Weakening eyesight can be sharpened with lenses, and impaired hearing can be improved with aids. What about a failing sense of smell?
Detecting and distinguishing the floral bouquet of fresh honey or the miasma of bad lunchmeat might not seem quite as critical for day-to-day existence as sight or hearing. But what the nose knows is clearly important for quality of life. Research has linked this diminution, which is common in people over the age of 60 and can be exacerbated by smoking and some diseases, to loss of appetite and even to depression.
Now sufferers might not have to give in to an odorless future, according to a new study, published online Sunday in Nature Neuroscience.
Researchers at the New York University Langone Medical Center have found that, with some simple training, over time lab rats could actually improve their brain's ability to distinguish smells. Without any practice rats could tell when one scent—in a mlange of 10—had been switched for another. (Researchers figured this out by waiting until the rodents were thirsty, then training them to look for water in one of a selection of holes based on what odor combination they had detected.) But their powers of discrimination were not perfect. If a scent was missing from the mix, the rats did not seem to be able to discern it from a full 10-scent combination.
Other rats, however, were trained to become extra-familiar with the different combinations through repeated exposures and rewards. "We made them connoisseurs," co-author Donald Wilson, a professor of psychiatry at NYU Langone, said in a prepared statement. Electrodes implanted in these rats' brains showed that each different scent combination sparked a unique electrical pattern in the olfactory (piriform) cortex. The group of rats that had not received this aromatic instruction, however, showed the same generic pattern for all of the scents that they didn't seem to recognize. This difference suggests that the trained rats' brains had developed specific signatures for the scents over time, effectively improving their sense of smell.
A third group of rats was conditioned to disregard the differences by being given water in the same place after all of the scents. These rodents ended up showing even more generic responses in their piriform cortexes—evidence that their brains could also change in the other direction—worsening their sense of smell.
"This bidirectional effect was not found in the olfactory bulb," an area of the brain that is located outside the piriform cortex, Wilson and his colleague Julie Chapuis, a postdoctoral researcher at NYU, noted in their paper. Instead, the changing sensory abilities seem to have come from changes in the olfactory cortex itself.
That plasticity, if it proves to be possible in humans as well, could be a way to hang on to smell—and its benefits—a little bit better. "Our findings suggest that, while olfactory impairment may reflect real damage to the sensory system, in some cases it may be a 'use it or lose it' phenomenon," Wilson said. So even if the sense of smell cannot be preserved entirely, the new findings suggest that the parts of it for which the olfactory cortex is responsible could be improved.
The new findings might give brain-training tactics a whole new dimension. "Training can improve the sense of smell," Wilson concluded. So when it comes out, perhaps best not to turn up your nose at that scratch-and-sniff sudoku.