Researchers revise popular understanding of the eye
Specialized ganglion cells in retina said to send series of sketchy ‘movies’ to the brain

By Robert Sanders, Public Affairs

(Top) The retina’s various cell layers all play a role in relaying visual messages to the brain. (Above) A simulation of seven of the dozen separate movies that the eye extracts from a scene and sends to the brain. Such simulations may help program a bionic eye. Frank Werblin graphics

04 April 2001 | 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 campus studies show, however, 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 Werblin, professor of molecular and cell biology in the College of Letters & Science. Werblin’s research is part of the 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 the March 29 issue of Nature, Werblin and doctoral student Botond Roska 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.

“These 12 pictures of the world constitute all the information we will ever have about what’s out there, and from these 12 pictures, which are so sparse, we reconstruct the richness of the visual world,” Werblin said. “I’m curious how nature selected these 12 simple movies and how it can be that they are sufficient to provide us with all the information we seem to need.”

While scientists have known that the eye transmits 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 bionic eye’s computer chip was invented in 1992 by Roska’s father, Tamás Roska, and Leon Chua, professor of electrical engineering and computer sciences.

“The biology we are learning is going into improving the chip, which is getting more and more similar to the mammalian retina,” Roska said.

Ganglion cells are the eye’s output cells, forming the optic nerve connecting it to the brain. By painstakingly measuring signals from more than 200 ganglion cells in the rabbit retina, Roska and Werblin determined that there are about a dozen different populations of ganglion cells, each spanning the full visual space and producing a different movie output.

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.

“Each representation emphasizes a different feature of the visual world — an edge, a blob, movement — and sends the information along different paths to the brain,” Werblin said.

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.