ALL OF US, even postmodern philosophers, are naive realists at heart. We assume that the external world maps perfectly onto our internal view of it—an expectation that is reinforced by daily experience. I see a coffee mug on the table, reach for a sip and, lo and behold, the vessel’s handle is soon in my grasp as I gingerly imbibe the hot liquid. Or I see a chartreuse-yellow tennis ball on the lawn, pick it up and throw it. Reassuringly, my dog appears to share my veridical view of reality: she chases the ball and triumphantly catches it between her jaws.
That there should be a match between perception and reality is not surprising, because evolution ruthlessly eliminates the unfit. If you routinely misperceive or even hallucinate and act on those misapprehensions, you won’t survive long in a world filled with dangers whose avoidance requires accurate distance and speed assessments and rapid reactions. Whether you are diving into rocky waters or driving on a narrow, two-lane road with cars whizzing by in the opposite direction, small mistakes can be lethal.
You probably believe that your eyes register high-fidelity information about the absolute size, speed and distance of visible objects and that you respond based on these impartial data. But although we build robots in this manner—equipping them with sensors and computers to plumb the metric properties of their environments—evolution has taken a more complex route.
As psychologists and neuroscientists have discovered over the past several decades, our consciousness provides a stable interface to a dizzyingly rich sensory world. Underneath this interface lurk two vision systems that work in parallel. Both are fed by the same two sensors, the eyeballs, yet they serve different functions. One system is responsible for visual perception and is necessary for identifying objects—such as approaching cars and potential mates—independent of their apparent size or location in our visual field. The other is responsible for action: it transforms visual input into the movements of our eyes, hands and legs. We consciously experience only the former, but we depend for our survival on both.
When driving in the mountains, have you ever noticed a discrepancy between the slope described on the yellow road sign and your sense that the incline is actually much steeper? Psychologist Dennis R. Proffitt of the University of Virginia and his then graduate student Jessica Witt did. Being scientists and not philosophers, they designed an experiment to find out why. Proffitt and Witt stood at the base of hills on campus and asked passing students to estimate their steepness in two ways. Subjects had to align the diameter line on a flat disk to the slant of the hill. They also were asked to place the palm of one hand on a movable board that was mounted on a tripod and then, without looking at that hand, to adjust the board’s slant until they felt it matched that of the hill.
In the first part of the test, which relied on visual cues alone, subjects badly overestimated, interpreting a 31-degree slant as a much steeper, 50-degree one. But when people’s eyes were guiding their hands, subjects judged accurately, tilting the board an appropriate amount. Perhaps even more striking was the finding that people’s tendency to overestimate on the strictly visual part of the test increased by more than a third when they had just run an exhausting race—but the hand estimates were unaffected. The same discrepancy occurred when subjects wore a heavy backpack, were elderly, or were in poor physical condition or declining health.
In another variant of the experiment, Proffitt had subjects stand on top of a hill on either a skateboard or a wooden box the same height as the skateboard. Participants were instructed to look down the hill and judge, both visually and manually, its grade. They were also asked how afraid they felt to descend the hill. Fearful participants standing on the skateboard judged the hill to be steeper than did the braver souls standing on the box. Yet the visually guided action measurement was unaffected by fear.