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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