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530 VISUAL PROSTHESES VISUAL PROSTHESES JEAN DELBEKE CLAUDE VERAART Catholic University of Louvain Brussels, Belgium INTRODUCTION Minute electrical stimuli delivered to the retina, the optic nerve, or the occipital cortex can induce light perceptions called phosphenes. The visual prosthesis aims at exploiting these phosphenes to restore a form of vision in some cases of blindness. Very schematically, a camera or a picture capturing device transforms images into electrical signals that are then adapted and passed on to some still functional part of the visual pathways, thus bridging the defective structures. The system has at least some parts implanted, including electrodes and their stimulator circuits. A photo- sensitive array in the eye could provide the necessary image input, but most approaches use an external minia- ture camera. Typically, the visual data handling requires an external processor and the power supply as well as the data are provided to the implant by a transcutaneous transmission system. Despite a first pioneering attempt by Brindley and Lewin as early as 1968 (1) only very few experimental visual prostheses have been implanted in humans so far. The limited accessibility of the involved anatomical struc- tures, the poorly understood neural encoding, and the huge amount of information handled by the visual nervous system have clearly hampered a development that can not yet be compared with the far more advanced evolution of cochlear implants (see article on Cochlear Implants in this encyclopedia). The visual prosthesis is still at a very Encyclopedia of Medical Devices and Instrumentation, Second Edition, edited by John G. Webster Copyright # 2006 John Wiley & Sons, Inc. VISUAL PROSTHESES 531 early stage, exploring different methodological directions, nate at the periphery. These very sensitive sensors play a and seeking minimal performances that would justify clin- major role in night vision, but do not contribute to high ical applications in the most severe cases of blindness. The resolution nor color perception. The cellular hyperpolari- first fully fledged clinical study has yet to begin and the zation generated by light impinging on the photoreceptors experimental character of existing systems limits all trials is carried over to other the neuronal cell layers ultimately to a restricted number of well-informed adult volunteers. connected to output layer represented by the ganglion cells. With the exception of the fovea (most central part) and especially in the periphery of the retina (i.e., of the visual Vision Basics and Retinotopy field), there is a great deal of convergence and encoding of A basic knowledge of the anatomy and physiology of the the visual signal that must be squeezed from the analog visual system is necessary for a proper understanding of modulation of 130 million photoreceptor potentials into the visual prosthesis in its various forms. the discharge bursts of little more than 1 million ganglion The eye can be compared to a camera with an adjustable cells. Roughly, the bipolar cells represent the main con- optics including the cornea and the lens, focusing inverted verging vertical link between the photoreceptors and the images of the external world onto the retina. The retina is ganglion cells. The horizontal cells at the junction between in fact a thin slice of brain that has expanded into the eye photoreceptors and bipolar cells and the amacrine cells at during the embryo development. It is made up of several the junction between bipolar cells and the ganglion cells cell layers. Among these, the photosensitive cells called provide sideways connections. Functionally, the retinal cones and rods form the external layer with, as a result that network can be subdivided in a number of parallel channels light entering the eye has to cross the complete retina each focusing on the transmission of one aspect of images. before to reach them (see Fig. 1c). These include a color coding system and movement detect- The 120 million or so photosensitive cells are not uni- ing circuits. Bipolar cells and ganglion cells also subdivide formly distributed. Their density is highest at the fovea, a in ON OFF types. The ON cells increase their activity on region that corresponds to the fixation point, near the exposure of the center of their receptive field to light center of the visual field. Cones outnumber the rods in while their ongoing firing slows down when light strikes the central retina and are the only kind of photosensitive in the periphery of that region. The OFF cells react in the cells at the fovea. They discriminate colors. Rods predomi- opposite way. Ganglion cells also respond more or less (a) (b) (c) Sclera Visual field coroid epithelium Eyes Rod Receptor cells Retina Cone Optic chiasma Optic nerve Bipolar cells Lateral geniculate nuclei Retinal Optic radiations ganglion cells Towards higher visual structures Light Visual cortex Figure 1. (a) Schematic representation of the visual pathways from the eye to the visual cortex with an indication of the hemifield segregation at the level of the chiasma. (b) Retinotopy mapping all along the visual pathways. Note also the cortical amplification, that is, disproportional cortical representation of the central visual field. (c) Enlarged view of a cut through the retina showing the various layers and their position in relation to light entering the eye. 532 VISUAL PROSTHESES transiently. The result is a spatiotemporal retinal filter they can be recognized. Reference patterns subtend 5 min providing the optic nerve with a complex and still not well of arc at a distance of 60 m and have features of 1 min of arc, understood neural code Ganglion cell axons first runs over corresponding to the standard normal acuity. A visual the inner surface of the retina to joint the optic nerve head acuity of 3/60 means that such a sign can only be recognized located slightly nasally from the center of the retina. There, at a distance of 3 m. For practical purposes, the Snellen all ganglion cell axons join before to leave the eye and form chart is made up of different sized letters such that the the optic nerve. examination can be performed at a single distance. In the orbit, the optic nerve is relatively slack, laying Because the normal acuity corresponds to 1 min of arc, between extraocular muscles, fat tissue and various blood the MAR value in arc minutes has the same numeric value vessels and nerves. It is protected by a strong sleeve of dura as the reciprocal of the Snellen fraction. mater as well as a very thin pia mater with cerebrospinal However, sight is a multidimensional ability that can- fluid in between. When the optic nerve enters the skull, it not be measured by acuity alone. The effect of a visual field looses its dura mater sleeve, which now lines the inner defect or a reduced sensitivity to light cannot be compared surface of the skull. After a little >1 cm, the two intracra- directly to acuity. nial optic nerves meet and exchange fibers at the level of The International Statistical Classification of Diseases the chiasma (see Fig. 1a). Ganglion cell axons of the nasal and Related Health Problems of the World Health Orga- hemiretina or temporal visual field cross the midline and nization (ICD-10) uses codes 1–5 to describe moderate, join the temporal axons of the other eye to form the optic severe, profound, near total, and total visual impairments, tract. Foveal axons split into one branch toward each side. respectively. Within that range, the label low vision (cate- Some fibers (not represented) corresponding to accessory gories 1 and 2 ) designates a visual acuity >6/18 (0.3), but functions leave the mainstream visual pathway to reach <3/60 (0.05) in the better eye with optimal correction or a the hypothalamus, pretectum and superior colliculus. The visual field between 10 and 308. The label blindness encom- axons directly involved in vision end in the lateral genicu- passes categories 3–5. Code 3 corresponds to a visual acuity late nucleus. This structure performs further signal pro- <0.05 on the Snellen scale for the best eye using appro- cessing and receives control signals from various parts of priate correction, or a central visual field diameter of <108 the brain. From the lateral geniculate nucleus, the visual in its largest diameter. An acuity of <0.02 or a visual field information reaches the occipital lobe of the brain through <58 is coded 4 (near total) while total visual impairment the optic radiation. Interestingly, corresponding inputs means deprived of light perception. from both eyes are arranged in close proximity, but remain To measure a visual acuity in near total visual impair- segregated up to the level of the primary visual cortex. ment, alternative methods are used including close range From there, signals corresponding to various aspects of the on the Snellen chart reading, finger counting, the detection visual stimulus are dispatched to different brain locations of hand motion, or the perception of light. for further processing. Blindness can result from any cause potentially affect- Consequently, of the fiber exchanges at the level of the ing the visual pathways: genetic abnormalities, infections, chiasma, except for the representation of the fovea, one metabolic diseases, trauma, vascular deficiency, or cancer visual cortex receives only information about the contra- for example. In one subgroup, mainly represented by reti- lateral visual field. In addition, albeit with much distortion, nitis pigmentisa (RP), age related macular degeneration, cells of the visual cortex tend to retain the topological and stargardt’s disease, it has been shown that blindness relationship of the retinal location from which they receive can result from a total destruction of the photosensitive cell their input. The resulting point-to-point correspondence layer while a proportion of other cells of the retina and the with the retinal locations, and hence the visual field is remaining of the visual pathways survive.
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