Retinal and transforming them into electric signals and

Retinal Protheses also commonly known as ‘bionic eye’ are FDA approved technological devices that emerged to provide photoreceptor function for people who lost their vision to profound vision loss as a consequence of retinal dystrophies such as retinitis pigmentosa. The Argus II is one of the most recent of the retinal prosthesis systems. These systems are dependant on remaining functional photoreceptors and optic nerve. How successful these systems are, depend on efficiency of capturing visual images and then transduction of these images into neural electric signals that will ultimately be sent through the optic nerve to the visual cortex. The retinal prosthesis are made of three external and three internal components. The external components consist of a pair of glasses with a video camera mounted on, a portable computer which processes the images captured and transforming them into electric signals and an external coil mounted onto the arm of the glasses which transmits the processed data wireless to the internal components via radio frequency.  The internal component are made of an internal coil, inbuilt Application-Specific-Integrated-Circuit (ASIC), and a sixty channel micro-electrode epiretinal array. The coil receives the frequency sent by the external coil. The ASIC stimulates the multi-electrode array and the a sixty channel micro-electrode epiretinal array is made of 60 platinum electrodes which have a diameter 1?4 200 mm with 575 mm apart from each other, lodged in a thin film of polyamide. The system is made that each microelectrode is attached to the ASIC in a parallel by a metallised polymer connecting cable, so that each micro electrode independently can be activated. The multi electrode array directly contacts the retinal surface, allowing the electrical charges locally to stimulate the retinal tissues. When the retinal prosthesis where made, they were based on a few assumptions. First assumptions were that the patient still has functionally intact retina or fictional photoreceptors which can transmit information to the visual cortex. Then the photoreceptors will be activated with a localized electrical stimulation without causing any further damage. The retinal mapping is partially preserved in the inner retina and the visual pathway, in order to have multi focal stimulations that form geometric patterns of phosphene according to retinotopic locations. The obtained geometric patterns of phosphenes are correctly arranged  spatio-temporally along the visual pathway and then are interpreted and recognized by the primary visual cortex as visual patterns. Even though the availability of the pixels is limited it can still provide useful visual information (Luo, 2016). The capture spatio-temporal pixelization mimics the functional eye movements in order to generate images for artificial retina by combining spatial and temporal information(Kim, 2017). One issue found with the microelectrode electrical signals is that they are pickup up only by “close point stimulation”, resulting in “gross retinotopic localization of retinal responses” which gives rise to geometric patterns and pixelated vision. This means partial vision for the patient (Geruschat, 2017). The continuous stimulation of multiple electrodes can produce recognizable patterns, and the patients are usually able to identify objects, locate and recognize direction of movement in the visual field. The majority of patients achieve outstanding visual improvement which allows them to partially visuals letter and potentially read short sentences (Lauritzen, 2012). Some patients were able to highly perform on optotype acuity tests using Argus II systems (Shivdasani, 2017). However, the benefits of being able to recognize objects, and to partially restore vision with the aid of a retinal prosthesis outweighs the limitations.


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