Have you ever seen the movie North by Northwest?

There's a moment, in the background, where a little boy plugs his ears right before a gun goes off. And in Pretty Woman, Julia Roberts is eating a croissant in one shot and a pancake in the next.

Little mistakes like these are in almost every movie you've ever seen, but you've probably never noticed them— even though they're right there on the screen in front of you. It's weird. You'd think noticing things that are out of place would be an evolutionary advantage.

But it turns out that our brains aren't actually that good at perceiving things. It's more than your eyes recording whatever's going on in front of them— your brain has to interpret the image they produce. And when it prioritizes what it thinks is important, sometimes you don't notice what's right in front of your face.

You can miss more than just small mistakes:. In a famous experiment conducted in 1999, subjects watched a short video of two groups of people tossing around a basketball. Some wore white clothes, and the others wore black clothes.

The subjects were asked to count the number of times the players wearing a particular color passed the ball. But while they were busy counting, a person wearing a gorilla suit walked right through the shot, thumped her chest, and walked off! About half of the subjects didn't notice the gorilla walk through at all.

If you've taken the test and didn't notice it either, don't feel bad— your brain was busy at the time. When you don't notice an unexpected change because you're focusing on something else, that's called inattentional blindness. And it can tell you a lot about how our brains process the world around us.

When you're focusing on something, your brain will automatically filter out information it thinks is irrelevant. So when you're concentrating on counting people passing a basketball, your brain ignores everything else— even a gorilla walking across the screen. Something similar happens when you're watching a movie where there's a mistake: your brain focuses your attention on the most important things, and a little boy plugging his ears in the back of the room isn't as important as the main character in mortal danger right in front of you.

Your brain kind of has to pick something to focus on, because even though our eyes have a pretty wide field of view, they can only focus on a very small area at a time. Eye tracking experiments have shown that we tend to focus our gaze on other people's hands and faces— two huge sources of information when we communicate with each other. And yet, in Pretty Woman, Julia Roberts was holding a croissant in her hand in one shot, which turned into a pancake in the next.

If your brain considers hands and faces important, you'd think you'd notice something changing in her hand. But you don't, because your brain also tries to correct for continuity errors. In 2014, researchers at UC Berkeley looked into this by asking subjects to match the orientation of a white bar to a black bar that flashed on the screen, over and over again.

But the subjects weren't very good at matching the angles of the bars. They tended to choose an angle that was an average of the three most recent bars they'd seen. The team suggested that this mistake is your brain trying to make sure your experience of reality has the same kind of continuity that you'd expect from a movie.

Sometimes, that involves blending your experience over about 15 seconds to be as smooth as possible, in what's known as a continuity field. So if the input from your eyes shows Julia Roberts' breakfast changing between shots, your brain will pretend it's just a glitch in the Matrix and ignore it to keep you from being confused. Focusing like this can be really helpful when, say, you're trying to have a conversation with someone at a crowded, noisy party, or follow the story and characters in a convoluted spy thriller.

But it also means that sometimes you'll miss things that are right in front of you. It's a little unsettling that you can miss something as significant as a gorilla walking across your field of vision, and it's important to know that our brains work that way, especially when it comes to things like eyewitness testimony in court cases. But it's normal not to notice little mistakes in movies, or even big, unexpected things.

Your brain is just trying to make you see what it thinks you're supposed to see. Thanks for focusing your attention on this episode of SciShow Psych, and thanks to our patrons on Patreon for making this show possible. For more on weird human quirks, you can go to youtube.com/scishowpsych and subscribe.