New ‘smell maps’ reveal how flies and humans decode odours
Key topics:
Scientists uncover “smell maps” showing how neurons process odours efficiently
Fly olfactory system uses paired neurons to guide behaviour and survival
Smell mapping could inspire faster computing and decision-making research
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By Adrian Ephraim
The next time you stop to smell the roses or take in a whiff of your favourite coffee aroma, know that it is because our sensory systems have evolved to decipher critical information about the world around us, including colours and sounds interpreted through our sight and hearing.
Through some painstaking work, scientists have now uncovered “smell maps” which up until now had never been discovered. These so-called “valence maps” depict how sensory neurons are arranged to help animals and humans process information more efficiently.
The foul stench of rotting meat or polluted water forces humans to stay away but will attract flies for different reasons. A fly will find the stench irresistible because it prefers to lay its eggs in rotting meat, and so the cycle of its life continues.
At the University of California, San Diego researchers set about trying to understand why and discovered that the fly’s olfactory system enables the insect to quickly assess odours and make a decision. A fly’s thousands of olfactory receptor neurons (ORNs) – the components that sense smell – are organised inside the sensory hairs and communicate through electrical interactions with nearby ORNs.
“We are constantly being bombarded by hundreds of odorous chemicals in our environment,” Biological Sciences Associate Professor and co-author of the study, Chih-Ying Su told Science Daily. “We have described a peripheral mechanism that has allowed the fly to make sense of such overwhelmingly complex stimuli.”
The university’s study reveals how compartments with two ORNs are arranged to detect cues with opposite meanings for the fly. These cues either promote or inhibit certain behaviour, allowing the fly to quickly assess complex odours in its environment.
According to the paper written by team lead, graduate student, Shiuan-Tze Wu, “This arrangement provides a means to both evaluate and shape the countervailing sensory signals relayed to higher brain centres for further processing,” meaning the fly’s ability to better assess potential danger or opportunity is heightened.
Researchers, with the help of Assistant Professor Johnatan Aljadeff from the University of California San Diego’s Department of Neurobiology, built a mathematical model that explains how electrical interactions help in extracting relevant information.
The new smell map was published in early 2022 in the Proceedings of the National Academy of Sciences. [https://www. pnas.org/doi/10.1073/pnas.2120134119]
“We found that nature has chosen a specific way of structuring this sensory assay,” said Aljadeff. “If we can understand the principle of this type of processing, there could be future engineering applications.” What those future applications could be we don’t know yet, but the potential for faster computing and better decision-making in humans is a discussion that is likely already under way.
The University of California San Diego team uncovered ORNs situated next to each other that “antagonistically regulate behaviours”. They’re located in the sensory hairs of the fruit fly. Scientists are enthralled by the “elegance” of the system of ORNs that flies use to compute these cues. They compare it to the way that visual systems contrast colour shades to help us differentiate between colours like red and green.
The significance of this olfactory mapping shouldn’t be lost on us. How paired ORNs behave determines animals’ and humans’ preferred habitat, who they mate with, and where they choose to give birth or lay eggs. Even when odours mix, experiments have shown that these smell maps may have evolved to compute those cues in a faster and more efficient way called synaptic computation.
According to the US National Library of Medicine, “Odours mediate both innate and learned behaviours such as attraction and aversion, governing decisions to eat, mate, attack or flee from aggressors and predators. A remarkable feature of the olfactory system is the extraordinary diversity of possible odour molecules that exist.”
The ability to make sense of one’s environment has been key to survival for every species on the planet. Our olfactory systems, as illustrated by smell maps, serve a critical function in determining how quickly an animal is able to make an informed decision when confronted with an odour – a matter of life and death in some cases.
Science has made ground-breaking strides in its understanding of smell maps and the sensory system. In 2004, Richard Axel at Columbia University in New York, and Linda Buck at the Fred Hutchinson Cancer Research Centre in Seattle, shared the Nobel Prize in Physiology or Medicine “for their discoveries of odorant receptors and the organisation of the olfactory system”.
Sadly, scientists don’t have all the answers yet. They haven’t been able to determine how and why the 300-400 smell receptors in human beings detect thousands of odours.