Abstract

1. 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.

2. 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.

3. Problem Statement The client requests a detection device to measure horizontal monkey eye movement in a functional Magnetic Resonance Imaging (fMRI) environment.

4. 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

5. 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)

6. 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)

7. Design #1: Copper Eye Coils Skalar Medical BV. www.skalar.nl.

8. 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

9. Eye camera & IR illuminator Head coil Design #2: Infrared Camera System Head coil Infrared camera and IR illuminator (Adapted from Gitelmen et al. 1999 )

10. 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

11. 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

12. Proposed Final Design

13. Analysis of Final Design • Advantages: • Compatible with MRI environment • Non-invasive • Inexpensive relative to other methods • Disadvantages: • Calibration time

14. Design Comparison Based on our design matrix, we selected the photodiode system

15. 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

16. 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

17. 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

18. Preliminary Testing: Sheep eye Top left: Dissection and preparation of sheep eye Bottom right: Cable arrangement during preliminary sheep eye testing

19. Testing Set-up Testing set-up consisted of an infrared laser box, two fiber optic cables, and an optical power meter.

20. 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

21. 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.

22. Standard Calibration Curve Voltage (in Volts) vs. angular displacement of rhesus monkey eye (in degrees). Calibration curve provided by Luis Populin’s Laboratory

23. 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.

24. 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

25. Proposed Budget

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

27. 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 FunctionalNeuroimaging of Cognition. Boston, MA: MIT Press. Bullwinkel, P.E., 2000. Fiber optic eye-tracking system utilizing out-of-band light source. US Patent, 6,079,829 Delgado-García, J.M. 1999. Oculomotor System. Nature: Encyclopedia of LifeSciences. Nature Publishing Group. Gazzaniga, M.S. (ed.). 1999. fMRI measurement methods. The New CognitiveNeurosciences. Boston, MA: MIT Press. Gitelman, D.R., Parrish, T.B., LaBar, K.S., Marsel Mesulam, M. 1999. Rapid Communication: Real-time monitoring of eye movements using infrared video oculography during functional magnetic resonance imaging of the frontal eye fields. NeuroImage. 11: 58-65. Judge, S.J., Richmond, B.J., Chu, F.C. 1980. Implantation of magnetic search coils for measurement of eye position: An improved method. Vision Research. 20: 535 538. Kimmig, H., Greenlee, M.W., Huethe, F., Mergner, T. 1999. MR-Eyetracker: a new method for eye movement recording in functional magnetic resonance imaging. Exp Brain Res. 126: 443-449. Kowler, E. (ed.). 1990. Eye Movements and Their Role in Visual and Cognitive Processes. Reviews of Oculomotor Research. 4: 1-70. Reulen, J.P.H., Marcus, J.T., Koop, D., de Vries, F.R., Tiesinga, G., Boshuizen, K., Bos, J.E. 1988. Med. & Biol. Eng. & Comput. 26: 20-26. Richards, J.E. 1990. Eye position prospectus for measuring eye movements. http://jerlab.psych.sc.edu/pdf/eyeposition.pdf Skalar Medical BV. www.skalar.nl.