Associative Learning Acquisition and Retention Depends on Developmental Stage in Lymnaea stagnalis

Abstract

Associative learning dependent on visual and vestibular sensory neurons and the underlying cellular mechanisms have been well characterized in Hermissenda but not yet in Lymnaea. Three days of conditioning with paired presentations of a light flash (conditional stimulus: CS) and orbital rotation (unconditional stimulus: UCS) in intact Lymnaea stagnalis results in a whole-body withdrawal response (WBWR) to the CS. In the current study, we examined the optimal stimulus conditions for associative learning, including developmental stage, number of stimuli, interstimulus interval, and intertrial interval. Animals with a shell length longer than 18 mm (sexually mature) acquired and retained the associative memory, while younger ones having a shell length shorter than 15 mm acquired but did not retain the memory to the following day. For mature animals, 10 paired presentations of the CS and UCS presented every 2 min were sufficient for the induction of a WBWR to the CS. Furthermore, animals conditioned with the UCS presented simultaneously with the last 2 s of the CS also exhibited a significant WBWR in response to the CS. Blind animals did not acquire the associative memory, suggesting that ocular photoreceptors, and not dermal photoreceptors, detected the CS. These results show that maturity was key to retention of associative learning.