Get a whiff of clove just as you sniff a rose, and the mix smells like carnation. How does our brain create such novel scents seemingly out of thin air? A new study of how the mouse brain responds to a variety of odors may offer an answer.
An enduring mystery of smell is how humans can describe far more odors than they have nose receptors to detect. Smells begin when receptors--there are 347 different ones in humans--in the nose latch onto molecules in the air. The receptor then transmits a signal to the brain, which tells us what we're sniffing. Ostensibly, we should be able to distinguish the different scents that make them up, but this isn't always the case. One theory is that the brain sometimes mixes and matches signals that come from the nose to encode unique combo-scents.
To test the idea, Linda Buck and Zhihua Zou, neuroscientists at the Fred Hutchinson Cancer Research Center in Seattle, Washington, and the University of Texas in Galveston, respectively, gave mice a whiff of chemicals that smell like either clove, chocolate, citrus, fish, vanilla, or apple. Some mice were exposed to individual scents while others sniffed two different ones at once. To see how the mice processed these smell signals, the researchers turned to the brain's smell center, the olfactory cortex. A gene called Arc is known to turn on when the neuron fires--so by tracking its expression in this region of the brain, the team could determine which neurons switched on in response to specific smells.
The mouse brains seemed wired to process some smell combinations as new odors. As expected, a specific set of neurons in the olfactory cortex responded to each of the smelly chemicals the researchers introduced. But about 30% of neurons in the region only fired up when two odors were mixed, the team reports 10 March in Science. The researchers conclude that these neurons are where novel scents are born, although exactly how the activity of these combined-scent neurons leads to our perception of the smell is unknown.
"This is a big step forward in understanding smell processing," says Eric Kandel, a neuroscientist at Columbia University. It isn't the final word, he says, but it does offer a glimpse at "how individual scents can add up to the immense complexity of our scent perception."