A Surprising Reason Why Two (Coordinated) Eyes Are Better Than One.
Seeing while moving; what eye exams don’t reveal.
Posted Jun 07, 2012
You are driving down the middle lane of a highway, perhaps late at night. The white markers on either side of the lane appear to move faster and spread apart as you drive between them. (See for example this video.) This apparent movement of the lane markers provides a cue called optic flow which helps you to keep your car centered in the middle of the lane. Automatically, you adjust the position of the car so that the lane markers appear to approach in a symmetrical fashion to your right and your left. All that’s needed is for you to keep aiming your two eyes straight ahead.
“Keep looking in the distance,” my husband used to say to me as he rode as a passenger in my car. But for me, looking in the distance was difficult. Since I was cross-eyed, I aimed one eye straight ahead while turning in the other. I rapidly switched which eye did the straight-ahead looking and which eye turned in. As a result, the images moving across my two retinas, such as the images of lane markers, provided confusing information to my brain. I could not make good use of optic flow and was not sure where on the road the car was. What’s worse, when driving along a very straight road, the whole scene would start to “grain out” like static or snow building up on a TV screen. When this happened, I’d shake my head to get a clearer picture. So I avoided highway driving, preferring narrower, (actually more dangerous) two-lane roads. On the narrower roads, I could steer by watching the edge of the road on my right side although this also caused me to veer a little too far to the right. In the end, I found ways to avoid driving altogether.
But then I had kids, and they needed to be chauffeured around town. I had to gain more confidence and competence behind the wheel. After some searching, I found Dr. Theresa Ruggiero, a developmental optometrist, who customized a program of optometric vision therapy that taught me to do what most babies learn in the first months of life – how to aim both eyes at the same point in space at the same time. About six weeks after starting vision therapy, I noticed something surprising. When I walked forward, the objects to my left and right appeared to be moving backward toward me. With more therapy, I noticed the same effect while driving. Since I could now aim both eyes straight ahead, I was able to appreciate optic flow and gain a better sense of the car’s position and speed on the road. What’s more, the ability to aim both eyes at the same spatial location allowed me to develop 3D vision. Although you can use optic flow information with just one eye (if the other eye doesn’t provide conflicting information), the stereovision provided by two eyes makes for even more accurate steering. Driving for me became much easier.
Many doctors and scientists assume that the everyday difficulties encountered by someone with strabismus (crossed eyes or wall-eyes) involve poor static depth perception and thus troubles with parking a car or threading a needle. But these tasks can be accomplished slowly and over many trials. What is much more challenging is making rapid, visually-based decisions, such as occurs with driving. These decisions are hindered when conflicting information comes into the brain from two misaligned eyes. It is harder to see and judge while moving. Vision exams in the eye doctor’s office (with the exception of exams done by developmental/behavioral optometrists) do not generally test how well a person sees while moving or how quickly a patient can make visually-based decisions. Moreover, perceptual tests done by scientists often contrast the ability to perform a task with one eye or two, but less frequently examine what happens when the two eyes don’t work well together. As a result, the wide-ranging effects of strabismus and other binocular disorders on driving and other everyday skills are not generally appreciated.
One day, about a year after starting vision therapy, I was driving with my son along a country road. I was relaxed and, enjoying the scenery. “Mom,” my son said to me, “do you know you’re going 50 miles per hour?” I was going 50 in a 35 mile-per-hour zone. I immediately slowed down but not without the satisfying thought that I had exceeded the speed limit for the very first time.