Late in a baby's development, before birth, the brain goes through a period of explosive growth, making more brain cells than can be used. For the next six or seven years, the actions of specialized cells within the brain lead to the destruction of some of those neurons, carving out a neural network much as a sculptor creates a statue by removing chunks of marble from the original block.

Studies suggest that some forms of mental retardation and other brain abnormalitites can arise when the brain's pruning mechanism fails to remove enough neurons.

Researchers at the National Institute of Child Health and Human Development may have identified some of the key players in this pruning process, they report in this month's Journal of Cell Biology.

Glial cells, an essential support cell in the brain that helps nourish the neurons, appear to produce an as yet unidentified substance that preserves the neurons. Without this substance, neurons die.

But it is a two-way street. The electrical activity of neurons causes them to secret a compound called vasoactive intestinal peptide, VIP. VIP in turn stimulates the glial cells to produce the unidentified neuron-sparing factor.

Neuron survival appears to be a matter of natural selection, said NICHD's Douglas Brenneman, who ran the study. Neurons in the wrong place, or that don't get the proper stimulation, will not produce VIP and will not stimulate glial cells to produce the unknown factor. Those neurons eventually will die.

The link between electrical activity, VIP and neuron survival means the "information coming into the brain may determine what the person's going to have to work with for the rest of his life," Brenneman said. "It's an exceedingly important thing to understand."