ABSTRACT
In damaged or
dysfunctional retina, the photoreceptors stop working, causing blindness. By
some estimation, there are more than 10 million people worldwide affected by
retinal diseases that lead to loss of vision.
The absence of
effective therapeutic remedies for retinitis pigmentosa (RP) and age-related
macular degeneration (AMD) has motivated the development of experimental
strategies to restore some degree of visual function to affected patients.
Because the remaining retinal layers are anatomically spared, several
approaches have been designed to artificially activate this residual retina and
thereby the visual system.
At present,
two general strategies have been pursued. The “Epiretinal” approach involves a
semiconductor-based device placed above the retina, close to or in contact with
the nerve fiber layer retinal ganglion cells. The information in this approach
must be captured by a camera system before transmitting data and energy to the
implant. The “Sub retinal” approach involves the electrical stimulation of the
inner retina from the sub retinal space by implantation of a
semiconductor-based micro photodiode array (MPA) into this location. The
concept of the sub retinal approach is that electrical charge generated by the
MPA in response to a light stimulus may be used to artificially alter the
membrane potential of neurons in the remaining retinal layers in a manner to
produce formed images.
Some
researchers have developed an implant system where a video camera captures
images, a chip processes the images, and an electrode array transmits the
images to the brain. It’s called Cortical Implants.
INTRODUCTION
The
retina is a thin layer of neural tissue that lines the back wall inside the
eye. Some of these cells act to receive light, while others interpret the
information and send messages to the brain through the optic nerve. This is
part of the process that enables us to see. The
device consists of a silicon chip inserted into the eye, which is designed to
act like a retina, receiving images captured by a pair of glasses worn by the
user. It will restore vision to people who have lost sight during their
lifetime. Intelligent eye work by
stimulating nerves, which are activated by electrical impulses. In this case
the patient has a small device implanted into the body that can receive radio
signals and transmit those signals to nerves. In the case of the intelligent
eye the device is a circle about the size of a five-cent piece, inserted into
the eye where the retina sits.
It is a silicon chip which decodes the radio signals and
delivers the stimulations. The chip sends messages to the retinal ganglion
cells through small wires. The camera feeds the visual information into a
separate image-processing unit, which makes ‘sense’ of the image by extracting
certain features. It might find a door, for example, by contrasting the bright
open door with a dark room.
The intelligent eye is a
“Bio-electronic eye” which is also called as bionic eye. It is an electronic
device used to replace the functionality of the eye.
The unit then breaks down the image
into pixels and sends the information, one pixel at a time, to the silicon chip,
which then reconstructs the image.
Future
'Intelligent eye' technology would be people with eye diseases like retinitis
pigmentosa and age-related macular degeneration, who were born with vision and
therefore have the necessary brain pathways established for processing visual
information, unlike those who were born blind.
Recently in London,
doctors have fitted a blind man with a Intelligent eye that has given the
73-year-old virtual eyesight.
TECHNOLOGY OVERVIEW
Ocular implants are those which are placed
inside the retina. It aims at the electrical excitation of two dimensional
layers of neurons within partly degenerated retinas for restoring vision in
blind people. The implantation can be done using standard techniques from
ophthalmic surgery. Neural signals farther down the pathway are processed and
modified in ways not really understood therefore the earlier the electronic input
is fed into the nerves the better.
There are two types of ocular implants are
there epi-retinal implants and sub retinal implants.
EPI-RETINAL
IMPLANTS:
In
the EPI-RETINAL approach scientists had developed a micro contact array which
is mounted onto the retinal surface to stimulate retinal ganglion cells. The
information in this approach must be captured by a camera system before
transmitting data and energy to the implant.
A tiny video camera is mounted on eyeglasses and it sends
images via radio waves to the chip. The actual visual world is captured by a
highly miniaturized CMOS camera embedded into regular spectacles. The camera
signal is analyzed and processed using receptive field algorithms to calculate
electric pulse trains which are necessary to adequately stimulate ganglion
cells in the retina.
This signal together with the energy supply
is transmitted wireless into a device which is implanted into the eye of the
blind subject. The implant consists of a receiver for data and energy, a
decoder and array microelectrodes placed on the inner surface of the retina.
This micro chip will stimulate viable retinal cells. Electrodes on microchip
will then create a pixel of light on the retina, which can be sent to the brain
for processing.
The main advantage of this is that it consists of only a
simple spectacle frame with camera and external electronics Communicates
wirelessly with microchip implanted on retina programmed with stimulation
pattern.
Chip Development:
EPI RETINAL ENCODER:
The design of an epiretinal
encoder is more complicated than the sub retinal encoder, because it has to
feed the ganglion cells. Here, a retina encoder (RE) outside the eye replaces
the information processing of the retina. A retina stimulator (RS), implanted
adjacent to the retinal ganglion cell layer at the retinal 'output', contacts a
sufficient number of retinal ganglion cells/fibres for electrical stimulation.
A wireless (Radio Frequency) signal- and energy transmission system provides
the communication between RE and RS. The RE, then, maps visual patterns onto
impulse sequences for a number of contacted ganglion cells by means of adaptive
dynamic spatial filters. This is done by a digital signal processor,
which, handles the incoming light stimuli with the master processor, implements
various adaptive, antagonistic, receptive field filters with the other four
parallel processors, and generates asynchronous pulse trains for each simulated
ganglion cell output individually. These spatial filters as biology-inspired
neural networks can be 'tuned' to various spatial and temporal receptive field
properties of ganglion cells in the primate retina.
Biocompatibility:
The material used for the chips and stimulating
electrodes should satisfy a variety of criteria’s. They must be
corrosion-proof, i.e. bio stable.
·
The
electrodes must establish a good contact to the nerve cells within fluids, so
that the
stimulating
electric current can pass from the photo elements into the tissue.
·
It must be
possible to manufacture these materials with micro technical methods and.
·
They must
be biologically compatible with the nervous system.
RF Telemetry:
In case of the epiretinal encoder, a wireless RF telemetry
system acts as a channel between the Retinal Encoder and the retinal
stimulator. Standard semiconductor technology is used to fabricate a power and
signal receiving chip, which drives current through an electrode array and
stimulate the retinal neurons. The intraocular transceiver processing unit is
separated from the stimulator in order to take into account the heat
dissipation of the rectification and power transfer processes. Care is taken to
avoid direct contact of heat dissipating devices with the retina.
SUB
RETINAL IMPLANTATION:
The sub retinal approach is based on the fact that for
instance of retinitis pigmentosa
the neuronal network in the inner retina is preserved with a relatively intact
morphology. Thus, it is appropriate for excitation by extrinsically applied
electrical current instead of intrinsically delivered photoelectric excitation
via photoreceptors. This option requires that basic features of visual scenes
such as points, bars, edges, etc. can be fed into the retinal network by
electrical stimulation of individual sites of the distal retina with a set of
individual electrodes.
Sub-retinal approach is aiming at a direct physical
replacement of degenerated photoreceptors in the human eye, the basic function
of which is very similar to that of solar cells, namely delivering slow
potential changes upon illumination. The quantum efficiency of photoreceptor
action, however, is 1000 times larger than that of the corresponding technical
devices. Therefore the intriguingly simple approach of replacing degenerated
photoreceptors by artificial solar cell arrays has to overcome some
difficulties, especially the energy supply for successful retina stimulation.
On the
‘back’ side of the retina, photoreceptors (rods and cones) are excited by the
incoming light and deliver gradual potential changes to the inner retina
layers. The path of the electrical signals is then opposite to that of the
incoming light. The main problem in diseases like retinitis pigmentosa or
macula degeneration is the loss of photoreceptors or photoreceptor function,
whereas the signal processing path in the inner retina is remaining intact.
This gives us the chance to place a micro photo diode array (MPDA) in the
subretinal space, which may then electrically stimulate remaining photoreceptor
or bipolar cells. Appropriate surgical techniques have recently been developed
and tested.
It’s believed that the so
evoked retinal activity leads to useful sensations if the retinal output
reveals the topography of the image feature and is projected retinotopically
correct to the visual cortex.
In addition, the sampling density of a sub retinal
device could be designed to match that of the remaining photoreceptor or
bipolar cell matrix, thereby providing a potentially high-resolution input to
the retina.
Implant chips have been tested both in vitro and in vivo
to assess their bio-stability. In vitro stability (in buffered saline solution)
is excellent even for periods as long as 2 years. In vivo, however, the
passivation layer could withstand the biological environment for up to about
six months only. In contrast, the electrodes made of titanium nitride showed
excellent bio-stability over more than 18 months in vivo. These are the results
of vitro and vivo tests conducted by the scientists in Retinal Implant Research
centre.
THRESOLD AND OPERATIONAL RANGE FOR
SUBRETINAL
STIMULATION:
Evoked retinal response related to the amount of injected
charge. (A) Raster plot (40 trials) and cumulative response histogram (bin
width 1 ms) to a single voltage pulse with 0.5 ms duration and increasing
amplitude, applied via a platinized gold electrode to a chicken retina sample.
In the histograms the number of spikes from 40 trials is given. (B) Relative
ganglion cell response in a 40 ms window after pulse onset plotted against
charge injected per pulse and electrode. At the upper axis the related voltage
level and peak current are given. The error bars indicate the standard
deviation of the number of spikes per trial within the analyzing window. The
colored triangle indicates the operational range between the 10% and 90%
response level. (C) Scatter diagram showing the charge thresholds for spot stimulation
(n = 10). The line represents the median value (0.43 nC).
The experiments revealed that in a partly degenerated
neuronal network information processing capabilities are present and can be
activated by artificial inputs. This open up promising perspectives not only
for the development of subretinally implanted stimulation devices as visual
prostheses but also for the entire field of neurobionics and neurotechnology.
In Vivo
Tests:
Electrical
signals from the brain - VEP
A
special part of the brain, the visual cortex, is believed to be the entrance
structure to visual perception and cognition Activity of nerve cells within the
brain's surface (the cortex) produce electrical fields that can be picked up at
some distance with electrodes (like ceiling microphones pick up sound from
instruments in an orchestra during a concert). In humans these electrodes are
simply "glued" on the scalp with a sticky paste on the back of the
head. In the pig model special arrays of electrodes fixed on a silicone-carrier
are placed under the scull bone above the duration by neurosurgeons and can be
left there for several months.
When a visual stimulus (e.g. a
blinking spot or a reversing checkerboard pattern) is presented within the
visual field the electrical fields arising from the visual cortex change over
time in a characteristic manner. These changes are measurable as voltage
changes across the electrodes. They are referred to as "visually evoked
potentials" or VEP.
VEP as an
objective measure for visual function
VEPs
are very informative about the visual system and its function. Each time a VEP
can be recorded most probably a visual sensation has occurred. In humans
VEP-curves vary in amplitude and time, dependent on the intensity, the location
and the type of visual stimulus that is used to evoke the VEP. VEPs are an
objective measure for central visual function. Since its anatomy and size is
very close to the human eye, the pig is an ideal model to develop implantation
techniques for subretinal devices or to test long-term stability and
biocompatibility.
White light flashes of varying intensity were repeatedly
presented to the anaesthetized pig. Electrical fields arising from the visual
cortex in response to the stimulation were recorded with special amplifiers and
further analyzed by computer. A) At high light intensities response amplitudes
of up to 200 µV of voltage could be recorded. B) Following electrical
stimulation with a subreitnal device evoked brain activity in the visual
cortex.
In the last years scientists could prove that
stimulation via a subretinal implant indeed led to a activation of the visual
cortex both in pigs and rabbits . These electrically evoked "VEP"
signals from the brain were similar in time course and amplitude to the VEP
obtained by light stimulation (white flashes) of an equivalent retinal area.
Stimulus amplitudes of 600 mV (trace at 600 mV) evoked
brain activity clearly above noise level (trace at 0 mV). The response
amplitude further increased on increasing stimulation amplitudes. In
comparison, the lowermost trace reflects the brain's response to a white light
flash. Although the shape of the responses to electrical stimulation and to
light stimulation matches closely, with the subretinal implant there is a much
lower implicit time (~time from onset of the stimulus green line to the onset
of a recordable response in the brain). This is probably due to the much faster
propagation of the "light signal" through the subretinal prosthesis
and the connected retinal cells.
The Chows originally tested
their chip in blind animals and successfully produced visual sensations. Their
device displays only black and white images and works best in well-lit rooms,
but they hope that the addition of more solar cells on the chip will eventually
improve the results. Much of this technology hinges upon the ability of the
human eye to accept silicon chip implants, and six retinitis pigmentosa
patients have undergone the procedure during the past year. Dr. Chow reports that,
as yet, there has been no sign of rejection, infection, inflammation, or
detachment, and that the patients (all affected by retinitis pigmentosa) are
reporting improved vision.
A recent press release from
Optobionics (May 2002) reported these positive results, and also that the chips
seem to be stimulating remaining healthy cells. Initial expectations were to
gain some light perception at the site of the implant, but improvement outside
the implant areas is also being seen: something Dr. Chow calls a "rescue
effect." His report was also presented at the 2002 meeting of the
Association for Research in Vision and Ophthalmology (ARVO) in Ft. Lauderdale,
Florida.
In addition to
continuing to follow up on these six patients, the Optobionics company is
planning more implants in the near future. This work by the Chows is for the
purpose of determining the safety of the procedure in humans under FDA
guidelines, and it will be several years before large-scale clinical trials will
prove the efficacy of their approach.
The micro chip
which should be designed for sub retinal implantation should small enough to be implanted in eye, supplied
with continuous source of power, and it should be biocompatible with the eye
tissues. To meet these requirements scientists in optobionic research centre
have developed a device called artificial silicon retina.
STRUCTURE AND WORKING OF ASR:
The
micro chip which should be designed for sub retinal implantation should small enough to be implanted in eye, supplied
with continuous source of power, and it should be biocompatible with the eye
tissues. To meet these requirements scientists in opt bionic research centre
have developed a device called artificial silicon retina.
The
ASR microchip is a silicon chip
2mm in diameter and 25 microns thick, less than the thickness of a human hair.
It contains approximately 5,000 microscopic solar cells called “micro
photodiodes,” each with its own stimulating electrode. These micro photodiodes are
designed to convert the light energy from images into electrical chemical
impulses that stimulate the remaining functional cells of the retina in
patients and rp type or devices.
The ASR microchip is powered solely by
incident light and does not require the use of external wires or batteries.
When surgically implanted under the retina—in a location known as the “sub
retinal space”—the ASR chip is designed to produce visual signals similar to
those produced by the photoreceptor layer. From their sub retinal location,
these artificial “photoelectric” signals from the ASR microchip are in a
position to induce biological visual signals in the remaining functional
retinal cells which may be processed and sent via the optic nerve to the brain.
In preclinical laboratory testing, animal
models implanted with the ASRs responded to light stimuli with retinal
electrical signals (ERGs) and sometimes brain-wave signals (VEPs). The
induction of these biological signals by the ASR chip indicated.
When a diode is
reverse biased the electrons &holes move away from PN junction. If the
photo diode is exposed to a series of light pulses the photon generated
minority carriers must diffuse to the junction &should be swept across to
the other side in a very short time. Therefore its decided that he width of the
depletion region is be large enough that
most of the photons are absorbed within the depletion region rather than
in the neutral PN junction region. Photodiode can work in two modes. One in
which the external circuit delivers power to the device other in which device
gives power to the external circuit. Therefore it can be called as a solar cell.
The ASR is
powered solely by the incident light &does not require the use of external
wires or batteries. When surgically
implanted under the retina in a location known as sub retinal space the ASR is
designed to produce visual signals similar to those produced by the
photoreceptor layer. Thus a photodiode produces a voltage corresponding to the
light energy incident on it. Solar cells
in the device's microchip are supposed to replace the function of the retina's
light-sensing cells that have been damaged by disease. The ASR microchip relies
on the ability to stimulate the remaining functional cells within a partially
degenerated inner or neuro retina. As a result, the ASR chip will not be able
to assist patients with conditions where the retina or visual pathway is more
substantially damaged.
Implant
design and fabrication
The current micro
photodiode array (MPA) is comprised of a regular array of individual photodiode
subunits, each approximately 20×20-µm square and separated by 10-µm channel
stops which is shown in the figure-4.1. Across the different generations
examined, the implants have decreased in thickness, from ~250 µm for the
earlier devices, to approximately 50 µm for the devices that are currently
being used. Because implants are designed to be powered solely by incident
light, there are no connections to an external power supply or other device. In
their final form, devices generate current in response to a wavelength range of
500 to 1100 nm.
Implants are comprised of a doped and
ion-implanted silicon substrate disk to produce a PiN
(positive-intrinsic-negative) junction. Fabrication begins with a 7.6-cm
diameter semiconductor grade N-type silicon wafer. For the MPA device, a photo
mask is used to ion-implant shallow P+ doped wells into the front surface of
the wafer, separated by channel stops in a pattern of individual micro
photodiodes. An intrinsic layer automatically forms at the boundary between the
P+ doped wells and the N-type substrate of the wafer. The back of the wafer is
then ion-implanted to produce a N+ surface.
Thereafter, an insulating layer of silicon
nitrate is deposited on the front of the wafer, covering the entire surface
except for the well openings. A thin adhesion layer, of chromium or titanium,
is then deposited over the P+ and N+ layers. A transparent electrode layer of
gold, iridium/iridium oxide, or platinum, is deposited on the front well side,
and on the back ground side. In its simplest form, the photodiode and electrode
layers are the same size. However, the current density available at each individual
micro photodiode subunit can be increased by increasing the photodiode
collector to electrode area ratio.
Implant finishing involves several
steps. Smaller square devices are produced by diamond sawing, affixed to a
spindle using optical pitch, ground, and then polished to produce the final
round devices for implantation. The diameter of these devices has ranged from
2-3 mm (for implantation into the rabbit or cat sub retinal space) to ~0.8 mm
(for implantation into the smaller eye of the rat).
Titanium
is sputtered at high pressure in a nitrogen atmosphere to obtain nano-porous
titanium nitride (TiN) stimulation electrodes on the implant. This enables
enhancement of electrode surface area by a factor of up to 100 which is a
critical prerequisite for efficient charge transfer from chip to tissue. SEM
micrograph of thin film electrode: nano-porous surface texture provides for
excellent charge transfer from chip to tissue.
The
electric performance of the interface between chip and tissue is critical for
the proper function of the implant. From the point of view of an electronics
engineer, this interface acts like a capacitor. For this reason, no DC currents
may be used in electro-stimulation but only current transients may be applied.
Micro-graph of cross-section through retinal tissue on micro-photodiode
obtained by transmission electron microscopy. The electrical properties of the
interface may be described by an equivalent circuit. Only transient current
pulses may be used to stimulate tissue.
ASR IMPLANT PROCEDURE:
The microsurgical procedure
consists of a standard vitrectomy plus an additional step. The surgeon starts
by making 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
surgeon removes the gel in the middle of the eye and replaces it with saline.
The surgeon then make an opening in the retina through which fluid is injected:
the fluid lifts up a portion of the retina from the back of the eye and creates
a small pocket in the “subretinal space” just wide enough to accommodate the
ASR microchip.
The surgeon then slides the
implant into the subretinal space, much as one might slip a tiny coin into a
pocket. Finally, the surgeon introduces air into the middle of the eye to
gently push the retina back down over the implant. Over a period of one or two
days, the air bubble is reabsorbed and replaced by fluids created within the
eye. The procedure takes about 2 hours and is done on a hospital outpatient
basis.
MULTIPLE-UNIT
ARTIFICIAL RETINA CHIPSET (MARC):
This is
the epiretinal approach implantation. The components of the MARC system are characterized,
from the extraocular video camera, video processing chip, and RF generator and
primary coil, to the intraocular secondary coil, rectifier and regulator MARC
processing and demultiplexing chips, and the electrode array.
Overall System
Functionality:
The MARC system,
pictured in Figures 1-4 will operate in the following manner. An external
camera will acquire an image, whereupon it will be encoded into data stream
which will be transmitted via RF telemetry to an intraocular transceiver. A
data signal will be transmitted by modulating the amplitude of a higher
frequency carrier signal. The signal will be rectified and filtered, and the
MARC will be capable of extracting power, data, and a clock signal. The
subsequently derived image will then be stimulated upon the patient’s retina.
As shown in Figures 4.2,
the MARC system would consist of two parts which separately reside exterior and
interior to the eyeball. Each part is equipped with both a transmitter and a
receiver. The primary coil can be driven with a 0.5-10 MHz carrier signal,
accompanied by a 10 kHz amplitude modulated (AM/ASK) signal which provides data
for setting the configuration of the stimulating electrodes. A DC power supply
is obtained by the rectification of the incoming RF signal. The receiver on the
secondary side extracts four bits of data for each pixel from the incoming RF
signal and provides filtering, demodulation, and amplification. The extracted
data is interpreted by the electrode signal driver which finally generates
appropriate currents for the stimulating electrodes in terms of magnitude,
pulse width, and frequency.
MARC Photoreceptor and Stimulating Pixel:
MARC consists of a photosensing,
processing, and stimulating chip fabricated in 2.0 ?m CMOS, which is endowed
with a 5x5 phototransistor array. Measuring 2x2 mm2 in dimension, it has been
used to demonstrate that current stimulation patterns corresponding to simple
images, such as the letter "E" presented optically to the chip which
is shown in the figure-4.3, can be generated and used to drive the 5x5 array of
electrodes. The chip delivered the requisite power for stimulation, as was
determined by our clinical studies conducted on visually-impaired patients with
RP and AMD. For each of the 25 individual photosensing pixels, we were able to
adjust VDD, the clock frequency, the sensitivity to light, and the current
output. The chip is fully packaged in a standard 40 pin DIP package,
facilitating interconnect via a ribbon cable from the chip to the stimulating
electrode array.
Each pixel on the
MARC1 is divided into two parts, the photoreceptor and the current pulse
generator, as shown in Figure 4.4. The photoreceptor converts light intensity
into a voltage level. This voltage level is then compared to the threshold voltage
Vth in the comparator, which controls the current pulse driver circuit. If
light intensity is stronger than the threshold value, a biphasic stimulating
signal is generated by that pixel. Timing of the current pulse is controlled by
external pulses and a clock signal. The chip has been designed with the
biological stimulating constraints determined in clinical studies conducted on
patients with RP and AMD. It successfully delivered the 600 microA current in 1
millisecond bipolar pulses, which were required for retinal stimulation.
By successfully fabricating
and testing a chip which receives an image and delivers the requisite currents
for human retinal stimulation to a 5x5 electrode array, we gained the necessary
insight and expertise which is allowing us to design the second and third
generation chips which will drive 10x10 and 25x25 photosensing arrays. The
design of MARC1 includes enhanced holed CMOS phototransistors [6]. This chip
was designed before it was suggested that we split the components into two
pieces, so as to facilitate the
transmission of energy into the eye. As RF powering became feasible, so
did telemetry and the transmission of visual information from an exterior
camera. Thus it was decided that all the video would be processed exterior to
the eye.
IMAGE
ACQUISTION OF MARC:
Standard
technology can provide that which is needed for an image acquisition setup for
a retinal implant. Usually flicker fusion of individual frames of a movie appears
continuous at a rate of 30 frames/second. This would lead one to believe that similar
frame rates would be necessary for a retinal implant. However, because electrical
stimulation bypasses phototransduction performed by the photoreceptors and stimulates
the remaining retinal neurons directly, the flicker fusion rates are higher. In
our group’s tests in blind humans we have found the rate to be between 40-60
Hz[3,12]. We will modify our camera input to accommodate this requirement, as a
flickering strobe image is not appealing, let alone useful. Although color is
not essential to be able to navigate or read, it is appealing and is missed by
blind patients. We are in the process of gaining further understanding as to
how different stimulus parameters elicit various colors, and we hope to convey
the color information that our imaging system will be capable of delivering.
The CMOS image sensor
offers the cost benefits of integrated device functionality and low power and
volume production, and due to its compact size, it may be easily mounted on a
pair of glasses. The extraocular video processor[30] will be implemented
discretely using SRAM frame-buffers, an ADC and a FPGA/EPLD. Reconfigurable
FPGAs afford the flexibility of investigating the impact of various image
processing algorithms (including artificial neural networks) on image
perception. This system will process the analog video signal from an off-chip
monochrome CMOS mini-camera. Color mapping and indicial mapping provide further
image data processing. An integrated, full-duplex RS232 asynchronous, serial
communications port within the video processor will link the retinal prosthesis
to a PC or workstation for configuration and monitoring. The video system will
be endowed with an alternate test mode of operation, in which image input can
be taken from the communications port instead of the camera. This will permit a
selected still image to be transmitted from the PC to the video buffer for
fixed pattern stimulation and biological monitoring in the early stages of
development. A block diagram of the video processor is shown in Figure 4.5. All
digital signals (Tx, Rx, and serial_data) are one bit wide.
CORTICAL
IMPLANTS:
Scientists have
created a device that allows them to communicate directly with large numbers of
individual nerve cells in the visual part of the brain. The device is a silicon
electrode array may provide a means through which a limited but useful visual
sense may be restored to profoundly blind individuals.
This shows the development of the first visual
prosthesis providing useful "artificial vision" to a blind volunteer
by connecting a digital video camera, computer, and associated electronics to
the visual cortex of his brain. This device has been the objective of a
development effort begun by our group in 1968 and represents realization of the
prediction of an artificial vision system made by Benjamin Franklin in his
report on the "kite and key" experiment.
This new visual
prosthesis produces black and white display of visual cortex
"phosphenes" analogous to the images projected on the light bulb
arrays of some sports stadium scoreboards. The system was primarily designed to
promote independent mobility, not reading. It has a battery powered, electronic
interface that is RF isolated from line currents for safety. This interface can
replace the camera, permitting the volunteer to directly watch television and
use a computer, including access to the Internet. Because of their potential
importance for education, and to help integrate blind people into the
workforce, such television, computer, and Internet capabilities may prove even
more valuable in the future than independent mobility.
First of all passing an electric current through a
single electrode into the visual cortex causes a blind subject to see a point
of light called a phosphene. The visual scene before the subject will be
encoded by miniature video camera attached to a pair of eye glasses. The
resulting video signals will be processed by custom circuitry. The processed
signals pass across the skull to an array of electrodes implanted in the
primary visual cortex.
Relaying the electric signals to the cortical implant
could be accomplished by two methods- conductive and inductive. In the former
connectors are attached to the cranium and provide access to the external
circuitry with the later a transformer is formed with one coil under the skin
and the other one on the outside.
A platinum foil ground
plant is perforated with a hexagonal array of 5 mm diameter holes on 3 mm
centres, and the flat platinum electrodes centred in each hole are 1 mm in
diameter. This ground plane keeps all current beneath the dura. This eliminates
discomfort due to dural excitation when stimulating some single electrodes
(such as number 19) and when other arrays of electrodes are stimulated
simultaneously. The ground plane also eliminates most phosphene interactions
when multiple electrodes are stimulated simultaneously, and provides an
additional measure of electrical safety that is not possible when stimulating
between cortical electrodes and a ground plane outside the skull. Each
electrode is connected by a separate Teflon insulated wire to a connector
contained in a carbon percutaneous pedestal.
When stimulated,
each electrode produces 1-4 closely spaced phosphenes. Each phosphene in a
cluster ranges up to the diameter of a pencil at arms length. Neighbouring
phosphenes in each cluster are generally too close to the adjacent phosphenes
for another phosphene to be located between them. Indicate the primary visual
cortex (area 17) would
permit placement of 256
surface electrodes on 3 mm centres on each lobe in most humans (512 electrodes
total).
Electronics
Package
The 292 X 512 pixel CCD black and
white television camera is powered by a 9 V battery, and connects via a battery
powered NTSC link to a sub-notebook computer in a belt pack. This f 14.5
camera, with a 69° field of view, uses a pinhole aperture, instead of a lens,
to minimize size and weight. It also incorporates an electronic
"iris" for automatic exposure control.
The sub-notebook computer
incorporates a 120 MHz microprocessor with 32 MB of RAM and a 1.5 GB hard
drive. It also has an LCD screen and keyboard. It was selected because of its
very small size and light weight. The belt pack also contains a second
microcontroller, and associated electronics to stimulate the brain. This
stimulus generator is connected through a percutaneous pedestal to the
electrodes implanted on the visual cortex. The computer and electronics package
together are about the size of a dictionary and weigh approximately 10 pounds,
including camera, cables, and rechargeable batteries. The battery pack for the
computer will operate for approximately 3 hours and the battery pack for the
other electronics will operate for approximately 6 hours.
This general architecture, in which one computer interfaces with the
camera and a second computer controls the stimulating electronics, has been
used by us in this, and four other substantially equivalent systems, since
1969. (8) The software involves approximately 25,000 lines of code in
addition to the sub-notebooks' operating system. Most of the code is written in
C++, while some is written in C. The
second microcontroller is programmed in assembly language.
Stimulation
Parameters:
Stimulation delivered to each electrode
typically consists of a train of six pulses delivered at 30 Hz to produce each
frame of the image. Frames have been produced with 1-50 pulses, and frame rates
have been varied from 1 to 20 frames per second. As expected, (3) frame rates of 4 per second currently seem best, even with trains
containing only a single pulse. Each pulse is symmetric, biphasic (-/+) with a
pulse width of 500 µsec per phase (1,000 µsec total). Threshold amplitudes of
10-20 volts (zero-peak) may vary +/-20% from day to day; they are higher than
the thresholds of similar electrodes without the ground plane, presumably
because current shunts across the surface of the pia-archnoid and encapsulating
membrane.
The system is calibrated each morning by recomputing the thresholds for
each electrode, a simple procedure that takes the volunteer approximately 15
minutes with a numeric keypad.
Although
stimulation of visual cortex in sighted patients (2) frequently produces coloured phosphenes, the phosphenes reported
by this volunteer (and all previous blind volunteers to the best of our
knowledge) are colourless. We speculate that this is the result of
post-deprivation deterioration of the cells and/or senaphtic connections
required for colour vision. Consequently, colour vision may never be possible
in this volunteer or in future patients. However, optical filters could help
differentiate colours, and it is also conceivable that chromatic sensations
could be produced if future patients are implanted shortly after being blinded,
before atrophy of the neural network responsible for color vision.
The problem kindling
of neural tissues or the triggering of seizures in those tissues by periodic
electrical stimulation has to be solved. Biocompatibility is another issue of
concern. This particular vexing problem has yet to be solved. A power supply to
the system has to be efficiently designed. The position of the implant within
the skull has to be decided upon. Lastly the implant should function flawlessly
for years.
ADVANTAGES
Implementation of
an Intelligent Eye has certain advantages. An electronic eye is more precise
and enduring than a biological eye and we cannot altogether say that this would
be used only to benefit the human race. In short successful implementation of a
bioelectronics eye would solve many of the visual abnormalities suffered by
human’s to date.
To be honest, the final visual outcome of a patient cannot be
predicted. However, before implantation several tests have to be performed with
which the potential postoperative function can be estimated. With this
recognition of large objects and the restoration of the day-night cycle are the
primary goals of the prototype
implant.
•
Compact
Size – 6x6 mm
•
Reduction of stress upon retina
CONCLUSION
The application of the research work done is directed
towards the people who are visually impaired. People suffering from low vision
to, people who are completely blind will benefit from this project. The
findings regarding biocompatibility of implant materials will aid in other
similar attempts for in human machine interface. Congenital defects in the
body, which cannot be fully corrected through surgery, can then be corrected.
There has been marked increase in research and clinical
work aimed at understanding low vision. Future work has to be focused on the
optimization and further miniaturization of the implant modules. Commercially
available systems have started emerging that integrates video technology, image
processing and low vision research.
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