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Vision research for years researchers have known that

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Vision ResearchFor years, researchers have known that electrical impulses administered to the brain of a blind person cause them to "see" things called phosphenes. From the descriptions given by these patients, phosphenes are points of illumination that appear three-dimensionally in different shapes—usually spherical—against a black background. Sometimes more than one is seen; and using one as a reference point, the patient can describe the other as being "behind" the first. William Dobelle has devised a scheme in which he uses these phosphenes to map out a patient's field of vision, and is then able to connect a camera to the patient's brain and channel signals to the correct areas—allowing the blind to "see."In both macular degeneration and retinitis pigmentosa, the photoreceptor cell layer of the innerretina (a.k.a. rods and cones) progressively degenerates. When these cells lose their function, the retina cannot convert light signals into electrical signals—causing a break in the visual chain.To restore this function, research dating back to the 1960s has simply sidestepped or shunted the cell layer. A grid of electrodes is implanted directly onto the visual cortex of the brain, bypassing the entire eye, with wires exiting the skull in a small bundle. This grid can be stimulated by signals received from an externally worn camera.Other systems, closer to what experts envision as the future of artificial vision systems, use implanted retinal microchips. Two types of retinal implants are under study: epiretinal implants—those positioned on the surface of the retina; and subretinal implants—those implanted behind the retina.Epiretinal implant systems generally receive light signals from external camera systems. The light is received by the first of two implants. The first microchip deciphers the signal and relays itto the second microchip. This microchip, positioned on the surface of the retina, uses the signal to stimulate ganglion cells. Once stimulated, the ganglion cells relay the signal to the eye's original and intact optic nerve. The nerve relays the signal to the visual cortex of the brain. In turn, the cortex generates perceptible flashes of light called phosphenes.Subretinal implants receive light directly from the environment, as a healthy retina would. In this approach, the light received is converted into electrical signals that stimulate remnant cell layers of the retina. This approach places the stimulation at the photoreceptor cell layer, which is the first layer to convert light signals to electrical signals in the normal scheme of things. In a healthy eye, the ganglion cell layer through to the photoreceptor cell layer is transparent. By placing the stimulation at the light- converting cell layer, researchers hope to produce the sensation of different shades of light and darkness. Subretinal implants require no external optics, possibly offer different shades of light, and are fully implanted, making them preferable to the epiretinal implants.

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Term
Winter
Professor
Beninger
Tags
Test, Lateral geniculate nucleus, prosopagnosia, visual cortex

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