Roska discovered, however, that this region of tangled axons and dendrites is really laid out in orderly strata. By staining the cells from which he recorded, he found that bipolar cell axons converge on 12 or so well-defined layers, where they synapse with the dendrites of the ganglion cells. Each layer of dendrites belongs to a specific population of ganglion cells.
Without interaction between layers, though, the signal emerging from the tangle would not be much different from the original 12-channel output of the bipolar cells. The critical element is another type of cell, the amacrine cells, which send processes to the various layers of dendrites and allow the layers to talk with one another. This cross-talk is what allows the layers to process the visual data and extract the sparse information that the ganglion cells send up to the brain.
"Previously, when people studied ganglion cells, they would look at the cell and flash lights. One of Botond's major contributions to this was, he thought about this not as the cell, but as the layer of processes from which the cell is reading. So, we began to think in terms of layers, and all of the activity we measured corresponded to what happened in a particular layer," Werblin explained. "Then it became clear that these layers were actually talking to each other. Previously no one had even thought that these layers talked to one another, even though 100 years ago the picture was there. No one had really looked at that picture."