Published online 26 August 1999 | Nature | doi:10.1038/news990826-5

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Partial recall

What is the role of conscious awareness in our ability to recall past events? A part of the brain called the hippocampus has been implicated in the processing of learning and memory, but, as a study in the September issue of Nature Neuroscience shows, the hippocampus plays an important part in processing memories of which we are not aware, as well as the ones that we can consciously call to mind. This work forms part of a change-of-mind among neuroscientists in which the hippocampus is not seen as the ‘seat’ of memory or awareness, but as a kind of master-of-ceremonies, coordinating various aspects of a memory - including our conscious awareness of an event having happened - and ‘presenting’ the package before our mind’s eye.

While they were both working at Yale University, New Haven, Connecticut, Marvin M. Chun - now at Vanderbilt University, Nashville, Tennessee - and Elizabeth A. Phelps, of New York University, worked out how to test the role of the hippocampus in conscious memory by comparing how healthy subjects performed in memory tests alongside amnesiacs whose problems of recall stemmed from damage specific to the hippocampus.

All the test subjects were asked to search for a ‘T’ shape hidden on a computer screen in a sea of ‘L’s, and to perform this test many times, with different configurations of ‘T’s and ‘L’s - except that some of the same patterns cropped up again and again. All the subjects, whether amnesiac or not, got better at performing the test each time. However, there was an important difference - healthy subjects were much quicker than amnesiacs to find the ‘T’ in a pattern they had seen before, but were consciously unaware of having done so. When asked directly, they were vaguely aware that some of the patterns might have been repeated, but were unable to be more specific. The improved performance on repeated patterns, therefore, stems from an unconscious memory of the earlier pattern. Because amnesiacs with damage to the hippocampus did not memorize the repetition, the hippocampus must be involved in processing this unconscious memory.