GUSTATORY NEURAL CODING IN THE MONKEY CORTEX - ACID STIMULI

1. We sought to define the gustatory neural code for acidic stimuli. T herefore we analyzed the responses of 44 single neurons in the insular cortex of four alert cynomolgus macaques in response to the oral appl ication of four basic taste stimuli (glucose, NaCl, HCl, and quinine H Cl) and fruit juice, and to a series of 20 additional acids. 2. Neuron s responsive to gustatory stimulation were encountered within a volume of 38.2 mm(3) (3.5 mm anteroposterior X 2.1 mm mediolateral x 5.2 mm dorsoventral). Taste cells constituted 81 (5.2%) of the 1,552 neurons whose sensitivities were tested. Of these, the activity of 44 was foll owed through at least one complete application of the stimulus series, and those responses compose the data of this study. Nongustatory cell s included those responsive to mouth movements (36.3%), tactile stimul ation within the mouth (2.1%), visual approach of the taste stimulus ( 1.4%), and extension of the tongue (0.1%). The functions of the remain ing 54.8% were not determined. 3. The mean spontaneous discharge rate of these cortical taste cells was 3.0 spikes/s (range 0.0-14.4 spikes/ s). The mean breadth of tuning coefficient was a moderate 0.72 (range 0.260.98). Most evoked activity was excitatory, although inhibition wa s a prominent response option for four (9%) taste cells. 4. There was no evidence that taste cells with similar functional characteristics w ere clustered within the cortex, i.e., there was no apparent topograph ic organization of taste quality. 5. Thirty-four of the 44 cells were divisible into three functional types on the basis of their response p rofiles to the four basic stimuli used here. The types could be charac terized as sugar oriented, salt oriented, and acid oriented. 6. A two- dimensional taste space was generated from correlations among the resp onse profiles evoked by the stimulus array. The 21 acids formed a diff use group that occupied the space through nearly the full extent of bo th dimensions. The mean response profile for the acids was most closel y related to that evoked by water, then, with increasing distance, to those representing quinine, NaCl, and glucose. 7. Within the acid grou p, six stimuli (propionic, nitric, ascorbic, butyric, acetic, and form ic acids) generated activity that matched this group profile, and so o ccupied the center of the space. Others lacked the typical relationshi p with NaCl (folic, hydrochloric, sulfuric, benzoic, and valeric acids ) or with glucose (citric, succinic, tartaric, and lactic acids). Thre e acids (glycolic, pyruvic, and aspartic) showed a nearly equal affili ation with all four basic stimuli. The remaining three diverged sharpl y from the main group: malic acid away from all nonacids, implying a p urely acid taste; and glutamic and tannic acid away from acids, implyi ng a dominant quality other than acidity. 8. For the subset of stimuli for which comparisons could be made, electrophysiological data from t he macaque are in accord with human psychophysical results.