S&P: Shots in the dark

It is said, but only Allah knows all, that "when there is a conflict between vision and other sensations, vision usually dominates – a phenomenon called visual capture."

You can demonstrate a remarkable exception through a simple procedure. Take a flash camera into a room with patterned wallpaper or a picture on the wall. It should be a room that can be darkened (an interior bathroom with a towel against the bottom crack is ideal). Turn off the lights and dark adapt for 5 minutes. Now, while holding your arm outstretched with an open hand toward the wallpaper or picture, aim and click the camera so the flash hits your hand. In a second or two, you will see a strong positive afterimage of your hand against the wallpaper. Now, move your hand downward. What did you see? What do you think happened?

When I carried out the kinesthetic capture observation above (not my design, but that of J. Seiver, or whoever JS got it from), I did as instructed, using a flashbulb to illuminate my hand against a visually interesting background. Here's what I observed:

In the moment of the flash, I didn't see much except "wow, that's bright!" Then I dropped my hand. Within a second, I saw a strong image of my hand and arm in a uniform magenta, as though my arm was still outstretched. I blinked, then saw my arm and hand outstretched in full color against the background, which also was in color but not as vivid as my hand and arm. After watching for a few seconds, the colorful scene faded, and I saw my hand and arm in cyan for a while before the scene faded away. I did the observation three times, too, just to enjoy it.

So, what happened? I think that by adapting to the darkness beforehand and then only having illumination for a moment, I set my visual system up to be fooled into seeing a false representation of the scene. I think the illusion is an outcome you'd expect when you essentially shut down the visual system then give it a brief flash of a scene then shut down the inputs again. It's reasonable to surmise the visual system has a feature analogous to the vestibular sense, and that the most recent output has an echo.

A corollary experiment involves just wiggling your fingers or making a fist with your arm in place after the flash. When I tried this, I found the afterimage blinked and never resolved completely as it did the scene did when I dropped my arm. I tried this again leaving my arm in as close to the same place as I could while turning my hand so that instead of having my palm outstretched I had the back of my hand turned outward. So I had my palm out when I hit the flash, then turned my hand inward. I saw, clearly but in dimmer light, my palm turned inward, then the afterimage of my arm and hand with the palm outward.

I have no explanation for being able to clearly see my hand with the palm inward. Maybe the flashbulb was still dying when I turned my hand? In any case, I am assuming that by turning my hand inward after that I disrupted the position of my arm enough to then see the afterimage, unlike when I wiggled my fingers. In that observation I never got a clear afterimage. In this one, I got the clear afterimage. This fits with what H Hogendoorn, M P M Kammers, T A Carlson and F A J Verstraten found in their 2008 study, "Being in the dark about seeing your hand: multisensory conflict resolved by proprioception erasing visual information," namely that moving your hand around in the place disrupts the afterimage effect but moving your hand elsewhere leads to the visual system taking over.

Magic eye pictures

File this under how stuff works for stereograms (aka Magic Eye pictures)
At a glance, the Magic Eye pictures (stereograms, like this one, which is in the portfolio of a designer whose Web site you can reach by clicking on the image) you see sometimes at stores in the mall, or in books or even postcards, look like a colorful abstract pattern or a collage of little pictures. But if you can learn to focus on a spot some distance behind the surface of the picture, you can get a 3-D image to show up. This all boils down to building into the image cues that let you perceive the 2-D image as having depth.

We have several mechanisms for perceiving depth. Some rely on having two eyes and others rely on just one. But although I have no problem resolving stereograms, I can't do it with only one eye, so that narrows the choices of explanations down considerably. I also observe that the pictures are harder to resolve if they are turned on their sides or upside down, which leads me to believe that what's really at work is that the artists have created images for the right and left eye that differ just enough to create the 3-D effect.

In Steven Pinker's "How the Mind Works," a couple of illustrations are a big help in understanding how we achieve depth perception using binocular (p. 218, if that book's on your shelf) and monocular cues (p. 226). The two-eye mechanism works because information arrives at different parts of each of your eyes and your visual processors use those differences (and therefore trigonometry, really) to build a 3-D representation of what you're looking at. The one-eye mechanisms are easier to comprehend: Larger items appear to be closer, for example, than smaller items. It is helpful, I think, to look at a simplified stereogram so you can actually pick out the differences on the left and right sides.

So how these pictures work is deceptively simple: They capitalize on our ability to see in stereo, using cues we rely on for 3-D vision to give us a perception of depth when looking at a 2-D object.