الفهرس 
An Overview of Human Visual SystemOnly 14 pages are availabe for public view
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Abstract
Once the topic of folklore and science fiction, the notion of restoring vision to the blind is now approaching a tractable reality. Technological advances have inspired numerous multidisciplinary groups worldwide to develop visual neuroprosthetic devices that could potentially provide useful vision and improve the quality of life of profoundly blind individuals. There are more than 40 million blind individuals in the world whose plight would be greatly ameliorated by creating a visual prosthetic. The ability to see is a complex phenomenon that involves neuroprocessing of images by the retina and conduction of this signal via the optic nerve to the primary visual cortex. We begin by outlining the basic operational characteristics of the visual system as this knowledge is essential for producing a prosthetic device based on electrical stimulation through arrays of implanted electrodes. Retinitis Pigmentosa (RP) and Age-Related Macular Degeneration (ARMD) are two of the leading causes of non-congenital blindness. These degenerative disorders primarily affect the photoreceptors in the retina while leaving the rest of the retina and visual pathway relatively intact. In chapter 2, we outline the basic diagnostic features of both diseases and recent treatment modalities available to date with emphasis on the emerging role for the retinal prosthetic devices in their management. While a variety of approaches and designs are being pursued, they all share a common principle of creating visual percepts through the stimulation of visual neural elements using appropriate patterns of electrical stimulation. We further outlined here the basic concepts of the prosthetic vision idea, different surgical approaches, advantages and disadvantages of each type, and the clinical trials to date being investigated. Among these trials, and being the most successful examples, was the Second Sight Medical Products Company in association with Doheny Eye Institute teamed by Dr. Mark S. Humayun. The so named Argus II epiretinal prosthesis was the first device to received CE and FDA approval for commercial use as a visual aid in Europe and USA. The device relies on basics of epiretinal prosthesis with the 60-elected chip implanted on the surface of the retina and with external components as spectacle-mounted camera and built video processing unit. The results were so promising and the group is actually planning for a 250-electrode chip aiming for much higher resolution and better of course quality of life visual parameters. On the other hand, the Artificial Silicon Retina was one of the earliest devices utilizing the subretinal approach for retinal prostheses. The trial, which was carried out by Optobionics Company and Dr. Alan Y. Chow team, has achieved good results regarding artificial vision. However, based on many findings, the idea of using microphotodiodes array as sole source for electrical stimulation have proven to be ineffective to stimulate remaining retinal cells and the improvement observed in these patients who received the implant was later on attributed to the neuroprotective effect of the device on the remaining dying photoreceptors. The device has now fallen in popularity with the Dr. Chow abandoning the notion of this approach. The alpha IMS subretinal device system combined both microphotodiodes array and 4 x4 electrodes for subretinal stimulation. The study on this device carried out by RI (Reutlingen, Germany) GmbH was the first to report letter reading. Based on these promising results a large multicenter study has been launched with an upgraded version of the device utilizing wireless connection to eliminate the percutaneous cables. Regarding other epiretinal devices, the EPIRET3 was the first system that made all its components to lies inside the eye with no cables crossing the eye’s wall. The energy source and data processing unit are placed in front of the eye in the capsular bag after phacoemulsification. However, the implant was only for investigative purposes and all the implants were removed from the subjects. Indeed, the idea of total intraocular implant is still in mind and possible if the technical difficulties were passed. The suprachoroidal transretinal stimulation STS system was a device that has been studies by investigators in Japan in order to achieve a device with a less invasive method for implantation. The surgery is less complicated and the electrodes are relatively easy to remove where there is any damage or complications. However, being far from the targeted neurons is still an issue and the clinical trials are still in their primitive stages. Finally, in the microfluidic system model of retinal prosthesis, the idea of using chemical neurotransmitters to stimulate the remaining neural retinal cells instead of electrical impulses is entertained. The advantage of this approach is that it can be combined with other methods of artificial vision aiming to reduce the amount of current needed for stimulation.