terms of evolutionary theory, when life moved from sea to land, the undifferentiated chemical receptor systems of taste and smell became differentiated and began to serve different functions where the taste system served as a "close-up" sense that provided the last check on the acceptability of food, and smell served as a useful "distance" sense, although it also retained an important function in dealing with food. The physical stimuli for the taste system are substances that can be dissolved in water and, as is common for physical stimuli, the amount of a chemical substance present is related to the intensity of the experienced taste (cf., A. Baradi & G. Bourne's enzyme theory of taste). However, which properties result in the various different taste qualities is still unknown in detail, even though there are several guesses, such as the size of the substances' individual molecules, how the molecule breaks apart when dissolved in water, or how molecules interact with cell membranes. Complete agreement on the basic dimensions of taste is still lacking, but there seems to be general agreement on at least four primary taste qualities [cf., H. Henning's taste theory/taste pyramid, or Henning's tetrahedron -a classification of tastes using a pyramid with a triangular base whose corners represent the primary tastes, named after the German psychologist Hans Henning (1885-1946)]: sweet, salty, sour, and bitter (L. Bartoshuk suggests a fifth quality: that of water). When considering the question of how taste quality is neurally coded, it was originally thought that there would be different receptors for different taste qualities. However, most receptor cells on the tongue seem to respond to all four of the basic kinds of taste stimuli but at different rates. One theory of taste, called the acrossfiber pattern theory [formulated by the American psychologist Carl Pfaffmann (1913-1994)] holds that if the condition of various neural units having different stimulus-specific response rates is met, then the code for taste quality could be an across-fiber pattern of neural activity. According to this theory, unique taste fibers respond in a different pattern to each taste quality, even though all of the fibers respond to all taste inputs to some extent. Another theory of taste quality encoding, called the labeled-line theory of C. Pfaffmann, suggests that each taste fiber encodes the intensity of a single basic taste quality. This theory states that to the extent that a stimulus activates the "sweet" fibers, for example, it tastes sweet, and to the extent that it activates the "bitter" fibers, it tastes bitter. The theory suggests, also, that "simple" stimuli could have a complex taste if they activate several types of fiber. The labeled-line theory is compatible with the across-fiber pattern theory except that in the former the code for taste quality is a profile across a few fiber types rather than a pattern across many thousands of unique fibers. Different gustatory fibers seem to be "tuned" to certain taste stimuli, much as auditory nerve fibers are tuned to certain sound frequencies. Such fibers respond most intensely to their "best" substances and less intensely to others. In the future, it may be possible to classify such taste fibers into a few classes, corresponding to the basic taste qualities. Although it is unknown at present whether labeled-lines exist along the entire taste pathway, cortical neurons most responsive to the four basic tastes seem to be localized in different parts of the taste cortex. Also, it is likely that some recoding of the taste information takes place in the cortex, where specific cortical cells give an "on" or "off" response to different taste stimuli, much like the feature-specific cells in the visual cortex. See also EVOLUTIONARY THEORY; GARCIA EFFECT; OLFACTION/SMELL, THEORIES OF; VISION/ SIGHT, THEORIES OF. REFERENCES
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