You are looking at columns of zebra-like ovals that swing back and forth in front of a diagonally striped background.
1. Look directly at the ovals. The ovals appear to swing horizontally.
2. Focus your gaze several inches above the screen, but pay attention to the ovals as you do so. The ovals should now appear to swing diagonally.
The difference between the two conditions is dramatic. If you don’t see it, try fixing your gaze a few inches higher above the screen.
Comments: If you’ve seen a diagram of a human eye, you know that in the back of the eye is a layer of cells called the retina, and in the center of the retina is an area called the fovea. The retinal area outside the fovea is referred to as the periphery. When you look directly at the ovals in the above display, the image of those ovals falls on the fovea. When you focus your gaze a few inches above your computer monitor, the image of the ovals falls in the visual periphery.
The machinery for foveal vision differs from the machinery for peripheral vision in numerous ways—too many ways, in fact, to list them all in a blog entry. As a result of these differences, we are able to see objects in much more detail when we view them directly (foveally) than when we view them indirectly (peripherally). Our representation of the world, therefore, is much different from the representation in a typical photograph, in which all objects in the same visual plane have the same amount of detail.
The different perceptions of the “peripheral escalator” illusion are partly due to the inability of peripheral vision to see fine detail. You can simulate this aspect of peripheral vision by removing fine detail from the display and then viewing the display foveally. Here are three simple ways to remove fine detail from the display: 1) view the display from about 6 feet away from the monitor; 2) scrunch up your eyes; or 3) if you can't (or can barely) see the words on the computer screen without corrective lenses, take them off and then view the display.
When you remove the fine detail, you will see a weird pattern of stripes that move up and down. This pattern is similar to what people see when they view the display indirectly, but it does not capture the peripheral perception of a diagonal sweeping motion that observers frequently report.
It seems, then, that relative to foveal vision, peripheral vision is missing not only the ability to represent fine detail but also “something else.” The nature of the “something else” is an important question for vision research. One possibility is that peripheral vision is not very good at alignment (in the trade, we say that the visual periphery is poor at representing “phase” information). Indeed, I created the peripheral escalator illusion as part of a search for the perceptual ramifications of the “poor-phase” hypothesis. While the poor-phase hypothesis may be correct, my colleagues and I suspect that central and peripheral vision may also differ in their capacity to segregate individual features. We have designated this possible capacity limitation “feature blur”… stay tuned.
As a final thought, primates differ from other mammals in the way cell types are distributed in the retina (see Masland, 2001, third paragraph). The primate fovea is dominated numerically by a particular group of post-receptoral cells, whereas in the retina of other mammals and in the primate periphery, cell types are distributed more evenly. It may be that most non-primate mammals see the world in a way that is closer to our peripheral vision than to our foveal vision. If this is so, then perhaps lions looking at a pack of moving zebras see something not unlike what we see when we look indirectly at the peripheral escalator illusion.
I will be on the road next week, so I may not be able to post a new illusion until after I return.
Note 6/3/08 8:25: For your reference, here is a link to a pdf of an excellent earlier study (1992) on “Misdirected visual motion in the peripheral visual field,” by R. Cormack, R. Blake, and E. Hiris (Vision Research, 32, 73-80).
As you will see, the principle is the same, but I think there may be interesting differences.
Here is a link to Randolph Blake’s website at Vanderbilt University. I encourage you to view the visual phenomena on his demonstration page.