"When Horace Barlow discovered that signals from some cells specified the direction of movement, he tried to explain it, and his explanation has been the reigning assumption," Werblin said "He was right in general outline, but his explanation contained a lot of black boxes. We've shown what's going on in the boxes."
The basic light detectors in the retina are the photoreceptors, which fire off signals to a layer of horizontal cells and thence to bipolar cells. The bipolar cells funnel signals down their axons and relay them to the dendrites, or input wires, of ganglion cells, which send the processed information to the brain. All these cell types are arrayed in unique layers, stacked one atop the other in the retina. At the bottom of the stack are the 12 or so different kinds of ganglion cells, including the directionally selective ganglion cells.
If there were no other cells types, light shining on a photoreceptor would initiate a signal that cascades unaltered through the cell layers to the ganglion cell, and then into the brain. But other cells, called amacrine cells, weave among the bipolar cell axons and alter their output. This is what makes some ganglion cells respond to motion in one direction only.
Fried, Mnch and Werblin showed that the key player in detecting motion is an amacrine cell dubbed a starburst cell, because its dendrites spread out from the cell center in a spoke-like pattern that suggests a figurative starburst.
The UC Berkeley team developed a technique to measure not only the firing of the directionally selective ganglion cells, but also the input these cells receive from starburst and bipolar cells. In contrast to a recent paper suggesting that starburst cells are not crucial to detecting directional motion, they found them to be the critical link.