Lab 4: Electrooculogram

• Electrooculogram (EOG)
  • The eye presents a dipole, positive in front, negative in back
  • Shifts in eye position can be detected as shifts in the angle of the dipole relative to sets of electrodes
  • Shifts in retinal potential during light adaptation and dark adaptation can be recorded and used to diagnose various conditions
  • Light adaptation – breakdown of photosensitive pigments due to intense light exposure decreases receptor sensitivity allowing normal contrast to be seen under conditions of intense light
  • Dark adaptation – regeneration of photosensitive pigments in absence of intense light increases receptor sensitivity allowing contrast to be seen in low light conditions
  • Arden ratio (light peak/dark trough) – can be used to detect some conditions (vitelliform macular degeneration, etc.)
• Eye Movements
  • Saccades – short, jerky (ballistic) movement of the eyes (saccadic movement).
  • Fixation: Locking the eyes onto an object. 80 – 90% of the time.
  • Voluntary fixation movement – conscious unlocking of the eyes from one object in order to move them to another object. Movement occurs in saccades. Signals from the retina are ignored/suppressed while the eye is in motion to avoid blurred images.
  • Involuntary fixation movement –Unconscious movement of the eyes to follow an object they are locked onto as it moves. The brain anticipates the object’s path and moves the eyes in saccades along that projected path ahead of the object in order to “keep up” with it. Track can become smooth (pursuit movement - slower and smoother than saccades) if the object is moving in a regular pattern that the brain can learn and predict.
  • Drift – slight movement of the eyes due to the inability of the ocular muscles to hold the eyes perfectly still.
  • Microsaccades – Very small jerky eye movements thought to compensate for drift, and return the “locked” object to the center of the field of vision.
  • Physiological nystagmus (tremor) – slight tremor of the eyes. Normal condition, imperceptible to the naked eye.
• Vision and Balance – brain uses information from vestibular apparatus and vision to determine body position in space
  • Binocular vision/depth perception used to determine position in space relative to objects in visual field
  • Vertigo can be due to inability of eyes to lock onto an object when the brain expects to be able to
  • Vestibular apparatus, saccule, and utricle detect head movement in relation to gravity and inertia
  • Rotational nystagmus – abnormal condition, “Bouncing” of the eyes back and forth (or up and down) in response to continued movement of fluid in the vestibular apparatus of the inner ear when the head has stopped spinning.
  • Vestibulo-ocular reflex – Eyes move opposite to of head movement to maintain fixation on an object

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