July 30, 2003.
By R. Colin Johnson,
Enabling the blind to see again is moving beyond the realm of Biblical prophecy as researchers around the world tackle the problem head-on. Recently, several researchers showed off artificial retina prototypes at the International Joint Conference on Neural Networks that are bringing human applications for machine vision closer to reality.
A Japanese team built a three-chip set "that performs the analog functions of a real retina," said Tetsuya Yagi, a physician and professor in the Electronic Engineering department at Osaka University. His work with fellow engineer Seiji Kameda is part of Japan's five-year "artificial vision system" effort at the New Energy and Industrial Technology Development Organization (NEDO) and Nidek Co. Ltd.
Most current prosthesis efforts to cure blindness concentrate on using a video camera — a CCD chip with lens — and a DSP. Together, they transform pixels into a pattern of electrical stimulation that can be delivered by electrodes to either remaining retinal cells, the optic nerve or the visual cortex itself.
Most recently, the University of Southern California successfully implanted a 16-electrode retinal array that gave partial sight to patients blinded by retinitis pigmentosa, which leaves "bipolar" retinal cells intact.
That approach works as long as the video camera and DSP are mounted on a pair of glasses connected to a belt-pack with DSP and batteries. However Yagi's research looks to the day when any kind of blindness can be corrected with an implantable chip that would replace any defective layer from the retina, to the optic nerve, to the wiring of the brain itself.
"We use analog chips [instead of digitizing a video signal and using a DSP] because analog uses so very little energy to operate and, more importantly, does not generate heat, which is very important when we start engineering implants," said Yagi.
Yagi's current chip provides a 184-pixel (40 x 46 pixels) image. It was fabricated in 0.6-micron CMOS using double-polysilicon and three metal layers on a 8.9 mm2 die. It and a second analog chip mimic two of the five layers between the eye and the brain. Yagi said he already has the other layers in sight. The next one is already on the drawing board.
Nor is he alone. Labs around the world are aiming to decode the neural patterns of electrical stimulation on each visual processing layer so that dead cells can eventually be bypassed with specialized electronics that precisely replicate the missing signals.
Visual circuitry computes higher-order visual functions, such as edge detection. Hope is rampant because the so-called retinal processing circuit is among the few tissues of the nervous system where the correlation between the physical structure and electrical property is well understood.
The overall effect of the retinal processing circuit is that without movement the signal goes to neutral grey because of persistent negative feedback. In the presence of motion the retinal circuit delivers a smoothed and contrast-enhanced image.
The chip circuitry replicates the first two layers of the eye with three chips — two analog chips for each layer plus an field-programmable gate array to sequence them.
The first chip is an array of loosely coupled photodiodes emulating the photoreceptor cell layer. The second chip is a variable resistive network emulating the horizontal cell layer. The second chip also computes the difference between the two layers to emulate the persistent negative feedback of the horizontal layer.
A FPGA supplied control signals to sequence the chip's processing tasks. Sample-and-hold circuits stored analog values between the separate computational layers, enabling analog voltage levels to be transferred between chips.
"I plan to build the next layer on another parallel operating chip, eventually connecting to the visual cortex itself," said Yagi.
By carefully characterizing and replicating the signal-processing capabilities at each layer of the retinal processing circuit, Yagi and his Japanese colleagues aim to someday help blind patients by substituting prostheses for whatever part of the visual circuitry is damaged.
Electronics Times and EETimes UK material Copyright © 2003 CMP Europe Ltd.
All other material Copyright © 2003 CMP Media LLC
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