Susan Barry by Rosalie Winard

Susan R Barry Ph.D.

Eyes on the Brain

Reflections on Christmas Lights

What Christmas lights reveal about our vision.

Posted Dec 22, 2012

The other day, I walked home from work in a heavy fog – that is, the fog was in the air, not in my head. What could have been a gloomy walk felt more like a fantasy. The fog should have made it hard to see, but it’s Christmastime. Trees and bushes along the roadside were covered with lights, and there’s nothing like Christmas lights to remind me that I now see in 3D.

 When I was cross-eyed and stereoblind, before my vision therapy, I would have seen the tree lights in a flat plane. But, on this walk, the lights appeared to float in mid-air, with little pockets of space between each light, and, together, they revealed the three dimensional structure of the tree. Lights strewn along bushes gave me a feel for the bushes’ texture and depth. When I looked through the window of one of the houses, I saw a lit Christmas tree inside. In my stereoblind years, the tree would have appeared in the plane of the window pane. Of course, I would have known that the tree was at some distance behind the window, but I would not have seen the volume of space between the window and the tree.

 It is very hard for someone who has always seen in 3D to imagine what it’s like to be stereoblind. Even if a person with normal binocular vision closes one eye to eliminate stereopsis, their brain will use a lifetime of past stereo views to fill in the missing stereo information. So, as a way of explaining stereoblindness, I ask people to recall how they see themselves in a mirror. Most will respond that they see their reflection behind the plane of the mirror. When I was stereoblind, however, I did not see my reflection behind the mirror; it was right on the plane of the glass. If there was a dirt mark on the mirror, I saw the mark as on my own clothes and tried to rub it off.

 It’s not just my own reflection in a mirror that has taken on a new view. Reflections produced by window glass are a constant source of both confusion and delight. A few years ago, during the Christmas season, I experienced the most wonderful illusion. Christmas lights, that were strung across the inside walls of a restaurant, were reflected by the window onto the street outside. “Why are there lights floating in the middle of the street?” I wondered until I realized that these lights were a reflection of the indoor lights and appeared at a distance from the window on the outside equivalent to their distance from the window on the inside. I would not have seen the reflected lights floating outside in space like that when I was stereoblind; I would have seen them on the plane of the window and would not have been confused. This view made me realize how far I have come with my stereovision – with this new skill, I can now see something that is not real.