This morning I went into the darkest room in our house (the kids’ bathroom), closed the door, and turned off the lights for 5 minutes. There was enough light coming in through the crack in the door that after a minute or two I could begin to make out shapes in the room: A towel rack, the shower curtain. My eyes had adapted to the dark condition. Then I closed my right eye and covered it with my hand. I turned the lights back on, for a minute, until my left eye had adapted to the light. Then I turned the lights off.
I could still see the towel rack and shower curtain with my right eye, which remained adapted to darkness. But my left eye could see nothing. In fact, my left eye felt as if it was closed. I made every effort to open the eye, but it seemed that some unstoppable force was keeping it closed. The only way to make my eye feel as if it was open was to cover it with my hand. I still couldn’t see anything with the eye, but at least I could convince myself it was open.
What I was experiencing was a fascinating illusion discovered by Uta Wolfe and her colleagues. When one eye is adapted to darkness and the other is not, then in a dark room, the light-adapted eye will feel as if it is at least partially closed. You can try it out for yourself, just as I did (You might want to bring an iPod [with screen dimmed] or some other form of non-visual entertainment — five minutes in the dark is a long time!). I’ll include a poll later on so we can see if our readers experienced the effect.
Although I experienced the effect after just 5 minutes, Wolfe’s team actually asked student volunteers to adapt to darkness for 30 minutes. A key to the experiment is that the room should not be completely dark. They used adjustable fluorescent lights to dim the room to 0.005 fc (footcandles — compare to about 30 fc in typical indoor lighting). Then the students closed one eye and covered it with their hand while the experimenter increased the lighting to 2 fc for one minute. The lights were dimmed again and the students uncovered their eyes. Then they rated the sensation in the light-adapted eye (the one that had NOT been covered and therefore was no longer adapted to darkness). Here are the results:
Thirteen of the fourteen students tested experienced the illusion, saying that the eye that had adapted to light felt like it was sagging, closed, and “droopy,” compared to the other eye. So what’s going on here?
Wolfe et al. say that the effect is related to other somatosensory illusions, such as the “phantom leg” effect that amputees feel. A milder version of this effect can be experienced by most people: if you see a rubber hand in a spot where your hand could be (e.g. in front of a screen that hides your hand), then if someone scratches the rubber hand, you feel your own hand being scratched. In fact, for a wide variety of physical sensations, when the visual sense contradicts the touch sense, usually vision will win out.
That appears to be what is going on in Wolfe’s team’s illusion. You don’t normally see your eyelid, so the only visual confirmation you get that your eye is closed is that things appear darker. The illusion takes advantage of the fact that everything appears darker when you first enter a darkened room. After a few minutes, your eyes adjust and you can see again. In the illusion, only one eye is adapted to the dark, and so while one eye sees normally in the darkened room, everything seems dark in the other eye. Even though our tactile sense tells us the eye is open, vision trumps touch, and we experience the illusion of the eye being closed.
In a second experiment, people who experienced the illusion covered up the light-adapted eye. The illusion disappeared, and the eye felt as if it was open again. Covering the dark-adapted eye also caused the illusion to disappear. Why?
Now the sense of touch and vision were aligned again. We expect to not be able to see when our eye is covered, so it makes sense that the eye could be open. When the dark-adapted eye was covered, neither eye could see anything, so visual and somatosensory systems were again aligned.
In a final experiment, people experiencing the illusion covered the light-adapted eye with a clear disk (they were unaware the disk was clear). The illusion disappeared, even though the visual input didn’t change at all. But when the dark-adapted eye was covered with a clear disk, the illusion did not disappear: at this point, the participants could still see out of the dark-adapted eye, and could not see out of the light-adapted eye, so the illusion persisted, as before. Fascinating stuff.
So, let’s see how many of our readers are willing to try this effect on themselves. It’s important to not be in a completely dark room, and not to look directly at light sources (even faint ones) while adapting to darkness. If you have five minutes to try this out, you’ll experience a truly extraordinary sensation. You can report your results in the poll below.
Wolfe, U., Comee, J.A., Sherman, B.S. (2007). Feeling darkness: A visually induced somatosensory illusion. Perception & Psychophysics, 69(6), 879-886.