Visual perception involves more than corneas, cones and choroids. According to research out of the University of Virginia, it's also influenced by a number of entirely non-visual factors.
A team of researchers led by psychologist Dennis Proffitt determined that the way humans interpret concrete properties of their environments is dependent on their specific physical and emotional states. The study's findings are published in the June issue of Perspectives on Psychological Science.
"It's a basic common sense assumption that everybody sees the world the same, and they think and reason about it differently; but they don't." Proffitt said. "We all live in a world that's defined by the physical reality that we're in, but also by the body that we live in."
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To test his hypothesis, Proffitt asked participants to estimate the slant of a hill on the UVA campus, while outfitting some of the subjects with a heavy backpack. They found that subjects donning backpacks estimated the slant to be much steeper than did those who had nothing on their backs.
According to Proffitt, these differences in perception have to do with energy efficiency and, by extension, human survival. In order to survive, we weigh the cost of taking an action, like walking up a steep hill, against the amount of energy gained from performing the action.
"The basic task of the brain is to support survival, and, as a hunter-gatherer, that means you need to be very good at acquiring energy at a rate that is faster than you're expending it," he said. "And this all happens without having to think—it happens in perception."
Proffitt adds that physical conditions aren't the only factors that influence perception: Emotions—especially fear—can have an effect, too.
To test the impact of fear, Proffitt and his team asked a group of participants to estimate the same hill's slant from its peak, while standing on a skateboard. Researchers found that anxiety over falling caused most of the subjects to overestimate the incline of the hill.
While energy efficiency was not at the root of perception in this scenario, human survival once again was involved. The cost of falling down the hill would likely be injury or even death, and an exaggerated sense of danger—manifested as an overestimation of the hill's slope—promotes caution and provides an adaptive advantage, Proffitt says.
According to Maggie Shiffrar, a psychology professor at Rutgers University, Proffitt's research challenges long-held assumptions about the mechanism behind perception; namely, that it is not influenced by the actions a person plans to take in the future.
"[Proffitt's] work reminds us that the brain is connected to the body," she said via e-mail. "I know that sounds pretty simple-minded, but it is surprisingly radical."
Shiffrar's colleague in Rutgers' psychology department, Alan Gilchrist, points to several facts that challenge Proffitt's theory that physical or emotional circumstances influence visual perception. He references research that indicates that vision in most animals, including frogs and goldfish, works very similarly to vision in humans. He also cited evidence suggesting that newborn babies perceive the world in the same manner as adults.
Jessi Witt, a graduate student in Proffitt's lab, says that despite evidence to the contrary, her group's findings are very logical.
"I think it makes sense that perception reflects the environment as it relates to the perceiver," she said.