Abstract
A fiber optic, photodiode design for
detecting horizontal eye movement in
the rhesus macaque monkey while in the
fMRI is developed and tested. Light from
an infrared laser source with an output
of 40 mW is directed via fiber optic cable
to reflect off of the eye. A photodiode
receives the reflected light by way of a
fiber optic cable and generates a voltage
based on the intensity of light received.
Maximum reflection achieved was 35%.
Output based on a single source cable
and single detector cable is not linear.
Motivation
Dr. Populin, a member of the
anatomy department at the
University of Wisconsin – Madison,
wishes to study the neural
mechanism behind the spatial
visual attention in the cerebral
cortex of macaque monkeys
through functional magnetic
resonance imaging, allowing for
analysis of behavioral processes.
Problem Statement
The client requests a
detection device to
measure horizontal
monkey eye movement in
a functional Magnetic
Resonance Imaging (fMRI)
environment.
Design Requirements



Measure horizontal saccadic eye
movement in one eye
Monkey’s view of stimulus
projection must not be blocked in
both eyes
MRI compatible
• No ferromagnetic material
• Limited electrical components
Background

Eye Movement
• Smooth pursuit eye movements are made when
the eye follows a moving object
• Saccadic eye movement consists of rapid jumps in
angular direction of eye to redirect line of sight
when viewing stationary objects
• Our device will record saccadic eye movements
(Kowler, 1990)
Background
fMRI
• Coil aligns atomic
nuclei with magnetic
field
• As brain activity
increases,
oxygenated blood
flow to active areas
increases
• Waisman center
fMRI has a magnetic
strength of 3 Tesla
(Kimmig et al. 1999)
Design #1: Copper Eye Coils
Skalar Medical BV. www.skalar.nl.
Analysis of Design #1

Eye Coils
• Copper wire search coils implanted on the eye
generate RF field and induce a current.
Current measurement determines eye
position.

Advantages:
• Increased accuracy due to placement
• Established record of successes

Disadvantages:
• Surgical procedure involved
• May alter monkey’s natural behaviors
• To date, not tested in an fMRI setting due to
magnetic constraints
Design #2: Infrared Camera System
Headcoil
coil
Head
Infrared
camera
Eye camera
&
IR illuminator
and
IR illuminator
(Adapted from Gitelmen et al. 1999 )
Analysis of Design #2



Infrared Camera System
• Infrared camera used to capture eye image
via infrared light projected by fiber optic
cable
Advantages:
• Non-invasive
• Devices currently in use in MRI on humans
Disadvantages:
• Camera electronics
• Frequent Detection Errors
• Expensive
Proposed Final Design:
Photodiodes System



Infrared radiation is projected onto the eye
via a fiber optic array attached to an LED
source outside of MRI’s Faraday cage
Two fiber optic cables carry radiation
reflected from the eye to photodiodes
located outside of Faraday cage
Photodiodes detect incident radiation and
produce current based on intensity of light
Proposed Final Design
Analysis of Final Design

Advantages:
• Compatible with MRI environment
• Non-invasive
• Inexpensive relative to other methods

Disadvantages:
• Calibration time
Design Comparison
Design 1
Eye Coils
Design 2
Camera
Design 3
Photodiodes
Invasiveness
0
5
4
Expense
2
1
5
MRI compatibility
1
3
5
Responsiveness
5
4
5
Totals
8
13
19
Based on our design matrix, we selected the photodiode system
Materials Used






2 fiber optic cables (50/125 micron)
Infrared laser box (output 850nm)
Power meter (in dB)
PVC (3/4 inch, 1 inch)
Oscilloscope (max voltage = 335
mV)
Sheep eyes, Rabbit eyes, Marble
Preliminary Testing: Marble


Goal: To obtain infrared reflection off a
comparable spherical surface
Results:
• Angle = 96° between cables
• 21.0 dB of reflected IR light
• Max output of IR laser
box is 81.5 dB
Preliminary Testing: Sheep eye

Goal:
• To obtain realistic reflective properties
similar to those of a monkey’s eye

Methods:
• Used a Sheep eye (2.7 cm in diameter)
placed in cup

Results:
• Variation in dB readings with rotation of
eye
• Unable to accurately measure angle of
displacement
Preliminary Testing: Sheep eye
Top left: Dissection and
preparation of sheep eye
Bottom right: Cable
arrangement during
preliminary sheep eye
testing
Testing Set-up
Testing set-up consisted of an infrared laser box,
two fiber optic cables, and an optical power meter.
Secondary Testing: Sheep Eye


Goal: Find a linear relationship
between angular eye displacement
and voltage
Methods:
• Used two half-cylinders to determine
angular displacement
• Plotted optical power output vs. angular
displacement
Secondary Testing: Sheep Eye
Multiple views of cable and eye orientation for
secondary testing with sheep eyes. Double cylinder
method allowed for accurate angular displacement
measurements vs. optical power output.
Standard Calibration Curve
Voltage (in Volts) vs. angular displacement of
rhesus monkey eye (in degrees). Calibration
curve provided by Luis Populin’s Laboratory
Secondary Testing Results
Vertical Eye Movement
Horizontal Eye Movement
30
25
20
20
Trial 1
15
Trial 2
10
Output (dB)
Output (dB)
25
15
Trial 1
10
5
5
0
0
-20
-10
0
10
Angle (Visual Degrees)
20
-25
-20
-15
-10
-5
0
5
Angle (Visual Degrees)
Graphs depict power output (in dB) vs. sheep eye angular
displacement (in degrees). Highest power outputs generally
found at smaller angles of displacement.
10
Problems Encountered




Maintaining a steady reflection across the
surface
Accurate angle measurement
Keeping the cables fixed in one position
Sheep eyes
• Preservation
• Shape interfered with angles of reflection
• Anatomically different from monkey eyes

Oscilloscope
• Obtaining a constant reading
Proposed Budget
Component
Price
Supplier
Fiber Optic
Cables (FC-FC)
Pigtailed
Silicone
Photodiodes
Fiber optic
Array
Infrared Light
Source
Total
$80
Fiberdyne Labs
$25
Hamamatsu
$120
Opticis
$70
Ion Optics
$295
Future Work



Order components and construct
functional prototype
Test system on monkey eyes
Integrate photodiode system with
client’s computer software system
Acknowledgements and References
Special thanks to:
Professor Leon McCaughan, ECE Professor
Seth McGee, Biocore Lab Manager
Applied Science Laboratories (ASL): www.a-s-l.com
Buckner, R.L. and Logan J.M. 2001. Functional Neuroimaging Methods: PET and fMRI. In R. Cabeza and A. Kingstone (eds.)
Handbook of Functional Neuroimaging of Cognition. Boston, MA: MIT Press.
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Skalar Medical BV. www.skalar.nl.