The tongue has distinct zones for different tastes: sweet at the tip, bitter at the back, sour and salty on the sides.

The diagram appeared in biology textbooks for most of the twentieth century: an outline of the tongue divided into four neat regions, each labeled for the taste it supposedly monopolized. Sweetness occupied the tip. Saltiness and sourness claimed the lateral edges. Bitterness guarded the rear. The map was clean and memorable, which is partly how it survived for so long, it had the visual authority of a scientific fact, something that had been measured and confirmed.

The original source was a 1901 German dissertation by D.P. Hanig, who had asked subjects to rate the intensity of salt, sweet, sour, and bitter solutions applied to different regions of the tongue. Hanig's data did show slight regional variations, the tip was marginally more sensitive to sweetness, the edges somewhat more to salt and sour. But the differences were small and overlapping, and Hanig himself made no strong claims about exclusive zones. The American psychologist Edwin Boring translated Hanig's work into English in 1942 and drew the diagram that most textbooks eventually reproduced, transforming a modest finding about slight threshold differences into a hard anatomical partition.

Virginia Collings, a psychologist at the University of Pittsburgh, ran the systematic test that should have been run in 1901. Her 1974 paper in Perception and Psychophysics, "Human taste response as a function of locus of stimulation on the tongue and soft palate," applied five taste compounds, sodium chloride, sucrose, quinine hydrochloride, urea, and citric acid, to four distinct tongue regions and the soft palate, measuring both detection thresholds and perceived intensity functions. Her finding was unambiguous: all five tastes could be detected at all regions tested. Threshold differences between loci existed, but they were minor and did not remotely support the idea of exclusive zones. Bitter, in particular, was detectable at the tip and soft palate as readily as at the back of the tongue, directly contradicting the map's most recognizable feature.

The map should have been removed from textbooks in 1974. Many editions kept it for another two decades. Some kept it longer. The stickiness of the error was partly pedagogical, the diagram illustrated the concept of taste receptor distribution in a visually satisfying way that was simply more teachable than the actual physiology, and partly institutional inertia. Publishers reprint what previous editions contained; teachers teach what the diagrams show.

The second blow came from an unexpected direction. In the 1990s, research on glutamate receptors in taste cells led to the formal recognition by Western science of a fifth primary taste: umami, the savory quality of glutamate and related compounds that Japanese chemist Kikunae Ikeda had described and named in 1908. Ikeda had proposed at the time that umami was a distinct basic taste, detectable by dedicated receptors distributed across the tongue, a claim Western taste science largely ignored for most of the twentieth century. When the molecular receptor responsible for umami detection, the T1R1/T1R3 heterodimer, was characterized in 2001 by Charles Zuker's group at the University of California San Diego, it was found throughout the tongue surface. Umami had no zone. Neither, it turned out, did any of the others.