Gustatory neural coding in the monkey cortex: the quality of saltiness

Abstract

1. We analyzed the activity of single neurons in the insularopercular cortex of four alert cynomolgus monkeys in response to the oral application of four basic taste stimuli (glucose, NaCl, HCl, and quinine HCl) and fruit juice and to a range of 17 sodium and lithium salts with a variety of anions. 2. Neurons responsive to gustatory stimulation were encountered in an area of 34.5 mm3 (2.0 mm A-P x 2.5 mm M-L x 6.9 mm D-V). Taste cells composed 46 (4.5%) of the 1,028 neurons whose sensitivities were tested. Nongustatory cells included those responsive to mouth movements (20.4%), tactile stimulation in the mouth (3.4%), and visual approach of the stimulus (0.7%). The functions of the remaining 71.0% could not be determined. 3. The mean spontaneous discharge rate of these cortical taste cells was 4.4 spikes/s (range, 0.1–23.8 spikes/s). The mean breadth-of-tuning coefficient was a moderate 0.72 (range, 0.15–1.00). Inhibitory responses were nearly nonexistent. 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 qualities. 5. The 46 taste cells were divisible into three functional types, based on their response profiles to the four basic stimuli used here. The types could be characterized as sweet-, salt-, and quinine-oriented. 6. A taste space was generated from correlations among the response profiles evoked by the stimulus array. The 17 salts formed a coherent group from which the other basic stimuli were separated. Glucose was closest to the salt group, followed by quinine and HCl. 7. Within the salt group, the four halides (NaCl, LiCl, NaBr, LiBr) formed a tight cluster; the 11 stimuli with acetate, citrate, phosphate, sulfate, and tartrate anions joined with monosodium glutamate and Na bicarbonate to form two closely related clusters; Na succinate was somewhat distinct from the others, and Na carbonate was most separate. 8. The relative qualities of the salts did not relate systematically to anionic size, promotion of sodium transport, or molar conductivity. 9. The configuration of stimuli in this taste space was compared with that in a space derived from human descriptions of the relative similarities of many of these same stimuli. Using the position of NaCl as a reference, the distances to all other stimuli common to the two studies was measured.(ABSTRACT TRUNCATED AT 400 WORDS)