Integration of the upper and lower lips in the postcentral area 2 of conscious macaque monkeys (Macaca fuscata)

The representation of the lip in area 2 of the postcentral somatosensory cortex was studied in conscious macaque monkeys by recording single-neurone activities. Seventy penetrations were made in the oral region of six hemispheres of four animals and 1157 neurones were isolated. The receptive field characteristics of 839 neurones were identified. Among them, 363 neurones along 47 penetrations responded to mechanical lip stimulation (lip neurones). A substantial number of lip neurones (17%, 62/363) had composite receptive fields that included not only the lip but also other oral structures. Although, the majority of lip neurones had receptive fields on either the upper or the lower lip (unilabial neurones), about 20% had receptive fields including both the upper and lower lips (bilabial neurones). Receptive field features of bilabial neurones were summarized as follows: (1) the receptive fields always included the corresponding sites of the upper and lower lips that would come into contact when the jaw closed; (2) the submodality preferences of the upper and lower portions of the receptive fields were identical in all cases; (3) if a light stroking stimulus in a specific direction was adequate, portions of the receptive field on the upper and lower lips responded with a common directional preference. Furthermore, bilabial receptive fields were unlikely to be the simple 'dimer' of unilabial receptive fields: the relative incidence of neurones with bilateral or composite receptive fields was much higher in bilabial than in unilabial neurones. That is, bilabial integration was accompanied by the integration of both sides of the lips, and of the lip and other adjacent oral structures. These features of bilabial neurones appear to be suitable for the form discrimination of objects held in the anterior part of the mouth. These neurones may be the prerequisite neural basis for the oral stereognosis that would take place in the neighbouring association cortices.