When it comes to odors, the brain has impeccable timing.

Researchers at Carnegie Mellon University have discovered that specialized brain cells called granule cells fire electrical impulses after precise delay periods that correspond to specific scents.

The novel findings were published in yesterday's Journal of Neuroscience.

If you think of a neuron as similar to a switch, then in the olfactory system "what seems to matter is not how often the switch is flicked, but at what time it is flicked," explained senior researcher Nathan Urban, who holds a doctorate in neuroscience.

So a rose would have one time delay signature, while cinnamon would have another. In other words, the granule cell might fire after 500 milliseconds in response to one scent, but after merely 50 milliseconds to some other odor.

"There's this very precise relationship between the kind of input cells are receiving and the timing at which they're firing," he said. "That, in our eyes, is the most interesting, most surprising finding."

Timing of the granule cell impulses in turn modifies the firing of other specialized neurons, ultimately leading to an electrical pattern in the brain's olfactory center that is unique to a particular odor.

Scientists are most familiar with neural networks in which the number of impulses conveys the most information, Dr. Urban said. But the timing strategy, called the temporal coding hypothesis, could play a role in other sensory systems.

In the experiments, the researchers mimicked odor by electrically stimulating mouse cells to trigger a response.

The ends, or axons, of neurons that carry specific odor receptors are grouped in a structure called a glomerulus in the brain's olfactory center. To simulate different smells, the researchers stimulated different glomeruli, each about 100 microns in diameter.

They are now examining what happens when the cells are exposed to the electrical equivalent of a combination of odors.

Vertebrate animals, which includes humans, have similar olfactory systems, Dr. Urban said. Mice have about 1,800 glomeruli and 3 percent to 4 percent of their genome, some 1,300 genes, make receptors for odors. Humans have roughly 500 glomeruli, and 1 percent to 2 percent of their genome, 400 to 500 genes, make odor receptors.