Spontaneous alternation

The question

Time to stretch our legs. Why does a rodent so often turn into the arm it did not visit last time? You're watching Morris Water Maze Software Guide, over at morriswatermazesoftware.com.

Meet the spontaneous alternation

Spontaneous alternation is the natural tendency of a rodent, in a T-maze or a Y-maze, to visit the arm it visited least recently. The animal does not have to be trained or rewarded. To choose the other arm, it has to remember the last choice, so the percentage of alternations is a quick score of spatial working memory. A 2006 methods paper set out how to run this in the T-maze, and a later Y-maze protocol is widely used in mice.

How it runs

How do you run one? First, place the animal in the start arm, or at the end of one Y-maze arm, and let it choose freely. Next, record the sequence of arm entries, using a fixed entry rule such as all four paws inside the arm. Then, score an alternation when the animal enters a different arm from the one it chose last, or, in a Y-maze, when three entries in a row cover all three arms. Finally, run enough entries for a stable percentage, and clean the maze between animals.

Strengths and limits

What's it good for? No training and no food restriction. It is quick and gentle. The same idea works in both the T-maze and the Y-maze. Now, the small print. Chance is high, so you need enough entries for the percentage to mean something. Side biases and odour trails can look like memory if you do not check for them.

What to measure

Let's open up the analysis. Alternation percentage: alternations divided by the number of opportunities, times one hundred. Total entries: a check on activity, because a high score on very few choices is noisy. Side bias: the share of left versus right choices, or of each Y-maze arm. Choice latency: hesitation at the junction, a sign the animal is working at the choice. A mouse alternates on 80 percent of its chances, but it only made six entries. Another alternates on 65 percent of 25 entries. The second score is the one you can trust.

The takeaway

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