"We wanted to find out whether inputs from receptors that recognized the same odorant are all targeted to the same places in the cortex, producing a distinctive spatial map for the odorant," she said. "Or, whether the inputs from these receptors are sent to different locations in the cortex, resulting in a more distributed representation of the odorant."
To explore this question, the researchers exposed mice to each of a wide range of odorants -- including apple, skunk, floral, fishy, urine, vanilla, musk, woody, garlic, and chocolate. After each mouse was exposed to an odor, the scientists then proceeded to isolate the animal's olfactory cortex and map neural activity by measuring the activity of a marker gene called c-Fos in individual neurons across this entire structure.
"We found that a single odorant does not just stimulate one or two spots in the cortex," said Buck. "Instead it stimulates a very small subset of neurons that are sparsely distributed over a relatively large area. We found that different odorants stimulate different patterns, but the patterns for different odorants partially overlap."
Importantly, said Buck, the research team found that, despite this very complex patterning, the odor representations are very similar among individuals. "This may explain why odors elicit similar responses in different individuals. For example, most people don't like the smell of skunk odor, but they do like the smell of chocolate," she said.
The researchers also found that the spatial representations of the odors in the cortex expanded with increased concentrations of the odorants. Buck said this phenomenon could explain why odors can smell different at different concentrations.
She said it was particularly intriguing that odorants with related chemical structures showed highly similar patterns of activation in the cortex. "This finding gives a hint that there is some logic to the w