Gustatory neural coding in the monkey cortex: acid stimuli.

1. We sought to define the gustatory neural code for acidic stimuli. Therefore we analyzed the responses of 44 single neurons in the insular cortex of four alert cynomolgus macaques in response to the oral application of four basic taste stimuli (glucose, NaCl, HCl, and quinine HCl) and fruit juice, and to a series of 20 additional acids. 2. Neurons responsive to gustatory stimulation were encountered within a volume of 38.2 mm3 (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 followed through at least one complete application of the stimulus series, and those responses compose the data of this study. Nongustatory cells included those responsive to mouth movements (36.3%), tactile stimulation within the mouth (2.1%), visual approach of the taste stimulus (1.4%), and extension of the tongue (0.1%). The functions of the remaining 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 0.26-0.98). Most evoked activity was excitatory, although inhibition was a prominent response option for four (9%) taste cells. 4. There was no evidence that taste cells with similar functional characteristics were clustered within the cortex, i.e., there was no apparent topographic organization of taste quality. 5. Thirty-four of the 44 cells were divisible into three functional types on the basis of their response profiles to the four basic stimuli used here.(ABSTRACT TRUNCATED AT 250 WORDS)