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Measures

Zones around the platform and other calibrated positions

Not quite a crossing, but close enough to count.

By the watermaze.org editors · Updated

Here's a familiar frustration. Your animal clearly knows roughly where the platform was. It swims back and forth right next to the spot, again and again, but the track never passes exactly over it. Count exact platform crossings and that animal scores the same as one swimming at random. Count time in the target quadrant and it looks no better than an animal with only a vague idea of the location.

A small circular zone around the platform position fixes that. HVS Image calls this zone the counter, and the measures built on it close passes or close encounters.

What the counter is

The counter is a circular area centred on the set platform position. By default it is twice the diameter of the platform. So with a 10 cm platform, being within 5 cm of the platform position counts as close.

Entering the counter is a near miss: more precise than "somewhere in the right quadrant", more forgiving than "exactly over the spot". It sits neatly between the two classic probe measures.

What you can measure with it

For each position, the close encounter measures are:

  • Close passes: the number of entries into the counter.
  • Time close: the percentage of trial time spent within the counter.
  • Latency to first close encounter: how quickly the animal almost reached the position, even if it just missed the exact spot.
  • Path to first close encounter: how far it swam before it first got that close.

Not just the platform: other calibrated positions

This is the part that's easy to miss. The counters aren't only drawn around today's platform. They are applied to every calibrated platform position, whether or not a platform is there:

  • the learned platform position in a probe trial, when the platform has been removed;
  • the old and new positions in reversal learning;
  • marked positions in dual-strategy trials;
  • the other positions in a design where the platform moves every day, as in reference versus working memory studies.

That makes direct comparisons possible. In a reversal probe you can ask how often the animal came close to the old position versus the new one, using the same yardstick for both. And because the counters follow the calibrated positions, they work wherever in the pool the platform sits, including designs where it isn't in the middle of a quadrant.

HVS Image also gives, for other calibrated positions, exact crossings, Gallagher proximity, and latency and path to first reach each position. Its Strategy level adds direct paths to close encounters with other calibrated positions, and search focused on them.

Choosing the counter size

The counter size sets how precise you're asking the animal to be. Make it smaller to count only very precise memory. Make it larger to include less precise but still focused searching, which is still more accurate than quadrant time. You can re-analyse the same trials with different counter sizes, which can reveal effects that the basic measures, or the default size, miss.

One honest caveat: if you try several sizes, say so in your methods, and decide in advance which one is your main measure. Otherwise it starts to look like fishing.

Learning trials too

Close encounter measures are given in ordinary learning trials as well as probes. That lets you track how precisely the animal is homing in across training, and compare learning and probe trials on the same measure.

How this differs from the other zone pages

  • Circular zones divide the whole pool into rings centred on the pool centre. They tell you how far from the wall or centre the animal searched.
  • Counters are small circles centred on each calibrated position. They tell you how close the animal came to a particular place.
  • The chaining annulus is a ring at the platform's distance from the wall, used to spot animals circling at the right radius without knowing where the platform is.

Quick answers

Is a close pass the same as a platform crossing? No. A crossing means passing over the exact position. A close pass means entering the counter around it, so near misses count too.

Do I have to set the counters up? Not in HVS Image. Close encounter measures around the set platform position are given automatically, and you can adjust the counter size later and re-analyse.

Why did HVS Image add these measures? The close pass measure was first created for Dr Sarah Wong-Goodrich, after the observation that subjects often passed close to the platform location early in a trial and then didn't return for some time.

Sources

For reversal and dual-strategy designs, HVS Image explains close passes and close encounters.

Spotted a mistake, or know a better source? Tell us and we'll fix it and say so on our corrections page.