All but bare essentials

THE EYE as a camera has been a powerful metaphor for poets and scientists alike, implying that the eye provides the brain with detailed snapshots that form the basis for our rich experience of the world.

Recent studies at the University of California, Berkeley, however, show that the metaphor is more poetic than real. What the eye sends to the brain are mere outlines of the visual world, sketchy impressions that make our vivid visual experience all the more amazing. "Even though we think we see the world so fully, what we are receiving is really just hints, edges in space and time," said Frank S. Werblin, professor of molecular and cell biology in the College of Letters & Science at UC Berkeley. Werblin is part of UC Berkeley's Health Sciences Initiative, a collaboration among researchers throughout the campus to tackle some of today's major health problems. The brain interprets this sparse information, probably merging it with images from memory, to create the world we know, he said.

In a paper in Nature, doctoral student Botond Roska and Werblin provide evidence for between 10 and 12 output channels from the eye to the brain, each carrying a different, stripped-down representation of the visual world. While scientists have known that the eye forwards several parallel representations of the world to the brain, what these are and how they are produced has been a mystery. "What we have done," Roska said, "is show that the retina creates a stack of image representations, how these image representations are formed and that they are the result of cross-talk between layers of cells in the retina."

The results are a big step toward producing a bionic eye employing a unique computer chip that can be programmed to do visual processing just like the retina. The chip, called a Cellular Neural Network (CNN) Universal Machine, was invented in 1992 by Roska's father. Over a period of nearly three years, Roska painstakingly measured signals from more than 200 ganglion cells in the rabbit retina as he flashed pictures of a featureless square or circle. Ganglion cells are the eye's output cells, forming the optic nerve connecting it to the brain.

One group of ganglion cells, for example, only sends signals when it detects a moving edge. Another group fires only after a stimulus stops. Another sees large uniform areas, yet another only the area surrounding a figure.

The two researchers shared these detailed findings with software designer David Balya in Hungary, who modelled the visual processing on a computer, a preliminary step before actually programming a CNN chip to simulate the image processing that goes on in the eye. The computer model precisely mimics the output of the ganglion cells of the retina, vividly showing the difference between the world we see and the information that actually is sent to the brain.

Though scientists realize that the eye is not merely a camera providing digital input to the brain, the general consensus has been that the world projected onto the retina and detected by cells called photoreceptors got sent to the brain after some relatively simple processing.

Roska and Werblin showed that retinal cells do a lot of processing to extract only the essence of the picture to send to the brain. The anatomy of the retina is layered to facilitate this. Light initially impinges on the light-sensitive cells of the eye, 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 - the outgoing wires of the nerve cell - 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. Biologists noted earlier that all ganglion cells were not alike and that they fired off different information to the brain, though the details were hazy. Part of the reason is that the axons from the bipolar cells synapse with or touch the dendrites of the ganglion cells in a tangled region (the inner plexiform layer) that made biologists despair of making sense of the connections. 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.

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Section  : Science & Tech
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