2007/5/5
We have memory and animals have memory, too. Some
people remember space or orientation well, some are good at recognizing faces or
objects, while some tend to re-experience painful feelings that happened decades
ago. Accordingly, we distinguish memory into space memory, cognitive memory and
emotional memory.
In order to study the cognitive memory of mice, a
method called object recognition test is used. In this method, two different
objects are put in an open field. A tested mouse in the open field can freely
move in the open field and access the objects. Normally, a mouse will spend
equal time to approach or explore the two objects. If a group of normal mice
prefer one of the objects and spend more time exploring one object than
exploring the other, these two objects are not suitable for this test. There are
two stages with the object recognition test. In stage I, two same objects, A1
and A2, are put in different positions of the open field. A tested mouse is
allowed to move in the open field freely for ten minutes. The duration that the
mouse explores the objects A1 and A2 is counted. Surely, the duration should be
similar or vary in a limited range. Stage II is performed 24 hours after stage
I. In stage II, objects A2 is replaced with object B. The mice that have gone
through stage I precisely 24 hours before and they had never seen object B, so
they are expected to spend more time to explore object B, except that they
forget they had seen objects A on the previous day. Normal mice can remember an
object for more than three days, and they have a tendency to explore new things.
If a tested mouse spends more time to explore the new object B, we say the mouse
has normal memory. If a test mouse spends equal time to explore object B and
object A1, the mouse is believed to forget that it had explore object A before.
It’s unlikely that it specifically dislikes object B because this test is based
on the fact that the tested mice have no preference to object A or B.
We use a step-down test to examine the emotional memory
of mice. A tested mouse is put in a steel-grid floor box. A minor electric shock
is delivered to the mouse through a steel grid. The electricity persists for 3
seconds. The mouse suffers and is frightened, but it is unable to escape. After
the electric shock the mouse remains in the box for a minute. Normally a mouse
will remember this kind of emotional shock for a week. During this period, the
mouse that had ever been shocked before will show a behavioral pattern of
freezing when it is put back to the same box. However, the behavior of freezing
is not easy to quantify. An alternative way to quantify the memory of fear of
the previously-shocked mouse is put a little wooden chair on the steel grid. If
the mouse remembers the experience of being shocked on the grid, it will try
hard to stay on the little wooden chair and prevent itself from stepping down on
the terrible grid. With an eye to determining the strength of a mouse’s
emotional memory, I put shocked mice back in to the grid box every day. I
expected to see some mice step down on the grid on the fifth or seventh day
because the memory of fear should wear off. Some with better emotional memory
would step down later. However, the fact was totally beyong my expectations.
None of the previously shocked mice showed hesitancy to maintain on the little
wooden stool on the seventh day. It was incompatible with previous reports. I
thought the everyday-test strategy might become an everyday reminder, which did
not allow the fear memory to fade away due to a natural course. I changed the
testing strategy without letting the shocked mice return to the grid box until
the eighth day. All six tested mice stepped down on the grid within a few
seconds. They all forget the fear on the eighth day. I tested another group of
shocked mice on the fourth day. A half of them stepped down in a minute. The
method was simple and the results were clear. I am going to use this method to
test if a mouse with stronger emotional memory tends to become helpless when
they are given inescapable shocks.
Hong CJ