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Taste is more complicated than we think, says Dr. Kathrin Ohla

Taste, as we think of it, doesn’t really exist, according to human gustation researcher Dr. Kathrin Ohla. Our expectations, past social experiences, other sensory systems and the sequence in which food comes into our mouths help to shape our taste judgements, Ohla told an audience of students and faculty as part of the joint neuroscience/biology colloquium series on Wednesday, Sept. 25.

To understand how tasting flavors works, Ohla—a team leader at the Institute of Neuroscience and Medicine in Jülich, Germany—had to manipulate her research participants’ experience by blocking off their ability to smell, see, swallow and hear their food. 

When she performs EEG experiments, which are tests that monitor electricity in the brain—and in this case measure gustatory responses—Ohla sprays the taste in liquid form onto the participant’s tongue rather than just giving them the food whose flavor she wants them to taste. When given a food, participants’ resulting EEG signal can include a lot of “noise” from an amalgam of non-flavor factors, including appearance, smell, sound and texture of the food. Our taste is influenced by other senses as well as our expectations of what something is going to taste like. 

Ohla sprays, rather than pumps, food solution onto the tongue of her EEG participants. This step, although seemingly small, is actually huge for achieving some control in experiments. 

The flavor solution is equally distributed along the parts of the sprayed tongue, so that the resultant EEG firing patterns, which are supposed to be showing the firing patterns of the five flavor receptors alone, aren’t actually showing differences in intensity of the stimulus. 

Second, there’s no swallowing of any of the stimulus liquid, which would give participants the liquid’s texture to think about, rather than just its taste.

The taste is also held at a constant temperature, to minimize thermal sensations that could impact the EEG recordings. 

Finally, all of the stimulus is given at roughly the same time. This gives the experiments some temporal control, which is really important when trying to establish what the stimulus causes, said Ohla. 

Ohla also put a plug on participants’ noses so that they didn’t experience smell, which influences the taste EEG. And, she puts headphones on the participants, so they don’t hear when she sprays liquid on their tongues. It turns out that participants hearing the stimulus being sprayed can affect the EEG, or in other words, sound can affect whether we like a food or not.

Westerners typically think of just four flavors: salty, sweet, bitter and sour, said Ohla. But there’s a fifth flavor that isn’t as commonly thought of: umami, or savory—a flavor more recognizable to people living in parts of the world which consume more meat products, as in Asia. Different parts of the world being exposed to different flavors more often is something that scientists may overlook, but is extremely consequential when doing lab work, said Ohla. 

Ohla said she was inspired to study taste when she realized how little we actually know about it. To demonstrate, she talked of E. Bruce Goldstein’s “Cognitive Psychology,” a classic for neuroscience students. There are 142 pages dedicated to vision, compared to just five on taste. And it isn’t the case that taste is “just that simple,” said Ohla.

Early on in her career, Ohla saw that scientists weren’t controlling for taste very well, making conclusions potentially unreliable and invalid. Knowing this, she set out to try to create a more controlled way to study human gustatory perception. 

Twenty years ago, it might have been easy to overlook taste as part of our sensory system. But as Ohla and others have shown, there is still much to be gleaned about the taste process.

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