Oral cavity representation at the frontal operculum of macaque monkeys

Receptive properties of neurons at areas 3 and 1-2, and the gustatory area (area G) in the frontal operculum (Fop) and the neighboring areas were investigated in the cerebral cortex of Macaca irus and Macaca fuscata by applying mechanical and taste stimulation to the oral cavity and lips. Tactile neurons with different receptive properties were noted in areas 3, 1-2 and G. Areas 3 and G were packed with tongue neurons, though a considerable number of palate neurons were also found in area G, while area 1-2 was involved in representation of the tongue, palate, periodontia and lips. Both the anterior and posterior parts of the tongue were represented on these areas, and of the palate, the posterior part (the soft palate) was largely represented. The anterior and posterior periodontia were represented, without separation. Various tissues were represented with different laterality: Periodontia and the lips were most frequently represented bilaterally or contralaterally, but the tongue and palate often ipsilaterally. In area 6, the insula and other areas surrounding areas 3, 1-2 and G, tactile neurons were also found to have receptive fields on the lips, tongue, palate or periodontium, and the receptive fields on the tongue or palate were mainly bilateral. On the other hand, most of the taste neurons were located in area G and some in area 1-2 and the insula. The present study demonstrated that in macaque monkeys, tactile sensation of the oral cavity is represented on areas 3, 1-2 and G in the Fop as well as on the surrounding areas (e.g., area 6 and the insula), and taste sensation on areas G and 1-2 and the insula.