Eyes on the Brain

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

Visual Snow

Seeing something that may not be there.

Do you experience visual snow? It’s as if you are looking at the world through the static or “snow” on a TV set. Some people experience it all the time. I never see visual snow outside in bright light, but I think I do experience it sometimes when looking at a blank wall, especially in dim light. So, in the last two days, I’ve been experimenting with this phenomenon.

Yesterday, I woke up before dawn. When I opened my eyes in the dim light, visual snow appeared everywhere, looking like little sparkles dancing in mid-air. I got out of bed and headed downstairs in the very dim light. When I got to the kitchen, I switched on the light. Bam! Suddenly, the room was filled with light, and it took me a few moments to adjust. In the light, I saw everything clearly, without a trace of visual snow. Curious, I turned the light off. Would the visual snow reappear? It did not, at least not initially. I saw inky blackness, though, over the next minute or so, the visual snow began to re-emerge and grow stronger. This made me wonder if the visual snow originated in my retina or deeper in my brain.

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So this morning, I woke up again before dawn, opened my eyes, and saw intense visual snow. Again, I made my way, in the dim, predawn light, down to the kitchen and made sure that I saw visual snow through either eye. Then, before turning on the kitchen light, I closed one eye hard and put my hand over that eye. On went the light. I could see the room clearly through the open eye with only a hint of visual snow. I turned off the light again and looked around with the one open eye. The visual snow was gone. Then I closed that eye and opened the other one. Through the previously closed eye, the one that had not been exposed to the bright light, I saw plenty of visual snow.

With this little experiment, I concluded that the origin of the visual snow, for me anyway, was in the eye itself. Perhaps, in the dark, the sensitivity of the retina is turned way up in order to see in very dim light. This may explain why I see the most visual snow when I’ve been asleep for a while in a dark room and then wake up while it’s still quite dark. Under these conditions, my eyes have been dark adapted for many hours, and the retina may be at its most light-sensitive.

But I am still left with lots of questions. Are the sparkles of visual snow the result of photons in the dim light hitting different visual pigment molecules in my photoreceptors or does the visual snow result from spontaneous background retinal activity? Did turning on the bright kitchen light eliminate the visual snow for the open eye by bleaching visual pigments? Why do some people see visual snow continuously, even in bright daylight? Perhaps, there is more than one type or cause of visual snow.

 

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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