GUSTATORY NEURAL CODING IN THE AMYGDALA OF THE ALERT MACAQUE MONKEY

1. Neurons in the amygdala are implicated in mediating hedonic appreci ation, emotional expression, and conditioning, particularly as these r elate to feeding. The amygdala receives projections from the primary t aste cortex in monkeys, offering a route by which it could gain access to the gustatory information required to guide feeding behavior. We r ecorded the activity of 35 neurons in the amygdala of alert rhesus mac aques in response to a range of gustatory intensities and qualities to characterize taste-evoked activity in this area. 2. The stimulus arra y comprised 26 chemicals, including four concentrations of each of the four basic taste stimuli, a series of other sugars, salts, and acids, monosodium glutamate, and orange juice. 3. Neurons responsive to tast e stimulation could be found in a 76-mm3 region of the amygdala, cente red 9.1 mm lateral to the midline, 14.9 mm anterior to the interaural line, and 25.7 mm below the surface of the dura. They composed 7.2% (3 5/484) of the cells tested for gustatory sensitivity in the amygdala. 4. The mean spontaneous activity of taste cells was 8.2 +/- 2.3 (SE) s pikes per second. This rather high level provided an opportunity for r eductions from spontaneous rate that was used regularly in the amygdal a. When these negative response rates were included, the mean breadth- of-tuning coefficient of this sample of taste cells was 0.82. There wa s no strong evidence for gustatory neuron types, nor were functionally similar cells located together in a chemotopic arrangement. 5. Respon ses across 1.5 log units of stimulus concentration were nearly flat, w ith increasing excitation in some neurons largely offset by increasing inhibition in others. Taking the absolute value of the evoked activit y, concentration-response functions rose monotonically to all basic st imuli except HCl, but were not sufficiently steep to account for human psychophysical data. The neural response to HCI did not rise with sti mulus concentration within the range used. 6. Neural patterns represen ting the taste qualities of the basic stimuli were less sharply separa ted in the amygdala than at lower-order gustatory relays. Glucose elic ited activity patterns that were most distinct from those of the nonsw eet chemicals; those associated with NaCl were next most distinct. The re was no clear separation between the patterns generated by chemicals that humans describe as sour and bitter. Monosodium glutamate evoked responses that did not correlate well with those of any basic stimulus , implying that its quality cannot be subsumed under the four basic ta stes. 7. Taste-related activity in the amygdala did not provide an ade quate neural basis for the discriminative capacity of humans or monkey s with regard to either stimulus quality or concentration. It is propo sed that the amygdala contributes to gustatory processes not by provid ing precise distinctions among chemicals, but by imparting hedonic app reciation and emotional significance to taste experience.