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The Strange Symptoms of Blindness to Motion
   
 

The patient had great difficulty pouring coffee into a cup. She could clearly see the cup's shape, color, and position on the table, she told her doctor. She was able to pour the coffee from the pot.

But the column of fluid flowing from the spout appeared frozen, like a waterfall turned to ice. She could not see its motion. So the coffee would rise in the cup and spill over the sides.

More dangerous problems arose when she went outdoors. She could not cross a street, for instance, because the motion of cars was invisible to her: a car was up the street and then upon her, without ever seeming to occupy the intervening space.

Even people milling through a room made her feel very uneasy, she complained to Josef Zihl, a neuropsychologist who saw her at the Max Planck Institute for Psychiatry in Munich, Germany, in 1980, because "the people were suddenly here or there but I did not see them moving."

The woman's rare motion blindness resulted from a stroke that damaged selected areas of her brain.

What she lost—the ability to see objects move through space—is a key aspect of vision. In animals, this ability is crucial to survival: Both predators and their prey depend upon being able to detect motion rapidly.

In fact, frogs and some other simple vertebrates may not even see an object unless it is moving. If a dead fly on a string is dangled motionlessly in front of a starving frog, the frog cannot sense this winged meal. The "bug-detecting" cells in its retina are wired to respond only to movement. The frog might starve to death, tongue firmly folded in its mouth, unaware that salvation lies suspended on a string in front of its eyes.

While the retina of frogs can detect movement, the retina of humans and other primates cannot.

"The dumber the animal, the smarter its retina," observes Denis Baylor of Stanford Medical School. The large, versatile brain of humans takes over the job, analyzing motion through a highly specialized pathway of neural connections.

This is the pathway that was damaged in the motion-blind patient from Munich. Compared to the complex ensemble of regions in the visual cortex that are devoted to perceiving color and form, this motion-perception pathway seems relatively streamlined and simple. More than any other part of the cortex, it has yielded to efforts to unveil "the precise relationship between perception and the activity of a sensory neuron somewhere in the brain," says Anthony Movshon, an HHMI investigator at New York University.

Consider what happens when we watch a movie, suggests Thomas Albright of the Salk Institute. Each of the 24 frames projected per second on the theater screen is a still photograph; nothing in a movie truly moves.

The illusion of movement is created by the motion-processing system, which automatically fuses, for instance, the images of legs that shift position slightly from frame to frame into the appearance of a walking actor. The Munich patient is unable to perform this fusion. In life or in the movie theater, she sees the world as a series of stills.

"The motion system must match up image elements from frame to frame, over space and time," says Albright. "It has to detect which direction a hand is moving in, for instance, and not confuse that hand with a head when it waves in front of someone's face."

— Geoffrey Montgomery