Operant conditioning that the pond snail, Lymnaea stagnalis, suppressed its naturally occurring behavior of escape from a water tank was examined by using a negative reinforcement (i.e. an aversive stimulus) prepared outside the tank. During the training period, the number of escapes from a tank was strongly suppressed. One of behavioral factors for this suppression was confirmed as the elongation of latency to the first escape after training. The effects on the memory retention were examined in the massed and spaced training procedures. The latter procedure interposes a rest interval between three sets of 20-min training sessions, whereas the former has the same number of training sessions with no rest interval within 60 min. The memory retention by the massed training was observed within 20 min after training. By the spaced training, the learning acquisition was found to be stronger, which was observed as the slower latency to the first escape, than by the massed training, but the longer-lasting memory retention, which had been expected first, was not formed. These results suggest that once Lymnaea recognize the external environment is safe after training, they may extinguish their memory of the past situation quickly, resulting in no or very little difference in the memory retention by two different training procedures in this operant conditioning. Together with the facts that classical conditioning and its neuronal mechanisms in Lymnaea were previously clarified, the present findings may help to address not only the neuronal basis of operant conditioning but also the relation between classical and operant conditioning.
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