Smell is a symphony

Just as a road atlas faithfully maps real-world locations, our brain maps many aspects of our physical world: sensory inputs from our fingers are mapped next to each other in the somatosensory cortex, the auditory system is organized by sound frequency and the various tastes are signaled in different parts of the gustatory cortex.

The olfactory system was believed to map similarly, where groups of chemically related odorants—amines, ketones or esters, for example—register with clusters of cells that are laid out next to each other. When Associate Investigator Ron Yu, PhD, and his team traced individual odor molecules’ signal deep into the brain, they found evidence that chemically related odorants mapped all over the olfactory bulb, which processes incoming olfactory information.

“From the animal’s perspective, that makes perfect sense,” says Yu. “The chemical structure of an odor molecule is not what’s important to them. They really just want to learn about their environment and associate olfactory information with food or other relevant information.”

To explain their observations, Yu and his team developed a “tunotopic” hypothesis of the olfactory system. Individual olfactory receptors are “tuned” not to one particular kind of odorant but to a variety of molecules, allowing the animal to better distinguish among the nuances of odors.

Imagine a roomful of musicians. In chemotopy, the musicians play one instrument at a time and never play with other instruments. The team’s tunotopic hypothesis is closer to an actual symphony: Different instruments overlap to create many more different sounds than the individual ones could.

The tunotopic hypothesis may help us understand visual, auditory and somatosensory processing as well. In the case of olfaction, tunotopy allows the animal to discriminate between tiny variations in odor. That precision, from an evolutionary perspective, would come in handy as the animal sorted through its environment.

The study was published in the March 19, 2012, issue of the Proceedings of the National Academy of Sciences.