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Pavlovian locust experiments provide new clues about how sense of smell works in the brain

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From the scent of brewed coffee to the smell of rain soaking the ground, distinguishing between scents is one of the reasons having a sense of smell is wonderful.

But how smells like coffee can be understood by our brains regardless of season, location or time is a complex question to answer. There are a large number of neurons that make up our olfactory bulb to help us make sense of the aromas around us.

So the researchers turned to an unlikely ally — locusts — to break down how smells like coffee still smell like coffee, despite myriad other smells or environmental factors that might get in the way.

“This pattern-recognition feat that is still elusive in artificial chemical sensing systems is performed by most sensory systems for survival,” the team from Washington University in St Louis wrote in their new report. item.

“How is this capacity achieved? »

The team first did a version of Pavlovian respondent conditioning on insects, specifically an American grasshopper (Schistocerca americana). They placed an “odorant” next to the hungry locusts’ antennae (this is how locusts smell) and then gave them a food reward. This caused the locusts’ outer mouthparts – called maxillary palpi – to open, which the researchers painted green for easier viewing.

After a few cycles of this conditioning, the locusts would open their maxillary palps after just smelling the odorant, no food required. This response means that the locusts have recognized the odorant, which is obviously an important first step.

While the locusts wore small sensors that monitored neural activity, the team then tested whether the locusts could pick up the same scent under different conditions, such as dry or wet or changing intervals. As the crickets’ palpi opened each time, the neurons fired in inconsistent patterns depending on the circumstances.

“The neural responses were highly variable,” said one of the team, biomedical engineer Barani Raman.

“It seemed to be at odds with what the locusts were doing, behaviorally.”

The researchers then used machine learning to figure out what was going on. It quickly became apparent that there were a number of “ON” neurons that fire when an odorant is present and “OFF” neurons that fire when the odorant disappears. But they don’t have to be perfect to work.

The locust brain only needed the approximate amount of ON neurons on and OFF neurons off to say the smell was there, with wiggle room to fight any changes for the different conditions that would alter the smells. .

Although we cannot relate these findings directly to humans, studying these responses in a simpler animal model like these insects can help us better understand some of the fundamentals of the general functioning of olfactory systems.

In this case, we now have a model of how a neural system can detect a specific odor among a cacophony of other obscuring environmental factors.

You can learn more about it in the video below.

Of course, we can’t take too much from this study to relate to the much larger olfactory bulbs of humans, but it’s fascinating research. Hopefully, with more study, we can figure out how our brains can also understand the plethora of smells that surround us in our daily lives.

The research has been published in PNAS.