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Neuroprosthetics Week 8 Visual Neuroprostheses History Brindley (Cambridge) tried a series of experiments in the 1950s – limited success, but opened the field Last 15 years – lots of initial tests Mostly animal studies – proof? Of concept Limited human studies First generation will be pixelated vision for the profoundly blind (avoid guide dog?) Mostly still speculation/experimental Physiology of Visual Pathway - 1 Best site for an implant yet to be resolved Different sites – different characteristics Anatomy – www.webvision.med.utah.edu Light falls on the retina – located at the back surface of the eye Photoreceptor neurons (in the retina) convert electromagnetic (light) energy into electrochemical signals These are first stage retinal neurons Physiology of Visual Pathway - 2 Output of retinal ganglion cells (last) collected together on the optic nerve Fibres reorganised at the optic chiasm Majority form synapses in the lateral geniculate nucleus (LGN) of the thalamus LGN neurons project to cerebral cortex Region called visual cortex Jargon Receptive field- type of visual stimulus that causes neuron to respond Visuotopic – map from visual to neural space Visual pathway – massively parallel? signal processing M (large) and P (small) are two segregated pathways thought to represent (M) where object is and (P) what object is Blindness Mainly age-related degeneration, retinitis pigmentosa (RP), accidents and cancers Also glaucoma, diabetes but treatable Age related leading cause (2M in USA) – mainly loss of fine detail – central photoreceptors degenerate RP – inherited, affects peripheral + night vision – leads to tunnel vision – rod photoreceptors go Accidents + cancer more difficult as whole eye may be lost or visual pathway affected Prosthesis - Key Elements Recipients must be aware that their resultant sight will not be perfect/normal Acceptable system must be almost invisible Components integrated into glasses etc First generation experimental systems may not satisfy these criteria Video Encoder Mimics photoreceptors in the retina For cortical or optic nerve based - CCD array or photodiode array: Conventional, cheap video cameras good for lab exp. For retinal based – could be integrated into the neural interface, so reside in the plane of the retina: Latter has advantage of using natural acquisition, so no robotic head movements Spatial resolution low – limited no. of electrodes Signal Processing Visual system organised as a hierarchical sequence of maps Visuotopy: close points in space excite close together neurons – low resolution conformal but locally random Prediction of light spots only ½ degree Signal encoded into discrete signals – one for each neural electrode Light adapted into range of stimulus levels – must not be affected by ambient light Image compression + remapping for perception Telemetry & Power Wireless link for power and video signals in implant RF or light transmission – cellphone tech Telemetry – bidirectional, circuitry informs external electronics of power needs Transmitter + Receiver only 1cm apart Receiving coil implanted in eye or scalp Frequency must be limited to avoid heat and radiation damage to tissues Implanted receiver small, high reliability Low BW better but more electrodes means more BW Neural Stimulator Video signals processed before stimulating neurons External stimulator electronics easier Preferable for complete implant – problems Requires on-chip memory locations Each location dedicated to each electrode VLSI device prototyped Hermetic sealing of electronics difficult Neural Interface Same problems as with other neural interfaces Biological Biocompatibility Physical Biocompatibility – implant density, barriers, mechanical compliance, wire tethering Percutaneous – v - implant Four Approaches Subretinal Epiretinal Optic Nerve Cortex Subretinal Approach Replication of photoreceptors – good approach for most cases, uses remnant bipolar cells Array of phototransducers is placed in the subretinal space (Artificial Silicon Retina) Each element is photodiode + electrode Resultant voltage gradient from light source stimulates bipolar cell dendrites No external power or control needed Little/no signal processing required Presently undergoing human trials Epiretinal Approach Stimulating electrodes on inner retina surface – excite remnant ganglion cells Array of electrodes attached to inner retina surface Patterns of electrodes stimulated electrically Simple, linear organisation Still just ideas – can it be permanently attached? Can useful signals be obtained at safe currents? Optic Nerve Approach Optic nerve sole visual conduit from retina to lateral geniculate nucleus in thalamus Only one human study – spiral cuff electrode array with four surface electrodes Biphasic pulses, thresholds 350 microA Stimulus rate 8 to 10 Hz Identify simple objects via a head mounted camera Method not good for high resolution – MEA better ? Cortically Based Approach Yet to be developed – practically?? Dobelle – stimulation via electrode arrays under the dura on visual cortex percutaneous connector behind the ear Each electrode connected to one of 64 pins Subjects able to perceive points of light Currents 1 to 10 mA (unsafe for chronic implant?) Local electrodes alter image – nonlinearities – so large spacings required Recently single electrodes – 10 microA MEA thought to be the way to go! Final Words Four poss sites – subretinal, epiretinal, optic nerve and visual cortex Passive photodiode arrays cannot produce currents that excite retinal neurons Stimulating electrodes must be positioned close to neurons to excite them – electrodes must have same dimensions Stimulation best with highly localised current injections – penetrating electrodes Electrode arrays felt to be the way ahead – first human trial was in 2002!!!!!! Next Week Motor Neuroprostheses