Dudai, a neuroscientist at the Weizmann Institute of Science in Rehovot, Israel: “In prescientific times, many people thought that the role of memory is not necessarily to remember the past but to enable you to imagine the future.” In modern times, Dudai says, the notion was resurrected by the memory researcher Endel Tulving, who speculated that the ability to predict the future was a major driving force in the evolution of memory. Even so, Dudai says, only very recently have studies like Maguire’s begun to provide experimental evidence that memory and imagination may share neural circuitry.
More evidence comes from a functional magnetic resonance imaging study now in press at Neuropsychologia. Cognitive neuroscientist Donna Addis and psychologist Daniel Schacter of Harvard University scanned the brains of healthy volunteers who had been asked either to recall a vivid memory or to envision a future experience. Both situations activated a similar network of brain regions, including the hippocampus, the researchers found.
If the hippocampus does turn out to be as important for imagination as it is for memory, that could have interesting implications for aging, Addis says. The hippocampus is one of the first brain regions to show signs of deterioration as we get older, and Addis has recently found evidence that the ability to envision future experiences declines in parallel with memory as people age. Meanwhile, Moscovitch is examining the work of famous artists and novelists to see whether the detail of their work declined in their later years. The picture of the hippocampus that’s emerging suggests yet another compromise facing us in old age, he suggests: “Age will contribute wisdom because you can draw on a lot of past experience, but that experience may not be quite as rich as it used to be.”
Published online before print January 17, 2007, doi: 10.1073/pnas.0610561104 PNAS January 30, 2007 vol. 104 no. 5 1726-1731
Patients with hippocampal amnesia cannot imagine new experiences
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.
Each 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).