Higher-order conditioning of taste-odor learning in rats: Evidence for the association between emotional aspects of gustatory information and olfactory information

Previous studies have shown that rats prefer an odor paired with saccharin solution to an odor paired with quinine solution (taste-odor learning). However, it remains unclear whether the odors are associated with the emotional (i.e., positive and/or negative hedonics) or qualitative (i.e., sweetness and/or bitterness) aspects of gustatory information. This study aimed to examine this question using higher-order conditioning paradigms: second-order conditioning (SOC) and sensory preconditioning (SPC). Adult Wistar rats were divided into SOC and SPC groups. Food flavors, purchased from a Japanese market, such as melon (0.05%), lemon (0.1%), vanilla (0.1%), and almond (0.1%), were randomly used as odors A, B, C, and D for each rat. The SOC group was exposed to 0.005 M saccharin solutions with odor A and 0.02 M quinine solutions with odor C in the first 5 days of learning. Additionally, they were exposed to water with a mixture of odors A and B, and water with a mixture of odors C and D in the next 5 days of learning. The order of these two learning sessions was reversed in the SPC group. We hypothesized that if odor was associated with the emotional, or qualitative, aspects of gustatory information, the SOC, or SPC groups, respectively, would prefer odor B to odor D. Our results showed that the SOC group preferred odor B to odor D, whereas the SPC group did not show any such preference. This suggests that odors may be primarily associated with emotion evoked by gustation in taste-odor learning.