How Does the Nose 'Know' What Odors Are and What They Mean?
What our noses "know," how we make sense of the different smells our environment offers up, is down to a surprisingly simple process in the brain, scientists have discovered.
It's not our noses that "know" and recognize odors, actually; that all happens in regions of our brains known as the olfactory cortex and olfactory bulb.
Researchers have been studying how those brain regions process data gathered by molecular receptors in our noses, to figure out exactly what we're smelling in case we need to take action -- like running away from a skunk.
While a lot of what our brains do is incredibly complex, a research team including U.S. and Indian scientists found smell processing is a basic function surprising in its simplicity.
The researchers on two continents, at the Cold Spring Harbor Laboratory in New York and the National Centre for Biological Sciences in Bangalore, studied the process in rats, whose olfactory systems are similar to ours in a number of key ways.
Neural processors in rat brains manage the complex task of identifying odors in the environment using a method known as linear summation, a process as simple as the way a child adds or multiplies numbers, they say.
"This result delighted us, and also surprised us, since the whole field of neurobiology has a bias --- and it is a fair one -- that the mammalian brain is fundamentally non-linear," says lead investigator Priyanka Gupta, a doctoral student from the Bangalore institution now working in the U.S. lab.
Processing odors would seem to be complex because they arrive in our noses from the surrounding environment in fluctuating and intermittent waves, what researchers call plumes, and will also be taken in at different rates depending on how deeply or shallowly we're breathing.
This suggests the neurons in a brain's olfactory bulb must perform complex calculations to compensate for these variable factors, but that proved not to be the case, the researchers report.
Rather, the calculation turned out to be a simple one: each neuron exhibits a "latency," a slight time delay, before it fires or is suppressed in response to a perceived odor signal.
That time delay does not vary, and is the same if an animal is breathing in short, rapid breaths or long, deep ones.
"And we found that once we could capture the response of that cell to the short stimulus, we could use that response to accurately predict how the same cell would respond -- whether to a long version of the same stimulus, or to any incoming odor, no matter the shape of its plume in spatial terms, or its duration in time," says Cold Spring Harbor researcher Florin Albeanu.
The key finding is that there's an important simplicity at the heart of the odor identification system in a rat's brain -- and no doubt in our own, the researchers conclude.