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As best we can tell, the scientific information we're about to give next is gathered independently - it involves the sense of smell. Scientists have been asking how when you stroll past a bakery you can tell the difference between the smell of French bread or cinnamon rolls. Now scientists are beginning to understand precisely how that happens, thanks to some extraordinary mice.

NPR's Jon Hamilton reports.

JON HAMILTON: They look just like other mice, and Venkatesh Murthy, a neurobiologist at Harvard, says they act like other mice when something interesting reaches their nose.

Professor VENKATESH MURTHY (Harvard University): When they encounter a new odor, like if they're just going about their business, something interesting or new comes along, they just sniff faster. They go (makes sniffing sound).

HAMILTON: But these mice also sniff faster when you shine a blue light on the part of the brain that processes signals from the nose. That's because the cells in this area have been genetically altered to respond to light as if it were an odor. And that means scientists can study the circuits involved in smell with a precision that's impossible using actual odors.

So Murthy and a team from Harvard and Cold Spring Harbor Lab decided to use these unusual mice to explain one particular feature of our sense of smell.

Prof. MURTHY: Let's say that there are three kinds of citrus fruits - you have an orange, lime and lemon. If you really are careful, you can distinguish each of those just from the smell. There's something slightly different about them. But you can also say that they are similar.

HAMILTON: The scientists expected to find that this was because brain cells responding to orange were physically close to the ones responding to lime and lemon. But Murthy says experiments using light showed that where the neurons were wasn't he only important factor. It also mattered precisely when different neurons responded by emitting an electrical signal.

Prof. MURTHY: So you can find neurons that emit right at the onset of sniff. Another one will emit a little bit later. So I think even within a sniff, the timing of when the neuron decides to then do its thing carries information now.

HAMILTON: Information about stuff like whether you're smelling a lime or a lemon. Murthy says this sort of research should help solve other mysteries too, like how odors are connected to memories and emotions. And he says the new ability to put light-sensitive cells just about anywhere might help people who have diseases that affect the retina.

Prof. MURTHY: Your photoreceptors are degenerated by your eye is perfectly fine otherwise - these people are blind. So people are working on putting this light-sensitive protein in those surviving cells. That will allow you to detect light.

HAMILTON: And maybe even patterns of light. This is all part of one of the hottest fields in neuroscience right now. It's called optogenetics. Dylan Clyne is a researcher who was involved in some of the first optogenetic experiments at Yale. He says hundreds of labs are now using the technique because it lets scientists control a wide range of brain circuits with just a pulse of light.

Mr. DYLAN CLYNE (Researcher): If you understand how different circuits work for memory or depression or motor problems, then you can get at how we can try and treat those circuits in a more sophisticated manner.

HAMILTON: The new research appears in the journal Nature Neuroscience.

Jon Hamilton, NPR News.

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