The notion that the tongue is mapped into four areas—sweet, sour, salty and bitter—is wrong. There are five basic tastes identified so far, and the entire tongue can sense all of these tastes more or less equally.
As reported in the journal Nature this month, scientists have identified a protein that detects sour taste on the tongue. This is a rather important protein, for it enables us and other mammals to recognize spoiled or unripe food. The finding has been hailed as a minor breakthrough in identifying taste mechanisms, involving years of research with genetically engineered mice.
This may sound straightforward but, remarkably, more is known about vision and hearing, far more complicated senses, than taste.