Gallagher proximity measure

The question

Time to stretch our legs. Why is the average distance from the goal such a powerful memory score? You're watching Morris Water Maze Software Guide, over at morriswatermazesoftware.com.

Meet the Gallagher proximity measure

In 1993, a team studying ageing rats described proximity measures for the water maze. Instead of a yes or no, such as crossing the platform, you sample the animal's distance from the goal throughout the trial and average it. A low average means the animal stayed near the goal. It was designed to capture individual differences, such as in ageing rats.

How it runs

How do you run one? First, sample the animal's position at regular intervals through the swim. Next, calculate the straight-line distance to the platform, or to its old spot in a probe, at each sample. Then, average those distances across the trial. Finally, compare training trials, probes and groups on the same scale.

Strengths and limits

What's it good for? It uses the whole trial, not one moment. It's sensitive even when an animal never crosses the exact spot. It works in rodent pools and in virtual mazes for people. Now, the small print. Start position affects training trial values, so starts must be balanced or corrected. A single average can hide a change in strategy partway through the trial.

What to measure

Let's open up the analysis. Average proximity in the probe: mean distance from the old platform spot. Average proximity in training: how close the animal stays while it searches. Proximity by time slice: early versus late search. Cumulative search error: the related sum of distances, corrected for the ideal path. An old rat never quite crosses the platform spot in the probe, so its crossing count is zero. Yet its average proximity is close to that of young rats. It remembered the area; it just searched a little wide.

The takeaway

If you remember only one thing, make it this: average proximity in training and its neighbours tell you how the task was done, not just how fast. Everything we covered is written up below.