IntroductionThe bionic eye, commonly found in science fiction may someday be common in real life. Over the past few decades this piece of science fiction has become a reality. The Argus II retinal prosthesis system, approved by the Food and Drug Administration, has begun restoring visual perception in individuals with severe to profound retinitis pigmentosa as well as other degenerative diseases of the retina. (Second Sight)
Functional Anatomical Review
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Saladin

A review of the EyeThe eye collects light from the surrounding world and transduces it into a signal that can be processed in the brain. Transduction takes place in the photoreceptors found in the retina. (Nelson 477) In order to get to the photoreceptors, light enters through the cornea, passes through the anterior chamber, then through the pupil and continues through the lens into the vitreous humor back onto the retina. The pupil and the lens refract the light in order to form a clear image on the fovea. (Saladin) The retina consists of several layers, the retinal pigment epithelium, the receptorlayer, the outer plexiform layer, the inner nuclear layer, the inner plexiform layer, the retinal ganglion cell layer, and the optic nerve layer. The photoreceptors consist of Rods and cones. The rods contain rhodopsin, a photopigment that breaks down when exposed to certain wavelengths of light. They function at low levels of light and are more sensitive than the cone cells. Rods are found more in the peripheral of the retina. The cones contain photopigments that are color sensitive. The cone cells are heavily concentrated in the fovea allowing for high visual acuity. The photoreceptors converge and synapse on bipolar cells, which then converge and synapse on ganglion cells. The ganglion cells connect the retina to the brain acting as a direct pathway with their axons forming the optic nerve. (Dragoi)
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Components of the Argus II Retinal Stimulation SystemArgus II.png
The Argus II Retinal stimulation system consists of both implants and external components The implanted component consists of a 6 by 10 electrode array (200 lm electrode diameter, 575 lm center-to-center spacing), which is tacked to the epiretinal surface. The implanted component also consists of a titanium case that is attached to the outside of the eye with a scleral band. The case contains a receiver coil and a microprocessor driven stimulator. The external components are comprised of a body-worn video processing unit, and a pair of glasses with a miniature camera and transmitter coil attached. The camera transmits the image to the video processing unit, which transforms the signal into a custom pattern of electrical stimulation. The transmitter coil then powers up and sends the commands to the implanted stimulator. The implanted stimulator is then able to activate the retinal electrodes. (Fornos)

Input & Output pathways


The input into the eye is photons of light. In a normal human eye the photons are directed by the cornea and lens to the retina with the target focused on the fovea. The photons pass through the layers of the retina all the way to the receptor layer. The light is absorbed by rhodopsin (in rod cells) this absorption converts 11-cis-retinal to all-trans-retinal activating the rhodopsin. The activated rhodopsin then activated transducin, which activated cGMP phosphodiesterase (PDE). PDE then reduces cGMP closing off cation channels and preventing the influx of sodium and calcium. This causes the photoreceptor to hyperpolarize. (Nelson 477) The photoreceptors then release less glutamate. The photoreceptors synapse on the bipolar cells, which respond to the change in glutamate by either depolarizing or hyperpolarizing depending on the type of bipolar cell. The bipolar cells synapse on the ganglion cells, which take the signal from the retina to the brain, their axons forming the optic nerve. (Dragoi)
Bionic Eyes

Second Sight

The company Second Sight has developed the Argus II Retinal stimulation system. The device functions as a prosthetic photoreceptor and has been tested on individuals suffering from retinitis pigmentosa and other degenerative diseases of the retina such as age related macular degeneration. (Fornos) These diseases result in the loss of the photoreceptive rod and cone cells but the bipolar and ganglion cells remain intact. It is the fact that the bipolar and ganglion cells remain that allows the Argus II to work. The Argus II serves as an artificial photoreceptor, capturing the image with the camera transducing the light signals and sending the signal to the electrodes on the retina. The electrodes are then able to transmit the signal to either the bipolar or ganglion cells depending on how it is placed in the eye. The perception of the electrical stimulation of the retina can vary greatly among different subjects. The differences can include different perception thresholds, shapes and colors perceived. The temporal aspects can also vary greatly some with quick flashes others perceiving the stimulus for several seconds. (Fornos)

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Surgery

The internal components require a surgery averaging just over four hours (Fornos) During the surgery the lens of the eye is removed. The implant coil is placed temporarily on the globe and centered under the lateral rectus which has been isolated. The electronics package is centered in the superior temporal quadrant and the inferior part of the scleral band is passed under the inferior and the medial rectus muscles, and the superior portion of the band is passed under the superior rectus muscle. The implant is then fixed to the eye with sutures. The vitreous humor is then removed from the eye and the electrode array is slipped into the eye through a cut made in the sclera. It is placed onto the retina and tacked into place. The extra-ocular portion of the cable is sutured in place. (Humayun)

Tests for Visual Acuity

Tests for visual acuity have been conducted on the individuals that have received the Argus II. These tests included “square localization” where the subjects were asked to localize a white square on a black background, and “Direction of motion” where they had to differentiate the orientation of black and white bars of different widths. Subjects were also tested with real world utility tests including the “Door Test” in which they had to find a door across the room and the “Line test” where they had to follow a white line along the ground. Data from these tests suggests that the Argus II improved the subjects object localization, motion discrimination, and discrimination of oriented gratings. (Humayun) At this point the Argus II only has 60 electrodes, which does not allow for a very detailed image. To date the best recorded visual acuity is 20/1260, meaning that person is able to see an image at 20 feet that the average person can see at 1260 feet.( Humayun)

Conclusion

There is now hope for individuals losing their sight to retinal degenerative diseases such as retinitis pigmentosa and macular degeneration with the development of devices like the Argus II. Devices like this that replaces the lost or damaged photoreceptors and create new synapses with the remaining bipolar and ganglion cells of the retina ultimately restoring some vision. The vision restored with devices like the Argus II is not the most detailed visual field, consisting of only 60 points, but it is still an improvement and has been proven to improve patient’s ability to guide fine hand movements, and localize objects. (Humayun) As the technology progresses the implants are expected to produce more detailed images.

Glossary


Retinitis Pigmentosa – A degenerative disease where the photoreceptors of the retina deteriorate affecting the peripheral visual feilds

Photoreceptors – Rods and Cones

Retinal pigment epithelium - provides metabolic and supportive functions to the photoreceptors

receptor layer – contains the light sensitive outer segments of the photoreceptors

outer nuclear layer – contains the photoreceptor cell bodies;

outer plexiform layer - where the photoreceptor, horizontal and bipolar cells synapse

inner nuclear layer - contains the horizontal, bipolar and amacrine cell bodies

inner plexiform layer - where the bipolar, amacrine and retinal ganglion cells synapse

retinal ganglion cell layer -contains the retinal ganglion cell bodies

optic nerve layer- contains the ganglion cell axons traveling to the optic disc.

Rhodopsin- a photopigment in rods, it undergoes a conformational change when it absorbs light

Bipolar cells – the intermediate neurons between the photoreceptors and the ganglion cells

Ganglion cells- the neurons that take visual information from the retina to the brain. They make up the optic nerve

Fovea – a cluster of cone cells on the retina were visual acuity is highest

Visual acuity – the sharpness of vision

Optic nerve – nerve connecting the retina to the brain

Electrode – a conductor that allows electricity to enter or leave an object

Epiretinal surface – outer surface of the retina

Scleral band – a band on the Argus II used to secure the implant onto the eye, the band goes around the sclera

Superior temporal quadrant – the upper portion of the eye on the side of the temple

Photon – a particle with no mass representing light

cGMP phosphodiesterase – An enzyme that reduces cGMP

cGMP – a second messenger found rods and cones

Hyperpolarize – when a cells membrane potential becomes more negative

Globe – the eye

Macular degeneration – condition where the center of the visual field is lost

Suggested Readings and links


Second Sights official website with information about their company and product.
http://2-sight.eu/en/home-en

Nano Retina is working on a bionic eye that will be wireless and be charged by an infrared laser. The user will be able to move their eyes and not rely on an external camera.
http://www.nano-retina.com/

This is a video of the implant surgery.
http://www.youtube.com/watch?v=4vz9SxEDnFI

This is another implant being developed. It sits behind the retina.
http://www.youtube.com/watch?v=vO6BMkWhDDc

Quiz

True or false
  1. Rhodopsin is the photopigment found in cone cells.
  2. Visual perception of electrical stimuli on the retina varies from person to person
  3. The Argus II improves the patients ability to guide fine hand movements
  4. The Argus II replaces the damaged ganglion cells
  5. The Electronics package was centered in the inferior nasal quadrant
Short answer
  1. Why wouldn’t the Argus II work for all forms of blindness?
  2. Is the lens of the eye necessary after the implant is put in place?
  3. Would using a higher definition camera improve the capability of the apparatus?
Answers
1-F, 2-T, 3-T, 4-F, 5-F
6 – The Argus II works with retinitis pigmentosa because the photoreceptors are not working but there are still bipolar cells and ganglion cells that are working as well as the visual pathways within the brain itself. If the ganglion cells were damaged or the source of blindness is within the brain itself the Argus II will not be effective.
7 – No, the lens is no longer necessary; in fact it is removed in the surgery. The camera functions as the new lens and photoreceptors.
8 – perhaps a camera that distinguishes contrast better could make a slight difference but a huge limiting factor is the number of electrodes attaching to the retina. Even if the camera is better there are still only 60 synapses.

References

Dragoi, Valentin, Ph.D. "Visual Processing: Eye and Retina." Visual Processing: Eye and Retina (Section 2, Chapter 14) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston. N.p., n.d. Web. 07 Dec. 2013. <http://neuroscience.uth.tmc.edu/s2/chapter14.html>.


Fornos, Angelica P., J¨org Sommerhalder, Lyndon Da Cruz, Jose A. Sahel, Saddek Mohand-Said, Farhad Hafezi, and Marco Pelizzone. "Temporal Properties of Visual Perception on Electrical Stimulation of the Retina." (n.d.): n. pag. Print.


Humayun, M. S., J. D. Dorn, L. Da Cruz, G. Dagnelle, J. A. Sahel, P. E. Stanga, A. V. Cideciyan, J. L. Duncan, E. Filley, A. C. Ho, A. Santos, A. B. Safran, A. Arditi, L. V. Del Priore, and R. J. Greengerg. "Interim Results from the International Trial of Second Sight's Visual Prosthesis." (n.d.): n. pag. Pubmed.gov. Web. 7 Dec. 2013. <http://www.ncbi.nlm.nih.gov/pubmed/22244176?dopt=Abstract>.

Nelson, David L., Albert L. Lehninger, and Michael M. Cox. Lehninger Principles of Biochemistry. New York: W.H. Freeman, 2013. Print.


Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. N.p.: McGraw-Hill Science Engineering, 2009. Print.


"Second Sight." Argus II Retinal Prosthesis System. N.p., n.d. Web. 07 Dec. 2013. <http://2-sight.eu/en/g-the-argus-ii-prosthesis-system-pf-en>.