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 imagining new experiences.
In the new study, published online this week by the Proceedings of the National Academy of Sciences, cognitive neuroscientist Eleanor Maguire at 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 occurred 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 commonplace scenarios they might reasonably expect to encounter in the future, 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. Compared to healthy subjects, the patients 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 findings challenge the traditional textbook view that the main job of the hippocampus is to encode new memories, 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," he says. Maguire's study fits with other recent work suggesting that the hippocampus binds together the elements of a scene to create a coherent mental picture, says Morris Moscovitch, a cognitive neuroscientist at the University of Toronto in Canada. "In order to have vivid constructions of the past, the future, or of imaginary events, you always need the hippocampus," he says.
For more on why amnesiacs can't imagine new experiences, stay tuned for the 19 January issue of Science.
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