Download Test.

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/.