[...]... to the surface of the retina facing the vitreous cavity and send mostly unmyelinated axons in a more superficial layer toward the optic disk As the human RGC axons exit the eye, they become myelinated and form the optic nerve The cell bodies (somas) of these ganglion cells are mapped over the surface of the retina in a manner that approximates the projection of the visual world onto the surface of the. .. between the design of an IRP and a device for direct stimulation of the brain or other sensory areas of the central nervous system (CNS) The first section of this chapter gives a brief description of the retina and some background on work in visual prosthetics The second section gives an overview of the concept for an IRP Electrical stimulation of the retina is discussed in the third section The fourth... concept for a retinal prosthesis A ray trace of photons incident on a retina without a prosthesis is shown in the top half of the figure Incoming photons pass through several layers of clear retinal cells before being absorbed by the photoreceptors In the bottom half of the figure, a retinal prosthesis is shown positioned against the retina retinal cells before being absorbed by the photoreceptors In the. .. ms) There are welldefined relationships between the threshold current and the duration of the stimulus pulse required for neuronal activation (West and Wolstencroft, 1983) As the duration of the stimulus pulse decreases, the threshold increases exponentially Also, as the pulse duration increases, the threshold current approaches a minimum value, called the rheobase A chronaxie is the pulse width for. .. properties of the image that are passed on to the higher visual centers (color, intensity, motion, etc.) Therefore it may be necessary to integrate some image-processing functions into a retinal prosthesis This issue is the subject of the next section Overview of an Intraocular Retinal Prosthetic Device The basic concept of an IRP is straightforward: Visual images can be produced in the brain by electrical... feasible Another type of neural prosthesis, the cochlear prosthesis for deaf patients, has been successfully developed and commercialized (Agnew and McCreery, 1990; Heiduschka and Thanos, 1998) Development of a retinal prosthesis is generally following in the footsteps of the cochlear prosthesis, but is a number of years behind at this point Although there are other approaches to a visual prosthesis, this... examples drawn from other fields and then try to relate this categorization to actual or potential neural prostheses in order to understand their technical feasibility, clinical potential, and strategic risk ´ The cliche question from the layperson is, ‘‘If we can put a man on the moon, why can’t we cure cancer?’’ Putting a man on the moon is in the category of engineering because all the laws of physics... (figure 2.1) The sensory retina is a delicate sheet of transparent tissue varying in thickness from 0.4 to 0.15 mm The anatomical site for detailed fine vision, called the fovea, is in the center of the macula The outermost layer of the sensory retina consists of photoreceptors (figure 2.2); in the macular region, the photoreceptors are mostly cones (color-sensitive) Other more inner layers of the sensory... objects The transduction systems of both the intact retina and the intact cochlea are built in this way It has long been known that the first sensations induced by weak electromagnetic fields are visual and auditory auras In the absence of this background activity from the receptors, however, the postsynaptic neurons that generate all-or-none action potentials to convey sensory information to the brain. .. that inputs electrical impulses to create the perception of an image The axons of the stimulated ganglion cells then transmit the image through the optic nerve to cells in the visual cortex This is in place of the normal phototransduction process that occurs in a healthy retina In a large percentage of blind patients, the photoreceptors are diseased but the other retinal layers are still responsive . h0" alt="" TOWARD REPLACEMENT PARTS FOR THE BRAIN IMPLANTABLE BIOMIMETIC ELECTRONICS AS NEURAL PROSTHESES TOWARD REPLACEMENT PARTS FOR THE BRAIN IMPLANTABLE. 02142 ht tp://mitpress.mit.edu 0-262-02577 -9 !7IA2G2-acfhhf!:t;K;k;K;k Toward Replacement Parts for the Brain Toward Replacement Parts for the Brain Implantable Biomimetic Electronics as Neural Prostheses edited by Theodore