How strange.

Psychologists who study point out that we have memories for things that have happened to us, and also memory for where we encountered the things that happened to us. That memory for where we encountered information is called source memory.

There are two ways that you can recognize that you are in a familiar situation:

One is to retrieve the previous situation from memory. For example, you might visit the town you grew up in. When you go to an old school you went to, you might remember a class you took and know that you had been there before.

But, you can also just get a feeling that you have been somewhere before. This feeling of knowing is related to knowledge about the source of a memory. So, when visiting the town where you grew up, you might pass the library and feel that it is familiar without remembering ever going there.

The experience of déjà vu involves having that feeling of knowing in a situation in which you are experiencing something totally new.

A paper by Anne Cleary, Anthony Ryals, and Jason Nomi in the December, 2009 issue of Psychonomic Bulletin and Review suggests one factor that leads to the experience. They had people study a number of drawings of scenes. Later, they were shown a number of new scenes and were asked whether they had seen them before. Some of those new scenes had a similar configuration to ones they had studied earlier, but all of the objects were different. For example, during the first phase of the experiment, people might have seen an alley between a fence and a building. Later, they might have seen an alley between a train station and a train. In this case, people often felt that the new scene was familiar, and participants often reported a strong feeling that they had seen the new scene before.

What is going on here?

We have good memory for objects. If you see a familiar object in an unfamiliar setting, you will often recognize that you have seen that object before. For example, if a friend has the same set of dishes that your parents have at their house, you might remark that it is odd that your friends and your parents have similar taste in dishes.

We are not so good at retrieving a memory based just on the configuration of objects. If you are in a place that has some unfamiliar objects, but they are set up similarly to a situation you have experienced before, you will get a feeling of knowing, but you won't actually retrieve any specific memory for the place. That feeling of familiarity is quite helpful, of course. If you walk into a new restaurant, but it is configured like many other restaurants that you have visited in the past, then it is good for you to feel like you are in a familiar place. Your knowledge about restaurants will help you figure out what to do next.

If the configuration is nearly identical to one that you experienced before, though, then you may get a powerful feeling of knowing. That is, you may get a sensation of déjà vu. In the end, though, the experience of déjà vu is just an extreme reaction of the system that your memory uses to tell you that you are in a familiar situation.

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