Eye Movement Recording Techniques Flashcards
How can you assess the peripheral vestibular system?
Encased in bone so hard to assess and image
Need to look at how the system behaves under certain conditions to assess it
Because of the close anatomic and physiologic connections between second order neurons in vestibular nuclei and central eye movement systems we can indirectly measure function of peripheral and central vestibular systems
What are the three main methods to transform eye movements into electrical signals that can be analyzed by computers?
Electro-oculography (EOG/ENG)
Infrared Video-oculography (VOG/VNG)
Scleral search coils
How does electro-oculography (EOG/ENG) work?
Corneo-retinal potential (CRP) = bioelectrical signal used for recording EOG/ENG
The eyeball has a dipolar orientation like a “battery” with the cornea being positively polarized and the retina negatively polarized (back of eye) - electrode becomes more positive when the eye rotates toward it and less positive when the eye moves away
This orientation creates a steady electrical potential field that may be detected with the eye in total darkness and/or with the eyes closed
This potential difference and the rotation of the eye are the basis for a signal measured at a pair of peri-orbital surface electrodes (known as electro-oculography, (EOG))
Commonly used for measuring nystagmus, with the resulting signal called electronystagmography (ENG)
Why is ENG less ideal?
Changes a lot based on if we are in a light or dark-adapted room - makes the test longer
More variables to control for
Two fold increase in light-adapted vs dark adapted
Doesn’t detect torsion well
Do most VNG systems have the capability to do ENG?
Yes
How do we want to set up the electrodes?
Must prep face much like we do for AEPs
Skin, oils and cosmetic residue create electrical resistance and reduce CRP
Individual impedances need to be <10,000 ohms (10 kohms); inter-electrode impedances <3000 (3 kohms) - good conductance
How does infrared video-oculography work?
Make use of pupil localization technology and reflective nature of corneal surface to calculate pupil position and angle of gaze
Goggles contain infrared diodes to illuminate the eyes and “mirrors” to reflect the image of the eyes into a pair of cameras for recording
Infrared light is invisible to the human visual system and does not affect pupil or allow fixation
Doesn’t use corneal potential at all, like the ENG
How do scleral search coil work?
Coils are embedded into a tight-fitting contact lens or a rubber ring that adheres to the eye
Typically used in animal studies (might be surgically implanted)
Alternatingmagnetic fieldsare generated by magnets positioned around the eye and throughelectromagnetic induction,electric currentsare generated in the search coils
Can measure very small eye movements
Contact lens cannot be worn for more than 30 minutes
