Neuroprosthetics

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Neuroprosthetics

• Week 8
• Visual Neuroprostheses

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Neural Interface

• Same problems as with other neural interfaces
• Biological Biocompatibility
• Physical Biocompatibility implant density,
  barriers, mechanical compliance, wire tethering
• Percutaneous v - implant

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Four Approaches

• Subretinal
• Epiretinal
• Optic Nerve
• Cortex

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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

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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?

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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 ?

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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!

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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!!!!!!

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Next Week

• Motor Neuroprostheses