Gustatory coding

Afferents in the VII nerve commonly display preferences for either salty or sweet stimuli, while most of those in the IX nerve, supplied by the posterior tongue, are tuned to acids or bitter stimuli. Most vagal (X) afferents respond to distilled water. Such neurons have their lowest firing rate in 154 mM NaCl, and raise firing as salt concentration increases or decreases from this value. Vagal afferents therefore appear to measure to what extent the pharyngeal contents vary in ionic concentration from extracellular fluid.

The fact that gustatory neurons are usually quite non-specific argues against the existence of labeled lines corresponding to the classical taste sensations. Therefore distinctive taste sensations occur from neurons with opponent receptive fields which compare the outputs of differently tuned populations of afferents. This is equivalent to how color vision arises from opponent processing which compares output from merely three populations of cone photoreceptors.