Classification of taste responses in brain stem: membership in fuzzy sets

1. Classification methods in sensory systems in general, and gustation in particular, tend to place each of the relevant objects, such as stimuli or neurons, into one class each. Some of these methods are based on the responsiveness of neurons to various stimuli; in these, each group must contain a variety of nonidentical members because of the individuality of each neuron or stimulus. 2. The "fuzzy" set method is appropriate for more accurate classification in such heterogeneous populations. In this method each member is given graded membership in several sets rather than membership in only one set. In the present paper we subjected previously published data on the responses of individual taste neurons to a variety of stimuli to fuzzy set analysis. 3. We found that the amounts of response of 46 neurons in the solitary nucleus of the rat to NaCl, HCl, sucrose, quinine HCl, and KCl could accurately be accounted for by giving each a grade of membership in three sets; the same held in the parabrachial nucleus of the rat for the responses of 41 neurons to the first four of these stimuli. The response was calculated as the sum of the products of the stimulus times neuron ratings in each set. 4. Temporal patterns of response have often been related, but with only moderate success, to the identity of the stimulus or neuron. These patterns could be accurately accounted for with the present method. Each of the products of designated parts of the stimulus ratings times the neuron ratings gave the basis for accurate description of the temporal course of the response of each neuron to each stimulus. 5. This method appears to account for the varieties of amount and temporal pattern of response of taste neurons with a simple mathematical process of few parameters. This suggests that within the known complexities of receptor mechanisms and mechanisms of neural processing, the neural message is reduced to a rather simple form. 6. The fuzzy set approach, which is based on disclosing underlying sets rather than placement of heterogeneous members into one of several essentialistic groups, may be useful in disclosure of the underlying mechanisms producing the neural responses in taste.