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Khadija S. Shahid, OD, FAAO
Ben Szirth, PhD
Academy 2011 Course Outline
The Bionic Eye: A Review of how close we are to Replacing the Human Eye.
A. Visual Pathway
a. 130 million Photoreceptors, 10 billion RPE absorb light photons to create a biochemical
signal
b. Horizontal, Bipolar, Amacrine Cells filter signals to enhance high contrast and edges, etc.
c. 1.2 Million Ganglion Cells, Axons, Optic Nerve will convert signal to an action potential
for transmission out of eye and towards Lateral Geniculate Nucleus
d. Optic Radiations, V1
i. Binocularity, Lines, and Velocity
ii. Faces, Language, Perception
B. Visual Prosthetic Target Sites
a. Visual Cortex
b. Optic Nerve
c. Photoreceptors-Subretinal Space
d. Ganglion Cells-Epiretinal Space
C. Three Physiologic Principles
a. Electric currents can substitute light photons in producing visual sensations
(phosphene).
b. Most etiologies of blindness leave upstream structure intact.
c. Retinotopic organization of target neural structure
D. Visual Prosthetic Systems
a. Overall results of all system trials show:
i. Patients with implants have a crude level of visual perception; Problems with
implants include availability of a power source, surgical complications, and long
term viability; No visual prosthesis has yet restored vision
b. Intra Cortical: Theory and limitations
i. As early as 1920s: German neurosurgeon Dr. Otfrid Foerster investigated direct
electrical stimulation of the visual cortex.
ii. Projects and Human Trial
1. Artificial Vision System with an Electrode Array
2. Percutaneous Pedestal and attached Camera Prototype
3. Computerized Edge Detection
c. Optic Nerve System
i. electrodes placed around the optic nerve
1. Retinitis Pigmentosa trial: phosphene elicitation
2. Localize objects and discriminate between them.
ii. OPTIVIP, 2000-2004, Spiral-Cuff Electrode
d. Hybrid Retinal System
i. Micro-electrode mechanical system
ii. Culture Neurons on Subretinal Array & guide their growing Axons to CNS
e. Subretinal System
i.
Micro photodiodes attached to microelectrodes. Electrode array implanted
between the retina and the RPE.
ii. Stimulated by light passing through the retina, the resulting electric current
excites adjacent retinal sensory neurons.
iii.
Varied specifications
1. No external electrical source (incident light is sufficient for stimulation).
iv. Ab Interno, Externo surgical procedures
v. Mechanism of action
1. Possible direct stimulation of retinal neurons
2. Possible neurotrophic effects on photoreceptors (retinal cell rescue
effect)
f.
Epiretinal System
i. Electrode array placed on the vitreal surface to stimulate ganglion or possibly
bipolar cells.
ii. Second Sight, Inc.
1. Camera/Radiofrequency, Transmitter, Micro Computer (Px’s belt),
Electronic Implant, Electrode Array
2. Human trial results
iii. Epi-Ret Project System (German)
E. Visual Prosthetic System Comparisons
F. Advantages: Intra Cortical Implant
a. Skull protection
b. Long-term studies
c. Not limited to outer retina
d. Intact optical functioning not necessary
e. Tumor, Trauma, etc.
G. Disadvantages: Intra Cortical Implant
a. Individual cortical maps
b. Color, Intensity, Spatial, Orientation, Ocular dominance.
c. What kind of perceptions will be evoked?
d. Convoluted Cortical Anatomy
e. Neurosurgery risks, seizures
H. Advantages: Optic Nerve Implant
a. Decreased surgical complications vs. cortical
b. Intact ocular structures not necessary
I.
Disadvantages: Optic Nerve Implant
a. Neurosurgery.
b. Functional optic nerve pathway.
c. Retinotopic organization.
d. Complex electrode array to provide useful patterned vision.
J.
Advantages: Hybrid Implant
a. Artificial connection: CNS and Neural surface.
b. Less surgical risk
c. Electrode-neuron stability: firm cultured attachment prior to implantation
K. Disadvantages: Hybrid Implant
a. No long-term studies
b. Difficulty guiding axonal growth
c. No prototype
L. Advantages: Subretinal Implant
a. Take advantage of retinal, cortical signal processing.
b. Retinotopic organization preserved.
c. Decreased surgical risk
d. Vacuum seal: RPE pumps
e. Eye movements
f.
No external camera
M. Disadvantages: Subretinal Implant
a. Functional retina and optic nerve to convey signals
b. Intact optics
c. Nutrient block
d. External trans-pupillary IR stimulation
e. Encapsulation
N. Advantages: Epiretinal Implant
a. Take advantage of retinal, cortical signal processing.
b. Decreased surgical risk.
c. Eye movements
d. Wireless transmission of image-no clear optics necessary
O. Disadvantages: Epiretinal Implant
a. External image processor.
b. Functional optic nerve.
c. GC bodies and axons: complicate retinotopic organization.
d. Eye movements-stability
e. Adherence- cellular proliferation.
P. Future Projects and Research
a. Transretinal stimulation
i. Transcleral, intrascleral, or suprachoroidal electrodes
b. Biocompatible carbon nanotubes (retinal prostheses)
c. Implants with photosensitive dyes to generate impulses
d. Neurotransmitters used to stimulate retinal neurons via a neural prostheses (instead of
direct electrical activity)
e. Other novel ideas