Going the Distance Takes More Than Meets the Eye

How do humans perceive distance? To say “with our eyes” seems obvious. But when gauging the distance to a destination one is walking or throwing something to — or performing any of the other countless daily activities that require knowing how far away things are — there’s much more to it than meets the eye.

New research in perceptual psychology is revealing that when we mentally judge distances, our eyes have help from the rest of our body, as well as from our emotions. There are also situations, such as when braking to avoid a collision on the highway, in which we may not actually even gauge distance at all.

In his perception laboratory at the University of Virginia, Dennis R. Proffitt has done a number of studies revealing that distance perception is a complex process in which people’s behavioral goals, as well as their emotions and physiological state, affect how far away things appear. His work leads him to suggest that what he calls the “embodiment of perception” has played an adaptive role in human evolution.

A series of studies conducted in Proffitt’s laboratory have shown that the amount of effort required to walk to a destination or throw an object to a specific target affects how far away the destination or target is perceived to be. Apparent distance decreases with fitness and increases with fatigue. A destination will also appear farther away after a person walks on a treadmill (due to the temporary illusion, produced by treadmill walking, that it takes effort just to stay in the same spot). Similarly, a person throwing a heavy object will perceive the target to be farther away than a person throwing a light object.

This finding also extends to the perceived slant of a hill: A person wearing a heavy backpack will view a hill as steeper than will someone who is unencumbered.

Interestingly, Proffitt has found that only the specific locomotor system (e.g., walking or throwing) involved in the task to be performed affects judgment of distance. Walking on a treadmill only increases apparent distance when one plans to walk to the target, not when one plans to throw an object there. According to Proffitt, “These studies show that people view intervening distances as ‘walkers’ or ‘throwers’ and that perceived distances are influenced by the energy required to perform these actions.”

A person’s attitudes or emotions can also affect their perception of distance. Study participants who listened to a minor-key (i.e., “sad”) piece by Mahler reported that a hill was steeper than those who listened to a major-key (i.e., “happy”) piece by Mozart. And research conducted by one of Proffitt’s doctoral students has shown that people tend to greatly overestimate a platform’s height off the ground when they look down from it (more so than when they look up at the platform from the ground) and that the apparent distance correlates with the person’s anxiety about falling (i.e., fear of heights).

Proffitt suggests that because emotional and energetic factors affect distance perception, the visual-perception system may play an adaptive role by helping us make cost/benefit judgments. Visual perception, Proffitt writes, “promotes survival by making people aware of both the opportunities and the costs associated with action.”

But other research is finding that, in many situations, the visual-perception system helps guide our actions without requiring us to mentally calculate how far we are away from objects at all.

Take, for example, baseball players catching fly balls. They need to get to the right place to make the catch, but how do they know where that will be? Researchers have suggested that the players don’t actually need to mentally calculate the distance to their target, but simply run on a trajectory so that the ball follows, to their eyes, a straight (rather than curved) path as it rises and falls.

Other studies have shown that something similar occurs when driving: If a car brakes on the road ahead, a driver knows when and how quickly to slow her own vehicle to avoid a collision, not by making a mental calculation of distance (such a calculation would be time-consuming, neurologically speaking), but simply by relating the projected size of the other car to the rate that its apparent size increases in her visual field.

In other words, the distance-perception system helps people assess the costs and benefits of actions without the need to explicitly calculate energy and distance. “A principle function of perception,” Proffitt writes, “is to defend people from having to think.”

To learn more about how (and when) people perceive distance, see “Distance Perception” in the June issue of Current Directions in Psychological Science; to learn more about the adaptive role of distance perception, see “Embodied Perception and the Economy of Action” in the June issue of Perspectives on Psychological Science.