Scents of Smell Rooted in Math
Nose knows different odors, no matter how powerful, thanks to predictable patterns transmitted to brain
By Jo Craven McGinty in the Wall Street Journal
Crushed coriander seeds burst with a lemony aroma. Golden turmeric smells like corn cakes. Cardamom gives off a hint of bitterness. And pulverized cumin seeds smell like moist, peppery earth. Combine them, and you have the fragrant beginnings of curry.
But how does a nose, bombarded with odors that arrive in different amounts and combinations, consistently identify each aroma?
It turns out that it is simpler than many other neurobiological processes, and can essentially be broken down into a predictable mathematical pattern.
Odors arrive in small packets—tiny bouquets of molecules—that are inhaled. Receptor cells inside the nose respond by producing a series of electrical spikes, which are communicated to the olfactory bulb in the brain, where the smell is decoded.
“It’s like Morse code,” said Upinder Bhalla, a professor of neurobiology at the National Centre for Biological Sciences in Bangalore, India, and lead supervisor of a recent study about the olfactory system that is the first to document the coding is linear. “The pattern and spacing of the clicks make different letters.”
In this case, the pattern of the electrical spikes translates to specific smells. But significantly, according to the study, which was published in the journal Nature Neuroscience, when the smell is repeated in the same dose, the pattern remains the same. And when the odor varies in duration, the neurons’ electrical response changes proportionately.
In other words, the response is orderly and predictable, rather than chaotic and irregular.
“Mathematically, it can’t be simpler than this,” said Priyanka Gupta, a graduate student from the National Centre for Biological Sciences who devised the study and is its lead author. “We can understand what the brain might be doing.”
Unlocking how the brain handles a simple process like this can help researchers chip away at how the organ responds to more-complex events, and it also can inform technology. Knowing how the brain processes odors, for example, could facilitate the design of artificial noses to detect bombs.
“We have the hope that if there is a glimmer of a simple event, we have the chance to understand complex devices through simple computation,” said Dinu Florin Albeanu, a neuroscientist at Cold Spring Harbor Laboratories and another co-author of the study. “It’s an entry point.”
Neurons are cells that carry messages between the brain and other parts of the body, and when they communicate, they talk to each other simultaneously with electrical pulses that flit hither and thither. Typically, when a stimulus is introduced—sound, for example—all hell breaks loose, and the communication becomes chaotic and more difficult to explain mathematically.
But a study of rats led by Ms. Gupta in Bangalore and at Cold Spring Harbor Laboratories in New York has demonstrated that when their brains decode smells in the first stage of the olfactory system, the process is linear. This allowed the researchers to learn how the brain responded to a smell once and then reliably predict on average how it would respond to the same smell in subsequent exposures.
Rats have many more types of olfactory receptors than humans—1,500 compared with 350—and they rely more heavily on their sense of smell for survival.
“Animals use their noses in different ways,” Mr. Bhalla said. “They sniff more when they are interested in something. There is a time component. The stimulus is patchy. The sample is patchy.”
Ms. Gupta controlled for these differences by designing a method to deliver a precise number of molecules of an odor such as mint or banana to the noses of lab rats at specific points in time. She documented the spikes in electrical voltage that occurred in the olfactory bulb in response to the odor. And then she aligned the different pieces of data.
“The spikes in time are not much information until we plot when the odor was on or off,” Ms. Gupta said. “We know very precisely when the spikes happened in time. We align that with when we turned on the odor.”
The result is the response pattern associated with a particular smell.
The researchers delivered the odors to anesthetized rats in a steady stream that, thanks to a tracheotomy, wasn’t interrupted by exhalation. It is possible that normally functioning, awake animals experiencing complex odors would respond differently.
Within the field, the results of the study have been met with surprise, and it is too soon to tell whether others will attempt to repeat the work or how it may be put to practical use. But if it holds up, it suggests some things may be easier to accomplish than expected.
“Nonlinear information is much harder to interpret,” said Venkatesh N. Murthy, a professor of molecular and cellular biology at Harvard who has read the study. “If you can make everything more linear, programs and decoding are more simple. The same analogy holds for the brain.”
For now, at least, the work appears to put scientists a step closer to understanding what the nose knows.