Differences in the taste quality of maltose and sucrose in rats: issues involving the generalization of conditioned taste aversions

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Abstract
The present study employed a conditioned taste aversion generalization paradigm to test the hypothesis that maltose produces taste sensations in the rat which are qualitatively distinguishable from sucrose. Since stimulus generalization can occur in both the quality and intensity domains, an intrachemical (across concentration) generalization gradient was established to aid in the interpretation of the interchemical (across molecules) generalization gradient. Moreover, since the commonly used intake test is vulnerable to nontaste post-ingestional influences, the present study measured immediate responses to 100 μl stimulus samples, thus increasing our confidence that the behavior was under orosensory control. In Experiment 1, naive water deprived rats were trained in a specially designed gustometer to maintain drinking-spout contact for intermittent water reinforcement. Following this, rats in the experimental group were given three exposures to 0.1 M sucrose on separate days, with the first two exposures immediately preceding an injection of LiCl. A control group was treated identically but received distilled water instead of sucrose. Rats were then tested in the gustometer for their avoidance of three equimolar concentrations of sucrose and maltose. Rats received ten trials of each stimulus quasi-randomly presented in two sessions. Results indicated that all sucrose concentrations were avoided (in experimental group only), but only the 0.3 M concentration of maltose was avoided. The lowest sucrose concentration was significantly less avoided than the higher concentrations. Intensity generalization gradients are such that intensities weaker than the conditioned stimulus (CS) produce just as much or less of a conditioned response (CR) and intensities stronger than the CS produce just as much or a greater CR than that elicited by the CS itself. Therefore, based on the results of Experimental, it was predicted that if 0.1 M maltose served as the CS, the order of avoidance should be: 0.3 M sucrose ≥ 0.1 M sucrose ≥ 0.03 M sucrose ≥ 0.3 M maltose ≥ 0.1 M maltose ≥ 0.03 M maltose, if it were true that maltose and sucrose produce identical sensations that differ only in intensity. Experiment 2 explicitly tested this prediction using the same procedure as Experiment 1 except that 0.1 M maltose served as the CS. The observed order of avoidance was 0.3 M maltose > 0.1 M maltose > 0.03 M maltose = 0.3 M sucrose = 0.1 M sucrose = 0.03 M sucrose. In both experiments the intrachemical generalization gradient broadened and the interchemical generalization gradient steepened upon retesting. In conclusion, qualitative differences between maltose and sucrose explain the outcomes of these experiments better than differences in the relative intensity of these sugars at isomolar concentrations.