The Morris probe essay in plain words

This is our summary of the full essay. It isn't a substitute for reading it, and where we've simplified, the original wins.

1. The "lucky bump"

During training an animal only has to bump into the platform once to escape. Was that bump memory, or luck? The probe trial is the cheapest and most convenient way to rule out luck, and the essay says it works very well for rats.

2. Rats and mice search differently

A rat that knows where the platform was trawls over that spot again and again. Mice, the essay says, are more adaptable: many go straight to the old spot, swim back to where they were put in, return, and later head for the side where they'll be lifted out. Some float.

3. Once they realise it's gone, the test changes

The essay's key point: the probe is a memory test "if, and only if" the animal doesn't realise the platform has gone. After that, what you're seeing is a survival strategy, which may differ between groups for reasons such as anxiety.

4. Test anxiety and activity first

Nervous or weak swimmers do worse for reasons that have nothing to do with spatial learning. The essay suggests testing exploration and anxiety (for example in the elevated plus maze, holeboard or open field) before the water maze, and getting mice used to being handled first.

5. The learning curve is the prime test

In Dr Morris's view, the best test of spatial memory is the initial search for the hidden platform, set up so the animal realises early that the problem is hard but solvable, with steady improvement over many trials, and without overtraining. He is clear that this wasn't the common view at the time.

: Day one of a reversal can look like a memory difference when it is really a difference in strategy.

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