Why Do We Like Certain Scents?

Summary: Researchers delved into understanding why certain smells appeal to us and others don’t by studying locusts.

Through a series of experiments, they analyzed how locusts’ brains reacted to various odors and the behavior exhibited in response. They discovered that locusts innately gravitated towards certain odors, such as those resembling grass and banana, while rejecting others.

This study provides valuable insight into how an organism’s preferences can constrain its learning ability, particularly in relation to the timing of rewards.

Key Facts:

  1. The study used locusts as their olfactory system is vital for survival, converting chemical cues into electrical signals that guide behavior.
  2. Locusts innately preferred scents resembling grass and banana, while rejecting almond and citrus-like smells.
  3. The ability of locusts to associate appealing scents with rewards shed light on how sensory signals are processed as soon as they enter the brain.

Source: WUSTL

Everyone has scents that naturally appeal to them, such as vanilla or coffee, and scents that don’t appeal. What makes some smells appealing and others not?

Barani Raman, a professor of biomedical engineering at the McKelvey School of Engineering at Washington University in St. Louis, and Rishabh Chandak, who earned bachelor’s, master’s and doctoral degrees in biomedical engineering in 2016, 2021 and 2022, respectively, studied the behavior of the locusts and how the neurons in their brains responded to appealing and unappealing odors to learn more about how the brain encodes for preferences and how it learns.

This shows bottles full of flowers and herbs.
Raman has used locusts for years to study the basic principles of the enigmatic sense of smell. Credit: Neuroscience News

The study provides insights into how our ability to learn is constrained by what an organism finds appealing or unappealing, as well as the timing of the reward.

Results of their research were published in Nature Communications Aug. 5.

Raman has used locusts for years to study the basic principles of the enigmatic sense of smell. While it is more of an aesthetic sense in humans, for insects, including locusts, the olfactory system is used to find food and mates and to sense predators.

Neurons in their antennae convert chemical cues to electrical signals and relay them to the brain. This information is then processed by several neural circuits that convert these sensory signals to behavior.

Raman and Chandak set about to understand how neural signals are patterned to produce food-related behavior. Like dogs and humans salivating, locusts use sensory appendages close to their mouths called palps to grab food.

The grabbing action is automatically triggered when some odorants are encountered. They termed odorants that triggered this innate behavior as appetitive. Those that did not produce this behavior were categorized as unappetitive.

Raman and Chandak, who earned the outstanding dissertation award from biomedical engineering, used 22 different odors to understand which odorants the locusts found appetitive and which they did not.

Their favorite scents were those that smelled like grass (hexanol) and banana (isoamyl acetate), and their least favorites smelled like almond (benzaldehyde) and citrus (citral).

“We found that the locusts responded to some odors and not others, then we laid them out in a single behavioral dimension,” Raman said.

To understand what made some odorants more likable and others not, they exposed the hungry locusts to each of the scents for four seconds and measured their neural response.

They found that the panel of odorants produced neural responses that nicely segregated depending on the behavior they generated. Both the neural responses during odor presentation and after its termination contained information regarding the behavioral prediction.

“There seemed to be a simple approach that we could use to predict what the behavior was going to be,” Raman said.

Interestingly, some of the locusts showed no response to any of the odors presented, so Raman and Chandak wanted to see if they could train them to respond. Very similar to how Pavlov trained his dog with a bell followed by a food reward, they presented each locust with an odorant and then gave them a snack of a piece of grass at different time points following the odor presentation.

They found that locusts only associated appealing scents with a food reward. Delaying the reward, they found that locusts could be trained to delay their behavioral response.

“With the ON-training approach, we found that the locusts opened their palps immediately after the onset of the odor, stayed open during the presentation of the odor, then closed after the odor was stopped,” Raman said.

“In contrast, the OFF-training approach resulted in the locusts opening their palps much slower, reaching the peak response after the odor was stopped.”

The researchers found that the timing of giving the reward during training was important. When they gave the reward four seconds after the odor ended, the locusts did not learn that the odor indicated they would get a reward. Even for the appealing scents no training was observed.

They found that training with unpleasant stimuli led locusts to respond more to the pleasant ones. To explain this paradoxical observation, Raman and Chandak developed a computational model based on the idea that there is a segregation of information relevant to behavior very early in the sensory input to the brain.

This simple idea was sufficient to explain how innate and learned preference for odorants could be generated in the locust olfactory system.

“This all goes back to a philosophical question: How do we know what is positive and what is negative sensory experience?” Raman said.

“All information received by our sensory apparatus, and their relevance to us, has to be represented by electrical activity in the brain. It appears that sorting information in this fashion happens as soon as the sensory signals enter the brain.”

About this olfaction research news

Author: Talia Ogliore
Source: WUSTL
Contact: Talia Ogliore – WUSTL
Image: The image is credited to Neuroscience News

Original Research: Open access.
Neural manifolds for odor-driven innate and acquired appetitive preferences” by Barani Raman et al. Nature Communications


Neural manifolds for odor-driven innate and acquired appetitive preferences

Sensory stimuli evoke spiking neural responses that innately or after learning drive suitable behavioral outputs.

How are these spiking activities intrinsically patterned to encode for innate preferences, and could the neural response organization impose constraints on learning?

We examined this issue in the locust olfactory system. Using a diverse odor panel, we found that ensemble activities both during (‘ON response’) and after stimulus presentations (‘OFF response’) could be linearly mapped onto overall appetitive preference indices.

Although diverse, ON and OFF response patterns generated by innately appetitive odorants (higher palp-opening responses) were still limited to a low-dimensional subspace (a ‘neural manifold’). Similarly, innately non-appetitive odorants evoked responses that were separable yet confined to another neural manifold.

Notably, only odorants that evoked neural response excursions in the appetitive manifold could be associated with gustatory reward. In sum, these results provide insights into how encoding for innate preferences can also impact associative learning.

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