| Scientists have long believed that
constructing memories is like playing with neurological toys. Exposed to a
barrage of sensations from the outside world, we connect 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 depletable resource, like petroleum or gold. We are each given a finite number of cells, 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. Maybe it's time for this notion to be forgotten-or at least radically revised. In the past two years, a series of confusing experiments has forced scientific researchers to rethink this and other assumptions about how memory works. The perplexing results of these experiments remind scientists 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. 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. This area 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. Until now our old view of brain functionality has been that these patterns ate constructed from the supply of neurons that have been in place since birth. New memories don't require new neurons-just new ways of connecting the old ones together. Retrieving a memory is a matter of activating one of these circuits, coaxing the original stimulus back to life. 6. The picture appears very 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 ad skin and bone from new cells, it would serve only to scramble memory's delicate ornamental pattern. Studies with adult monkeys in the mid-1960s seemed to support the belief that the supply of neurons is fixed at birth. Therefore the surprise when Elizabeth Gould and Charles Gross of Princeton University reported last year that the monkeys they studied seemed to be producing thousands of new neurons a day in the hippocampus of their brain. Even more surprising, Gould and Gross found evidence that a steady stream of the fresh cells may be continually moving to the cerebral cortex. No one is quite sure what to make of these findings. There had already been hints that spawning of brain cells, a process called neurogenesis, occurs in animals with more primitive nervous systems. For years, Fernando Nottebohm of Rockefeller University has been showing that canaries create a new batch of neurons every time they learn a song, then slough them off when it's time to change tunes. But it was widely assumed that in mammals and especially primates this manufacture of new brain parts had long ago been phased out by evolution. With a greater need to store memories for a long time, these creatures would need to ensure that the engrams weren't disrupted by interloping new cells. |