Our nose is organized and hardwired to separate the stinky from the fragrant, says new research.
Long viewed as disorganized, with receptors of all different kinds mushed up against each other, this new study suggests something entirely different.
The smell receptors lining the inside of the nose are not scattered at random, but rather are organized by how pleasant or repulsive we feel about certain smells. Receptors for smells that repel us tend to cluster together and the same goes for smells we like.
"The best organization we could find for the organization was the perception of odorant pleasantness," said Noam Sobel of the Weizmann Institute of Science in Rehovot, Israel, who led the new study, published in Nature Neuroscience.
Not only are smell receptors arranged in patches, research shows, but the patches are organized along an axis of yucky to pleasant.
These findings across the senses show that we begin to organize our sensory world even at the first instant where the stimulus encounters the body. "The earliest stage of perception is already organized to match the perceptual world," Sobel said.
For smells, Sobel explained, odor pleasantness is related to the structure of the molecules that we smell. "Molecules that are small and dense tend to be unpleasant," Sobel said. "Molecules that are large and floppy tend to be pleasant. There is a correspondence between the principle axis of structure and pleasantness."
This all means that our smell preferences are, to some extent, hardwired.
"Newborns have odor preferences straight up," Sobel said. Lab rats bred in captivity for 1000 generations still have a fear response to fox odor, he added.
Researchers previously thought that smell receptors were randomly and uniformly spread over the nasal epithelium, the surface high up inside the nose that detects smells, said Sobel.
He and his team used specialized electrical probes extending almost three inches up into the nasal cavity to monitor neural signals from particular smell receptors as subjects smelled in real time.
The researchers piped odorants into the noses of subjects and asked them to rate how pleasant or unpleasant they found each smell. By measuring activity at different spots on the surface, the team was able to map the nasal epithelium according to a scale of pleasantness.
"If the location responded strongly to one pleasant odorant it was likely to respond strongly to other pleasant odorants," Sobel said, and the same applied to unpleasant ones.
"Although previous studies (mainly from the rodent olfactory epithelium) have demonstrated the non-uniform and non-random distribution of odor-induced responses, this is the first study to directly link this phenomenon to a key axis of perception, i.e., the pleasantness of odors," said Minghong Ma of the University of Pennsylvania.
In some sense, the findings are not surprising. Researchers know that other senses are also organized along particular axes. "The primary perceptual dimension in vision is spatial," Sobel said.
Things near each other in space are perceived by spots near each other on the retina. Likewise, in hearing, receptors for similar tones are located near each other on the cochlear membrane, Sobel said.