SENSORY NEURAL PATTERNS AND GUSTATION 207 



example, only three response magnitudes {including zero) could result from 

 stimulation with any given color. The three values which would be 

 obtained with stimulus Q are indicated in the bar graphs in Fig. 1b. In 

 these three bar graphs are represented the activity which would be 

 recorded from fiber types 1, 2 and 3. In each of these three graphs, the 

 bar for Q indicates the amount of neural activity that would be recorded 

 from that fiber type with the stimulus Q. Similarly, stimulus P (or R, or 

 S) would evoke characteristic response magnitudes in each of these fiber 

 types as indicated. Whenever we record from fiber 1, then, using these 

 four stimuli, we will get a bar graph with the " profile " indicated under 1. 

 Similarly, fiber types 2 and 3 will yield characteristic profiles. If a large 

 number of optic nerve fibers are sampled using these four stimuli, one of 

 these three bar graph profiles will be obtained from each fiber. Since 

 only three " response profiles " could be derived from these fibers, the 

 conclusion would be reached that there are only three fiber types. Note 

 that this conclusion could be reached without having knowledge of the 

 relevant parameter of the stimulus {wavelength), and without being able to 

 vary the stimulus systematically. 



The same kind of analysis could be accomplished in audition demon- 

 strating the effect of a multiplicity of fiber types. Again assume that the 

 significant aspect of the stimulus for pitch is not understood, but four 

 tones are available as stimuli. If these many fiber types were represented 

 in Fig. 1 there would be very many curves in Fig. 1a, and a corresponding 

 number of bar graphs in Fig. 1b. It would be concluded from the number 

 of bar graph profiles of responses obtained from auditory neurons that there 

 were very many fiber types, even if we did not understand the nature of the 

 stimulus, and did not vary it systematically. 



DETERMINATION OF THE NUMBER OF 

 GUSTATORY FIBER TYPES 



The above considerations give us some clues concerning the sensory 

 neural message for taste quality. Many investigators have collected the 

 type of bar graph for taste shown in Fig. 1b. Some examples are shown 

 in Figs. 2 and 3. Those in Fig. 2 were obtained from single second-order 

 neurons in the nucleus of the solitary tract of rats anesthetized with pento- 

 barbital sodium. KCl-filled glass micropipettes were used. Similar bar 

 graphs describing the responsiveness of first-order neurons in the chorda 

 tympani nerve of the rat are shown in Fig. 3. With reference to Fig. 1b, 

 it would not be easy to place these graphs into a few groups on the basis 

 of the profiles they present, but rather there would be very many groups. 

 (It is, of course, not necessary that these groups be represented by smooth, 

 uninterrupted curves on uninterrupted continua ; e.g., it would be 



