In 2010, I worked up a post on what I dubbed The Spelunker Illusion (see also the last endnote of my 2011 book). Now, hot off the press at Psychological Science, Kevin Dieter and colleagues offer empirical confirmation.
The Spelunker Illusion, well-known among cave explorers, is this: In absolute darkness, you wave your hand before your eyes. Many people report seeing the motion of the hand, despite the absolute darkness. If a friend waves her hand in front of your face, you don't see it.
I see three possible explanations:
(1.) The brain's motor output and your own proprioceptive input create hints of visual experience of hand motion.
(2.) Since you know you are moving your hand, you interpret low-level sensory noise in conformity with your knowledge that your hand is in such-and-such a place, moving in such-and-such a way, much as you might see a meaningful shape in a random splash of line segments.
(3.) There is no visual experience of motion at all, but you mistakenly think there is such experience because you expect there to be. (Yes, I think you can be radically wrong about your own stream of sensory experience.)
Dieter and colleagues had participants wave their hands in front of their faces while blindfolded. About a third reported seeing motion. (None reported seeing motion when the experimenter waved his hand before the participants.) Dieter and colleagues add two interesting twists: One is that they add a condition in which participants wave a cardboard silhouette of a hand rather than the hand itself. Under these conditions the effect remains, almost as strong as when the hand itself is waved. The other twist is that they track participants' eye movements.
Eye movements tend to be jerky, jumping around the scene. One exception to this, however, is smooth pursuit, when one stabilizes ones gaze on a moving object. This is not under voluntary control: Without an object to track, most people cannot move their eyes smoothly even if they try. In 1997, Katsumi Watanabe and Shinsuke Shimojo found that although people had trouble smoothly moving their eyes in total darkness, they could do so if they were trying to track their ("invisible") hand motion in darkness. Dieter and colleagues confirmed smooth hand-tracking in blindfolded participants and, strikingly, found that participants who reported sensations of hand motion were able to move their eyes much more smoothly than those who reported no sensations of motion.
I'm a big fan of corroborating subjective reports about consciousness with behavioral measures that are difficult to fake, so I love this eye-tracking measure. I believe that it speaks pretty clearly against hypothesis (3) above.
Dieter and colleagues embrace hypothesis (1): Participants have actual visual experience of their hands, caused by some combination of proprioceptive inputs and efferent copies of their motor outputs. However, it's not clear to me that we should exclude hypothesis (2). And (1) and (2) are, I think, different. People's experience in darkness is not merely blank or pure black, but contains a certain amount (perhaps a lot) of noise. Hypothesis (2) is that the effect arises "top down", as it were, from one's high-level knowledge of the position of one's hand. This top-down knowledge then allows you to experience that noisy buzz as containing motion -- perhaps changing the buzz itself, or perhaps not. (As long as one can find a few pieces of motion in the noise to string together, one might even fairly smoothly track that motion with one's eyes.)
Here's one way to start to pull (1) apart from (2): Have someone else move your hand in front of your face, so that your hand motion is passive. Although this won't eliminate proprioceptive knowledge of one's hand position, it should eliminate the cues from motor output. If efferent copies of motor output drive the Spelunker Illusion, then the Spelunker Illusion should disappear in this condition.
Another possibility: Familiarize participants with a swinging pendulum synchronized with a sound, then suddenly darken the room. If hypothesis (2) is correct and the sound is suggestive enough of the pendulum's exact position, perhaps participants will report still visually experiencing that motion.