Do you remember what your mom looked like when you were 4?

Are you sure?

A study published today in the Journal of Neuroscience sheds new light on when memories remain stable and when they get overwritten with new information.

Lead author Donna Jo Bridge, a postdoctoral fellow in medical social sciences at Northwestern University Feinberg School of Medicine, and colleagues guided 17 participants through an experiment involving remembering where objects were placed on a computer screen with different backgrounds.

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Participants were asked to try to remember where an object was on the original background and place it in the same spot on a new screen. Time after time, they got it wrong.

Then, when they were shown the object in three different locations on the original screen and asked to place it where they first saw it, they couldn't do that, either. They put the object where they themselves had placed it.

But, when researchers changed the experiment slightly, results improved: Researchers told the participants to put the object in a new location -- NOT the original spot. Somehow, this triggered the subjects to remember the original location.

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That's an important distinction, Bridge said, in cases when you want to preserve original memories. Somehow, the hippocampus (an area of the brain critically important to memory) is deciding what's important and either building on the original memory with new information or modifying the original memory.

The participants recalled the original location, only in context of being told not to place the object there -- when a layer of instruction about that original spot was added.

"The most important function of memory is to guide future decisions and inform the present," Bridge said. "We know that memory is adaptive, changing with new circumstances, and we think it's a good thing that it doesn't stay crystallized."

Think about an ex-boyfriend or girlfriend, Bridge said. If you remembered all the positives about that person, it would be hard to move on and be happy. Instead, our memory may help us get past breakups by revising our history with the person.

The MRI scan used in the study shows that the hippocampus plays an important role both in when memories stay stable and when they're more likely to change, said Tali Sharot, principal investigator at the Affective Brain Lab at the University College London, who was not involved in the study.

"If they're able to identify the mechanisms that result in memory change or stability, there would be real life applications," Sharot said. "We know eyewitness testimony is many times inaccurate, so if the system knew what was more likely to make it change or stay stable, that would be important to know."

Eyewitness testimony, even when given under sworn oath with the best of intentions, can't compete with a video for factual accuracy.

Same with that memory of your mom, Bridge said: the image in your brain has likely been overwritten many times. Flip through a photo album to see how your mom really looked in 1974.