Somethink to Chew On: stereo vision and stereo photos

Oliver Sacks (author of Awakenings and The Man Who Mistook His Wife for a Hat, among others), wrote a great essay in the latest New Yorker about stereo vision. Unfortunately, the essay is not available online, but a brief letter to the editor of Nature is available. The essay is about stereo vision, which is a property of our having two eyes, rather than one. In addition to other cues about the physical position of objects (focus, occlusion, size, motion parallax), we can directly tell the distance of objects up to a certain distance away (I think I read maybe 50 feet?) by the angle of our two eyes relative to each other as they focus on an object. After reviewing the history of how people discovered this property in the 19th century, and the stereo photograph craze of the Victorian era, Sacks then focuses on a unique medical case.

Many skills have what is known as a critical or sensitive period. If you don't learn a language in the first 5 years or so of life, you'll never really be able to learn one. If you're a kitten raised in the darkness for the first couple months of life, you'll never be able to really see, even though your eyes may be perfectly normal. The brain self-organizes when it's young, and if it doesn't have the right kind of stimulus during early development, it usually can't develop it later on.

The case that Sacks reports on may (or may not) be an interesting exception. A woman was born with severely uncoordinated eyes. Both eyes worked fine, but they didn't work together at all. She could see, but she didn't have stereo vision. (She may have had stereo vision for things a couple inches in front of her face, which is why it's not clear to what extent this case is as remarkable as it seems.) Her perception was very much like it would be as if she wore an eye patch over one eye. When she was an adult, a few years back, she started having worse problems with her eyes and went to a doctor who prescribed prism glasses that would help her eyes work together better. After just a few weeks of simple exercises, she developed stereo vision, a skill she (probably) never had, and a skill that, according to theories of critical periods, she should not have been able to develop as an adult. The woman writes eloquently about the experience:

"After almost three years," she wrote, "my new vision continues to surprise and delight me. One winter day, I was racing from the classroom to the deli for a quick lunch. After taking only a few steps from the classroom bulidng, I stopped short. The snow was falling lazily around me in large, wet flakes. I could see the space between each flake, and all the flakes together produces a beautiful three-dimensional dance. In the past, the snow would have appeared to fall in a flat sheet in one plane slightly in front of me. I would have felt like I was looking in on the snowfall. But now, I felt myself within the snowfall, among the snow flakes. Lunch forgotten, I watched the snow fall for several minutes, and, as I watched, I was overcome with a deep sense of joy. A snowfall can be quite beautiful -- especially when you see it for the first time."

It almost unimaginable to, well, imagine what it must be like to experience such a sense for the first time. Like imagining color if you've never seen red. But this particular experience, going from monocular to stereo vision, can be easily reproduced in photographs. It's easy to take a stereo photograph with a decent camera. Just set the focus and exposure to manual, so you're taking two photographs with all the same settings. Then, take one photo while leaning on one leg, then do a little "cha-cha" step to move the camera laterally a few inches, and take a second photo while leaning on the other leg. This works very well with architecture and sculpture and things that aren't going to move in between the exposures. Here are some 3-D photos I've taken over the years. To view them, just let your eyes diverge, like those random-dot stereographs that were popular about 10 years ago, and let the two images in the middle line up.