How the brain stores long-term memory is a mystery, but some researchers think it involves permanent changes in the gene expression of brain cells. So animals like songbirds that have small brains and relatively long lifespans would run out of neural "space" to store new memories if they didn't grow a constant supply of new cells. Songbirds do grow new neurons, though most of these die within three to five weeks and so can't store memories for long. But those that survive may provide space for new long-term memories.
Fernando Nottebohm of Rockefeller University in New York and his colleagues decided to study adult zebra finches to see whether their social conditions affect the survival rate of these new neurons. They injected the birds with a radioactive form of thymidine-a marker to track new neurons-and placed the birds in three different settings: either alone, with another bird of the opposite sex, or in a large group of about 45 other birds. After 40 days the researchers examined three specific regions of the birds' brains to check development.
The researchers found that compared with the other birds, those living in large groups had about 30 per cent more new neurons in a region of the brain involved in sound processing. Even more impressively, the male zebra finches, who do all the singing, had twice as many new neurons in areas of the brain involved in communication when living in large groups. That could be simply because the birds are trying to remember every other bird's distinctive song, say the researchers in a paper to be published in the journal Behavioural Brain Research.
Researchers have noticed before that social animals such as elephants tend to have better memories than loners. But no one had actually seen a change in the survival of neurons caused solely by the number of companions. "This is exciting stuff," says Arturo Alvarez-Buylla, an expert on neuronal growth from the University of California, San Francisco. There is evidence that adult humans also produce new neurons in their brains, so these results raise the possibility that social interaction could help our neurons survive too, he says. And perhaps that would even boost our memories.
But we can't yet be sure. "We really don't know exactly where neurogenesis occurs in humans, how much there is, or if it's active throughout our life," says Alvarez-Buylla.
Author: Anil Ananthaswamy, San Francisco
New Scientist issue: 23rd February 2002
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