In the last post, I showed you that you could see an image of your own finger that you could not touch. By looking in the distance while holding your finger in front of your face, you saw two images of your finger. If you tried to touch each image of the finger, while still looking in the distance, you could only touch one of the images. Only one of the images was "real."
This may seem like a pretty artificial situation, but actually there are plenty of people who suffer from double vision and deal with this problem every day. They may, for example, have developed strabismus or misalignment of their eyes. How do they cope? I posed this question to one of my students, Sarah, who saw with constant double vision. Sarah had just begun optometric vision therapy with the same developmental optometrist who had taught me to straighten my crossed eyes and see in 3D. I took Sarah to her vision therapy sessions each week. As we drove along to the first session, I asked her how many roads she saw. She answered that she saw two roads in front of her. In fact, she saw two of everything: every car, every sign, every telephone pole. One image came from her right eye and one from her left. "Whoa," I thought, "good thing I'm driving."
A few days later, Sarah and I were having coffee together. She told me that in class she saw two of me standing at the blackboard. "How would you shake hands with me?" I wanted to know. She said she would shake the hand that she saw with her right eye. Using the right eye image was automatic, not something she thought about. Then, I asked her,
"Does my voice come from the image of me seen by the right or left eye?"
Sarah told me that the voice came from the image of me seen by her right eye. I was intrigued.
"From which of your two coffee cup images," I asked pointing to her mug on the table, "does the delicious coffee aroma come from."
Sarah thought a moment, "Now that you mention it," she said, "it only comes from the cup image seen by the right eye."
This may seem pretty weird to you, but it makes sense. When most people look at their coffee cup, they aim both eyes at the coffee cup and its image falls on the corresponding, central region of both retinas. Their brain fuses the two coffee cup images into one. But Sarah pointed one eye at the coffee cup and turned the other eye in. The coffee cup image fell on distant, non-corresponding points of the two retinas. As a result, she saw the coffee cup in two places in space. If she was going to pick up the cup, she needed to rely on only one of the images. So even though she was aware of two images, she moved according to what her right eye saw. It was this image that she imbued with non-visual properties such as smells and sounds.
Later that year, I gave a talk about stereovision and I invited Sarah to attend. By this time, she had completed several months of optometric vision therapy. At the end of the evening, I asked Sarah what she thought of the talk. "It was great," she said "and do you know why?"
I felt a surge of joy go through me as I waited for Sarah to answer her own question.
"There was only one of you on the podium," she said.
(Note: Sarah's story as well as stories of other people with different ways of seeing are described in my book Fixing My Gaze.)
