Pulfrich's Pendulum

The teaching of psychology listserve I subscribe to presented an interesting vision demo that I haven't tried yet, but want to.

Here's how it goes:
Slowly swing a golf ball attached to a string from side to side. The string should be about 3.5 feet long. You'll need to stand on a chair or table for this to work well. Students hold a shaded filter over one eye while viewing the moving ball. A 2 inch square of colored transparent material will work just fine. They will perceive the ball moving in a circular fashion. When you have them move the filter to the other eye, the ball will move circularly in the opposite direction.

Why does this happen?

Lower contrast stimuli are perceived by our visual system to more slower than higher contrast stimuli. The filter reduces the contrast in just one eye so the subjective speed of the ball in that eye is slower. It appears to lag behind. To the brain, that difference in speed is perceived as a disparity between the two eyes in the distance of the object. The resulting impression is a ball moving along an elliptical path. The illusion is known as Pulfrich's pendulum

More interesting apparent motion information can be had here.

That website suggests that a similar effect can be achieved by watching a TV set to a vacant channel with one eye covered by sunglasses. The "snow" will appear to swirl. Switch the filter to the other eye and it will swirl in the other direction.

Stereoblind people, who can't fuse random-dot stereograms (i.e. MagicEye pictures), still perceive the apparent motion of the Pulfrich pendulum. The authors conclude that stereoblind people retain some residual binocular mechanism for depth perception.

The apparent motion effects caused by difference in contrast arise from the different speeds that neural impulses have when conducted down the optical nerve. High contrast stimuli travel faster. The symptoms of multiple sclerosis result from slower neural conductivity brought on by the deterioration of the myelin that insulates the axons of neurons. Without insulation, electrical impulses travel slower. It's possible that people with MS, cataracts, or some other neurological condition that affects the speed of neural impulses in just one eye could experience Pulfrich-like effects.