Pool & room
The escape platform: size, depth and position
The whole test hangs on one small, hidden object. Place it with care.
The platform is the one thing your animal is looking for, so its size and position quietly decide how hard the task is. The original notes say it "should not be too close to the side, or the subject finds it just by circling around the pool-side", and that for rats it can sit 1/4 in below the water, while for mice it should be very close to the surface. The full text is on the pool page.
Questions people ask
What size platform, and how deep below the surface?
About 10 cm across (or smaller) and 1–1.5 cm below the surface, placed halfway between the wall and the centre. Use a larger platform if you test mice in a rat pool. Your local ethics committee, licence conditions and vet always take precedence.
More detail
Use a small platform, about 10 cm across, hidden 1–2 cm below the surface (less for mice) and placed halfway between the wall and the centre. 10 cm or smaller, clear or matching the interior, 1–1.5 cm below, halfway between wall and centre [Vorhees & Williams 2024]; 10–12 cm, 1–2 cm below (0.5–1 cm for mice) [Vorhees & Williams 2006]. HVS: larger platform for mice in a rat pool [HVS].
Going deeper
Smaller platforms (7 cm, 5 cm) can be used for reversal and shift phases to keep the task sensitive [Vorhees & Williams 2024].
Where should I put the platform in the water maze?
The platform is the small escape platform the animal swims to in the water maze; it is hidden just under the water so the animal has to remember where it is. Put it in the middle of one quadrant, halfway between the pool wall and the centre, not in the centre and not against the wall, with its top about 1–1.5 cm below the surface. 1) Keep it in the same place for every learning (acquisition) trial. 2) Take it out for a probe trial. 3) Move it to the opposite quadrant for reversal. 4) Tell HVS Image where it is: on the calibration screen, calibrate the pool centre first (platform 0), then each platform position (1–4, platform 1 is the north-east quadrant), or click 'Suggested MWM Platform Positions' to fill in one position per quadrant.
More detail
The platform is the hidden escape platform of the Morris water maze: a small (about 10 cm) clear or pool-coloured platform whose top sits 1–1.5 cm below the water, so it cannot be seen and the animal must use the room cues to find it. Place it in the centre of the target quadrant, halfway between the wall and the pool centre (about half the radius from the centre). A platform near the wall is found by animals swimming round the edge, and one near the centre by chance circling, so both weaken the test. Keep the position fixed through acquisition, counterbalance the target quadrant across groups, and avoid release points right next to it. Remove it for probe trials and move it to the opposite quadrant for reversal (a smaller platform, 5–7 cm, keeps reversal sensitive). In HVS Image, calibrate the pool centre as platform 0, then platforms 1–4, one per quadrant (platform 1 = north-east). 'Suggested MWM Platform Positions' fills them in: in the Cartesian boxes, centre 0.5/0.5 and quadrant centres 0.67/0.33, 0.67/0.67, 0.33/0.67 and 0.33/0.33; in the Polar boxes, an angle from North and a distance of about 0.5 of the radius. Choose the platform number for each trial so the analysis measures the right target.
Going deeper
Why halfway: 0.67/0.33 is simply the point half a radius out along the quadrant's diagonal (0.5 ± 0.177 of the pool width), so quadrant time, platform crossings and the annulus around the platform all measure the same target. Mark the position on the rim or measure it from the wall so the platform goes back to exactly the same place each day; if the pool or camera moves, recalibrate before the next trial. Use the same water level every day so the platform depth does not change, and keep the water opaque so the platform stays hidden.
Should I use a rising (Atlantis) or a fixed platform?
Use the rising platform to make sure spatial learning is needed; use a fixed one to see which strategy animals prefer; use both to learn preference and capability. 1) With a fixed platform, animals can find it by circling at the right distance from the wall. 2) If you train with dwell-triggered Atlantis, use it in probes too (slightly longer dwell).
More detail
A non-rising platform allows non-spatial strategies, so falling latencies can be misread as spatial learning, and in probes strong learners may leave once it is gone. The Atlantis sits well below the surface and rises when the subject reaches or dwells at the target, so it won't be found by chance. It can act as a non-rising platform in non-automatic mode at regular height. Use analyses beyond latency and target-quadrant time: heading, Gallagher proximity, close passes, pool circling, slices.
Going deeper
The target area diameter can be set independently of the platform diameter. The regular platform (10 cm top, 29.5 cm high for 30 cm water) can be used for visible trials by lowering the water or adding a flag.
If you want the platform to appear only after a set time, or only when the animal has searched in the right spot, read about the Atlantis (rising) platform.
For a platform that rises on demand, see the Atlantis platform.
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