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by subhankar karmakar

How We See Things that Move:
A Hot Spot in the Brain's Motion Pathway

Researchers have now traced the path of neural connections that make up the motion pathway and tested the responses of cells at different steps along this path.

Starting in the retina, large ganglion cells called magnocellular neurons, or M cells, are triggered into action when part of the image of a moving hand sweeps across their receptive field—the small area of the visual field to which each cell is sensitive. The M cells' impulses travel along the optic nerve to a relay station in the thalamus, near the middle of the brain, called the lateral geniculate nucleus.

Then they flash to the middle layer of neurons in the primary visual cortex. There, by pooling together the inputs from many M cells, certain neurons gain a new property: they become sensitive to the direction in which the hand is moving across their window of vision.

Such direction-sensitive cells were first discovered in the mammalian visual cortex by David Hubel and Torsten Wiesel, who projected moving bars of light across the receptive fields of cells in the primary visual cortex of anesthetized cats and monkeys. Electrodes very close to these cells picked up their response to different moving lines, and the pattern of activity could be heard as a crackling "pop-pop-pop" when the signals were amplified and fed into a loudspeaker.

The keystone of the motion pathway was discovered by Semir Zeki of University College, London, in an area of the cortex that lies just beyond the primary and secondary visual areas (V1 and V2), further from the back of the brain—a vast unexplored wilderness vaguely known as the "sensory association cortex."

"It was thought that somewhere in this mishmash of association cortex visual forms were recognized and associated with information from other senses, says John Allman of the California Institute of Technology. But studies in the owl monkey by Allman and Jon Kaas (who is now at Vanderbilt) and in the rhesus monkey by Semir Zeki revealed that the area was not a mishmash at all.

Instead, much of it was made up of separate visual maps, each containing a distinct representation of the visual field. In 1971, Zeki showed that one of these visual maps was remarkably specialized. Though its cells did not respond to color or form, over 90 percent of them responded to movement in a particular direction. American scientists usually call this map MT (middle temporal area), but Zeki called it V5. He also nicknamed it "the motion area."

"This very striking finding of this little hot spot, this little pocket, in which almost all the cells are sensitive for the direction of movement," says New York University's Anthony Movshon, was the impetus for many vision researchers to turn their attention to motion. Nowhere else in the visual cortex was there an area that seemed so functionally specialized.

The cells of this motion area, MT, are directly connected to the layer of direction-sensitive cells in the primary visual area, V1. And the two areas have a remarkably similar architecture. Hubel and Wiesel had discovered that V1 is organized into a series of columns. The cells in one column may fire only when shown lines oriented like an hour hand pointing to one o'clock, for instance, while the cells in the next column fire most readily to lines oriented at two o'clock, and so on around the dial.

Amazingly, MT has the same kind of orientation system as V1, but in addition the cells in its columns respond preferentially to the direction of movement.

"When you see that an area, like V1 or MT, has this highly organized columnar structure," says Wiesel, "you get a sense of uncovering something fundamental about the way the cells in the visual area work."

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