Why scientists trained locusts to recognize odors

A few seconds after the odor puff is given, the locust gets a piece of grass as a reward, as a form of Pavlovian conditioning. Instead of salivating, they open their palps, or finger-like projections close to the mouthparts, when they predict the reward. Their response was less than half of a second. (Credit: Lynette Schimming/Flickr)

To learn more about how the brain can process multiple odors all at once, scientists trained locusts to respond to a specific smell.

Locusts have a relatively simple sensory system, which is ideal for studying brain activity.

Barani Raman, of the School of Engineering & Applied Science at Washington University in St. Louis, found that odors prompted neural activity in the brain that allowed the locust to correctly identify the stimulus, even with other odors present.

How to train a locust

The team used a computer-controlled pneumatic pump to administer an odor puff to the locust, which has olfactory receptor neurons in its antennae, similar to sensory neurons in our nose.

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A few seconds after the odor puff is given, the locust is given a piece of grass as a reward, as a form of Pavlovian conditioning. As with Pavlov’s dog, which salivated when it heard a bell ring, trained locusts anticipated the reward when the odor used for training is delivered.

Instead of salivating, they opened their palps, or finger-like projections close to the mouthparts, when they predicted the reward. Their response was less than half of a second.

The locusts could recognize the trained odors even when another odor meant to distract them was introduced prior to the target cue.

“We were expecting this result, but the speed with which it was done was surprising,” says Raman, 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.”

“There were some interesting cues in the odors we chose,” Raman says. “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 has spent a decade learning how the human brain and olfactory system operate to process scent and odor signals. His research could lead to a device for noninvasive chemical sensing that takes inspiration from the biological olfactory system. Such a device could be used in homeland security applications to detect volatile chemicals and in medical diagnostics to test blood-alcohol level.

This study is the first in a series focused on the principles of olfactory computation, Raman says.

“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 says. “We want to determine what kinds of computations have to be done to make those predictions.”

The results were published in Nature Neuroscience.

Source: Washington University in St. Louis