Sensory inputs from the oral region to the cerebral cortex in behaving rats: an analysis of unit responses in cortical somatosensory and taste areas during ingestive behavior

The responses of 90 cortical neurons in the somatosensory and gustatory areas were recorded with chronically implanted fine wires in freely moving Wistar rats. The responses were analyzed mainly while the animals were freely licking solutions and eating dry pellets. Cortical neurons were classified into several groups according to their response properties. 'Mechanosensitive' neurons (n = 20) showed rhythmic phasic activity in different phases of the licking cycle, depending on the location of their receptive field in the peripheral orofacial region. 'Movement-related' neurons (n = 27) changed their activities tonically during licking, chewing, or grooming behavior. The responses were either excitatory or inhibitory. Receptive fields and adequate stimuli could not be identified. These neurons might receive somatosensory (except light tactile) inputs from wide or deep areas of intra- or perioral regions, or might be related to orofacial active movement. 'Taste' neurons (n = 35) increased or decreased their discharge rates during licking of particular taste solutions. Some taste neurons received convergence from somatosensory inputs. 'Temperature' neurons (n = 2) responded exclusively to water of temperatures lower or higher than room temperature. The responses were opposite in direction between cold and warm stimuli. 'Anticipation' neurons (n = 4) increased their impulse discharges before the start of licking in the situation in which the animal expected access to the drinking tube. 'Attention' neurons (n = 2) responded to arousal stimulation such as sound, a flash of light, and body touch. These neurons showed only a slightly increasing response during ingestive behavior. The locations of 56 of 90 units were histologically identified. Mechanosensitive neurons were located in the appropriate parts of the somatotopic pattern within the primary somatic sensory area in the granular cortex. Taste neurons were found evenly in the dysgranular cortex and the agranular insular cortex. Other types of neurons were located mainly in the dysgranular cortex between the granular cortex and agranular insular cortex, and some were intermingled with taste neurons in the agranular insular cortex. The present study has shown that cortical neurons in the orolingual somatosensory and taste areas have different response characteristics related to each aspect of ingestive behavior. It is suggested that these neurons with different functions are well organized anatomically within the cortex, which may subserve the background cortical neural mechanisms involving integration of orolingual sensory inputs, perception of taste, and control of ingestive behavior.