(ISNS) – When your job is to be vigilant and spot things that are out of place, as is the case for an airport security screener or a film editor, you are trained to have laser-sharp attention to detail. If most people noticed every little change in their field of view at every moment of every day, they might go insane.
Then there are the people who can watch a movie and notice that in one scene the top button on a character's shirt is buttoned, and in the next scene it isn't. These are called "continuity errors" because they break the audience's attention and the illusion of realism. One famous example is in the 1990 film "Pretty Woman." In one scene Julia Roberts is eating a pancake that in the next shot it turns into a croissant and then back to a pancake.
The goal of any good film is to immerse the audience in the story and suspend their disbelief. "The editor’s job is to make sure that any breaks in continuity are invisible enough that they do not disturb the audience’s involvement in the story," said Norman Hollyn, a professor of cinematic arts at the University of Southern California in Los Angeles. "There are a multitude of problems that come from the fact that we must assemble a film from pieces shot at different times and some of the problems have to do with performances that vary from day to day."
Film editors can use these visual cues to their advantage. "If actors are angrier in a close-up than a wider shot, if we cut to them as they are standing up, the audience will tend to allow the change more easily," said Hollyn. "We use many different techniques to disguise these mismatches and distractions and we use the fact that, like magic, audiences tend to be distracted by differences in size, color and movement."
According to scientists, this idea also holds true in everyday life. "How an object looks at the present moment is biased toward what it looked like in the recent past and we refer to that bias as 'perceptual serial dependence,'" said Jason Fischer, who completed research on this topic while at the University of California, Berkeley and is currently a postdoctoral fellow at MIT. "We do not perceive a fresh snapshot at every moment."
A paper based on the research was published on March 30, in Nature Neuroscience.
Visual scientists have discovered what they call a "continuity field," which refers to how far an object can move from one moment to the next and still have perceptual serial dependence.
"For example, imagine a circular region of space around a coffee cup on your desk," said Fischer. "If the coffee cup was located somewhere within that circular region -- or continuity field -- five or ten seconds ago, then your perception of the cup at this moment will be biased toward what it looked like at those previous times."
Move the cup out of the continuity field and your brain sees the cup as a "new" object with no bias. The time between glances at an area is also a factor.
"We found that things seen up to about 15 seconds ago can still bias perception at the present moment," said Fischer. "Something seen 10 seconds ago will have an even stronger influence on our present perception, but something seen 30 seconds ago will not have any influence on our present perception."
Our visual systems are constantly trying to strike the perfect balance between what we actually see and what is important for us to see. "We are sensitive to important changes, but not so sensitive that we notice every minor fluctuation," said Fischer. "Continuity fields stabilize our visual experience by obscuring those minor changes that are irrelevant most of the time."
Without a continuity field, we would be sensitive to every fluctuation in the image that we see and our visual experience would appear to be a constant hallucinogenic trip. For example, if you and a friend walked along a tree-lined street together, your friend would pass in and out of the sunlight and shadows from the trees, and his or her face would constantly change in appearance.
"That would be a jarring experience," said Fischer. "Your friend should look like the same person from one moment to the next."
This stabilization makes sense when observing the real world where there is constant visual stimulation all around us, but in a dark movie theater, viewers are focused -- for the most part -- on the screen. So why do so many people miss continuity errors in films?
"There is evidence that people are blind to lots of film edits," said Joseph Magliano, a research psychologist at Northern Illinois University, in DeKalb, Ill. "People's attention is focused on making sense of the things that they are experiencing but in rare circumstance they will notice errors."
When a film editor uses a strategy such as cutting to a new shot of the actor outside of the continuity field, the majority of the audience would not notice a change. "I think this speaks to the delicate balance that the visual system achieves between stable perception and sensitivity to changes," said Fischer. "Serial dependence in perception occurs in just the right measure so that we enjoy stable, fluid vision without missing most of the important changes in the world."
The next time a friend points out a continuity error in a film that you missed, it means that your eyes and brain are doing exactly what they are supposed to do.