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.

MotivationMotivation 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 Measure horizontal saccadic eye movement in one eye Monkey’s view of stimulus projection must not be blocked in both eyes Monkey’s view of stimulus projection must not be blocked in both eyes MRI compatible MRI compatible No ferromagnetic materialNo ferromagnetic material Limited electrical componentsLimited electrical components

Background Eye Movement Eye Movement Smooth pursuit eye movements are made when the eye follows a moving objectSmooth 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 objectsSaccadic 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 movementsOur device will record saccadic eye movements (Kowler, 1990)

Background fMRI Coil aligns atomic nuclei with magnetic fieldCoil aligns atomic nuclei with magnetic field As brain activity increases, oxygenated blood flow to active areas increasesAs brain activity increases, oxygenated blood flow to active areas increases Waisman center fMRI has a magnetic strength of 3 TeslaWaisman center fMRI has a magnetic strength of 3 Tesla (Kimmig et al. 1999)

Design #1: Copper Eye Coils Skalar Medical BV.

Analysis of Design #1 Eye Coils Eye Coils Copper wire search coils implanted on the eye generate RF field and induce a current. Current measurement determines eye position.Copper wire search coils implanted on the eye generate RF field and induce a current. Current measurement determines eye position. Advantages: Advantages: Increased accuracy due to placementIncreased accuracy due to placement Established record of successesEstablished record of successes Disadvantages: Disadvantages: Surgical procedure involvedSurgical procedure involved May alter monkey’s natural behaviorsMay alter monkey’s natural behaviors To date, not tested in an fMRI setting due to magnetic constraintsTo date, not tested in an fMRI setting due to magnetic constraints

Design #2: Infrared Camera System Eye camera & IR illuminator Head coil (Adapted from Gitelmen et al ) Infrared camera and IR illuminator Head coil

Analysis of Design #2 Infrared Camera System Infrared Camera System Infrared camera used to capture eye image via infrared light projected by fiber optic cableInfrared camera used to capture eye image via infrared light projected by fiber optic cable Advantages: Advantages: Non-invasiveNon-invasive Devices currently in use in MRI on humansDevices currently in use in MRI on humans Disadvantages: Disadvantages: Camera electronicsCamera electronics Frequent Detection ErrorsFrequent Detection Errors ExpensiveExpensive

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 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 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 Photodiodes detect incident radiation and produce current based on intensity of light

Proposed Final Design

Analysis of Final Design Advantages: Advantages: Compatible with MRI environmentCompatible with MRI environment Non-invasiveNon-invasive Inexpensive relative to other methodsInexpensive relative to other methods Disadvantages: Disadvantages: Calibration timeCalibration time

Design Comparison Design 1 Eye Coils Design 2 Camera Design 3 Photodiodes Invasiveness054 Expense215 MRI compatibility 135 Responsiveness545 Totals81319 Based on our design matrix, we selected the photodiode system

Materials Used 2 fiber optic cables (50/125 micron) 2 fiber optic cables (50/125 micron) Infrared laser box (output 850nm) Infrared laser box (output 850nm) Power meter (in dB) Power meter (in dB) PVC (3/4 inch, 1 inch) PVC (3/4 inch, 1 inch) Oscilloscope (max voltage = 335 mV) Oscilloscope (max voltage = 335 mV) Sheep eyes, Rabbit eyes, Marble Sheep eyes, Rabbit eyes, Marble

Preliminary Testing: Marble Goal: To obtain infrared reflection off a comparable spherical surface Goal: To obtain infrared reflection off a comparable spherical surface Results: Results: Angle = 96° between cablesAngle = 96° between cables 21.0 dB of reflected IR light21.0 dB of reflected IR light Max output of IR laserMax output of IR laser box is 81.5 dB

Preliminary Testing: Sheep eye Goal: Goal: To obtain realistic reflective properties similar to those of a monkey’s eyeTo obtain realistic reflective properties similar to those of a monkey’s eye Methods: Methods: Used a Sheep eye (2.7 cm in diameter) placed in cupUsed a Sheep eye (2.7 cm in diameter) placed in cup Results: Results: Variation in dB readings with rotation of eyeVariation in dB readings with rotation of eye Unable to accurately measure angle of displacementUnable 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 Goal: Find a linear relationship between angular eye displacement and voltage Methods: Methods: Used two half-cylinders to determine angular displacementUsed two half-cylinders to determine angular displacement Plotted optical power output vs. angular displacementPlotted 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 Graphs depict power output (in dB) vs. sheep eye angular displacement (in degrees). Highest power outputs generally found at smaller angles of displacement.

Problems Encountered Maintaining a steady reflection across the surface Maintaining a steady reflection across the surface Accurate angle measurement Accurate angle measurement Keeping the cables fixed in one position Keeping the cables fixed in one position Sheep eyes Sheep eyes PreservationPreservation Shape interfered with angles of reflectionShape interfered with angles of reflection Anatomically different from monkey eyesAnatomically different from monkey eyes Oscilloscope Oscilloscope Obtaining a constant readingObtaining a constant reading

Proposed Budget ComponentPriceSupplier Fiber Optic Cables (FC-FC) $80 Fiberdyne Labs Pigtailed Silicone Photodiodes $25Hamamatsu Fiber optic Array $120Opticis Infrared Light Source $70 Ion Optics Total$295

Future Work Order components and construct functional prototype Order components and construct functional prototype Test system on monkey eyes Test system on monkey eyes Integrate photodiode system with client’s computer software system Integrate photodiode system with client’s computer software system

Acknowledgements and References Applied Science Laboratories (ASL): Buckner, R.L. and Logan J.M Functional Neuroimaging Methods: PET and fMRI. In R. Cabeza and A. Kingstone (eds.) Handbook of Functional Neuroimaging of Cognition. Boston, MA: MIT Press. Bullwinkel, P.E., Fiber optic eye-tracking system utilizing out-of-band light source. US Patent, 6,079,829 Delgado-García, J.M Oculomotor System. Nature: Encyclopedia of Life Sciences. Nature Publishing Group. Gazzaniga, M.S. (ed.) fMRI measurement methods. The New Cognitive Neurosciences. Boston, MA: MIT Press. Gitelman, D.R., Parrish, T.B., LaBar, K.S., Marsel Mesulam, M Rapid Communication: Real-time monitoring of eye movements using infrared video oculography during functional magnetic resonance imaging of the frontal eye fields. NeuroImage. 11: Judge, S.J., Richmond, B.J., Chu, F.C Implantation of magnetic search coils for measurement of eye position: An improved method. Vision Research. 20: Kimmig, H., Greenlee, M.W., Huethe, F., Mergner, T MR-Eyetracker: a new method for eye movement recording in functional magnetic resonance imaging. Exp Brain Res. 126: Kowler, E. (ed.) Eye Movements and Their Role in Visual and Cognitive Processes. Reviews of Oculomotor Research. 4: Reulen, J.P.H., Marcus, J.T., Koop, D., de Vries, F.R., Tiesinga, G., Boshuizen, K., Bos, J.E Med. & Biol. Eng. & Comput. 26: Richards, J.E Eye position prospectus for measuring eye movements. Skalar Medical BV. Special thanks to: Professor Leon McCaughan, ECE Professor Seth McGee, Biocore Lab Manager