A new study by a team of neuroscientists at the University of California, Berkeley, has determined that we learn new smells in an area of our brains, not just in our noses, which have neural receptors previously thought to be solely responsible for a person's ability to detect new odors.
This finding has prompted the team, led by graduate student Joel Mainland and Noam Sobel, assistant professor of psychology, to conclude that the adult brain has more capabilities to change than previously thought. The study appears in the Oct. 24 issue of Nature.
The discovery may have implications for how the brain recovers from injury. Lately, there has been a lot of evidence that activity in damaged regions of the body results in regeneration in the brain. For example, in stroke patients, tying down the unaffected limbs to force patients to try use their affected arms or legs has resulted in recovery of some use of those limbs.
The researchers conducted their work through a very simple mechanism using the chemical androstenone. Androstenone cannot be detected by approximately 30 percent of the population.
However, about half of such non-detectors can develop the capability to detect the odorant following repeated exposure to it. For those who can smell it, there is a wide range of reactions to its odor. The people who are most sensitive to it find the smell extremely foul and reminiscent, Sobel said, "of dirty laundry."
Sobel and his colleagues conducted an extensive screening to find subjects who could not detect this smell. The screening yielded 12 people. In the test subjects, one nostril was completely blocked, and the open nostril was exposed to androstenone every day for 21 days. After the 21 days, both nostrils were tested for detection. Both nostrils doubled their detection accuracy due to this exposure.
The unexposed nostril detected the androstenone at the same level as the exposed nostril. Because there is no neural link between the nostrils at the peripheral level, the researchers concluded that this exposure-induced learning must have occurred in the olfactory structures in the brain that share information from both nostrils.
"Since the unexposed nostril learned just as well, the brain is definitely involved. This contradicts a previous theory that olfactory learning occurred in the nose only," said Sobel, a member of UC Berkeley's Health Sciences Initiative, a broad effort bringing together campus researchers from many disciplines to work on health problems of the 21st century.
"Our results suggest there must be a central component in the brain at work," Sobel said, though he added the researchers have not ruled out peripheral neural changes occurring as well. Ongoing research is being conducted to determine if peripheral neural plasticity - the nervous system acquiring a capability it didn't have before - is involved.
In children, the nervous system is constantly changing and developing, "but in adults, it's a question as to how much it can change," Sobel said. "If you want to repair a damaged nervous system, the best way to go about doing this is to figure out how it regenerates on its own."
Further studies by the team will investigate the difference between people who can learn to detect an odor through exposure to that odor and those who cannot. The researchers also will use Magnetic Resonance Imaging to localize regions in the brain to see where learning and change is occurring.