Magnetic Eye Tracking using a Planar Transmitter Timothy Alberg Biomedical Engineering University of Rhode Island

Index Introduction Vestibular-Ocular Reflex Sclera Search Coil Cubic Transmitters Planar Transmitters Conclusion References

Introduction Eye tracking is very useful in understanding and monitoring many psychological, neurological, ophthalmologic, and vestibular disorders. It also has uses in flight simulations, music, language, Human-to-Computer Interactions, and rehabilitation.

Vestibular-Ocular Reflex The Vestibular-Ocular Reflex (VOR) allows one to turn their head and have their eyes fixated at a particular point. As the head moves, the eyes move in a counteracting direction to maintain the visual focal point at one spot. It is one of the fastest reflexes of the body, and is maintained by signals sent from the semicircular canals to the eye muscles via the three neuron arc. To measure the effectiveness of the VOR, Rotational tests are performed to find the change in angle per second for the eyes, and the change in angle per second for the head. The gain is then computed by dividing the eye number by the head number. For horizontal and vertical eye/head rotations, the gain is approximately 1 for a person who is healthy.

Sclera Search Coils A small coil that uses magnetic induction to track where the eye is looking. External horizontal and vertical magnetic fields are applied to vary the voltages generated on the coil. The voltages are proportional to the sine of the horizontal and vertical eye positions, and can thus be used to find their location and orientation. There are two types of magnetic field transmitters: Cubic and Planar.

Cubic Transmitters It is a large cubical transmitter that is positioned above the head of the patient. Contains a high area of transmitting coils, resulting in a powerful magnetic field and a high SNR. Uses simple tracking algorithms, allowing for fast computations and a high real-time tracking speed. Due to its bulky complex design, it is difficult to transport, difficult to maintain, and serves as a visual distraction for the patient. Due to inhomogeneity of the magnetic fields, the tracking error can be as large as 1%.

Planar Transmitters Utilizes an arrangement of 8 coplanar transmitter coils on a thin square board. Each electrode is excitable at specific frequencies to prevent overlapping of sidebands and to allow for continuous and simultaneous excitation of the coils. Uses a unique tracking algorithm to prevent crosstalk by mutual magnetic couplings, reduce random and systematic tracking errors, and to cut back on the hardware. Due to the lack of hardware components, it is easy to transport, maintain, and not visually distracting.

The Two Transmitters

The Test A VOR test was conducted on two volunteers: One who was healthy, and one who had bilateral Meniere’s Disease in the left and right ears. Both Volunteers were required to wear a SSC with a radius of 9mm and 7 turns, and mouth coil with a radius of 5mm and 200 turns. This would allow for eye and head tracking. The test required both volunteers to stare at a point 4 meters ahead of them as a medically trained professional rotated their heads 20 to 30 degrees at speeds ranging from 150 to 500 degrees per second. Once the experiment was completed, the VOR gain was computed for both eyes of both patients. For the left and right eyes of the healthy volunteer, the gains were 0.96 and 1.02 respectively. For the bilateral Meniere’s disease volunteer, they were 0.58 and 0.21 respectively.

Results Ideally, the best gain for the VOR test would be 1, which is demonstrated by the healthy volunteer. The volunteer with bilateral Meniere’s disease showed gains that were relatively low, which showed the presence and magnitude of the disease. From the test, the researchers came across a few limitations for the planar transmitter. 1) The SNR and random tracking errors increased as the distance between the SSC and transmitter increased. 2) Since the planar transmitter requires more time consuming algorithms, more computational power is required to achieve an update rate similar to the cubic transmitter. However, from this experiment, many optimal values were determined to significantly lower statistical and tracking error. Also, given the extra computing power, it can match the performance of the cubic transmitter.

Conclusion From the experiments, the tracking errors were found to be within 3millidegrees rms for angle, and 6μm rms for location. Despite the complex tracking algorithms, the computational power can be increased by upping the processor. The cubic transmitter can only track the orientation of the head and eyes, where the planar transmitter can track the location and orientation of the head and eyes. Compared to the cubic transmitter, the planar transmitter is highly transportable, easy to maintain, not visually unsettling, and able to be mounted on a bed for bedside observations. This can prove useful for monitoring the VOR of comatose patients, as well as providing more clinical locations for eye tracking. With more advanced eye tracking technologies, scientists will have the opportunities to observe the relations between eyes and neurological, ophthalmologic, and vestibular disorders.

Work Cited Plotkin, Anton, Oren Shafrir, Eugene Paperno, and Daniel M. Kaplan. "Magnetic Eye Tracking: A New Approach Employing a Planar Transmitter." IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING (2009): 1-7. Web. 2 Apr 2010. <http://0- ieeexplore.ieee.org.helin.uri.edu/stamp/stamp.jsp?tp=&arnumber=5415595>. Wikipedia contributors. "Eye tracking." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 31 Mar. 2010. Web. 3 Apr. 2010. Wikipedia contributors. "Vestibulo-ocular reflex." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 19 Mar. 2010. Web. 3 Apr. 2010. "Fundamentals and applications." Scleral Search Coil Systems. N.p., n.d. Web. 2 Apr 2010. <http://www.skalar.nl/epmintro.html>. Photo Courtesy of: