Sense Of Smell May Improve With Training

Featured Article
Academic Journal
Main Category: Neurology / Neuroscience
Also Included In: Ear, Nose and Throat
Article Date: 21 Nov 2011 - 3:00 PST


People who notice their sense of smell is not as good as it used to be may wish to take note of what scientists training laboratory rats concluded: a failing sense of smell can improve, however, it can also get worse, depending on the type of training. Drs Julie Chapuis and Donald A Wilson from New York University (NYU) Langone School of Medicine write about their findings in the 20 November online issue of Nature Neuroscience. They hope their discovery will help develop new ways to reverse the loss of smell that occurs with age or disease.

Humans detect smells because odor molecules interact with nerves in the nose that send impulses to the olfactory bulb, a structure beneath the frontal cortex at the front of the brain. The olfactory bulb also connects directly with other parts of the brain such as the amygdala that controls emotions, and higher-order regions like the prefrontal cortex, the seat of important "executive functions" like planning, thinking, organizing, and making choices.

Failing sense of smell is linked with diseases like Alzheimer's disease, Parkinson's disease, schizophrenia, and even normal aging, but why this happens is somewhat of a mystery. However, Chapuis and Wilson report they have found where this loss occurs in the brain, and show that with training, it can also improve, and get worse.

Wilson, a professor of child and adolescent psychiatry at NYU Langone Medical Center and senior research scientist at the Emotional Brain Institute at Nathan S. Kline Institute for Psychiatric Research in Orangeburg, New York, led the study. He told the press, that of all the senses, smell is unique because:

"Unlike information from your eyes and ears that has gone through many connections to reach the frontal cortex, the olfactory system is just two connections away. The result is an immediate pathway from the environment through our nose to our memory."

Wilson and co-author Chapuis, a post-doctoral fellow, put thirsty rats in individual boxes with a snout-sized hole in each of three walls. They then ran a series of experiments, each using slight variations on a blend of 10 smells, based on chemicals from fruits, oils and cleaning agents. The first variation used all 10 chemicals. The second had one chemical removed and a new one substituted. The third had one chemical removed with no substitute.

They exposed the rats to a quick blast of the mix through the hole in the middle wall of the box. Then, depending on what the mix was, they rewarded the rats by giving them a sip of water through the hole in the left wall, or the hole in the right wall. They repeated the experiments with the different odors, and observed which hole the rats went to for their rewards.

The results showed the rats were easily able to distinguish one odor from another when a chemical was replaced in a mix, but they could not tell the difference when one chemical was simply removed, with no substitute.

In the next phase of the study, the researchers anesthetized the rats and inserted electrodes into their brains while they exposed them to the smells, so they could see the patterns of electrical activity produced in the affected parts of the brain.

They could see that each smell produced a different pattern of activity in the olfactory bulb, but in the piriform (olfactory) cortex, a one-centimetre area of the cerebral cortex, the patterns were different when the rats were able to differentiate the smells, but they were identical when they were not able to differentiate them.

Wilson and Chapuis then "trained" a new group of rats to notice the difference between the smells that the first group were not able to distinguish: they just kept rewarding them over and over again by giving them sips of water through the same hole.

Wilson said the training turned the rats into "connoisseurs", and when they looked at the electrical activity in the animals' piriform cortex, they could see this reflected there: this time the patterns made by sensing the smells where one chemical was simply removed were different.

And in a final phase, they trained yet another group of rats to ignore the difference between the smells that the first rats were easily able to tell apart: they just gave them sips of water through the same hole every time, no matter which odor they exposed them to.

The researchers said this training effectively dulled the rats' sense of smell, and it was reflected in the patterns of electrical activity in their piriform cortex, which showed an identical response to both smells.

They write:

"Rats given extensive training with overlapping complex odorant mixtures showed improved behavioral discrimination ability and enhanced piriform cortical ensemble pattern separation. In contrast, behavioral training to disregard normally detectable differences between overlapping mixtures resulted in impaired piriform cortical ensemble pattern separation (enhanced pattern completion) and impaired discrimination."

Wilson said their findings suggest while losing one's sense of smell may reflect real damage to the sensory system, in some cases it could be a matter of "use it or lose it".

"Odor training could help fix broken noses," he added.

Written by Catharine Paddock PhD
Copyright: Medical News Today
Not to be reproduced without permission of Medical News Today

• Additional
• References
• Citations