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Artificial Implants and the Field of Visual Prosthesis
Brittney J. Pfeifer
University of Missouri – St. Louis
Senior Seminar, Section 001
Charles Granger, Ph.D.
March 18, 2013

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Artificial Implants and the Field of Visual Prosthesis
“Blindness affects over 40 million people around the world” (Ong and da Cruz 7). 15 million of those
people are suffering from blindness due to a degenerative retinal disease (Zrenner et al. 1489). Fortunately, the
technological advancements,developed within the field of visual prosthesis,are changing the way in which people
see the world. In fact, the field was established for the sole purpose of restoring vision to those who became blind
due to a retinal disease,and had no alternative treatment options.A visual prosthesis, or artificial retina, is a device
in which a signal is received and transmitted into an image. Once implanted, the device must be able to: (1) detect
and capture ‘light-based’ images, (2) convert these images into electrical stimuli, (3) deliver the stimuli to adjacent
remaining retinal cells, such as ganglion, bipolar, or amacrine cells, and (4) evoke a response in the visual cortex of
the brain to produce visual perception (Ong and da Cruz 7). There is a variety of prostheses,categorized by where
they are implanted along the visual pathway, and they include, cortical, optic nerve, and retinal prostheses.
Common Retinal Diseases
Many eye diseases cause blindness from early childhood into adulthood.For example, age-related macular
degeneration (AMD) is a common hereditary disease that results in the loss of rod and cone photoreceptors (cells
within the retinal layers that provide night and color vision, respectively) among people age 50 or older. Some
factors that influence the development of AMD includes, “age, gender, family history of the disease, smoking, and
high body mass index” (O’Brien et al. 473). The dry form of the disease occurs when the macula (part of the retina)
slowly degrades overtime. This causes people to lose their central vision. People can still see using their peripheral
vision, but the images have poor detail quality. In more severe cases,the blood vessels within the macula can start
leaking blood rapidly, which damages it and causes complete vision loss in the form of wet AMD (O’Brien et al.
474). Retinitis Pigmentosa (RP) is a common autosomal dominant disease that results in the gradual loss of
photoreceptorfunction during childhood or late 40s to early 50s. It is important to note that in both of these diseases,
“although photoreceptors gradually die, the ganglion cells and remaining visual pathway remain largely intact”
(O’Brien et al. 474). These diseases are currently incurable, but by knowing that there still may be visual
information in the remaining parts of the visual pathway,devices have been developed to try to restore the lost
vision.
History of the Field of Vision Prosthesis

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In 1929, a German neurologist and neurosurgeon,Otfrid Foerster, discovered that by electrically
stimulating the occipital lobe within the visual cortex of the brain of one of his patients,the patient could visually
detect spots oflight known as phosphenes (Ong and da Cruz 7). Then in 1931, neurosurgeons FedorKrause and
Heinrich Schum performed the same experiment as Foerster by electrically stimulating the left occipital lobe of a
patient who had been blind for eight years. The patient, too,could see phosphenes.These experiments confirmed,
“the visual cortex does not wholly lose its functional capacity despite years of deprivation of visual input” (Ong and
da Cruz 7). In 1956, Graham Tassicker decided to take this idea one-step further and surgically place a
photosensitive cell behind the retina of a blind patient in hopes that the device would restore function to the
photoreceptorcells and allow the patient to see phosphenes (Ong and da Cruz 7). Fortunately, the experiment was a
success and years later during the 1960s and 1970s, Giles Brindley and William Dobelle established the Field of
Visual Prosthesis utilizing the ideas put forth by the great minds before them. For the first time, there was a field of
medicine established with the idea of implanting electronic prosthetic devices into different target sites along the
visual pathway in order to restore lost vision, and to this day, the field has continued to grow.
Cortical and Optical Nerve Prostheses
Cortical and optical nerve prostheses,as the names imply, are implanted onto the visual cortex of the brain
and optical nerve, respectively. The cortical prosthesis was the first device developed to artificially induce
phosphenes,but because ofits location, the device caused poorresolution quality and discomfort from stimulation
(Ong and da Cruz 13). The optical nerve prosthesis has also proved challenging in achieving detail quality and
getting the eyes to focus (Ong and da Cruz 12). Fortunately, more developments and on-going research is being
conducted to counteract these side effects. The main advantage ofthese devices, however, is that they are the only
therapeutic treatments with the potential of restoring visual function to individuals with severely damaged retinas or
optic nerves.
Retinal Prostheses
Retinal prostheses function by “producing small localized currents that alter the membrane potential of
adjacent retinal neurons” (Ong and da Cruz 6). By altering the membrane potential of these neurons,function can be
returned to the photoreceptorcells. There are two types of retinal prostheses,epiretinal prostheses and subretinal
prostheses.Epiretinal prostheses are implanted into the inner surface of the retina while subretinal prostheses are
implanted between the retinal layers. The epiretinal prosthesis works by utilizing a camera mounted on a pair of

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glasses,which captures light images. The images are then converted into electrical stimuli by a visual processing
unit worn by the patient. After that,the electrical stimuli are transferred to the implanted prosthesis (sometimes
called a microelectrode array) using telemetry to stimulate the remaining retinal cells (Ong and da Cruz 8). An
external rechargeable battery pack is worn by the patient to provide continuous power to the system. Subretinal
prostheses contain microphotodiode arrays (MPDA), which consist of pixel-generating elements and electrodes for
electrical stimulation. The MPDAs are capable of generating signal currents directly from incoming ‘light-induced’
images through the eye into the retina and then sending the current to the electrodes,causing activation of bipolar,
horizontal, and amacrine cells (O’Brien et al. 474-75). These cells, located within the retinal tissue layers enhance
the visual perception of the image. Subretinal implants also do not require any external battery pack because the
pixel-generating elements serve as the energy for the implant. However, some subretinal implants do utilize an
external power supply because it allows the individual patient to adjust the amplification frequency thereby
“adjusting the overall brightness and contrast of the perception according to the particular luminance conditions”
(Stingl et al. 2).
Examples of Epiretinal and Subretinal Prostheses
An example of epiretinal prosthesis is the Argus II developed by Second Sight Medical Products Inc, in
California. Argus II is powered by an externally worn battery pack and works by capturing images through a camera
mounted on glasses,translating those images into pixilated images (electrical stimuli), and then transferring the
stimuli by a telemetry-linked band across the sclera (white of the eye) to the implant for final proces sing and image
perception (Ong and da Cruz 9). Anotherexample of epiretinal prosthesis is the EPI-RET3 developed by researchers
at The University of Marburg in Germany, and works in a similar way to the Argus II. However, the electrical
stimuli are transferred to a receiver placed in the anterior chamber of the eye between the cornea and lens. The
receiver then “stimulates the epiretinal implant via a connecting micro-cable” (Ong and da Cruz 10). An example of
a subretinal prosthesis is the Alpha-IMS developed by Retina Implant AG in Reutlingen, Germany. The Alpha-IMS
consists ofa MPDA that has 1500 pixel-generating elements and 16 electrodes. The whole unit consists ofthree
parts: a subretinal part, an extrocular portion, and a subdermal portion. The subretinal part is the MPDA implant.
The extraocular portion consists ofa foil strip carrying connection lanes to the implant and external connection.
Lastly, the subdermal portion consists ofa silicone cable leading from the extraocular portion to behind the ear

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where it penetrates the skull and skin and ends in a plug that is plugged into a power supply worn by the patient
(Ong and da Cruz 10).
Advantages and Disadvantages of Epiretinal and Subretinal Prostheses
Epiretinal and subretinalprostheses have many advantages and disadvantages.Forexample, epiretinal
prostheses are easy to surgically insert because the surgery is well understood and only takes four hours,whereas
subretinal prostheses can take up to six to eight hours to implant (Stingl 7). The site of implantation for both devices
also offers many advantages and disadvantages.For example, the open space surrounding the epiretinal implant
allows for minimal disruption to the retina and heat dissipation (Ong and da Cruz 12). On the other hand,the
confined space of the subretinal implant limits the size of the device and can potentially cause “thermal injury to the
neurons and consequently limits the thermal budget of the implant” (Ong and da Cruz 12). Subretinal implants can
also stimulate bipolar, amacrine, and ganglion cells due to the implants location, which allows for more visual
information processing,such as motion and contrast (Stingl et al. 7). The epiretinal prostheses only have the
capability of stimulating ganglion cells. Epiretinal devices also provide easy upgrades without having to perform
further surgery because the circuitry of the systemis located on the external devices (i.e. the camera and battery
pack). The camera is also capable of zooming, which helps to magnify the visual field of perception. Unfortunately,
this part of the systemeliminates the use of natural eye movements, “which are important not only for visual search
but also for preventing image fading on the retina through small involuntary eye movements that refresh images
during visual perception” (Stingl et al. 7). Luckily, the subretinal prosthesis does use naturaleye movements.
Research Study Utilizing the Subretinal Implant Alpha-IMS
Researchers at Retina Implant AG in Reutlingen, Germany conducted a study and published their findings
in the paper, “Artificial vision with wirelessly powered subretinal electronic implant alpha-IMS.” The researchers
wanted to see if they could restore visual function in people who have degenerative eye diseases by means of
subretinaly implanting a microelectronic device that converts light into electrical signals and then stimulates visual
neurons to transmit images (Stingl et al. 2). The researchers hypothesized that if it is possible to replace
photoreceptive function using a technical device, there may be a treatment for degenerative retinal diseases. To
begin the study,the researchers implanted the Alpha-IMS into one eye of nine patients:four females and five males
between 35-62 years of age. Eight of the patients had retinitis pigmentosa and one patient had cone-rod dystrophy,
which caused the patient to go completely blind due to the deterioration of the cone and rod photoreceptorcells. A

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written informed consent was obtained from all patients before participating in the study (Stingl et al. 2-3). After the
implantation surgery,the subjects were put through a series of efficacy tests overthe course of 3 to 9 months to see
how effectively and efficiently the implant could provide visual perception. The efficacy tests included,standardized
screen tasks, table tasks of activities of daily living, letter recognition, and reports of daily life experiences. “As a
control, all tests (except for reports of daily life experiences) were administered with the implant power source
turned ‘ON’ or ‘OFF’ in a randomized order; the subjects were masked to the condition” (Stingl et al. 3).
During the standardized screen tasks,the subjects were put through multiple tests to measure light
perception and visual acuity. They were subjected to a full field illumination test,light source localization test,and
motion detection test with a moving random dot pattern. The full field illumination test was used to see if the
subjects could perceive any phosphenes.The light source localization test was used to see if the subjects could
locate a source of light and then track it using the motion detection test. The subjects also performed a Basic Grating
Acuity (BaGA) test to measure their spatial frequency resolution. This was done by identifying the direction of
white stripes on a black background. Finally, the subjects performed a visual acuity test using Landolt C-rings to
assess at what distance they could see, and then the “spatial and visual resolutions were calculated for the
corresponding eye distance” (Stingl, et al. 3). All of these tasks were performed at least 12 times. The table tasks of
activities of daily living consisted of identifying, localizing, and discriminating between several geometrical and
tableware objects. A subject would sit at a table and then attempt to identify, locate, and name four of six white
geometrical objects placed on a black tablecloth and four of six white tableware objects placed around a large plate
on a black tablecloth (Stingl et al. 4). Subjects were given a score between 0 to 4 depending on how many objects
they could correctly identify, locate, or name. The subjects were then asked to identify letters of the alphabet that
were placed in front of them in a random order. Lastly, the subjects were given the taskof reporting experiences in
their daily life. During the first few days of the task, a mobility trainer accompanied the subjects to make sure they
could safely use the device in daily life. After that,the subjects kept daily accounts oftheir experiences by videotape
or oral reports.
Once the research was completed, all nine subjects declared that they were able to detect phosphenes
generated by the implant. Unfortunately, two subjects had to have their implants removed due to technical instability
of the implant and were omitted from the study. Subject (S8) developed post-operative subretinal bleeding in the
implantation area, which caused the eye’s internal pressure to increase, and luckily, was resolved with topical and

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general medication (Stingl et al. 4). During the implant surgery for subject (S1), the tip of the implant touched the
optic nerve head, which short-circuited the implant rendering it useless and unable to perceive any light (Stingl et al.
4).
After the standardized screen tasks, subjects (S2-S9) could perceive light, subjects (S2 and S4-S9) were
able to localize light, and subjects (S4 and S6-S9) detected motion of dot patterns. In addition, the BaGA test
successfully measured spatial frequency resolution in subjects (S4-S9) and visual acuity in subjects (S5 and S8).
Subject (S3) was only capable of perceiving light, which was believed to have been caused by the subject’s
degenerative disease’s inability to process electrical signals within the retina, and because the subject (along with
S7) had the implant set at only 1-2 Hz (Stingl et al. 5). At higher frequencies, the images tended to fade more
quickly, which may have been a result of the visual pathway’s stimulation processing time for each individual
(Stingl et al. 6). The other subjects had their implants set at 5 or 15 Hz. After the table tasks of activities of daily
living, recognition and localization were easier to perform than discriminating against object shapes.Furthermore,
only subjects (S2, S6, and S8) could read several letters during the letter recognition test. Finally, subjects (S2, S4-
S6, and S8) could report various visual perceptions in their daily life experiences. The subjects reported this as “the
most important and rewarding aspect” of the study (Stingl et al. 6). In near-vision range, subjects could recognize
facial characteristics, differentiate between people’s body shapes and clothing patterns,and localize or distinguish
between office supplies, parts of meals, etc (Stingl et al. 5). In far-vision range, subjects could localize cars and glass
windows based on reflections from their surfaces, recognize stopped ormoving cars at night due to headlights, and
recognize letters on restaurant and store signs (Stingl et al. 5).
Critique of Research Study
What this study showed was that the Alpha-IMS implant “can restore useful vision in daily life for at least
2/3 of the blind patients investigated” (Stingl et al. 7). Furthermore, this is the only visual prosthesis “that has
successfully mediated images in a trial with freely moving blind persons by means of a light sensorarray that moves
with the eye” instead of requiring the use of an extraocular camera like other prostheses (Stinglet al. 5). However,
there needs to be a much larger sample size in order to make a generalized statement that the subretinal implant can
restore visual function to blind patients suffering from degenerative retinal diseases.The subjects also need to have a
variety of degenerative retinal diseases so comparisons can be made to see how the implant affects the visual
circuitry that has been degraded by these diseases.In other words, the researchers need to find out how the implant

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restores visual function in a person with one disease versus another.There also needs to be safety precautions put in
place that let future participants be aware of the risks associated with the study.Furthermore, because several
patients were unable to participate in this particular study due to technical instability of the implant device,
researchers need to start devising ways to make the product safer to use.After all, the device is being implanted into
one of the most sensitive areas of the body. Finally, there needs to be information provided regarding cost for the
implant surgery as well as how long the device will continue to function once they become commercially available.
Social Impact of Visual Prostheses
The visual prostheses are sure to take society by storm because they will be able to provide a treatment
option for people suffering from degenerative retinal diseases with no alternative therapy options. Even though the
device does not restore full vision or color vision, the device “will aid in the detection of light and dark and the
ability to identify the location and movement of people and objects” (Roberts 1). Ethical issues may arise because
humans are the subjects for the research behind the devices, but it is up to the person to decide whether they want to
participate. After a person volunteers,then they are told of the risks associated with the procedure and study and
asked to sign a written informed consent prior to inclusion in the study. Moreover, the U.S. Food and Drug
Administration approved the Argus II for treatment of RP in adults (Roberts 1). Unfortunately, restoring eyesight
does come at a price. In Europe, the device and surgery costs about $116,000 while the cost is also likely to exceed
$100,000 in the U.S. (Reinburg 3). Fortunately, Second Sight Medical Products, Inc has started a process to get
insurance to cover the cost of the implant and operation (Reinburg 3). All negatives aside,the implant is sure to
provide people with the hope to be able to see again. Therefore, what it boils down to is how much a person values
their eyesight,and what they are willing to do to get it back.
Conclusion and Outlook
Visual prostheses are an innovative approach to restoring vision in people suffering from degenerative
retinal diseases that cause blindness. The field of visual prosthesis has grown to include a variety of designs that can
be utilized on different areas of the visual pathway. This is important because degenerative retinal diseases can
affect different parts of the eye, so it is necessary to have a device that can be used if one part were to be damaged
over another. For example, optic nerve and cortical prostheses are still in development because there will be cases
“where the retina is destroyed or the optic nerve is severely damaged” and thus,doctors will need to use these
devices (Ong and da Cruz 6). The most advanced prostheses are the retinal prostheses.The Argus II by Second Sight

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is close to being commercially available while the Alpha-IMS by Retina Implant AG is still undergoing test trials to
improve resolution and detail quality for the patients.One idea on how this might be achieved is that “light sensors
could be programmed with built-in feedbacks to sharpen the edges on objects or clump them togetherto provide
more detail at the center” (Zimmer 3-4). What the Alpha-IMS does provide is “an alternative concept with a
subretinal and photodiode-based device” (Ong and da Cruz 14). Although these technologies still have a few
limitations that need to be resolved, they ultimately will set the stage for a bright future in the field of visual
prosthesis.

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Works Cited
O'Brien, Emily E., et al. "Electronic Restoration of Vision in those with PhotoreceptorDegenerations." Clinical and
Experimental Optometry 95.5 (2012): 473-83. SCOPUS. Web. 8 Mar. 2013.
Ong, Jong Min, and Lyndon da Cruz. "The Bionic Eye: A Review." Clinical & Experimental Ophthalmology 40.1
(2012): 6-17. Academic Search Complete. Web. 8 Mar. 2013.
Reinberg, Steven. "FDA Approves 'Bionic Eye' to Help Against Rare Vision Disorder." HealthDay Consumer News
Service 14 Feb. 2013: Consumer Health Complete - EBSCOhost. Web. 15 Mar. 2013.
Roberts, Scott. "Retinal Implant Approved for Inherited Eye Disease." HealthDay Consumer News Service 14 Feb.
2013: Consumer Health Complete - EBSCOhost. Web. 15 Mar. 2013.
Stingl, Katarina, et al. "Artificial Vision with Wirelessly Powered Subretinal Electronic Implant Alpha-IMS."
Proceedingsof the Royal Society B: Biological Sciences 280.1757 (2013): 1-8. SCOPUS. Web. 8 Mar.
2013.
Zimmer, Carl. "The Brain." Discover 32.7 (2011): 30-31. Academic Search Complete. Web. 8 Mar. 2013.
Zrenner, Eberhart, et al. "Subretinal Electronic Chips Allow Blind Patients to Read Letters and Combine them to
Words." Proceedingsof the Royal Society B: Biological Sciences 278.1711 (2011): 1489-97. SCOPUS.
Web. 8 Mar. 2013.