GUSTATORY NEURAL CODING IN THE MONKEY CORTEX - MIXTURES

1. Psychophysicists have shown that the intensity and quality of a tas te stimulus, as perceived by humans, is modified by including that sti mulus in a mixture. Gustatory neurons in the primary taste cortex (ant erior insula and frontal operculum) of the cynomolgus macaque are invo lved with the coding of stimulus intensity and quality, and so should reflect the impact of these stimulus interactions. 2. We recorded the activity of 48 neurons in primary taste cortex in response to the oral application of each of the four basic stimuli, their six possible dya ds, the four triads, and the tetrad of all four. Stimuli were maintain ed at a constant intensity in all mixtures by increasing their concent rations as the number of components rose. 3. Glucose was the most effe ctive basic stimulus, followed by quinine HCl, NaCl, and HCl. The mean response to dyads was suppressed by 50% from the sum of responses to the two unmixed components. The response to triads was 62% lower than the sum of responses to their three components, and activity evoked by the tetrad was suppressed by 74% from the sum of all four individual responses. Therefore there was nearly total suppression in the sense t hat the responses to the mixtures were similar to 1/2, 1/3, and 1/4 th e sums of responses to two, three, and four components, respectively. 4. Neurons could be divided into four subtypes: those that responded b est to each of the basic stimuli. All subtypes except HCl cells were a bout equally suppressed when their preferred stimulus was included in a mixture. HCl was a particularly ineffective stimulus, such that this subtype responded poorly and so was less susceptible to mixture suppr ession. 5. Taste quality, as indexed by correlation coefficients among profiles of activity, was quite predictable for dyads. If the mixture included HCl, the profile it generated correlated poorly (about +0.20 ) with that of HCl and rather well (about +0.60) with that of the othe r component. If HCl was not included, the mixture's profile correlated about +0.40 with that of each component. 6. The profile generated by the mixture of three stimuli was predictable only if one of the compon ents was HCl. In that case, the triad elicited a profile midway betwee n those of the other two components, i.e., the contribution of HCl was largely ignored. When HCl was not involved, or when all four basic st imuli were combined, the resulting profiles were poorly correlated wit h those of all basic stimuli. 7. The contribution made by each basic t aste to human perception and to the macaque's neurophysiological respo nse was compared for all mixtures. The contribution was often quite si milar for human and macaque, but when differences occurred, they were typically due to lower activity from HCl cells in the macaque, a loss that was replaced mainly by larger responses from glucose neurons. 8. The magnitude of responses to mixtures in the macaque taste cortex mat ches well with expectations from human psychophysical studies. The pre sumed quality of the response to mixtures is also similar, except that HCl is less effective in monkeys and sugars more so.