Pavlov's Locust: Insects Trained to Associate Odor with Food, Leads to New Insights in Olfactory System

After researchers trained locusts to anticipate a food reward when confronted with a specific smell, the insects were able to quickly detect and react to the smell, even when other distracting odors were present, leading to new insights on how olfactory systems process odor signals.

Writing in the journal Nature Neuroscience, Washington University in St. Louis researchers say their study is the first in a series of seeking to understand the principles of olfactory computation.

To train the locusts, researcher Barani Raman used Pavlovian conditioning. Locusts in a chamber were exposed to a specific odor administered by a computer-controlled pneumatic pump. The locusts have olfactory receptors in their antennas. A few seconds after the odor puff was administered, the insects were given a piece of grass as a food reward. As the locusts learned to predict the food reward, Raman observed the insects moving their palps, the finger-like projections close to their mouths. The locusts learned to recognize the trained odors and respond accordingly, even when other odors were introduced as distractions.

"We were expecting this result, but the speed with which it was done was surprising," said Raman, who is an assistant professor of biomedical engineering. "It took only a few hundred milliseconds for the locust's brain to begin tracking a novel odor introduced in its surrounding. The locusts are processing chemical cues in an extremely rapid fashion."

Raman noted that the way locusts perceive odor could be different than how humans do.

"There were some interesting cues in the odors we chose," Raman said. "Geraniol, which smells like rose to us, was an attractant to the locusts, but citral, which smells like lemon to us, is a repellant to them. This helped us identify principles that are common to the odor processing."

Raman said follow-up research in the future will lead to even better understanding the principles of olfactory computation.

"There is a precursory cue that could tell the brain there is a predator in the environment, and it has to predict what will happen next," Raman said. "We want to determine what kinds of computations have to be done to make those predictions.

"Neural activity in the early processing centers does not terminate until you stop the odor pulse," he said. "If you have a lengthy pulse -- 5 or 10 seconds long -- what is the role of neural activity that persists throughout the stimulus duration and often even after you terminate the stimulus? What are the roles of the neural activity generated at different points in time, and how do they help the system adapt to the environment? Those questions are still not clear."

Raman, who has been studying how the olfactory system processes scents and odor signals for a decade, hopes to one day develop a device for noninvasive chemical sensing.