Eyes on the Brain

A neurobiologist explores the amazing capacity of the brain to rewire itself at any age

Thinking in Three Dimensions

To know is not the same as to see.

In a video about me on the NOVA website, The Secret Life of Scientists, I mention that gaining stereovision was one of the most empowering, liberating experiences of my life. This is a very strong statement to make and one that would certainly have surprised me during my stereoblind years. After all, I had long known the mechanism behind stereopsis, so I thought that I knew what seeing in 3D would be like. But I was wrong.

Here’s just one example:

One common vision therapy tool is the “lifesaver card.” The card provides four pairs of lifesavers, one member of each pair being red, the other green. You can fuse the red and green images in one row either by crossing your eyes or by looking “through” the card. When you first attempt to fuse the images in one row, you may see four lifesavers—two images from each eye. One of the middle images comes from your right eye; the other from your left. With practice, you can fuse the two middle lifesavers into one so that you see three, not four, images.

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When I was first able to do this, I saw the fused lifesaver as sitting on the sheet in between the two outer images. I thought I had successfully completed the task. But then, as my stereovision improved, and I began to see real objects pop out or recede from me in stereo depth, something remarkable happened. One day, as I practiced with the lifesaver card, the fused third image did not appear on the plane of the sheet. If I crossed my eyes, the fused lifesaver popped out toward me, and if I looked through the lifesaver card, the fused image of the lifesaver appeared to be floating behind the sheet! I practically fell out of my chair.

Of course, the fused image should float off the sheet! This view resulted from the merging together of two lifesaver images, one from my right and one from my left eye. Since these two images were cast on slightly different regions of my two retinas, I should see the fused image as floating in empty space rather than on the lifesaver sheet. This is how stereopsis works. I knew this on an intellectual level but had not transferred this knowledge to my own experience with the lifesavers. I had to see the float first and then work backward to explain to myself what I was seeing. With continued practice and experience, the fused image appeared to float at ever greater distances from the card. Even some of the little letters on the lifesaver images popped out or receded in space.

Work with the lifesaver card made me realize just how ignorant I had been, before vision therapy, about this concept of “float” or the way empty space takes on volume. Knowing that objects are separated by volumes of space and perceiving those empty volumes are very different experiences. Seeing the little lifesaver float off the card was one of the many epiphanies that I experienced while learning to see and think in 3D.

Susan R. Barry, Ph.D., is a professor of neurobiology in the Department of Biological Sciences at Mount Holyoke College and the author of Fixing My Gaze (June, 2009).

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