Under the Hood

Chemosensory System: Neuroscientist Don Katz Believes Humans Have 1 Sense, Not 5

5 Senses
We may only have one sense, according to this neuroscientist. Moyan Brenn CC BY 2.0

Everyone knows there are five senses: sight, smell, sound, taste, and touch. Neuroscientist Don Katz, however, believes humans only have one. His paper, published in Current Biology, is meant to backup his claim that, instead of the traditional five senses, all of our senses are actually just one big “chemosensory system.”

The current study builds on previous research from Katz, an associate professor of psychology at Brandeis University. In a 2009 study published in Nature Neuroscience, Katz and his team described how the brain’s taste center influences its smell center. To demonstrate this, the team used optogenetics — a technique that allows for control of neural cells with light — to turn off a rat’s primary gustatory (taste) cortex. After feeding another rat a banana, they had the altered rat smell its breath, then presented it with the choice of eating either a banana or avocado.

They found it preferred to eat the banana because it recognized the smell from the other rat, which indicated the bananas were safe to eat. When its sense of taste was reactivated, however, it chose between both the banana or the avocado. This led them to believe rats use their taste system for smelling, and that disabling their taste altered their sense of smell. They said this happened because foods smell differently when the sense of taste isn’t there to supplement sensory input.

Katz’s current study was inspired by the 2009 paper, with the aim of testing the hypothesis they put forward that year. It used optogenetics again, but this time the researchers shutdown neurons associated with taste in the rat’s primary olfactory cortex, an area primarily related to smell. The switch immediately impacted the way in which the rat’s smell neurons fired, so much so that the rat forgot even the most familiar of odors. “The way that neurons respond to smells — the ‘codes’ for those smells, if you will — depends” on whether the taste system is unobstructed, Katz told Medical Daily in an email.

In his 10 years of researching the senses, Katz has been led to believe asking someone how their food tastes or smells is meaningless. Instead, we should be asking how the experience of eating the food is, since he believes the senses are interdependent on each other.

“I think that taste and smell are a single system with two sense organs, just as a single house can have two separate doors,” Katz said. “I also think that the intimate connection between taste and the other classically defined ‘sensory systems’ may all be in service of an animal’s need to learn and act upon the connection between what a relatively distant object feels/smells/looks/sounds like, and how it will taste and nourish/poison.”

Despite his evidence, this single-sense theory has still not been proven. With rats, Katz said, “we are continuing to examine how activity in many circuits determines how taste and smell input are handled, how this plays out through time — both real time and learning time — and the implications for perception.” If his research is successful, one day we might just be teaching our kids about the one, all-encompassing sense.

Source: Katz D. Multisensory Perception: The Building of Flavor Representations. Current Biology. 2015.

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