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Retinal and Cortical Implants. Stat 19 SEM 2. 263057202. Talk 4. The brain is a network of nerve cell, neurons. The neurons communicate by electrical and chemical signals. Light to Electricity. • The input to the visual system is light rays/photons. • These are received at the retina and converted into electrical signals. • These signals are transmitted and processed by networks of neurons. • Idea: Input the same information directly by using electrodes to stimulate the neurons. Retinal & Cortical Implants. • First attempt in 1956. • Excitement and hype. Stevie Wonder. • Currently – cautious long term optimism. • “To impart a coarse level of vision that would expand a blind person’s autonomy is an ambitious but plausible goal”. John Wyatt (MIT). Retinal and Cortical Implants. • Where to Implant? • Visual Information • • received at Retina. Information sent by the Optic Nerve. Information decoded in the Visual Cortex. All three places have been tried. Implants in the Visual Cortex. • First attempts were in the visual cortex. • But: • (I) The retina acts like a sophisticated camera. • (II) The cortex is involved in processing information. • The cortex is more complicated and less understood. • “Easy to implant electrodes and stimulate an epileptic fit”. Visual Cortex. • Complexity of the Cortex. Retinal Implants. • Currently: most attempts are made to stimulate the retina. • Two alternative types: (i) Epiretinal (MIT, USC,…) (ii) Subretinal (Chicago, Tubingen). • Light perception, but can patients process images (e.g. read a letter)? • USC far more optimistic than MIT. Retina. Background. Light enters the Pupil. It is received and converted to electricity in the retina. The signal is transmitted down the Optic Nerve to The Cortex. Effect of Eye Diseases. • • • • Normal. Glaucoma Retina Pigmentosis, Macular Degeneration. Retinal Pathway. • Retina is organized 1. 2. 3. 4. 5. 6. backwards. Rods and Cones. Horizontal Cells. Bipolar Cells. Amacrine Cells. Ganglion Cells. Optic Nerve. Photoreceptors. • Structure of a photoreceptor. Photoreceptors: Light to Electricity. Processing in Retinal Pathway. Processing in the Retinal Pathway is fairly well Understood. Two Types of Implants. • I. Subretinal Implants (Chicago, Tubingen) • replace damaged rods and cones by Silicon plate carrying 1000’s of light-sensitive microphotodiodes Each with a stimulation electrode. • • Light from image activates the microphotodiodes, the electrodes inject currents into the neural cells (horizontal, bipolar, etc.) Two Types of Input. • II. Epiretinal Implants (MIT, USC). • The implants receive input from a camera and processing unit. (E.g. on glasses). • Electrodes from the implants electrically stimulate the ganglion cells and axons at the start of the optic nerve. Two Types: Tradeoffs. • Subretinal Implant uses all of the retina (except the rods/cones). • Epiretinal Implant does not, it must replace the function of entire retina and convert light to neural code. • But the input to the Epiretinal Implant is more easily controlled (external camera). • Fixing the Subretinal Implant is comparatively easier. Can Patients see with Implants? • USC and MIT groups attached Epiretinal Implants to human patients. • Patients perceived light patterns. • Only rarely did patients perceive geometric patterns. • Chicago group attached Subretinal implants to patients. No results reported yet. Will Implants be Stable? • “The eye (and brain) is full of water. electronic devices do not like water”. • Implants in animals can function for at least one year. The electronics can be encased in inert polymers. • Long term stability is unclear. Implants held in place by cellular contacts, microtacks. Will Resolution be High Enough? • The image resolution will depend on the density of electrodes. • Ideally one electrode to stimulate each neuron. • Current Epiretinal Implants are much lower than this (many orders of magnitude). MIT Presentation. • MIT Movie. Summary. • Cautious Optimism for partial sight. • Unknowns: Resolution, Long term stability of implants. • Practicality – probably only for patients who had fully functioning visual systems before disease (Glaucoma). • Readings: (i) Cohen. “Mix of Hype and Hope”. (ii) Zrenner. “Will Retinal Implants Restore Vision”? • Webpage: yuille/teaching/.