Brain cells communicate with their neighbors by sending a chemical burst from their axonal endings, across a space called the synapse, to stimulate the spine on the next cell. If the chemical bath is strong enough, the receiving spine bulges forward  strengthening the connection between the spines. This is thought to be the fundamental process underlying learning.

But the researchers, Christopher D. Harvey and Karel Svoboda, found something unusual when they stimulated a single spine. Not only did the spine bulge, but it also somehow made its neighbors more sensitive to chemical signals  standing ready, in effect, to digest any spillover of information. Imagine every neighbor on the block calling up to offer a corner of his basement for storage, just in case.

The combined effect of these helpers multiplies the capacity of any single brain cell, the authors concluded. Neuroscientists had theorized that this effect, called clustered plasticity, might help account for the tremendous capacity of the brain, but they had not seen it in action.

“The traditional view was that each synapse functioned independently, and the strength of individual connections modulated memory storage,” said Mr. Harvey, a graduate student at the Cold Spring Harbor Laboratory on Long Island. “What we’ve shown is that neighboring synapses may function together, which leads to the idea that information is stored in a clustered manner, with related things concentrated in the same neighborhood.”

The ability to watch a synapse in action is itself a scientific accomplishment. The average human brain has about 100 billion neurons, and about 1,000 times that many synapses. To zero in on a single one, the researchers used mice that were genetically engineered so their brains produced a fluorescent protein that glowed only in specific cells of the hippocampus. Peering through a high-powered microscope at a slice of this tissue, the researchers could zero in on a single synapse.