On the Road to Sweetness: A Clear-Cut Destination?

Margaretta Midura
By Margaretta Midura November 4, 2012 04:55

Imagine putting a piece of candy on your tongue. To taste its full sweetness, you would most likely follow the taste map you have seen in biology and psychology textbooks and place it at the tip of your tongue. After all, the well-known taste map shows that the tip of the tongue is responsible for tasting sweetness, the surrounding area for saltiness, the tongue’s edges for the sourness, and the back end of the tongue for bitterness. This map has become so widespread that it is believed wine glasses are shaped to cater to it. Despite how popular this idea may be, current research asserts it is nothing more than a myth.

The idea of a taste map dates back to a study by the German scientist D.P. Hanig during the early 1900s. After tasting a food, volunteers were asked to determine which part of the tongue they tasted it, and this was repeated for each of the four main “taste groups” (sweetness, saltiness, bitterness, and sourness). While Hanig showed that certain areas of the tongue did in fact vary with regards to sensitivity for the four tastes, he was not implying that these areas were insensitive to any of the tastes.

The original tongue map depicting which areas of the tongue sense the four primary tastes. Photo courtesy of James Beard Foundation.

Edwin G. Boring, a Harvard psychologist, contributed to the misinterpretation of the data. He quantified Hanig’s original findings by substituting subjective volunteer responses with numerical data, then used these numbers to create the tongue map with which we are familiar today. Boring’s map was supposed to document relative sensitivities. However, his unclear notation misled many scientists of the era to believe that the areas he designated on the map could only sense one particular taste, thus propagating the deceptive idea of a taste map.

This taste map concept spread at a surprisingly fast pace. It could have been easily disproved by a few simple taste tests, but people held on to the idea of a map. “People like having discrete classifications. It’s a system that is easy to be locked into,” explains Dr. Aidan Kiely, a postdoctoral fellow who is currently researching taste and smell mechanisms in the Carlson Lab at Yale University. Another potential reason is that certain animals do in fact have taste maps. Kiely noted that fruit flies have 32 taste hairs and each has receptors that are sensitive to different tastes, but this does not necessarily mean such a trait exists in humans.

The real mechanism by which our taste buds pick up on individual tastes elucidates why the taste map is a myth. Projections called papillae are spread all across the tongue. The papillae contain the majority of the taste buds. These taste buds house a mixture of elongated taste cells that are capable of responding to salty, sweet, bitter, sour or umami tastes. (Umami is a distinct fifth taste that was neglected in the original studies. It is the taste of glutamate and gives meat its savory flavor.) The taste cells have approximately 50 to 150 receptors for each taste, which allow specific molecules to bind and eventually translate the signal into the taste we perceive. A “sweet” cell, for example, will have receptors that can bind to sugar molecules. Therefore, each region of the tongue can sense each of the five primary tastes.

The process of sensing taste involves papillae that contain taste buds. The taste buds are made up of taste cells with receptors that bind to specific molecules (such as the sugar molecule pictured). Photo courtesy of Scientific American.

Virginia Collings, a researcher at the University of Pittsburgh, offered evidence for this reconstructed layout of the tongue in 1974. She found that even though there was a slight difference in concentrations of certain taste receptors in certain areas of the tongue, the overall effect this had on sensing taste was negligible. Her research also showed that taste receptors are not confined to the tongue. They are also present in areas surrounding the tongue, such as the soft palate of the mouth and on the epiglottis.

“Even though the commercial aspect of taste is greater than that of hearing or smell, it has been greatly understudied,” says Kiely. The fact that so little is known about taste definitely contributed to the dominant taste map myth. Fortunately, there has recently been a great deal of research done on taste. Studies done on mice suggest that there may be unique maps of neurons in the mammalian brain (rather than on the tongue) that are found in specific areas for each primary taste. Other studies are trying to determine whether there are receptors for other tastes, such as for fat. Research in this field is quickly progressing and myths such as the taste map are being brought to light.

So next time you put that piece of candy into your mouth, do not worry. You will taste it regardless of where it lands.

Margaretta Midura
By Margaretta Midura November 4, 2012 04:55