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.