Path length and swim speed

The question

Right, let's get our feet wet. How do we tell a shorter search from a faster swim? You're watching Morris Water Maze Software Guide, over at morriswatermazesoftware.com.

Meet the path length and swim speed

Path length is the distance the animal swims in a trial. Swim speed is that distance divided by the time spent moving, or by the whole trial time if you say so. Together they stop a simple mistake: a fast animal can look as if it has learned, because its latency is short, even when its path is still long and wandering. A 2006 review of water maze methods treats path length as a core acquisition measure, with speed as the motor and motivation check.

How it runs

So, what does a session look like? First, track the animal's position at a steady sampling rate throughout each trial. Next, sum the distances between points to get path length, after a light filter for tracking noise. Then, divide path length by time, using either total time or time spent moving, and report which you used. Finally, compare path length and speed across days and groups, next to latency.

Strengths and limits

Why do labs keep coming back to it? Path length is far less fooled by speed than latency is. Speed flags motor, age or drug effects early. Both measures work in rodent pools and in virtual mazes. And the catch? Tracking gaps and jitter can inflate path length if the data are not cleaned. A single mean speed can hide bouts of floating and bursts of sprinting.

What to measure

Let's open up the analysis. Path length per trial: how far the animal travelled to reach the goal, or during a probe. Average swim speed: a motor and motivation check. Path efficiency: actual path length divided by the direct line to the platform; one is a perfect swim. Speed in moving bouts only: so floating does not drag the average down. Two rats share a 20 second latency. One swam a short, fairly straight path. The other dashed around the pool at high speed and bumped into the platform. Path length and path efficiency tell them apart.

The takeaway

One thing to take away: average swim speed and its neighbours tell you how the task was done, not just how fast. Scripts, chapters and sources live on this page.