Wednesday, October 24, 2007

Decoding the Human Eye

Superdense arrays of electrodes will bring scientists closer to an artificial retina that approximates normal vision.

By Emily Singer

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Test bed: A 512-electrode array (gold circle), modeled after detectors used to capture particles in high-energy physics, is helping to decipher the neural code of the retina. The findings will aid in the design of future retinal prostheses.
Credit: Alan Litke

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•	View images of an artificial retina and its effect on vision.

Artificial retinas are already in human clinical trials at the University of Southern California, where they have helped blind patients distinguish walls from doorways and even watch soccer games, albeit as blurs of motion. But approximating normal vision--and possibly enabling people to read--will require devices that can deliver electrical current with much greater control and precision. A new chip densely packed with electrodes, developed by scientists at the University of California, Santa Cruz (UCSC), is the first step in that direction.

Currently being used in research, the chip can stimulate and record from individual cells in retinal samples. The technology will provide insight into how the retina codes information and how to mimic that coding--lessons that will be crucial in developing the next generation of retinal implants. Further down the road, some version of the technology might be used to send visual information down the optic nerve.

"The retina is a very sophisticated visual-information-processing device," says Alan Litke, a physicist at UCSC who is applying his expertise to neurobiology. "To have a human patient someday approach normal visual functioning, such as reading, you need to have a very accurate level of control."

The retina is a thin layer of cells at the back of the eye; photoreceptor cells in the retina detect light and send signals to the retinal ganglion cells, which then transmit the signals to the brain through the optic nerve. In macular degeneration and retinitis pigmentosa, two leading causes of blindness, photoreceptor cells are damaged, but the remaining retinal ganglion cells are left largely intact. Artificial retinas, which rely on an external camera to capture visual information, consist of a processor that translates that information into an electrical code intelligible to the nerve cells of the eye, and a chip dotted with tiny electrodes that transmit the electrical signals to the retinal ganglion cells.

Litke and his collaborators modeled their chip after the silicon microchip detectors that line supercolliders to capture signs of elusive, high-energy, subatomic particles, such as the Higgs boson. Using common integrated-circuit fabrication techniques, the researchers custom-built more than 500 electrodes and amplifiers onto a small glass strip. "There are other commercial, multi-electrode recording systems available, but the team at UCSC has really pushed the technology forward by coming up with a system with the capability to record many more neural responses," says Matt McMahon, a scientist at Second Sight, the company based in Sylmar, CA, that's developing the retinal prostheses used in the USC study. Second Sight is using Litke's device to inform the design of future prostheses. The company's first-generation device had 16 electrodes, the second-generation device currently in human trials has 60, and a 200-electrode version is under development. (See "Next-Generation Retinal Implant.")

Decoding the Human Eye

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