New understanding is emerging of memory and forgetfulness
IRENEO FUNES, a character in a story by Jorge Luis Borges, has a strange affliction. He forgets nothing, a condition that makes him incapable of analytical thought. Fiction, of course. But it emphasises a point. The memory is an evolved structure with a job to do. That job is to preserve its owner and help him or her reproduce. Perfection is not required, only adequacy. Indeed, selective forgetting of the useless is as important as selective remembering of the useful. And much of this winnowing takes place during sleep, as two papers in this week’s Nature Neuroscience observe.
One of these papers is a review of previous work, by Robert Stickgold of Harvard University and Matthew Walker of the University of California, Berkeley. They propose that the process of sleep acts as a form of triage—first choosing what to retain, and then selecting how it will be retained. The other paper, by Dr Walker and his colleague Bryce Mander, compares the process in the young and the old.
One of the studies Dr Stickgold and Dr Walker examine in their review (a study which was, as it happens, led by Dr Walker) found that sleep does indeed help people discard information they have been told to forget. The more frequently someone experiences waves of brain activity known as sleep spindles, the more his brain scraps items that it is supposed to. Rather than forget passively, then, the brain seems to shed memories actively.
Sleep also helps guide memories intended to be retained down particular paths—remembering patterns, for example, as opposed to facts. In two other studies the reviewers examined, some 15-month-old babies had been exposed to patterns of fake grammar in which the first syllable of a nonsensical word predicted the last. Only those who napped within four hours were able to recognise the pattern later that day or the next.
The paper by Dr Walker and Dr Mander looked further into the matter of forgetting, by comparing the process in the young and the old. Eighteen people in their teens and 20s, and 15 in their 60s and 70s, were taught nonsensical word pairs, tested on them almost immediately, and then tested again after a night’s sleep. The oldsters scored worse than the youngsters in the immediate test, which was no surprise. What was notable was that, after sleep, oldsters’ brains seemed to retain even less material than youngsters’. Those who retained the least had slept less deeply. This poor sleep, in turn, was linked to the shrinking of part of their brain, brought on by ageing.
Old people do not, of course, need to remember as much new material as the young do, because they are already familiar with so much of what they experience. So it may be that their inability to form new memories is not a bug, but a feature. In Borges’s story that matter is never put to the test. Ireneo Funes dies at the age of 21.
From the print edition: Science and technology