Artificial Human vision - KSOS

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424 Kerala Journal of Ophthalmology Vol. XXI, No. 4 Table1. Details of work on Epiretinal prosthesis Surgeon University Country Company Status Dr M Humayun University of California USA Second sight Chronic trials Dr J Rizzo, Wyatt Harvard medical school, USA Boston retinal Abandoned Massachusetts eye and ear implant due to inconsistent reports infirmary after acute trial and started subretinal project. Dr Eckmiller, Dept of computer science, Germany Learning retinal Concentrating on Uty of Bonn implant information processing requirements. Dr G Richard University of Hamburg Germany IMI implants Chronic Implantation trials Dr Walter University of Aachen Germany IBT group Epi ret phase III. The work on successful epiretinal prosthesis is guided by the requirements like, preserving as much the normal anatomy/physiology of the eye as possible while minimizing the amount of implanted electronics required to power the device. Several groups worldwide have developed different designs of epiretinal implants that vary in terms of the intraocular and external elements. Listed in table 1. Dr Mark Humayun at the Doheny Eye Institute and University of Southern California started working with artificial vision initiative, IRP; since early 1990 with Second Sight Medical Products, Inc (Sylmar, CA.) Their system consisting of an externally mounted camera visual processing unit and magnetic coils was implanted in the temporal skull, which provide the inductive link telemetry system. The microelectrodes on the array use these pulses to stimulate any viable inner retinal neurons. The array is positioned just temporal to the fovea and is attached to the inner retinal surface using a single tack, which is inserted through the electrode array into the sclera 13,14 . After it was demonstrated in several different animal models that epiretinal stimulation could reproducibly elicit neural responses in the retina, preliminary tests of acute (
December 2009 Arshad Sivaraman et al. - Artificial Human Vision 425 glasses and the stimulator chip then delivers this information to the microelectrode array on the epiretinal surface of the eye 20 . In order to study the acute effects of electrical stimulation on visual perception, they implanted this device in 5 blind patients with RP and 1 normal-sighted patient who was scheduled for enucleation due to orbital cancer. Three different types of electrode arrays, varying in the number, size and spacing of the peripheral electrodes, were tested. Similar to the results found by the IRP group, they observed that higher charge densities were required to stimulate the retinas in patients with worse vision. No apparent damage as a result of electrical stimulation of the retina was evident in histological specimens from the retina of the enucleated eye of the normal-sighted patient. But their results were mixed and inconclusive. While stimulating a single electrode above threshold levels, multiple phosphenes were often perceived by the blind subjects. Simple pattern vision was not achieved by either the blind or the normal sighted patients when multi electrode patterns of electrode stimulation were applied in trials with multiple electrodes. On average, 3 of the 5 blind RP patients accurately described the percepts that corresponded to the correct stimulation pattern only 32 % of the time, compared to 43 % for the normalsighted patient. Driving the same electrode with the same stimulus parameters at different times showed relatively good reproducibility, which was achieved 66% of the time in the 5 patients 19,20 . Later, due to the inability of getting good or consistent results with epiretinal stimulation, Rizzo and Wyatt have abandoned the epiretinal approach and then started developing a subretinal approach very similar in nature to the Zrenner group( commented later in this article). In 1995 a consortium of 14 expert groups in Germany directed by Rolf Eckmiller has started working for the development of the Learning Retina Implant. Like the previous 2 epiretinal prostheses, their implant also consists of intraocular and extra ocular components. Their retina encoder (RE) (processor), which approximates the typical receptive field properties of retinal ganglion cells, replaces the visual processing capabilities of the retina by means of 100 to 1000 individually tunable spatiotemporal filters. The processing of visual information that occurs in the RE simulates the filtering operations performed by individual ganglion cells. The RE output is then encoded and transmitted via a wireless signal and energy transmission system to the implanted retina stimulator (RS). The REs not only simulate the complex mapping operation of parts of the neural retina, but also provide an interactive, perception-based dialogue between the RE and human subject. The purpose of this dialogue is to tune the various receptive field filter properties with information “expected” by the central visual system to generate optimal ganglion cell codes for epiretinal stimulation 21 . They successfully tested their retina encoder/stimulator in several different animal models as well as normally sighted subjects 22, 23 ( later this group spread up and started working as independent groups). At University of Hamburg Professor G Richard is working on another epiretinal prosthesis with Intelligent medical implant system in their European trial. This device, similar to the other epiretinal designs has got a visual interface( spectacle) a pocket processor, the retinal stimulator and 49 electrodes 24 (Fig. 3). The Visual Interface which looks like a standard pair of glasses consists of a camera to capture images and the electronics to provide energy to the Retinal Stimulator/ electrodes implanted in the eye via wireless transmission. The Pocket Processor with the size of a walkman, contains rechargeable batteries that supply energy for the entire system and a microcomputer that translates the image data into stimulation commands for the Retinal Stimulator. The acute trial results of this group in 4 subjects with an implanted 49-electrode epiretinal array in a trial designed to last 18 months5 (clinicaltrials.gov identifier: NCT00427180; recruiting) were encouraging regarding the phosphene production and the feasibility of a long term intra ocular implant 25 . For the chronic trials the company has come Fig.3. Epi retinal prosthesis of IMI.( image courtesy Dr Hornig)
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