Gone but not forgotten: the lingering effects of intermediate-term memory on the persistence of long-term memory

Aerial respiratory behaviour can be operantly conditioned in Lymnaea stagnalis and, depending on the interval between the training sessions, memories of significantly different durations are produced. In naïve snails, a 15 min training procedure with a 30 min interval between three training sessions results in memory that persists for only 3 h (intermediate-term memory, ITM); whilst if the three 15 min training sessions are separated by a 1 h interval memory persists for 48 h (long-term memory, LTM). We found that if ITM training preceded LTM training, then LTM would persist for 24 h longer. This augmenting effect on LTM persistence could be demonstrated for up to 5 h following the last ITM training session, even though ITM was not observed at that time. However, if LTM training ensued 8 h after the last ITM training session, an augmented LTM did not occur. Extinguishing the memory produced by the ITM training procedure also prevented augmentation of LTM. That is, if an extinction procedure was given to the snails after the ITM training procedure, LTM did not persist longer than 48 h. Thus, at the behavioural level, ITM and LTM are interconnected.