But simple optical illusions (or looking at “The Dress”) quickly reveal that your brain sometimes alters what you perceive based on context. Two lines of the same size can appear to be different, depending on the other lines that frame them. Your brain can fill in visual information that falls into your eye’s blind spot. If you look at “The Dress” and your brain thinks it’s cast in shadow, you’ll find it to be a different color than a friend standing next to you—even though you’re both receiving the same sensory information.
“People tend to have this impression, first of all, that they see things accurately, that they see the world as it really is,” says Jessica Witt, a cognitive psychology researcher at Colorado State University. “And there’s a lot of research in vision to show that’s not true.”
Witt started working on visual perception with psychologist Dennis R. Proffitt at the University of Virginia, where Proffitt was finding that people often overestimate how steep hills are—and do so even more if they are tired, elderly, out of shape, or wearing a heavy backpack. He found that if people had just gone for a run, or were positioned at the top of a hill on a skateboard, they were also more likely to report seeing a hill as steeper than it really was. Their findings seemed to be showing that actions, past and potential future ones, could impact what people saw.
Witt has continued, over the past 15 years, to study the effects of actions on perception, turning at first to athletes, who regularly combine the two. For a 2005 study, she went to softball fields with a poster board with different-sized circles on it, asking players to pick the circle that matched the size of the ball. She found that the players with better batting averages reported the ball as bigger, and those who didn’t play as well that day thought the ball was smaller than it really was. In a similar study with golf players, she found that golfers who were playing better judged the size of the holes to be larger.
It’s not always that things look larger, she says—tennis players returning more hits judged the nets to be lower, and the ball to be moving more slowly. Obstacle courses look smaller for those who have more experience with them. When she studied athletes kicking field goals, she found that those who kicked more goals perceived the height of the cross bar to be shorter compared to those who kicked less successfully. At the same time, they perceived the uprights to be farther apart. If a participant kicked the ball too wide, they perceived the uprights as narrower, and if they missed because they couldn’t kick it high enough, they saw the cross guards as taller. Perception, she says, seemed to be link to the specific actions an individual was taking.
She’s applied this finding to non-athletic people as well, showing that giving people a tool to reach for something can decrease a perceived distance, giving flippers to swimmers made underwater targets appear closer, and that chronic pain and being obese can both increase your perception of distance.
Witt thinks that actions provide a filter through which the brain processes visual information. “If you were looking at the world through the context of ‘I’m tired, my energies are depleted,’ then the brain produces a very different kind of visual, a different perception, than if you feel high energy and you have all those energetic resources available to you,” she says. Witt says that in many cases—like for steep hills, and perceived distances—the brain may be trying to conserve that energy. Studies have shown that when stairs are perceived as steeper, people are more likely to find an alternative, like an escalator.
“Another way to think of it, is the visual system is incorporating action to help make its best guess,” she says. “We don’t really know how steep that hill is, but I’m going to make my best guess. And my guess is going to be a lot steeper when I’m tired.”