Scientists implant mice brains with synthetic odors to study smell

Scientists in the United States have for the first time ever created an electrical signature perceived as an odor in the brains of mice -- even though the smell did not exist.

The purpose of the experiment was to decode how mammals' brains perceive odors and distinguish smells from each other. The study was published in the journal Science on Thursday.

Edmund Chong, a doctoral student at NYU Grossman School of Medicine and the lead investigator, said there was much that scientists didn't yet understand about our sense of smell, and what aspects are most important.

"In facial recognition, for example, the brain can recognize people based on visual cues, such as the eyes, even without seeing someone's nose and ears," he said.

"But these distinguishing features, as recorded by the brain, have yet to be found for each smell."

To explore this question, the researchers designed experiments around mice that had been genetically engineered so their brain cells could be activated by shining a light on them using an optical fiber -- a technique called optogenetics.

The experiments centered on the olfactory bulb, a structure that is located behind the nose in animals and in humans.

Molecules linked to scents trigger cell receptors in the nose, which then pass on an electrical signal to nerve-ending bundles in the bulb called glomeruli. These signals are finally sent on to neurons in the brain.

The team trained the mice to recognize a synthetic smell signal they created by using light to activate six nerve bundles in a particular order -- rewarding them with water when they pushed a lever after perceiving the "odor."

If they pushed the lever after a different set of nerve bundles were activated, they received no water.

Using this experimental design, the scientists were able to subtly change the timing and mix of activated nerve bundles to tease out which were most important -- in other words, what aspects of a smell most define it and differentiate it from other smells.

Another way of thinking of this is a familiar musical melody -- like, for example, the opening of the Beatles' song "Hey Jude."

You may still recognize it if a few notes are changed, or if their timing is changed slightly -- but after too many modifications you may no longer be able to identify the tune.

Specifically, the team found that switching around the order of nerve bundles that are activated early in a smell sequence led to as much as a 30 percent loss in smell recognition.

Making changes to the nerves that are activated later in the sequence had a much weaker effect, leading to only a five percent drop in smell recognition.

Going back to the musical analogy -- the nerve activations worked "like the notes in a melody," said the researchers, with delays or interruptions in early notes degrading accuracy.

"Our results identify for the first time a code for how the brain converts sensory information into perception of something, in this case an odor," said the study's senior author Dmitry Rinberg.

He added: "Now that we have a model for breaking down the timing and order of glomeruli activation, we can examine the minimum number and kind of receptors needed by the olfactory bulb to identify a particular smell."

The questions may be easier to decipher among humans than mice -- we have only 350 kinds of odor receptors to the rodents' 1,200.