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Seeing in Depth

September 17, 2011

c. 1987

An apple sits on my desk rather precariously balanced atop my grandmother’s recipe box. The apple is a Cortland, more green than red, a second by virtue of its misshapen sides, a remnant of the half-peck bought in Wisconsin six weeks ago, now mostly vanished by way of applesauce, apple crisp, and picnicking.

Without touching it I know that this apple is round (relatively speaking); that I could pinch its stem between my thumb and first finger and pick it up; that the fruit would snug into the curve of my palm; that my teeth could bite into it and taste apple, not Kodak paper.

My eyes were crossed when I was born, both pupils drawn protectively toward my nose by too-short muscles. My eyes stared in opposite directions, transmitting two very different visions of the world to my brain. By the time surgeons were done trying to correct my eye muscles, my brain had already learned to ignore the stimuli from one eye or the other rather than try to reconcile images as disparate as our family’s poodle on my left with the toy in my right hand.

Even as an adult I am able to see with only one eye at a time, a phenomenon called monocular vision.

People’s first reaction to the idea of monocular vision is a flash of pity followed closely by curiosity. Almost invariably listeners ask, “So everything you see is flat? Like a picture?” Their eyes drift to the nearest painting or photograph, dwelling there momentarily to gather some insight before turning their gaze back to me, glancing (casually, they hope) at my eyes with new knowledge while waiting for me to answer the question.

“No,” I reply. “The only things that look flat to me are flat things. Most people are born with the ability to develop depth perception. Mine never developed. But I’ve learned to compensate so well that I really only miss being able to see in depth during broom ball season.”

Physiologically, depth perception in humans arises from the fact that two eyes are set in close proximity on the face.

Even though humans have two ears, they are located on opposite sides of the head. This means a trained listener can discriminate between sounds heard simultaneously in opposite ears. The sound of traffic predominates in the ear closest to the street while a NPR headlines register in the ear directed toward the radio.

Typically directed eyes, however, see the same object from perspectives that are only slightly different. These nearly identical pictures converge in the occipital lobe at the back of the brain where the visual cortex reconciles the differences and produces the perception of a single, unified object.

The left cornea transmits the image of a Cortland apple, and the right cornea, an image of the same apple seen from 30 degrees to the right, across the bridge of the nose. The brain reconciles the two images and identifies one apple balancing atop a recipe box 15 degrees to the right of the left eye and 15 degrees to the left of the right, or as seen from an imaginary single eye with 3-D powers located above the bridge of the nose between the eyebrows.

In combining the two perspectives, the brain remembers having seen 15 degrees around each side of the apple unaccounted for in the visualized image. These ‘missing’ degrees of sight are experienced as depth perception.

My occipital lobe works fine. Both eyes work fine. My brain just missed the critical window for learning to coordinate input from both eyes simultaneously in order to see depth.

Even so, I perceive depth. I know my apple is round because I placed it on top of the recipe box yesterday and I have been experiencing apples –holding them, eating them, painting them –all my life. My apple is round because dusk has fallen and the light of my desk lamp casts a pale shadow of apple that spills off the box and puddles on my desk. I don’t need depth perception to tell me a flat apple would cast a flat shadow. Apples are round because apples are round.

Please don’t pity me because you cannot imagine the world appearing in 2-D. The limitation may be your imagination; it is not mine.

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