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Office of Neuroscience Research > WUSTL Neuroscience News > What a locust’s nose taught engineers about monkeys’ ears

What a locust’s nose taught engineers about monkeys’ ears

From the WashU Newsroom...

Is there an opposite for the smell of a rose? Is silence simply the absence of sound? The results of a recent study by a team of biomedical engineers in the School of Engineering & Applied Science at Washington University in St. Louis offer profound implications for how sensory information may be encoded in the brain.

When you cradle your morning cup of coffee in your hands and inhale its rich aroma, certain neurons in your brain, or “coffee-present neurons,” are turned on by the coffee molecules to tell you the coffee is present. When you remove your nose from your coffee cup, is the disappearance of the coffee aroma the result of the absence of coffee molecules that deactivate the “coffee-present neurons,” or is the absence of coffee also a sensory stimulus similar to its presence? The Washington University team, led by Barani Raman, associate professor of biomedical engineering, found the answers to these questions using a surprising combination of animals and sensory modalities in research published May 23 in Nature Communications.

Using the nose of a locust and the ears of a primate, the team investigated how the presence and absence of an odor or a sound is processed. Not surprisingly, they found that a chemical activated an ensemble of neurons in the insect’s brain that uniquely encoded the identity and intensity of the sensory cue. In the locust, when the sensory cues were turned off, a completely different set of neurons, “coffee-absent neurons,” was activated. When the “coffee-present neurons” are active, they suppress the “coffee-absent neurons.” Conversely, after the coffee cup is removed, the “coffee-absent neurons” are activated, and they suppress the “coffee-present neurons.”

“When the ‘on’ neurons are active, they suppress the ‘off’ neurons,” Raman said. “And when you withdraw your nose, the ‘off’ neurons become active and shut down the ‘on’ neurons to tell you the scent is no longer there. It’s not a passive unsensing, but an active process.”

To test this interpretation that “off” responses are necessary for unsensing, Raman and members of his lab used Pavlovian methods to train locusts to recognize certain odors. When the trained locusts were presented with the scent they had been conditioned to recognize, they opened their palps — sensory appendages near their mouths — and kept them open as long as the stimulus was presented, or during the time the “on” neurons would be responding. Raman and his team found that trying to distract the locusts with another cue did not terminate the behavioral response, even though the “on” neurons responding to the odor they had been conditioned to recognize were no longer active.

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