24-07-2012, 03:00 PM
SILICON RETINA USING VLSI TECHNOLOGY
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INTRODUCTION:
A silicon retina that reproduces signals in Optic Nerve:
Prosthetic devices may someday be used to treat lesions of the central nervous system. Similar to neural circuits, these prosthetic devices should adapt their properties over time, independent of external control. Here we describe an artificial retina, constructed in silicon using single-transistor synaptic primitives, with two forms of locally controlled adaptation: luminance adaptation and contrast gain control. Both forms of adaptation rely on local modulation of synaptic strength, thus meeting the criteria of internal control. Our device is the first to reproduce the responses of the four major ganglion cell types that drive visual cortex, producing 3600 spiking outputs in total.
How Our Retina Works:
To understand how artificial vision is created, it's important to know about the role that the retina plays in how we see . Here is a simple explanation of what happens when you look at an object:
• Scattered light from the object enters through the cornea.
• The light is projected onto the retina.
• The retina sends messages to the brain through the optic nerve.
• The brain interprets what the object is.
Age old technique of implantation:
The eye implant-a 3 millimeter -wide chip that would fit behind the retina – could be a dramatic step above the currently available technique. For the device to work the microchip would have to be implanted behind the retina of a blind person. The patient would wear goggles mounted with a small video camera, which then sends the image to a wireless wallet-sized computer for prossesing. The computer trasmitts this information to an infrared LED screen on the goggles.
The new surgical procedure:
The microsurgical procedure starts with three tiny incisions in the white part of the subject's eye, each incision no larger than the diameter of a needle. Through these incisions, the surgeons insert a miniature cutting and vacuuming device that removes the gel in the middle of the eye and replaces it with saline. They then make a pinpoint opening in the retina through which they inject fluid to lift up a portion of the retina from the back of the eye, creating a small pocket in the "subretinal space" just wide enough to accommodate the ASR.
Description:
The ASR contains about 3,500 microscopic solar cells that are able to convert light into electrical pulses, mimicking the function of cones and rods. To implant this device into the eye, surgeons make three tiny incisions. Through these incisions, the surgeons introduce a miniature cutting and vacuuming device that removes the gel in the middle of the eye and replaces it with saline. Next, a pinpoint opening is made in the retina through which they inject fluid to lift up a portion of the retina from the back of the eye, which creates a small pocket in the subretinal space for the device to fit in. The retina is then resealed over the ASR.
Methodology
We implemented modulation by exploiting the exponential I(V) relationship of the MOS (metal-oxide-semiconductor) transistor. In the subthreshold regime, the current from the drain erminal to the source terminal is the superposition of a forward component that decreases exponentially with the source voltage (Vs ) and a reverse component that decreases similarly with the drain voltage (Vd); both components increase exponentially with the gate voltage (Vg). That is, Ids = I0 eκ Vg (e−Vs − e−Vd ) where κ ≈ 0.7 is a nonideality
factor; voltages are in units of UT = 25 mV, at 25 ◦ C (this equation describes the n-type device; voltage and current signs are reversed for a p-type [14]). Hence, the transistor converts voltage to current exponentially and converts current back to voltage logarithmically. Modulation occurs by changing the source voltage, which changes the transistor’s transconductance. Current mirrors are added to reverse the direction of current when necessary.
. Light and contrast adaptation:
The silicon retina’s ganglion cells adapt to mean luminance and encode stimulus contrast. They maintain contrast sensitivity over at least one and a half decades of mean luminance. This intensity range was limited on the low end by leakage currents; these transistors pass a few picoamperes even when their gate voltage is zero. And it was limited on the high end by the projector in our experimental set-up (could not exceed 200 cd m–2 ) and by stray photocurrents (lightinduced leakage currents) in the silicon chip. To obtain the results presented here, we compensated for the effect of these photocurrents by changing two externally applied voltages
that would otherwise require no adjustment (see section 2).
Conclusion
Silicon retina has became the emerging industry in the field of VLSI technology. The photo diodes , photo receptors, the rods and cones used for the artificial visiual perception in silicon retina are the major contributions from electronics industry.the use of silicon retina is not only confined to the biological field, but also, many innovative applications like walking robots, spatial sensors, traffic sensors etc are emerging out. The silicon retina helped blind people to rebuild their vision upto certain extent, so that they could visualise this world, which wasn’t possible before.