From Bitter to Sweet – The Tongue Map

The concept of a “tongue map” is likely familiar to anyone who has taken a science class. You’ve probably seen the image of a human tongue divided into different zones representing the four basic tastes: sweet at the tip, salty and sour along the sides, and bitter at the back. Perhaps you’ve even conducted the classic classroom experiment where you placed various substances on different parts of your tongue, only to be left puzzled and disappointed when the results didn’t match the map. This may have led you to develop a negative view of science, possibly even causing you to question the entire field and miss out on potential career opportunities and accolades.

But fear not, for I bring you essential news—the tongue map, a fixture in scientific textbooks for nearly a century, is, in fact, inaccurate. This article will unravel the truth about taste perception, shedding light on a fact that has remained hidden for far too long. The reality is that there are more than just four basic tastes, and taste receptors on the tongue do not neatly conform to distinct zones. Surprisingly, scientists have known this for around half a century. So, how did this erroneous map persist for so long? The answer lies in a significant misunderstanding within the scientific community.

The Origins of Misconception

Our exploration began in 1901 when German scientist David P. Hänig published a groundbreaking paper titled “The Psychophysics of Taste.” Hänig described an experiment in which he sought to determine the stimulus threshold—the smallest amount of a substance required for a person to perceive a specific taste—across various regions of his subjects’ tongues. He used sucrose for sweetness, quinine sulfate for bitterness, dilute hydrochloric acid for sourness, and salt for saltiness. The results revealed that the stimulus threshold varied slightly but significantly across the tongue’s surface, with lower thresholds at the outer edges and higher thresholds at the center.

Hänig presented his findings in a graph illustrating the change in sensitivity, the inverse of the stimulus threshold, for each taste across different tongue regions. This pivotal work remained hidden from the public eye for nearly four decades, known only to a select group of experts in the field of psychophysics.

Fast forward to 1942 when American psychologist Edwin G. Boring, despite his name, made significant contributions to experimental psychology. Boring was known for popularizing a fascinating cartoon in 1930, depicting a woman who could be seen as either young or old, with the viewer’s brain constantly switching between the two interpretations. These intriguing images, known as “Boring figures,” were used to study the brain’s response to ambiguous stimuli.

In his extensive 1942 textbook, “Sensation and Perception in the History of Experimental Psychology,” Boring revisited and reinterpreted Hänig’s taste sensitivity graphs. Unfortunately, Hänig had made a critical error in data presentation by failing to label his graph axes. This led Boring to misinterpret Hänig’s data in two significant ways. Firstly, he assumed that the lowest values on the graph indicated zero sensitivity to a particular taste. Secondly, he wrongly assumed that Hänig had measured the change in sensitivity relative to other flavors.

As a result, Boring’s reinterpreted graphs conveyed the misleading impression that the four primary tastes were concentrated in distinct regions of the tongue, with each region capable of detecting only one taste, excluding others. This appeared to validate the findings of German researcher A. Hoffmann, who, in 1875, observed that the central region of the tongue had few fungiform papillae, small bumps covering the tongue. This misconception ultimately led to the creation of the now-familiar tongue map, with one of its earliest iterations appearing in a 1952 Scientific American article by A.J. Haagen-Smit.

Debunking the Myth

However, the scientific community took only a short time to confirm what countless students had suspected for years. As early as 1974, experiments conducted by American researcher Virginia Collings revealed that while sensitivity to different tastes varied across the tongue’s surface, these differences were minimal. Importantly, every part of the tongue was capable of detecting all tastes.

David Hänig’s nearly century-old research had a critical omission; it did not include the tongue’s sensitivity to the taste of salt, which was found to be consistent across the tongue’s surface. Collings’ groundbreaking work debunked Edwin Boring’s claims and unveiled that taste receptors were not limited to papillae. These receptors were distributed throughout the tongue, the soft palate at the back of the throat, and even in the epiglottis—the flap of tissue that prevents food from entering the windpipe.

But the revelations didn’t stop there. Numerous other findings dismantled the traditional tongue map. Signals from taste buds are transmitted to the brain through two cranial nerves: the glossopharyngeal nerve, connected to the back of the tongue, and the chords, tympani, connected to the front. If the tongue map were accurate, damage to the chorda tympani would eliminate one’s ability to taste sweetness. However, in 1965, surgeon T.R. Bull’s experiments proved otherwise. Similarly, Linda Bartoshuk’s research at the University of Florida in 1993 showed that anesthetizing the chorda tympani did not interfere with subjects’ ability to taste sweetness; it heightened their perception of sweetness.

Expanding the Taste Palette

As if debunking the tongue map wasn’t groundbreaking enough, both Boring and Hänig were also mistaken about another fundamental aspect—there are more than just four basic tastes. In 1907, Kikunae Ikeda, a chemistry professor at Tokyo Imperial University, made a fascinating discovery while enjoying a family dinner. He noticed that the dashi broth in his soup had a richer and more flavorful taste than usual. After some experimentation, Ikeda identified that this enhanced flavor came from adding kombu, a type of seaweed, and katsuobushi, dried fish flakes.

Ikeda postulated the existence of a fifth fundamental taste, which he called “umami.” Within a year, he successfully isolated monosodium L-glutamate (MSG) from kombu, the chemical responsible for umami flavor. In 1909, Ikeda patented a method to mass-produce MSG from wheat and soy and founded Ajinomoto Co., Inc., which translates to “essence of flavor” in Japanese. Today, Ajinomoto is a multinational giant generating nearly $10 billion in annual revenue, while MSG ranks among the world’s most popular flavor enhancers, alongside salt and pepper.

Ikeda’s quest continued with MSG; he explored other foods and found high glutamate concentrations in meat, seaweed, and tomatoes. He hypothesized that humans evolved taste receptors for glutamates—a vital component of an omnivore’s diet, often signaling the presence of proteins. While umami’s existence has been debated for years, it is now widely accepted as the fifth fundamental taste. Moreover, its perception serves as a signal for the digestive tract to produce protein-digesting enzymes.

As we navigate the ever-evolving realm of scientific knowledge, embracing the notion that our initial school lessons may only sometimes align with the latest discoveries is crucial. The debunking of the tongue map serves as a testament to the dynamic nature of science, urging us to continue exploring, questioning, and expanding our horizons.