Locusts may be pretty small creatures, but a group of researchers think that an investigation into how their brains process multiple smells will allow them to gain more insights into the human brain too.
In a recent study published in Nature Communications, a team of biomedical engineers at Washington University in St. Louis took some locusts and trained them to recognize odors in order to gain some insights into how their brains responded to different stimuli.
Each locust's antennae have sensory neurons that convert "puffs of odor" into electrical signals, which are transmitted around the circuits of neurons in its brain. What Raman and his team wanted to find out was whether the neurons in a locust's brain responded differently to an odor if the creature was exposed to a different activity beforehand.
To test this out, the researchers set up a clever experiment that took cues from a circus master in a ring. They used Pavlovian conditioning, where they "trained hungry locusts to respond to a puff of odor by rewarding them with a piece of grass." Pavlovian conditioning was discovered by Russian physiologist Ivan Pavlov in the 1890s. It involves teaching a subject to associate one particular stimulus that induces a specific response with a new stimulus. This means that the new stimulus, when presented to the subject, would induce the same automatic response.
Once the locusts were trained up, the researchers found that the creatures moved their palps—small appendages near their mouth—when they detected an odor puff that they'd been trained to recognize.
"We found that the locusts recognize the odor and move their palps within roughly 500 milliseconds," said Raman in a press release. The locusts turned out to be good trainees too. "Once trained, the locusts did not forget the learned association easily," added Raman, who noted that the locusts continued to respond even when there was no promise of a grassy tidbit.
But how could the researchers be sure of this? William Padovano, a researcher in Raman's lab, doused the locust palps in green paint so that they stood out from the creatures other body parts. He then filmed the locusts' responses to the odor puffs, and found that they quivered their palps in anticipation of a reward.
And what goes on in a locust's brain when its made to sniff different odours? The researchers found that when two similar smelling puffs were given in succession, the locust' neural activity didn't spike as much when it came to processing the second odor. "The locusts robustly recognized and responded to the trained odor whether it was presented alone or after another odor, but their response time and behavior were less predictable when the trained odor followed a similar odor that evoked highly overlapping neural activity," said Raman.
But when two radically different odor puffs were presented to the locusts one after another, the researchers found that the locust responded better. "The more novel the stimulus, the more preserved the pattern of spiking activity becomes and more predictable the behavioral responses were," said Raman.
Now that the researchers have found a way to correlate locust behaviour with neural activity, next up, they aim, said Raman, "to make very fine calculations with neural activity to understand how each aspect of neural response is related to behavior." Let's hope they've got a plentiful supply of grass for those voracious bugs.