It's not just some humans that find it easier to learn than others, snails too can be memory champions or scatterbrains. The key question is why some snails are better at forming long-term memories than others: it might help researchers understand how long-term memories form, too. Etsuro Ito and his Japanese and Canadian colleagues scrutinised memory champions and scatterbrains – or good and bad learners – in the pond snail(Lymnaea stagnalis), to find out how good snails are at forming memories, and what might affect their formation().
First the team had to train their snails to remember a specific event. They used a technique called conditioned taste aversion, where snails are fed a yummy treat – sucrose or carrot juice – followed by a horrible taste, in this case bitter potassium chloride (KCl) solution. If the snails have learned, they will avoid the same sweet treat in the future, knowing that it is followed by a bitter aftertaste.
To see how well the snails had learned, the team tested their trainees 9 min 30 s after training, counting how many bites they took of sucrose solution. The team found that 42% of the snails were good learners, not feeding on sucrose. The remainder fed on the sucrose solution, showing that some snails had remembered the bitter aftertaste, while others had not.
Having shown that some of the snails could form memories, the team next wanted to test if the memories were long-term. Testing both the good and poor learners again seven days later, they found that only the good learners remembered their training, showing that the memories were long-term. Even though the memories had lasted a week, there was always the possibility that the memory could be wiped out. This is called extinction, and the team wanted to know if their training was resistant to extinction or not, and if the snails had formed strong memories. They gave trained snails extra extinction training to try and make them forget the link between the sucrose and bitter KCl: each snail dined on sucrose, without KCl, on three separate occasions. Testing the snails' response to sucrose 1 h and 24 h later, the snails still rejected the sweet substance, showing that the memory had resisted extinction.
But when were the memories forming? The team knew that memories would form sometime during or directly after training, and would involve protein synthesis and altered gene activity in the brain. Immediately after training,the team cooled a group of snails down to 4°C for 30 min, testing them again once they had rewarmed them to 20°C. None of the snails had remembered their training, suggesting that memories were formed in this 10 min period. If the team delayed cooling until 10 min after training, the snails remembered their training, supporting this idea.
Knowing that some snails learn better than others, and that memories are formed in the first 10 min after learning, researchers will be able to focus their attention on this crucial time window, which might give them clues as to what separates the good learners from the bad.
