Mouse / Water maze Hints 12/98
Copyright (C) 1998 R.J.Morris and HVS Image.
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Editor's notes:
a) Rats are natural swimmers and if handled well will cooperate 
easily in Water Maze. They don't have the option to float or jump - 
they are solid citizens who just swim for that platform and get 
onto it.

b)Mice are also good swimmers, and if handled well, will also take
part easily. But they are more temperamental, and they do have the 
option to stop swimming and just float, and to jump. These notes 
suggest ways to reduce stress, so your mice will feel confident, 
and cooperate. 

The following notes are edited from material kindly supplied by
Dr. Roger Morris, UK.....

INTRODUCTION
The swimming-pool (Morris Water Maze) is a delicate balance of
motivation provided by stress/reward, and a difficult task. It is 
an unnatural for rodents, for whom sight is a minor sense. If the 
stress is too high, or the reward too slight, the task is not 
attempted.

PLATFORM & WATER
1. Cover the platform with a fibrous mesh into which the mice
can dig their paws and feel secure when they get there. We found
a synthetic underlay (to stop carpets slipping) ; it is whitish
and blends with the water (milk). Others have used pot-scouring
pads. 

2. Make the water just cover the platform by ~5mm. Rats are big
enough to feel they are out of the water. Mice are smaller; 1-2
cms is too much. 

3. Water-temperature has to be accurate to 1 deg; we use 24 deg
C. A couple of degrees colder and they just climb out.

TIME ON PLATFORM & RESCUE
A) Time on platform has to be sufficient for them to adjust to the 
feel & location, and to see where they are. However the prime reward 
is not to climb onto the platform, but to leap into your arms when 
you intervene to rescue them. So keep it to 10 secs before you are 
seen or heard coming to the rescue. 

B)And they do leap ! You need the pool to be sufficiently high-sided
to stop them leaping out into the room. The high-side requirement is 
a major determinant of water level & location of cues.

C)Because rescue by you is the reward, you will never train them
while you are detectable by sight or sound. The task has to clearly
direct them first to the platform.

CUES
i) Must be visible, & useful to mice. They must be far enough
away to require the mice to use spatial analysis, rather than
association, to solve the task. But near enough & positioned
appropriately, for the mice to see and use them. And spatially
specific enough to solve a spatial task. Feel-good psychedelic
& animal posters stuck on the far walls of a large room ,below the 
line of vision of mice swimming in a high sided pool, do not qualify!

ii) We use a 2m pool, sufficiently large that I only have to place
the cues just outside the walls of the pool, and the platform
~0.5m in from the side. That makes the task difficult - impossible
to solve by association, and the cues are maximally visible.
Although it is still obvious that mice placed near the walls have
to swim to the centre to get their full bearings. There are places
near the wall of the pool, where they can see 1 or 2 cues, but
need all 3 to triangulate the position of the platform.

iii) For a 1m pool you have to place the cues some distance from
the pool, for the task to be spatial not association. I use board
5-15 cm wide, 1m long, & put different patterns of black stripes
(vertical, horizontal, triangles) of different widths. 

They need to stand out to a mouse, be positioned where they are
optimally visible & angled towards the centre of the pool so they 
appear perpendicular to the mouse. And instantly distinguishable
from each other. You can see if the mice are using them; they
need 3 specific cues, and diffuse cues in the room, such as sources
of light, as general orientaters.

TIME & INTERVALS OF SWIMMING

1. We use 2 mins in a 2m pool with a 15 cm platform*. If your
pool is as small as 1m, I would swim them for 1 min, & when they 
fail to find the platform, guide them to it so they climb on. 
Ideally you leave them there for 10 secs, so they assess the
surroundings. In practice they will make a leap for it if you let 
them. So as they climb on, step well-back & try to ensure they stay 
on for 10 secs. If they immediately take to the water & keep 
swimming, intervene again to guide them back. They have to learn 
that rescue is dependent on staying on that platform.

2. If they leap for you before you retreat, you may as well catch
them, as at least they have got the idea. You should find that
on the first day your mice mostly fail to find the platform. Then
on subsequent dates their efforts are greeted with increasing
success. If you leave them too long at the beginning, swimming
around without success, they will associate the swim with failure,
& panic.

3. We find that mice given 2 trials per day learn nearly as quickly
as 4 times a day. If you are worried about the stamina of elderly
mice*, cut back to 2 per day. Our mice look forward to their swim
(they rattle the tops of their cages when my postdoc comes to
collect them). We suspect they possibly enjoy it too much, and
extend the swim-time because the alternative is to go back in
a boring cage. Then we tried lowering the temperature of the pool
by 2 deg, and they went on strike ! What I was leading-up to is
that if the mice are not panicking, I suspect the swim is not
traumatic, and they enjoy doing laps of the pool. 

4) Intervals between swims is important. We use 30 mins, as being
a good way to reinforce learning.

HANDLING & ACLIMATIZING

a) We give them 2 days in the test room, playing with them in
& out of their cages, so they are comfortable with us and the room, 
before starting swimming. They have also been handled by us 
intermittently since birth, so they are happy with people.

b)Smells are very important. Even subtle amounts of perfume can
confuse ! "bad" smells are totally out, eg rats & especially cats. 
If you have a pet cat, it might just work if you shower & change into 
fresh clothes at the lab.

EDITOR'S NOTES
1. HVS can supply a plexiglass platform, good for white & black pools. 
Diameter 10 cms, with a cross-hatch of lines on the top to reduce 
slipping (so hopefully you don't need pot-cleaning pads.

2. 24 deg C is cooler that the average rat-pool (26deg C). 

3. More than one Morris! R.J.Morris (Dr Roger Morris) indly 
provided this mouse-info. R.G.M. Morris (Prof Richard Morris) -
invented the Water Maze technique.

4. Metric/imperial conversions: 1m~39ins;. 10cm~4.1ins