Stephen G. Lisberger, an HHMI investigator at the University of California, San Francisco, is investigating a complementary phenomenon. "I'm interested in how you take a visual sensory signal and convert it into a command for movement," he says. For a window into this critical area, Lisberger has long studied the neuronal circuitry that enables monkeys to move their eyes smoothly while "pursuing"—that is, tracking—an object in motion.

Visual pursuit is a highly developed faculty in primates, and Lisberger likes to point out the virtuosity with which it performs in, for example, an outfielder turning and sprinting to the exact spot where a flying, curving baseball will come to earth. This feat depends on two separate faculties within the brain: keeping the eyes locked on the speeding ball, and compensating for the jerky, bouncing movements of the running fielder's head.

Scientists used to think that smooth pursuit was a straightforward reflexive action. But over the past several years, Lisberger has discovered that pursuit is actually a "complex voluntary behavior that comprises many components." The eye and brain must choose which moving object to track, estimate the direction and speed of the target with respect to the moving eye, and command the eyeball to rotate along the object's path at the correct speed.

One of the most interesting components Lisberger discovered is an "online volume control" that selectively dials up or down the strength of visual inputs to the motor system. Analogous to Newsome's experiments, where stimulation in the brain area labeled "MT" changed what the monkey reported he "saw" for a given moving stimulus, Lisberger's laboratory demonstrated that stimulation in a part of the frontal motor cortex can change how the monkey's pursuit system responds to a given visual stimulus. The effect of stimulation seems to be mediated by altering the setting of the volume control.

Another longtime interest of Lisberger's is how the proverbial out-fielder, despite running along and turning his gaze rapidly from place to place, manages to perceive the world as stable. It's due to the vestibulo-ocular reflex, or VOR, which occurs when, for instance, in watching someone pass by, you turn your head to the right: This action produces a smooth eye rotation to the left. Remarkably, even though the VOR is a simple reflex, it is capable of learning, so that any errors in stabilizing the world are quickly eliminated. Lisberger's lab has pinpointed the neural loci of learning to two places in the cerebellum and has begun to explain how learning at specific loci in the brain can be converted into organized changes in motor output.

		Visual inputs flow from the retina through a series of processing centers in the brain, climbing a ladder of increasingly higher-level stages until the image is in a "finished" form that, John H.R. Maunsell says, has been "edited to suit the immediate goals of the viewer." Maunsell's aim is to understand how changes in attention alter the responses of visual nerves involved in this editing process. Photo: Paul Fetters

Visual information from the outside world falls on our retinas in an overwhelming jumble of stimuli, like the incoherent babble of voices at a cocktail party. Fortunately, the brain is equipped to focus on small, important parts of a scene while screening out what is irrelevant. Attention, as this filtering process is called, sharpens our perception of the target and enables the brain to make better-informed decisions about responding.

Visual inputs flow from the retina through a series of processing centers in the brain, climbing a ladder of increasingly higher-level stages until the image is in a "finished" form that, HHMI investigator John H.R. Maunsell says, has been "edited to suit the immediate goals of the viewer." Maunsell's aim is to understand how changes in attention alter the responses of visual nerves involved in this editing process.

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