Probe trial

The question

Right, let's get our feet wet. What happens when we take the platform away, and why is that the real memory test? You're watching Morris Water Maze Software Guide, over at morriswatermazesoftware.com.

Meet the probe trial

A probe trial is a water maze trial with the escape platform removed. The animal swims for a fixed time, often 60 seconds, and we watch where it searches. Because there's nothing to find, the search pattern shows what the animal remembers about the platform's place, not how good it is at bumping into it.

How it runs

So, what does a session look like? First, train the animal until latencies level off. Next, remove the platform, often 24 hours after the last training trial. Then, release the animal from a start point it hasn't used as the nearest one to the old platform, and record a fixed swim time. Finally, keep probe trials few, because repeated empty swims can teach the animal that the platform has gone.

Strengths and limits

Why do labs keep coming back to it? It separates place memory from general swimming skill. It's quick to add to any water maze protocol. It works with every proximity and quadrant measure. And the catch? Too many probes can cause extinction of the search. A short swim time can miss a slow, careful searcher. Quadrant time alone can hide a tight search just off the spot.

What to measure

Let's open up the analysis. Time in the target quadrant: the classic measure; one quarter of the time is chance, so higher means a preference for the right area. Platform crossings and annulus entries: how often the animal passes over, or close to, the exact old platform spot. Average proximity to the old platform: the mean distance from the spot across the probe; a 2009 analysis found distance-based measures tended to be more sensitive than quadrant time. Time slices: splitting the probe into early and late parts, because search often spreads out once the animal finds nothing. Two mice spend the same 35 percent of the probe in the target quadrant. One circles the exact spot again and again; the other wanders the whole quadrant. Proximity and crossings tell them apart.

The takeaway

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