01-11-2012, 04:21 PM
Artificial Eye Seminar Report
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INTRODUCTION
The retina is a thin layer of neural tissue that lines the back wall inside the eye. Some of these cells act to receive light, while others interpret the information and send messages to the brain through the optic nerve. This is part of the process that enables us to see. In damaged or dysfunctional retina, the photoreceptors stop working, causing blindness. By some estimates, there are more than 10 million people worldwide affected by retinal diseases that lead to loss of vision.
The absence of effective therapeutic remedies for retinitis pigmentosa (RP) and age-related macular degeneration (AMD) has motivated the development of experimental strategies to restore some degree of visual function to affected patients. Because the remaining retinal layers are anatomically spared, several approaches have been designed to artificially activate this residual retina and thereby the visual system.
At present, two general strategies have been pursued. The “Epiretinal” approach involves a semiconductor-based device placed above the retina, close to or in contact with the nerve fiber layer retinal ganglion cells. The information in this approach must be captured by a camera system before transmitting data and energy to the implant. The “Sub retinal” approach involves the electrical stimulation of the inner retina from the sub retinal space by implantation of a semiconductor-based micro photodiode array (MPA) into this location. The concept of the sub retinal approach is that electrical charge generated by the MPA in response to a light stimulus may be used to artificially alter the membrane potential of neurons in the remaining retinal layers in a manner to produce formed images.
VISUAL SYSTEM
The human visual system is remarkable instrument. It features two mobile acquisition units each has formidable preprocessing circuitry placed at a remote location from the central processing system (brain). Its primary task include transmitting images with a viewing angle of at least 140deg and resolution of 1 arc min over a limited capacity carrier, the million or so fibers in each optic nerve through these fibers the signals are passed to the so called higher visual cortex of the brain.
BLOCK DIAGRAM OF VISUAL SYSTEM
The nerve system can achieve this type of high volume data transfer by confining such capability to just part of the retina surface, whereas the center of the retina has a 1:1 ration between the photoreceptors and the transmitting elements, the far periphery has a ratio of 300:1. This results in gradual shift in resolution and other system parameters.
At the brain’s highest level the visual cortex an impressive array of feature extraction mechanisms can rapidly adjust the eye’s position to sudden movements in the peripherals filed of objects too small to se when stationary. The visual system can resolve spatial depth differences by combining signals from both eyes with a precision less than one tenth the size of a single photoreceptor.
THE EYE
The main part in our visual system is the eye. Our ability to see is the result of a process very similar to that of a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed.
EPI RETINAL ENCODER
The design of an epiretinal encoder is more complicated than the sub retinal encoder, because it has to feed the ganglion cells. Here, a retina encoder (RE) outside the eye replaces the information processing of the retina. A retina stimulator (RS), implanted adjacent to the retinal ganglion cell layer at the retinal 'output', contacts a sufficient number of retinal ganglion cells/fibers for electrical stimulation. A wireless (Radio Frequency) signal- and energy transmission system provides the communication between RE and RS. The RE, then, maps visual patterns onto impulse sequences for a number of contacted ganglion cells by means of adaptive dynamic spatial filters. This is done by a digital signal processor, which, handles the incoming light stimuli with the master processor, implements various adaptive, antagonistic, receptive field filters with the other four parallel processors, and generates asynchronous pulse trains for each simulated ganglion cell output individually. These spatial filters as biology-inspired neural networks can be 'tuned' to various spatial and temporal receptive field properties of ganglion cells in the primate retina.
SUB RETINAL IMPLANTATION
The subretinal approach is based on the fact that for instance of retinitis pigmentosa; the neuronal network in the inner retina is preserved with a relatively intact morphology. Thus, it is appropriate for excitation by extrinsically applied electrical current instead of intrinsically delivered photoelectric excitation via photoreceptors. This option requires that basic features of visual scenes such as points, bars, edges, etc. can be fed into the retinal network by electrical stimulation of individual sites of the distal retina with a set of individual electrodes.
Subretinal approach is aiming at a direct physical replacement of degenerated photoreceptors in the human eye, the basic function of which is very similar to that of solar cells, namely delivering slow potential changes upon illumination. The quantum efficiency of photoreceptor action, however, is 1000 times larger than that of the corresponding technical de-vices. Therefore the intriguingly simple approach of replacing degenerated photoreceptors
by artificial solar cell arrays has to overcome some difficulties, especially the energy supply for successful retina stimulation.
CONCLUSION AND FUTURE SCOPE
The application of the research work done is directed towards the people who are visually impaired. People suffering from low vision to, people who are completely blind will benefit from this project. The findings regarding biocompatibility of implant materials will aid in other similar attempts for in human machine interface. Congenital defects in the body, which cannot be fully corrected through surgery, can then be corrected.
There has been marked increase in research and clinical work aimed at understanding low vision. Future work has to be focused on the optimization and further miniaturization of the implant modules. Commercially available systems have started emerging that integrates video technology, image processing and low vision research.
Implementation of an Artificial Eye has advantages. An electronic eye is more precise and enduring than a biological eye and we cannot altogether say that this would be used only to benefit the human race. In short successful implementation of a bioelectronic eye would solve many of the visual anomalities suffered by human’s to date.
To be honest, the final visual outcome of a patient can not be predicted. However, before implantation several tests have to be performed with which the potential postoperative function can be estimated. With this recognition of large objects and the restoration of the day-night cycle are the primary goals of the prototype implant
[attachment=37782]
[attachment=37783]
INTRODUCTION
The retina is a thin layer of neural tissue that lines the back wall inside the eye. Some of these cells act to receive light, while others interpret the information and send messages to the brain through the optic nerve. This is part of the process that enables us to see. In damaged or dysfunctional retina, the photoreceptors stop working, causing blindness. By some estimates, there are more than 10 million people worldwide affected by retinal diseases that lead to loss of vision.
The absence of effective therapeutic remedies for retinitis pigmentosa (RP) and age-related macular degeneration (AMD) has motivated the development of experimental strategies to restore some degree of visual function to affected patients. Because the remaining retinal layers are anatomically spared, several approaches have been designed to artificially activate this residual retina and thereby the visual system.
At present, two general strategies have been pursued. The “Epiretinal” approach involves a semiconductor-based device placed above the retina, close to or in contact with the nerve fiber layer retinal ganglion cells. The information in this approach must be captured by a camera system before transmitting data and energy to the implant. The “Sub retinal” approach involves the electrical stimulation of the inner retina from the sub retinal space by implantation of a semiconductor-based micro photodiode array (MPA) into this location. The concept of the sub retinal approach is that electrical charge generated by the MPA in response to a light stimulus may be used to artificially alter the membrane potential of neurons in the remaining retinal layers in a manner to produce formed images.
VISUAL SYSTEM
The human visual system is remarkable instrument. It features two mobile acquisition units each has formidable preprocessing circuitry placed at a remote location from the central processing system (brain). Its primary task include transmitting images with a viewing angle of at least 140deg and resolution of 1 arc min over a limited capacity carrier, the million or so fibers in each optic nerve through these fibers the signals are passed to the so called higher visual cortex of the brain.
BLOCK DIAGRAM OF VISUAL SYSTEM
The nerve system can achieve this type of high volume data transfer by confining such capability to just part of the retina surface, whereas the center of the retina has a 1:1 ration between the photoreceptors and the transmitting elements, the far periphery has a ratio of 300:1. This results in gradual shift in resolution and other system parameters.
At the brain’s highest level the visual cortex an impressive array of feature extraction mechanisms can rapidly adjust the eye’s position to sudden movements in the peripherals filed of objects too small to se when stationary. The visual system can resolve spatial depth differences by combining signals from both eyes with a precision less than one tenth the size of a single photoreceptor.
THE EYE
The main part in our visual system is the eye. Our ability to see is the result of a process very similar to that of a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract, or focus the light and a film (retina) on which to focus the rays. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed.
EPI RETINAL ENCODER
The design of an epiretinal encoder is more complicated than the sub retinal encoder, because it has to feed the ganglion cells. Here, a retina encoder (RE) outside the eye replaces the information processing of the retina. A retina stimulator (RS), implanted adjacent to the retinal ganglion cell layer at the retinal 'output', contacts a sufficient number of retinal ganglion cells/fibers for electrical stimulation. A wireless (Radio Frequency) signal- and energy transmission system provides the communication between RE and RS. The RE, then, maps visual patterns onto impulse sequences for a number of contacted ganglion cells by means of adaptive dynamic spatial filters. This is done by a digital signal processor, which, handles the incoming light stimuli with the master processor, implements various adaptive, antagonistic, receptive field filters with the other four parallel processors, and generates asynchronous pulse trains for each simulated ganglion cell output individually. These spatial filters as biology-inspired neural networks can be 'tuned' to various spatial and temporal receptive field properties of ganglion cells in the primate retina.
SUB RETINAL IMPLANTATION
The subretinal approach is based on the fact that for instance of retinitis pigmentosa; the neuronal network in the inner retina is preserved with a relatively intact morphology. Thus, it is appropriate for excitation by extrinsically applied electrical current instead of intrinsically delivered photoelectric excitation via photoreceptors. This option requires that basic features of visual scenes such as points, bars, edges, etc. can be fed into the retinal network by electrical stimulation of individual sites of the distal retina with a set of individual electrodes.
Subretinal approach is aiming at a direct physical replacement of degenerated photoreceptors in the human eye, the basic function of which is very similar to that of solar cells, namely delivering slow potential changes upon illumination. The quantum efficiency of photoreceptor action, however, is 1000 times larger than that of the corresponding technical de-vices. Therefore the intriguingly simple approach of replacing degenerated photoreceptors
by artificial solar cell arrays has to overcome some difficulties, especially the energy supply for successful retina stimulation.
CONCLUSION AND FUTURE SCOPE
The application of the research work done is directed towards the people who are visually impaired. People suffering from low vision to, people who are completely blind will benefit from this project. The findings regarding biocompatibility of implant materials will aid in other similar attempts for in human machine interface. Congenital defects in the body, which cannot be fully corrected through surgery, can then be corrected.
There has been marked increase in research and clinical work aimed at understanding low vision. Future work has to be focused on the optimization and further miniaturization of the implant modules. Commercially available systems have started emerging that integrates video technology, image processing and low vision research.
Implementation of an Artificial Eye has advantages. An electronic eye is more precise and enduring than a biological eye and we cannot altogether say that this would be used only to benefit the human race. In short successful implementation of a bioelectronic eye would solve many of the visual anomalities suffered by human’s to date.
To be honest, the final visual outcome of a patient can not be predicted. However, before implantation several tests have to be performed with which the potential postoperative function can be estimated. With this recognition of large objects and the restoration of the day-night cycle are the primary goals of the prototype implant