Issues of gustatory neural coding: Where they stand today

The basic issues of gustatory neural coding an revisited. Questions address ed and conclusions drawn are: (1) what is the physical dimension across whi ch gustatory neurons are sensitive, and upon which taste perceptions are ba sed? The dimension that unites the Various taste qualities is not physical, but physiological: a dimension of well-being, bounded by toxins at one ext reme and nutrients at the other. (2) How broadly tuned are taste cells acro ss the dimension? There are instances of specificity, but most mammalian ta ste cells respond to a range of qualities. (3) Are there basic taste qualit ies? Sweet, salty, sour, and bitter are widely accepted as basic tastes. Um ami and starch tastes are considered basic by some. (4) Is taste topographi cally organized? There is some degree of physical separation among neurons most responsive to different taste qualities, but this does not appear to b e sufficient precision to act as a meaningful coding mechanism. (5) Are the re gustatory neuron types? Neurons, separated into categories according to their response profiles, respond as members of their category to the challe nges of conditioned aversions and preferences, sodium deprivation, hypergly cemia, and receptor blockade, while cells from other categories react diffe rently. This indicates the existence of functionally distinct types of tast e cells. (6) Is the quality signal coded within the activity of the single most appropriate category of neurons, or is it carried by the pattern of re sponse across neuronal categories? Both the breadth of responsiveness and t he logical ambiguity of the signal in any one category of neurons argue tha t the taste message is carried by a pattern of activity across gustatory ne uron types. (C) 2000 Elsevier Science Inc. All rights reserved.