On a neural mechanism for cortical processing of taste quality in the rat.

The responses of 31 chorda tympani fibers and 47 cortical neurons were recorded in response to 6 concentrations of NaCl, and single concentrations of sucrose, HCl, and quinine hydrochloride solutions applied to the anterior portion of the tongue in rats. The neural responses were analyzed in terms of the two hypotheses of quality coding: across-neuron response pattern and across-region response pattern notions. In a behavioral experiment, animals were given a conditioned taste aversion to one of 5 concentrations of NaCl solution by pairing it with a gastrointestinal illness caused by i.p. injection of 0.15 M LiCl. Behavioral taste profiles were constructed for each stimulus from the suppression of drinking, which indicates the extent of generalization of aversion to each of the 4 basic taste stimuli. Among the two neural analyses employed for the chorda tympani and cortical units, across-region correlation coefficients for cortical neurons that were derived from the across-region response pattern theory showed the highest correlation (r = 0.89) with the behavioral suppression rates. Across-neuron correlation coefficients in the chorda tympani fibers also showed a good correlation (r = 0.81) with the behavioral data. However, the taste profile for 1.0 M NaCl in chorda tympani fibers was very similar to that for the lower concentrations of NaCl, in spite of the suggestion from the behavioral experiment and the neural analyses of cortical responses that 1.0 M NaCl has HCl and quinine components besides the NaCl component. The present result confirmed the idea that processing of taste information in the cortex involves differences in both response magnitude across neurons and the spatial localization of those neurons.