Enabling the blind to see and the deaf to hear was once a biblical tale of divine intervention, but today silicon chips are enabling such miracles. The core problems of such "neural implants"-discovering biocompatible-materials and life-like signal-encoding-have enabled man-made electronics to fool the brains of the afflicted into accepting sensory data from artificial sources. So far, hybrid analog/digital sensor systems seem to work best, with analog chips used for the neural implants inside the body and digital chips used for outboard sensory-preprocessing, but for the future, who knows? When the formerly divine becomes man-made, no mortal can predict what miracles may emerge next, but on the drawing board already are such miracles as curing blindness and deafness, enhancing senses, annotating experience, augmenting memory and enabling artificial telepathy.
"Our DSPs are enabling neural implants to convert our natural senses into electronic input channels for the brain. We don't understand enough about the brain to interface with it directly yet, but we are beginning to understand the neural encoding of the eye, ears, nose, tongue and skin senses. They are becoming our peripheral inputs to the brain, a kind of natural USB channel into the brain," said Texas Instruments Fellow Ray Simar.
As Advanced Architecture Development Manager for TI's Digital Signal Processing Semiconductor Group, Simar claimed his DSPs are converting the raw sensory inputs from video cameras, microphones, pheromone detectors, spectrographs and pressure sensors, then encoding it in realtime into a facsimile of the pulse-coded, neural spike-trains that naturally emit from the sensor neurons of the eye, ear, nose, tongue or skin, respectively.
For the ear, the cochlea is the analog mechanism that transduces the sound waves vibrating the eardrum by virtue of Inner-ear Hair Cells. IHCs transduce an acoustic signal into spike train down a nerve directly into brain. Recently, Johns Hopkins researchers have shown that before a newborn "hears" outside sounds, the cochlea receives a modulated test signal from the brain, which it echoes back to the brain in an initialization stage that lasts several days. After initialization, the newborn begins accepting inputs from the outside world.
Consequently, people who have gone deaf after this period are "pre-wired" for an artificial cochlea. Today, physicians implant a small encased device that has a capacitive connection to the nerve that sends aural pulses to the brain. It communicates with RF to an electronic ear-clip with a microphone, DSP and RF transmitter.
The success of artificial cochlea is already enabling the deaf to hear again, results directly enabled by a close cooperation between medical researchers, who logged and analyzed the neural behaviors of our senses, and engineers who crafted the DSP algorithms that encode the neural pulse trains that fool the brain into accepting artificial inputs.
"We have a large interdisciplinary group with expertise in both physiology and software algorithms," said Mark Humayun, a professor of ophthalmology and associate director of research at the Doheny Retina Institute at USC's Keck School of Medicine.Latent blindness
In a manner similar to the already successful artificial cochlea's, last year Humayun and his colleague at USC, Eugene de Juan, implanted an artificial retinal into the eye of patients with retinitis pigmentosa and macular degeneration, both conditions that cause blindness long after the "initialization stage" of newborns that prewired the optic nerve connection to the brain. Their artificial retina, a mostly analog chip measuring 3 mm2, stimulates the ganglion cells, those prewired input channels to the optic nerve, resulting in new stimulation to the visual cortex that, after several weeks of adaptation, enables the formerly blind to see again, albeit blurrily.
The entire system consists of a video camera mounted on a pair of eyeglasses that sends a mega-pixel scene to a belt-pack, where the DSP and microcontroller encode the image into a pulse-train similar to that of the natural retina. A high radio-frequency (RF) transmitter then transfers the coded signal to a receiver on the implanted artificial retina, where platinum electrodes stimulate the ganglion cells below the dead retina. (Simultaneously, a low RF transmitter sends a constant ac signal to the chip, which it rectifies into the dc to power the chip.)
"Obviously, we are not perfectly replicating the original optical signal to the brain, because if we were, people would be able to see right away after the operation. What happens is that at first their brains don't know what to make of this new input, but within a period of weeks people begin to see. What we've found is that the brain can make do with very crude input and begins to use it rapidly. Almost like seeing a child learn to talk or to walk," said Humayun.
His current artificial retina prototype has only 16 electrodes, which makes its input to the optic laughably crude, compared with the original signal from the millions of neurons in the normal eye. Nevertheless, patients report being able to see blurrily, and one could reliably identify two different objects. The next implant prototype being readied for preclinical trials will have from 64 to 100 pixels, organized as 8-by-8 or 10-by-10 pixels, respectively.
"For our third-generation implant, we want to use a chip with 1,024 electrodes," said Humayun about his proposed 32-by-32 pixel retinal implant, now on the drawing board. "There are some hardware problems with making the electrodes small enough, but our main obstacle will be encoding the signal fast enough. I don't know yet if we will need TI's 1-GHz DSP or not, but we will certainly need a very fast one."
Today, there are 40 million blind people worldwide, but currently retinal prosthetics only aid a small percentage of those whose optic nerve still links to the brain, which includes macular degeneration (blind spots) and retinitis pigmentosa (genetic disorder of night- and peripheral-vision). Today, the "curing blindness" industry only nets about $5 million annually, but as the technology comes to cure more and different kinds of blindness, it will eventually be measured in billions of dollars instead of millions-perhaps before the end of the decade.
Blindness cures will stem from three divergent research directions-genetic research, which aims to regress DNA in the eye so that it regenerates its own dead cells; transplantation efforts to repopulate dead cells by implanting cultured cells; and electronic prostheses. The genetic solution is estimated to be 20 years away, and implanting cultured cells at least 10 years away. But electronic prosthesis are already here.
"I was originally against the idea of prosthesis for the retina-I was in favor of the other medical options, such as implanting cultured cells or using genetics to regenerate dead ones. But when I discovered how long it will take-15 or 20 years-I embraced prosthesis as a first step," said medical doctor Tetsuya Yagi, also a professor in the electronic engineering department at Osaka University (OU; Japan).
An implantable device acquires power and data from external hardware and electrically stimulates the retina using an array of electrodes.
According to Yagi, today the implant itself is the weak link, where problems with the body rejecting foreign matter have not been completely solved. Also, the optic nerve degenerates if disturbed, according to Yagi, making it impossible to directly stimulate individual ganglion cells.
"I think that the interface between the living tissues and the prosthetic device is the weak link-that's where we need real breakthroughs right now. For instance, we've discovered [from cat experiments] that if you stimulate the optical nerve for 20 minutes after you sever it, it doesn't degenerate-which will surely happen if you don't stimulate it," said Yagi.
There are as many as 1 million ganglion cells within the eye's 2-mm-round foveon-where "in-focus" images form, each measuring just microns in diameter, but today's electrodes are measured in millimeters-so large that they can't pinpoint any particular ganglion cell. "Today, we don't even know which ganglion cells we are stimulating; in fact, we can only really stimulate the tissue, indirectly stimulating any nearby ganglion cells," said Yagi.
Yagi's work is under the auspices of the Japanese government and its five-year (2001-2005) "artificial vision system" effort by its New Energy and Industrial Technology Development Organization (NEDO; http://www.nedo.go.jp/english]. With the Nidek Co. Ltd. [see www.nidek.com], NEDO sponsors scientists like Yagi in not only fabricating artificial retinas, but also in creating prosthetics for every other kind of blindness, including direct brain implants (eventually) for the people with no functioning eyes at all.
In the United States, five national labs, two universities and a few companies are jointly developing a MEMS electrode for an artificial retina. The DOE project has allocated $9 million to fund its "Artificial Retina" project. Oak Ridge National Laboratory is managing the project as well as performing the dynamic and static testing services for the electrode arrays.
The Lab plans to develop special "ocular sensors" for military service and has enlisted the help of Lawrence Livermore and Sandia National Laboratories to develop the advanced electrode itself. Los Alamos National Laboratory is cooperating to provide specialized optical imaging for the project.
Instead of augmenting human senses, Motorola's MC33794 Electric-Field IC senses the size of passengers inside a car. Electrodes inside seats and door panels form a 3-D "image" of the electric field revealing passenger size. The chip drives a capacitive load with a 100-picofarad upper limit between its electrode and their nearest grounds. The object being sensed effects the strength of the 120-kHz electric field (measured in volts per meter) in proportion to its dielectric constant, thus indicating its composition.
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