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Fig 4.8 (far left) EOG (Electro Oculography) Silver chloride electrodes are used to sense the proximity of the corneal apex. Fig 4.9 (left) Principal of EOG. Charge on electrodes varies as proximity of cornea changes during secondary gaze. |
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EOG is probably the most widely used clinical objective eye movement recording device. It is sensitive to one degree eye movement and it works on non-communicative patients like infants. Its disadvantage is that it is also sensitive to muscle action potentials coming from the orbicularis oculus when the child cries and from the temporal lobe muscles when the child sucks from a pacifier or bottle. You can get around these problems by recording as soon as the child wakes, and has been fed and changed. You have about 20 more minutes before it becomes tired and cranky again. You will need some kind of record or data storage apparatus like a strip chart or computer that digitizes the voltage analogs of eye movements. Typically you will measure OKN and fixational eye movements in the infants under 4 months of age.
Infrared Reflection (limbal trackers): This technique is much more sensitive than EOG. It senses 1/4 of a degree of horizontal movement by sensing infrared light reflected from the limbal region of the eye. Light reflected from the limbus moves with the eye because the center of curvature of the cornea is anterior to the center of rotation of the eye. Thus as the eye rotates, the cornea is translated. (If the center of the cornea was concident with the center of rotation of the eye, the eye would be a perfect sphere and the reflection of an external light source would not move when the eye rotated.) The reflected light is sensed by a light sensitive diode that varies its resistance as it receives light. This variation in resistance shunts various amounts of voltage and it is possible to get a voltage analog of eye position. |
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Fig 4.10 (far left) Spectacle-mounted infrared limbal sensors. Fig 4.11 Record of eye movements using a limbal sensor. |
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Infrared limbal tracker devices are used mostly in adults because the infrared sensors must be attached near the eyes with spectacle frames. The ìOber 2î device works similarly to this, but it is mounted in swim goggles and may be attached to a child's face. The main disadvantage of this type is system is it can't measure vertical eye position well because the limbus is occluded by the eye lids. |
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Fig 4.12 Principal of operation of a limbal tracking system, illustrating the illumination of the limbus and detection of the reflected infrared light. |
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Video based systems (pupil trackers): These devices are analyzing the image of the corneal light reflex and pupil center. They track the change in shape of the pupil and the displacement of the center of the pupil or position of the corneal light reflex. They are more remote than the infrared systems and they also are equally good at measuring horizontal and vertical eye movements because they analyze the pupil center rather than the eye limbus. |
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Fig 4.13 Head-mounted video-based pupil tracker. |
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Video cameras can be mounted in a helmet and move with the head. They can also combine the position of the eye with the scene viewed by the subject to indicate where in the scene someone is fixating. This type of analysis is useful in analyzing information used in heading perception such as with pilots or drivers. It is also useful in virtual reality machines that need to know where the user is directing his/her interest.
Laboratory eye measurement techniques
Search Coil: It is also possible to get extremely precise measures of eye position of 1 arc min using a coil of copper wire mounted in a contact lens, much like an erg electrode. A subject sits in a magnetic field and as the coil on the eye rotates with the eye a small voltage is generated in the coil, much like it was in your elementary physics class. The voltage can be calibrated to indicate eye position. Such a device is used to measure saccadic eye movements with a high degree of spatial and temporal resolution. The disadvantage of this device is that the contact lens can only be worn for 20 minutes before corneal swelling occurs. It fits tightly to prevent slippage when the eye rotates. A related corneal contact lens device attaches a small mirror to the eye on a contact lens. This technique has been used primarily to stabilize the retinal image. An image is reflected off the mirror and back into the eye through an optical system that moves the image with the eye. The stabilized image is unaffected by eye movements and researchers are able to measure visual perception without retinal image smear. |
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Fig 4.14 Search coil. (copper wire mounted in a contact lens) |
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SRI Eye tracker: It is possible to get very good position accuracy without the contact lens with an infrared reflection sensor that measures the position of both the first and fourth Purkinje images. The first image is influenced by both rotation and translation of the eye, and the fourth image is mainly influenced by translation. The difference between the motion of the two is a good measure of pure eye rotation. This device is excellent for long term non-invasive measures of small eye movements. Its main disadvantage is that it has a whiplash artifact of ocular lens motion during saccades. In addition, the tracker itself is very large, and the head must remain fixed inside the apparatus. However, it is excellent for laboratory measurements of slow eye movements (like pursuits and vergence) and for steady eye position. Review Questions: 1. List two entoptic measures of eye position. 2. Define angles Kappa, Lambda (classical definition). 3. Which objective measure of eye position utilizes the dipole characteristic of the eye? 4. Define the 4 Purkinje images, describe their relative size and orientation. 5. Explain the difference between the unilateral and alternate cover tests.
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