Taste quality and neural coding: implications from psychophysics and neurophysiology.
Historically, taste research has often been guided by the concept that there are only four (or possibly five) basic taste qualities (sweet, sour, salty, and bitter, and possibly "umami"). All other tastes have been presumed to be combinations of these basic tastes. This psychophysical concept has been extended to electrophysiological data. That is, the neural code for each basic taste is hypothesized to be coded by a dedicated channel of neurons (the "Labeled-Line" theory); i.e., one group of neurons signals "salty" and another separate group signals "sweet." Numerous psychophysical and electrophysiological findings, however, cannot be accomodated by this quadripartite theory, which limits taste to four basic qualities and four basic neuron types. Rather, the data described in this article suggest that the range of taste is more extensive than four or five basic tastes, and that this breadth of taste quality results initially from the activation of a broad array of ion channels, receptors, and second messengers associated with taste cell membranes. These findings have implications for neural organization and provide support for the "Across-Fiber Pattern" theory in which the neural code for taste is represented by the pattern of activity across all of the neurons, i.e., neurons are not exclusively labeled for a particular sensation but cooperate with the others in the ensemble to encode taste quality.