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UC Berkeley scientists detail neural circuit that lets eye detect directional motion

oblem is that there are directionally selective cells and starburst cells everywhere, and you can't tell which are connected," Mnch said.

By probing this section of the retina, they discovered that directionally sensitive cells are connected to only half of the 60-70 starburst cells within their reach. All of these are on one side only, an asymmetry that allows them to veto firing when a stimulus comes from that direction.

Starburst cells not only affect directionally sensitive ganglion cells, they also reach out and touch bipolar cells. This allows starburst cells to alter the firing of bipolar cells, too.

As a result, when a target moves in the "preferred" direction - the direction that causes directionally sensitive ganglion cells to fire - the ganglion cells get a strong stimulus from bipolar cells and reduced inhibition from starburst cells. The cumulative effect is a stimulus that triggers the ganglion cell to send a signal to the brain.

When a target moves in the "null" direction, starburst cells inhibit firing and also veto the excitation from bipolar cells. The net effect is strong inhibition and no firing of the directionally selective ganglion cells. Also, because the inhibition from starburst cells arrives before any excitation from the bipolar cells, firing is diminished even more.

"Starburst cells not only shut off the output of bipolar cells, they also deliver negative input to the ganglion cells," Fried said.

One unexpected finding was that starburst cells themselves are sensitive to the direction of a moving stimulus. They emit more neurotransmitter when a target moves outward along a dendrite than when it moves inward toward the cell body, creating a third mechanism by which they can signal direction of movement.

"The original problem was, how do directionally selective cells work," Werblin said. "Now we want to know how the processes of starburst cells are selective."


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Contact: Robert Sanders
rls@pa.urel.berkeley.edu
510-643-6998
University of California - Berkeley
27-Nov-2002


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