Abstract1. In order to determine whether the responsiveness of neurons in the caudolateral orbitofrontal cortex (a secondary cortical gustatory area) is influenced by hunger, the activity evoked by prototypical taste stimuli (glucose, NaCI, HCI, and quinine hydrochloride) and fruit juice was recorded in single neurons in this cortical area before, while, and after cynomolgous macaque monkeys were fed to satiety with glucose or fruit juice.2. It was found that the responses of the neurons to the taste of the glucose decreased to zero while the monkey ate it to satiety during the course of which his behaviour turned from avid acceptance to active rejection.3. This modulation of responsiveness of the gustatory responses of the neurons to satiety was not due to peripheral adaptation in the gustatory system or to altered efficacy of gustatory stimulation after satiety was reached, because modulation of neuronal responsiveness by satiety was not seen at earlier stages of the gustatory system, including the nucleus of the solitary tract, the frontal opercular taste cortex, and the insular taste cortex. monkey had been fed to satiety. For example, in seven experiments in which the monkey was fed glucose solution, neuronal responsiveness decreased to the taste of the glucose but not to the taste of blackcurrant juice. Conversely, in two experiments in which the monkey was fed to satiety with fruit juice, the responses of the neurons decreased to fruit juice but not to glucose.5. These and earlier findings lead to a proposed neurophysiological mechanism for sensory-specific satiety in which the information coded by single neurons in the gustatory system becomes more specific through the processing stages consisting of the nucleus of the solitary tract, the taste thalamus, and the frontal opercular and insular taste primary taste cortices, until neuronal responses become relatively specific for the food tasted in the caudolateral orbitofrontal cortex (secondary) taste area. Then sensory-specific satiety occurs because in this caudolateral orbitofrontal cortex taste area (but not earlier in the taste system) it is a property of the synapses that repeated stimulation results in a decreased neuronal response. the caudolateral orbitofrontal cortex taste projection area, in that neuronal responses here to water were decreased to zero while water was drunk until satiety was produced.4. The decreases in the responsiveness of the neurons were relatively specific to the food with which the 6. Evidence was obtained that gustatory processing involved in thirst also becomes interfaced to motivation in

1. In recordings made from 3,120 single neurons, a secondary cortical taste area was found in the caudolateral part of the orbitofrontal cortex of the cynomolgus macaque monkey, Macaca fascicularis. The area is part of the dysgranular field of the orbitofrontal cortex and is situated anterior to the primary cortical taste areas in the frontal opercular and adjoining insular cortices. 2. The responses of 49 single neurons with gustatory responses in the caudolateral orbitofrontal taste cortex were analyzed using the taste stimuli glucose, NaCl, HCl, quinine HCl, water, and blackcurrant juice. 3. A breadth-of-tuning coefficient was calculated for each neuron. This is a metric that can range from 0.0 for a neuron that responds specifically to only one of the four basic taste stimuli to 1.0 for one that responds equally to all four stimuli. The mean coefficient for 49 cells in the caudolateral orbitofrontal cortex was 0.39. This tuning is much sharper than that of neurons in the nucleus of the solitary tract of the monkey, and sharper than that of neurons in the primary frontal opercular and insular taste cortices. 4. A cluster analysis showed that at least seven different groups of neurons were present. For each of the taste stimuli glucose, blackcurrant juice, NaCl, and water, there was one group of neurons that responded much more to that tastant than to the other tastants. The other groups of neurons responded to two or more of these tastants, such as glucose and blackcurrant juice. In this particular region neurons were not found with large responses to HCl or quinine HCl, although such neurons could be present in other parts of the orbitofrontal cortex. 5. On the basis of this and other evidence it is concluded that in the caudolateral orbitofrontal cortex there is a secondary cortical taste area in which the tuning of neurons has become finer than in early areas of taste processing, in which foods, water, and NaCl are strongly represented and where motivation dependence first becomes manifest in the taste system.

1. In recordings made from 2,925 single neurons, a region of primary taste cortex was localized to the rostral and dorsal part of the insula of the cynomolgus macaque monkey, Macaca fascicularis. The area is part of the dysgranular field of the insula and is bordered laterally by the frontal opercular taste cortex. 2. The responses of 65 single neurons with gustatory responses were analyzed in awake macaques with the use of the taste stimuli glucose, NaCl, HCl, quinine HCl (QHCl), water, and black currant juice. 3. Intensity-response functions showed that the lowest concentration in the dynamic part of the range conformed well to human thresholds for the basic taste stimuli. 4. A breadth-of-tuning coefficient was calculated for each neuron. This is a metric that can range from 0.0 for a neuron that responds specifically to only one of the four basic taste stimuli to 1.0 for one that responds equally to all four stimuli. The mean coefficient for 65 cells in the taste insula was 0.56. This tuning is sharper than that of neurons in the nucleus of the solitary tract of the monkey, and similar to that of neurons in the primary frontal opercular taste cortex. 5. A cluster analysis showed that at least six different groups of neurons were present. For each of the taste stimuli, glucose, NaCl, HCl, QHCl, water, and black currant juice, there was one group of neurons that responded much more to that tastant than to the other tastants. Other subgroups of these neurons responded to two or more of these tastants, such as glucose and black currant juice, or NaCl and QHCl. 6. On the basis of this and other evidence, it is concluded that the primary insular taste cortex, in common with the primary frontal opercular taste cortex, represents a stage of information processing in the taste system of the primate at which the tuning of neurons has become sharper than that of neurons in the nucleus of the solitary tract, and is moving toward the fineness achieved in the secondary taste cortex in the caudolateral orbitofrontal taste cortex, where motivation-dependence first becomes manifest in the taste system.

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SUMMARYAND CONCLUSIONS1. The responses of 165 single taste neurons in the anterior operculum of the alert cynomolgus monkey were analyzed. Chemicals were deionized water, blackcurrant juice, and the four basic taste stimuli: glucose, NaCl, HCl, and quinine HCl.2. Taste-evoked responses could be recorded from an opercular region that measured -4.0 mm in its anteroposterior extent, 2.0 mm mediolaterally, and 3.0 mm dorsoventrally. Within this area, taste-responsive neurons were sparsely distributed such that multiunit activity was rarely encountered and neuronal isolation was readily achieved.3. Intensity-response functions were determined for nine cells. In each case, the lowest concentration of the dynamic response range conformed well to human electrophysiological and psychophysical thresholds for the basic taste stimuli.4. There was some evidence of chemotopic organization. Cells that responded best to glucose tended to be distributed toward the anterior operculum, whereas most acid-sensitive neurons were located more posteriorly. The proportion of cells responding best to NaCl peaked in the middle of the area, whereas quinine sensitivity was rather evenly distributed throughout.5. Opercular neurons in the monkey showed moderate breadth of sensitivity compared with taste cells of other species and at other synaptic levels. A breadth-of-tuning coefficient was calculated for each neuron. This is a metric that can range from 0.0 for a cell that responds specifically to only one of the four basic stimuli to 1.0 for one that responds equally to all four stimuli. The mean coefficient for 165 cells in the operculum was 0.67 (range = 0.12-0.99).6. Efforts were made to determine whether neurons could be divided into a discrete number of types, as defined by their responsiveness to the stimulus array used here. It was concluded that most taste cells may be assigned to a small number of groups, each of which is statistically independent of the others, but within which the constituent neurons are not identical.7. An analysis of taste quality indicated that the sweet and salty stimuli evoked patterns of activity that were significantly intercorrelated. Similarly, patterns representing HCl, quinine HCl, and water were related.