Eyewitness testimony often contributes heavily to criminal cases. In situations where the event was not caught on video or otherwise recorded, what someone saw at the scene may be the only account of what took place — at least, the only account that does not come from the person accused of breaking the law.

But is that testimony reliable? Can you count on it? That may be a bit less clear.

Researchers have found that people change their own memories simply through the act of recalling them. They can influence those memories with new thoughts and ideas. It alters them slightly. The next time they bring the memory up, they do it again. Eventually, after thinking about it enough times, they could have changed the memory to become something they never saw in real life.

“Say you’ve been questioned by an investigator and you recall the event,” explained one researcher. “In the next 15-20 minutes, you could run into another eyewitness or overhear investigators talking to each other. Some inaccurate information could update your memory.”

When you keep updating the memory like this, it twists it into something new. However, you do not necessarily realize that it is happening, and you may no longer be able to bring up the original memory. To you, the new one feels real and you may be very convinced that it is.

In criminal cases, a convincing witness can pass the eye test and a lie detector test, but that doesn’t mean they are necessarily right. You have to know all of your legal defense options in this delicate situation.