Making 'Sense' of Scents: How Animals Survive Based on Smell

Many animals' lives greatly depend on making "sense" of a cluster of scents they come in contact with. 
For instance, how animals separate prey from predators can essentially determine the length of their lifespan. Yet just how animals make those distinctions was relatively a mystery--until now. 

A new study published in the journal Nature Neuroscience showed that while mice can be trained to detect specific odorants embedded in random mixtures, their performance drops steadily with increasing background components.

"There is a continuous stream of information constantly arriving at our senses, coming from many different sources," said lead study author Venkatesh Murthy, a Harvard professor of Molecular and Cellular Biology, in a news release. "The classic example would be a cocktail party - though it may be noisy, and there may be many people talking, we are able to focus our attention on one person, while ignoring the background noise.

For the study, researchers presented the animals with a combination of smells. Though previous studies have shown that many creatures are not the best at differentiating between individual scents, and instead, perceive mixtures as a single smell, findings revealed that mice were able to identify the target scent with 85 percent accuracy or higher.

"Although the mice do well overall, they perform progressively poorer when the number of background odors increases," Murthy explained.

To further determine how two odors relate to one another, researchers used fluorescent light proteins to create images that showed each of the 14 odors stimulated neurons in the olfactory bulb. This revealed that the animals had the ability to identify a particular smell that was markedly diminished if background smells activated the same neurons as the target odor.

"This study is interesting because it first shows that smells are not always perceived as one whole object - they can be broken down into their pieces," Murphy concluded. "This is perhaps not a surprise - there are in fact coffee or wine specialists that can detect faint whiffs of particular elements within the complex mixture of flavors in each coffee or wine. But by doing these studies in mice, we can now get a better understanding of how the brain does this. One can also imagine that understanding how this is done may also allow us to build artificial olfactory systems that can detect specific chemicals in the air that are buried amidst a plethora of other odors."