It is necessary to determine whether, in the same species and for the same behavior, aversive and appetitive conditioning yield different strengths and periods of either acquisition or retention. To this end, we first examined the effects of various chemo-sensory and physical stimuli on feeding and avoidance behavioral responses in the pond snail, Lymnaea stagnalis. Then, using these findings, we constructed classical-conditioning paradigms with aversive and appetitive stimuli. In the aversive conditioning paradigm, an appetitive stimulus (sucrose), which increased the feeding response, was paired with an aversive stimulus (KCl, quinidine sulfate or electric shock), which inhibited the feeding behavior. Upon presentation of KCl, the first type of aversive conditioning, which is generally called "taste-aversion learning with cessation of feeding response", was acquired quickly and persisted for up to a month. When using a noxious stimulus (quinidine sulfate or electric shock) inducing pain we additionally found the second type of aversive conditioning, in which the previously appetitive stimulus (sucrose) not only failed to increase the feeding response, but came to elicit an avoidance response. This second type conditioning took longer to acquire and persisted for a shorter period of time than the first type. On the other hand, the appetitive conditioning paradigm paired a neutral stimulus (vibratory) with an appetitive stimulus (sucrose). The strength and period for acquisition and retention of this appetitive learned response were very similar to those of the second type aversive conditioning but not to the first one. On the basis of these behavioral analyses, the neuronal mechanisms of the two types of aversive and appetitive conditioning were discussed.
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