Gustatory intensity discrimination in rat NTS: a tool for the evaluation of neural coding theories

Summary The neural coding of taste quality in vertebrates currently is addressed by the labeledline and the across-fiber pattern theory. Experimental tests that have tried to distinguish between these theories by manipulating taste quality have been problematic in that both are fairly successful. However, the two theories maintain that different numbers of neurons participate in coding the presence of a substance. In the labeled-line theory, only those neurons labeled by that substance are supposed to be involved, whereas in the across-fiber pattern theory, all neurons responding to the substance are taken into account. We therefore expected that the theories might predict different intensity discriminating abilities among the four classical taste stimuli, which could provide a less equivocal test between the theories.A previous paper (Maes 1984) described a model for the neural basis of intensity discrimination, based on signal-detection theory. In the present paper, experimental data obtained from single neurons in the nucleus of the solitary tract of rat are analyzed in terms of this model. Whole-mouth stimulation with the four classical taste substances was used. For both theories, predictions of the intensity discriminating acuity for the four substances were derived, using different response analysis intervals, and including two plausible non-linearities in neural processing.The acuity profile across the four stimuli (consisting of the reciprocals of the four differential thresholds) turned out to be sensitive to the choice of coding theory and non-linearity. The interval chosen for response analysis had a strong effect. The predicted profiles should be checked against behavioral data on intensity discrimination in rat, but since such data are not yet available, a comparison was made with human psychophysical data, for illustrative purposes. It appeared that a unique combination of variables (tonic part of the response, across-fiber pattern coding with a contrast enhancement type of non-linearity) could be selected to match the psychophysical profile.In the Discussion, special attention is paid to the fundamental differences between the two coding theories, to the relevance of whole-mouth stimulation, and to the informational significance of various parts of the phasic/tonic response.Abbreviations AFP across-fiber pattern - CT chorda tympani - DA discrimination acuity - DT differential threshold - H sour - LL labeled line - N salty - NTS nucleus tractus solitarius - PTA pontine taste area - Q bitter - S sweet