Environmental stimuli often trigger our sense of smell before we exhibit any other response. Smells trigger neurons in our brains that alert us to take action, but there are often more than one odor in our environments at any given time.
Barani Raman, PhD, of the School of Engineering & Applied Science at Washington University in St. Louis, wanted to answer the question of how our brain processes multiple odors received simultaneously.
The researchers used locusts, which have a relatively simple sensory system ideal for studying brain activity. They found that odors prompted neural activity in the brain that allowed the locust to correctly identify the stimulus, even with other odors present. The findings were published in a recent issue of Nature Neuroscience.
A computer-controlled pneumatic pump was used by the team to administer an odor puff to the locust, which has olfactory receptor neurons in its antennae, similar to sensory neurons in our nose. The locust receives a piece of grass as a reward a few seconds after the puff in a form of Pavlovian conditioning. Much like Pavlov‘s dog salivated when it heard a bell ring, the locust anticipated the reward when the odor used for training was delivered.
Rather than salivating, when anticipating the reward the locust opened their palps, which are finger-like projections close to the mouth-parts. Their response time was less than half of a second, and the locusts were able to recognize trained odors 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 been researching how the human brain and olfactory system operate to process scent and odor signals for over a decade seeking to take inspiration from the biological olfactory system to develop a device for noninvasive chemical sensing. Such a device would have applications, for example, in homeland security to detect volatile chemicals and in medical diagnostics, to test blood-alcohol level.
Raman said the current study would be the first in a series designed to understand 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 says. “We want to determine what kinds of computations have to be done to make those predictions.”
The researchers are also trying to answer other questions.
“Neural activity in the early processing centers does not terminate until you stop the odor pulse,” he says. “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.”
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