Humans’ sense of smell has evolved into a sophisticated system that processes scents through several intricate stages and most times they can pick up something like a dangerous level of fumes from a toxic chemical.

After all, the brains of mammals have billions of neurons at their disposal to recognize odors.

On the other hand, insects such as fruit flies, have a mere 100,000 neurons to work with. Yet, their survival is dependent upon their ability to decipher the meaning of complex odor mixtures around them to locate food, seek potential mates and avoid predators.

That concept has left scientists pondering how insects are able to smell, or extract information from odors, with a much smaller olfactory sensory system compared with mammals.

Looking at Fruit Flies
There may now be an answer as scientists at the University of California San Diego uncovered how fruit flies use a simple, efficient system to recognize odors.

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“Our work sheds light on the sensory processing algorithms insects use to respond to complex olfactory stimuli,” said Palka Puri, a physics Ph.D. student at the University of California at San Diego and the first author of a paper on the subject. “We showed that the specialized organization of insect sensory neurons holds the key to the puzzle – implementing an essential processing step that facilitates computations in the central brain.”

Previous investigations of the odor processing system in flies focused on the central brain as the main hub for processing odor signals. But the new study shows the effectiveness of the insect’s sensory capabilities relies on a “pre-processing” stage in the periphery of their sensory system, which prepares the odor signals for computations that occur later in the central brain region.

Flies smell through their antennae, which are replete with sensory hairs that detect elements of the environment around them. Each sensory hair usually features two olfactory receptor neurons (ORNs) that activate via different odor molecules in the environment. ORNs in the same sensory hair end up strongly coupled by electrical interactions.

“This scenario is akin to two current-carrying wires placed close together,” Puri said. “The signals carried by the wires interfere with each other through electromagnetic interactions.”

Understanding Odor’s Meaning
In the case of the fly olfactory system, however, this interference is beneficial. The researchers showed as flies encounter an odor signal, the specific pattern of interference between the receptors helps flies quickly compute the “gist” of the odor’s meaning: “Is it good or bad for me?”

As flies encounter an odor signal, the specific pattern of interference between the receptors helps flies quickly compute the “gist” of the odor’s meaning.
Source: Palka Puri, UC San Diego


The result of this preliminary evaluation in the periphery is then relayed to a specific region in the fly’s central brain, where the information about odors present in the outside world translates to a behavioral response.

Researchers constructed a mathematical model of how odor signals end up processed by electrical coupling between ORNs. They then analyzed the wiring diagram (“connectome”) of the fly brain, a large-scale dataset generated by scientists and engineers at Howard Hughes Medical Institute’s research campus. This allowed Puri and his team to trace how odor signals from the sensory periphery integrate in the central brain.

“Remarkably, our work shows that the optimal odor blend – the precise ratio to which each sensory hair is most sensitive – is defined by the genetically predetermined size difference between the coupled olfactory neurons,” said Assistant Professor Johnatan Aljadeff in the UCSD School of Biological Sciences. “Our work highlights the far-reaching algorithmic role of the sensory periphery for the processing of both innately meaningful and learned odors in the central brain.”

Aljadeff describes the system with a visual analogy. Like a specialized camera that can detect specific types of images, the fly has developed a genetically driven method to distinguish between images, or in this case, mixtures of odors.

“We discovered that the fly brain has the wiring to read the images from this very special camera to then initiate behavior,” he said.

Combining Efforts
To arrive at these results, the team integrated research from previous findings from UCSD Associate Professor Chih-Ying Su’s lab that described the conserved organization of ORNs in the fly olfactory system into sensory hairs. The fact signals carried by the same odor molecules always interfere with each other, in every fly, suggested to the researchers this organization has meaning.

“This analysis shows how neurons in higher brain centers can take advantage of balanced computation in the periphery,” Su said. “What really brings this work to another level is how much this peripheral pre-processing can influence higher brain function and circuit operations.”

This work may inspire research into the role of processing in peripheral organs in other senses, such as sight or hearing, and help form a foundation for designing compact detection devices with the ability to interpret complex data.

“These findings yield insight into the fundamental principles of complex sensory computations in biology, and open doors for future research on using these principles to design powerful engineered systems,” Puri said.

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