The little electrical shocks that Wen Li gives subjects at her Northwestern University neuroscience lab feel less painful than a bad jolt of static electricity, and more like the mild snap of a rubber band on skin.
But getting just seven of those shocks trained people in a new study to distinguish between extremely similar odors, offering a new and perhaps potent way of changing a person’s power of perception.
Psychologists have used the basic conditioning technique for decades to alter the behavior of people and animals. But experts said the Northwestern study went further by actually modifying how the brain processes the sensation of smell.
The new report, published in Friday’s issue of the journal Science, illustrates the power of emotions like fear to shape how we learn and even perceive the world, said Dana Small, a neuroscientist who studies odor and taste at the John B. Pierce Laboratory in New Haven, Conn.
“I think the study is important for understanding learning in general,” Small said. “We often don’t appreciate just how malleable the brain is, and how much perception depends on learning.”
One possible implication of the work could be new insights into phobias and conditions like post-traumatic stress disorder, which can increase a person’s sensory sensitivity. The group’s conditioning regimen could even lead to new therapies to alleviate the sensory overload of such patients.
The study used 12 college-age people who were hooked up to tubes that brought scents to their noses. They were exposed to two smells emanating from substances that are virtually identical in their chemical structure. Each person took the test while in a functional magnetic-resonance imaging machine, to show how their brain activity might change in response to the training.
At first, none of the subjects could tell the difference between the two smells.
Then the researchers gave some of the smell-testers a small shock from electrodes attached to the leg whenever they were exposed to one of the scents. The brain scanner showed that the brain area that process and perceives smells changed its activity pattern as a result of the shocks. Once the conditioning was finished, the subjects who had received shocks were much better at distinguishing the two similar odors, the report states.
“These subjects have moved to a level of sensory processing that we normally don’t have. That’s pretty unheard of,” said David Zald, an associate professor of psychology and psychiatry at Vanderbilt University in Nashville.
The findings may reveal some of the tools that evolution has given humans and other creatures to adapt quickly to new surroundings, said Li and one of her co-authors, Northwestern researcher Jay Gottfried.
In nature, an animal’s survival might depend on learning the difference between two similar scents — say, the smell of an ordinary cat and that of a leopard. Normally those sensations might be indistinguishable, but an emotional event, like nearly being killed by a leopard, could quickly train an animal to tell the difference.
Humans have a relatively weak sense of smell compared with animals such as dogs and rats, but the shock could be an urgent signal for the brain to handle certain smells differently, said Pamela Dalton, an olfaction researcher at the Monell Chemical Senses Center in Philadelphia.
“The olfactory system probably evolved more as a warning system than an attracting system, so it’s biased toward detecting things with negative consequences,” Dalton said.
Brain scans showed that as people were conditioned to discern the smells, they had more activity in brain areas linked with emotional responses. The Northwestern team believes that may prompt the part of the brain that identifies smells to process signals from the nose in a more complex way than without conditioning.
People often train themselves to enhance their senses without the help of a lab conditioning experiment. Wine aficionados can learn small differences between vintages, while musicians can pick out instruments and melodic lines from a cacophony.
Yet smells can be uniquely tied with memory and learning experiences, and our perception of smell may be more flexible than other senses, experts said.
Phobias and post-traumatic stress disorder demonstrate how emotional experiences can powerfully distort the perception of smells. Dalton said she once talked with an emergency response worker who developed PTSD after dealing with decomposing bodies. In the aftermath of that traumatic experience, the worker had a fear of all dank basements and other places that recalled the smell of corpses.
That type of disorder is the flip side of the Northwestern experiment. Instead of using pain to develop a finer sense of different odors, traumatic experiences can remove sensory distinctions so that everything with a remote resemblance to the feared scent triggers the same dread.
Li said she plans to study ways of using conditioning to restore the subjects’ original sensitivity to smells. She said that also could allow new therapeutic approaches for PTSD patients, who might benefit from less sensitivity to sensations linked with painful memories.
———-
jmanier@tribune.com
