Gustatory coding in the precentral extension of area 3 in Japanese macaque monkeys; comparison with area G.

The precentral extension of area 3 as well as the transition between the frontal operculum and insula (area G) comprises the primary gustatory cortex in the subhuman primate, receiving projections from the thalamic taste relay. However, in contrast to the extensive studies that have been carried out on the latter area, only a few taste units in the former area have been recorded. To clarify gustatory coding in area 3, we investigated the taste response properties of neurons in area 3 compared with those in area G in alert monkeys by infiltrating into their mouths seven taste stimuli [0.3 M sucrose (S), 0.1 M NaCl (N), 0.01 N HCl (H), 0.003 M quinine-HCl (Q), 0.1 M monosodium glutamate (MSG), distilled water (W), and orange juice (OR)] and artificial saliva (SA). A larger number of HCl-best units and a smaller number of quinine-best units were found in area 3 than in area G. The onset latency and response duration were significantly shorter in area 3 than in area G. Weighted multi-dimensional scaling showed that area G divided eight stimulants into four classes, i.e. two groups (H-Q-W and S-MSG-OR), N and SA, whereas area 3 divided them into three classes (N-MSG-W-OR, S-Q, and H-SA). This suggested that tastants not separated in area G were separated in area 3, and vice versa. This indicates that both areas complement each other in the representation of taste stimuli, each contributing to taste information processing in a different manner.