When you were a child, did you ever bend over and look between your legs to see what the world looked like upside down? If you were like me, you were disappointed: for me, anyways, the world didn’t look as different as I had hoped. Though turning things upside down does make it more difficult to get around, we’re actually quite good at adapting to changing our head position. You can do an experiment to see just how good you are at it. Fix your eyes on an object ten feet or so away. Now tilt your head to the left and to the right. Does it appear as if the image is tilting back and forth as you tilt your head? Perhaps a bit, but the effect is nothing like what it would be if you took a movie camera and filmed a scene while similarly tilting the camera.
Now try another experiment. Look at a wall or some other large object ten feet or so away. Now rapidly scan the object with your eyeballs so as to get a good view of the whole thing. Again, the wall doesn’t appear to move as you do this, even though clearly the image projected on the back of your eyeball (the retina, in case you’ve forgotten that eye-dissection lab from high school) is constantly changing. Again, imagine filming a movie scene the same way: the film would be a jumbled mess.
Clearly our mind is doing something to counteract the constant movements we make with our heads, bodies, and eyes. David Whitney, David A. Westwood, and Melvyn Goodale (of CIHR Group, the University of Western Ontario, and Dalhousie University, respectively) devised a set of experiments to determine what inputs we use to straighten out those crooked images on our retinas (“The Influence of Visual Motion on Fast Reaching Movements to a Stationary Object,” Nature, 2002).
There really are two possibilities: either our minds detect when our bodies are moving and compensate accordingly, or we look for visual cues that motion is occurring and use that information to compensate.
In their first experiment, Whitney et al. showed participants a computer screen with the following image:
The two striped rectangles displayed animated motion: the black bars on the right slowly moved up, and those on the left slowly moved down. Then a small rectangle flashed to the right of the rectangles. Viewers were instructed to move their hand as rapidly as possible and poke the computer monitor where the small rectangle had appeared (don’t ever do this to one of Greta’s computers). Viewers consistently poked the screen above where the rectangle flashed, as if it was moving along with the larger rectangles.
Periodically, the movement of the large rectangles changed directions. When the rectangle on the right showed downward movement, viewers poked below where the small rectangle appeared. If the motion changed direction near the time the small rectangle appeared, then viewers started to move their hand in one direction, and shifted their motion to match the shift in the large rectangles.
Whitney and his colleagues had been using a chin rest to make sure participants didn’t move their heads, so these results suggested that people do use visual cues to see if the “world” is moving.
Next they strapped volunteers in a chair and rotated it back and forth in a dark room. They flashed an LED in front of viewers and asked them to reach for it. This was essentially the same experiment they had done before, but the sensation of motion was used instead of a visual indicator of motion. Unlike in the first experiment, participants were unable to compensate for the motion of their bodies. In another condition, a set of background LEDs was also included, so that participants had a visual cue that showed them how far they were moving. In this condition, the results were similar to the first experiment.
So it appears that we actually use visual cues to help us see objects as stable, despite the fact that the image on our retina moves around all the time, both due to the movement of our bodies, and our eyeballs. It seems that we constantly examine the data coming into our eyes and adjust it so that we experience a stable world. Perhaps most amazing of all is the fact that we do all this unconsciously, from the time we are small children.