Learning isn't easy, and learning how we learn is even more difficult. Every human memory is made up of complex neural networks across millions of nerve cells. But Lymnaea stagnalis' memories are much simpler. Ken Lukowiak realised that although their brains are simple, they are ideal for learning how memories are formed. For some snail memories three neurones are sufficient to store life's lessons. Training the snails with a simple behavioural program, has already taught Lukowiak a lot about memory, but in this issue of J. Exp. Biol. he describes how combining a training protocol that doesn't build good memories with another that does, results in a memory that is more than the sum of its parts(p. 1171).
Lymnaea stagnalis has many talents, including two ways of breathing. If you put Lymnaea stagnalis in a beaker of oxygenated water, it happily breathes through its skin. Transfer it to hypoxic water, and it rushes to the surface to breathe air with its lung through a tube called a pneumostome. Unless it's been taught to keep its pneumostome shut. With the right training, it can remember for days to keep breathing through its skin,even if it is a little breathless. But memories can be lost too, and with the right training, the best breath holder can be made to forget.
Lukowiak's team of trainers used two approaches to train the snails to keep their pneumostomes closed in hypoxic water. They taught one group of snails by touching their pneumostomes every time they tried to open them to breath(continuous training). The second group of snails only got a training touch every alternate time they opened the pneumostome (partial training). After three 30 minute lessons the first group of snails learned to keep their pneumostomes shut. But after the same amount of partial training, that group of snails hadn't got a clue and kept coming up for air when they should have known better. But how good were the snails' memories after they'd been taught both lessons?
Lukowiak's team alternated the lessons; one set of snails had partial training that was followed up with continuous training, while the second group were trained the other way round. A few days later, the snails were tested to see what they remembered.
The first group who'd had partial training followed by continuous training,had memories that were as strong as the snails who'd only had continuous training; no surprise there. But the second group of snails, who had followed continuous training with partial training, had longer memories than all the other snails. More remarkably, the snails had learned a lesson that they couldn't forget; all attempts to extinguish the memories failed. Instead of having a detrimental effect on the snail's memories, partial training had given them a boost. Which means that in this regard, snail's memories are just like humans!
With just three neurones holding the key to this type of snail memory,Lukowiak has a real chance of getting to the root of how memory is stored. Although the snails can always remember to keep their pneumostomes closed,different lessons last for different lengths of time. Lukowiak hopes that learning how memories of different strength are stored in the snails' tiny brains could help us to learn how our own memories are wired too.