Visual prosthesis

For the non-functional prosthesis or glass eye see Ocular prosthesis and Craniofacial prosthesis.

A visual prosthesis, often referred to as a bionic eye is an experimental visual device intended to restore functional vision. Many devices have been developed, usually modeled on the cochlear implant or bionic ear devices, a type of neural prosthesis in use since the mid 1980s.

History

Scientific research since at least the 1950s has investigated interfacing electronics at the level of the retina, optic nerve, thalamus, and cortex. Visual prosthetics, which have been implanted in patients around the world both acutely and chronically, have demonstrated proof of principle, but do not yet offer the visual acuity of a normally sighted eye. It has recently been claimed that after the accidental death of a human volunteer in the late nineties, much research on visual prostheses has been halted, however, this claim was based on an unpublished source that recent correspondence to the journal Perception has questioned.

Biological considerations

The ability to give sight to a blind person via a bionic eye depends on the circumstances surrounding the loss of sight. For retinal prostheses, which are the most prevalent visual prosthetic under development (due to ease of access to the retina among other considerations), vision loss due to degeneration of photoreceptors (retinitis pigmentosa, choroideremia, geographic atrophy macular degeneration) is the best candidate for treatment. Candidates for visual prosthetic implants find the procedure most successful if the optic nerve was developed prior to the onset of blindness. Persons born with blindness may lack a fully developed optical nerve, which typically develops prior to birth.

Technological considerations

Visual prosthetics are being developed as a potentially valuable aid for individuals with visual degradation. The visual prosthetic in humans remains investigational.

Ongoing projects

Argus Retinal Prosthesis

Drs. Mark Humayun and Eugene DeJuan at the Doheny Eye Institute (USC) were the original inventors of the active epi-retinal prosthesis and demonstrated proof of principle in acute patient investigations at Johns Hopkins University in the early 1990s. In the late 1990s the company Second Sight was formed to develop a chronically implantable retinal prosthesis. Their first generation implant had 16 electrodes and was implanted in 6 subjects between 2002 and 2004. Five of these subjects still use the device in their homes today. These subjects, who were all completely blind prior to implantation, can now perform a surprising array of tasks using the device. More recently, the company announced that it has received FDA approval to begin a trial of its second generation, 60 electrode implant, in the US. Additionally they have planned clinical trials worldwide, all getting underway in 2007. Three major US government funding agencies (National Eye Institute, Department of Energy, and National Science Foundation) have supported the work at Second Sight and USC.

Microsystem-based Visual Prosthesis (MIVIP)

Designed by Claude Veraart at the University of Louvain, this is a spiral cuff electrode around the optic nerve at the back of the eye. It is connected to a stimulator implanted in a small depression in the skull. The stimulator receives signals from an externally-worn camera, which are translated into electrical signals that stimulate the optic nerve directly.

Implantable Miniature Telescope

Although not truly an active prosthesis, an Implantable Miniature Telescope is one type of visual implant that has met with some success in the treatment of end-stage age-related macular degeneration. This type of device is implanted in the eye's posterior chamber and works by increasing (by about three times) the size of the image projected onto the retina in order to overcome a centrally-located scotoma or blind spot.

Tübingen MPDA Project

A Southern German team led by the University Eye Hospital in Tübingen, was formed in 1995 by Eberhart Zrenner to develop a subretinal prosthesis. The chip is located behind the retina and utilizes microphotodiode arrays (MPDA) which collect incident light and transform it into electrical current stimulating the retinal ganglion cells. As natural photoreceptors are far more efficient than photodiodes, visible light is not powerful enough to stimulate the MPDA. Therefore, an external power supply is used to enhance the stimulation current. The German team commenced in vivo experiments in 2000, when evoked cortical potentials were measured from Yucatán micropigs and rabbits. At 14 months post implantation, the implant and retina surrounding it were examined and there were no noticeable changes to anatomical integrity. The implants were successful in producing evoked cortical potentials in half of the animals tested. The thresholds identified in this study were similar to those required in epiretinal stimulation. The latest reports from this group concern the results of a clinical pilot study on eight participants suffering from RP. The results were to be presented in detail on the ARVO 2007 congress in Fort Lauderdale.

Harvard/MIT Retinal Implant

Joseph Rizzo and John Wyatt at the Massachusetts Eye and Ear Infirmary and MIT began researching the feasibility of a retinal prosthesis in 1989, and performed a number of proof-of-concept epiretinal stimulation trials on blind volunteers between 1998 and 2000. They have since developed a subretinal stimulator that sits on the outside of the eye and receives image signals beamed from a camera mounted on a pair of glasses. The stimulator chip decodes the picture information beamed from the camera and stimulates retinal ganglion cells accordingly.

Artificial Silicon Retina (ASR)

The brothers Alan Chow and Vincent Chow have developed a microchip containing 3500 photo diodes, which detect light and convert it into electrical impulses, which stimulate healthy retinal ganglion cells. The ASR requires no externally-worn devices.

Optoelectronic Retinal Prosthesis

Daniel Palanker and his group at Stanford University have developed an optoelectronic system for visual prosthesis that includes a subretinal photodiode array and an infrared image projection system mounted on video goggles. Information from the video camera is processed in a pocket PC and displayed on pulsed near-infrared (IR, 850-900 nm) video goggles. IR image is projected onto the retina via natural eye optics, and activates photodiodes in the subretinal implant that convert light into pulsed bi-phasic electric current in each pixel. Charge injection can be further increased using a common bias voltage provided by a radiofrequency-driven implantable power supply Proximity between electrodes and neural cells necessary for high resolution stimulation can be achieved utilizing effect of retinal migration.

Dobelle Eye

Main article: William H. Dobelle

Similar in function to the Harvard/MIT device, except the stimulator chip sits in the primary visual cortex, rather than on the retina. Many subjects have been implanted with a high success rate and limited negative effects. Still in the developmental phase, upon the death of Dr. Dobelle, selling the eye for profit was ruled against in favor of donating it to a publicly funded research team.

Intracortical Visual Prosthesis

Main article: Intracortical Visual Prosthesis

The Laboratory of Neural Prosthesis at Illinois Institute Of Technology (IIT), Chicago, is developing a visual prosthetic using Intracortical Iridium Oxide (AIROF) electrodes arrays. These arrays will be implanted on the occipital lobe. External hardware will capture images, process them and generate instructions which will then be transmitted to implanted circuitry via a telemetry link. The circuitry will decode the instructions and stimulate the electrodes, in turn stimulating the visual cortex. The group is developing a wearable external image capture and processing system. Studies on animals and psyphophysical studies on humans are being conducted to test the feasibility of a human volunteer implant.

Virtual Retinal Display (VRD)

Main article: Virtual retinal display

Laser-based system for projecting an image directly onto the retina. This could be useful for enhancing normal vision or bypassing an occlusion such as a cataract, or a damaged cornea.

Visual Cortical Implant

File:ImplantSawan.JPG

The Visual Cortical Implant

Dr. Mohamad Sawan, Professor and Researcher at Polystim neurotechnologies Laboratory at the Ecole Polytechnique de Montreal, has been working on a visual prosthesis to be implanted into the human cortex. The basic principle of Dr. Sawan’s technology consists in stimulating the visual cortex by implanting a silicium microchip on a network of electrodes made of biocompatible materials and in which each electrode injects a stimulating electrical current in order to provoke a series of luminous points to appear (an array of pixels) in the field of vision of the sightless person. This system is composed of two distinct parts: the implant and an external controller. The implant lodged in the visual cortex wirelessly receives dedicated data and energy from the external controller. This implantable part contains all the circuits necessary to generate the electrical stimuli and to oversee the changing microelectrode/biological tissue interface. On the other hand, the battery-operated outer control comprises a micro-camera which captures the image as well as a processor and a command generator which process the imaging data to select and translate the captured images and to generate and manage the electrical stimulation process and oversee the implant. The external controller and the implant exchange data in both directions by a powerful transcutaneous radio frequency (RF) link. The implant is powered the same way.

Other projects

Other note-worthy researchers include Richard Normann (University of Utah) and David Bradley at University of Chicago, Eduardo Fernandez and the European Consortium CORTIVIS (http://cortivis.umh.es), Ed Tehovnik at MIT, Tohru Yagi in Japan Visual Prosthesis Project, and the Bionic Vision Australia multi-institute partnership.

See also

• Bionic contact lens

External links

• ARMD, Retinal Electronic Prosthesis and RPE Transplantation - eMedicine.com
• 'Bionic' eye implants look ahead - BBC
• NewScientist.com: 'Bionic eye' may help reverse blindness
• BBC: 'Artificial eye'
• How does a "bionic eye" allow blind people to see? - HowStuffWorks.com
• Research opens way for bionic eye; BBC, 24 April 2007.
• Visual Prosthesis Links; Japan Visual Prosthesis Groupde:Retina-Implantat

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