Scientists have long believed that constructing memories is like playing with neurological Tinkertoys. Exposed to a barrage of sensations from the outside world, we snap together brain cells to form new patterns of electrical connections that stand for images, smells, touches and sounds.
The most unshakable part of this belief is that the neurons used to build these memory circuits are a depletable resource, like petroleum or gold. We are each bequeathed a finite number of cellular building blocks, and the supply gets smaller each year. That is certainly how it feels as memories blur with middle age and it gets harder and harder to learn new things. But like so many absolutes, this time-honored notion may have to be forgotten--or at least radically revised.
In the past year, a series of puzzling experiments has forced scientists to rethink this and other cherished assumptions about how memory works, reminding them how much they have to learn about one of the last great mysteries--how the brain keeps a record of our individual passage through life, allowing us to carry the past inside our head.
"The number of things we know now that we didn't know 10 years ago is not very large," laments Charles Stevens, a memory researcher at the Salk Institute in La Jolla, Calif. "In fact, in some ways we know less."
This much seems clear: the traces of memory--or engrams, as neuroscientists call them--are first forged deep inside the brain in an area called the hippocampus (after the Latin word for seahorse because of its arching shape). Acting as a kind of neurological scratch pad, the hippocampus stores the engrams temporarily until they are transferred somehow (perhaps during sleep) to permanent storage sites throughout the cerebral cortex. This area, located behind the forehead, is often described as the center of intelligence and perception. Here, as in the hippocampus, the information is thought to reside in the form of neurological scribbles, clusters of connected cells.
It has been considered almost gospel that these patterns are constructed from the supply of neurons that have been in place since birth. New memories, the story goes, don't require new neurons--just new ways of stringing the old ones together. Retrieving a memory is a matter of activating one of these circuits, coaxing the original stimulus back to life.
The picture appears eminently sensible. The billions of neurons in a single brain can be arranged in countless combinations, providing more than enough clusters to record even the richest life. If adult brains were cranking out new neurons as easily as skin and bone grow new cells, it would serve only to scramble memory's delicate filigree.
Studies with adult monkeys in the mid-1960s seemed to support the belief that the supply of neurons is fixed at birth. Hence the surprise when Elizabeth Gould and Charles Gross of Princeton University reported last year that the monkeys they studied seemed to be minting thousands of new neurons a day in the hippocampus of their brain. Even more jarring, Gould and Cross found evidence that a steady stream of the fresh cells may be continually migrating to the cerebral cortex.