Eye Movements

• Why do we move our eyes?

  • Gaze Shifting = to acquire a target
  • Gaze Holding = to maintain gaze on the target

  *movements ensure image is always on fovea

  Troxler Effect

  • Pink ring around dot disappears if we fixate on cross in the middle
  • We must continually move our eyes to keep fovea refreshed
  • Microsaccades can only refresh edges

  Damage to Cranial Nerves or Extraocular Muscles:

  • Diplopia - 2 different locations of same image on retina
  • Through stroke or tumour
• 6 Extraocular Muscles:

  • Superior Oblique (CN IV - TROCHLEAR) - rotates clockwise
  • Inferior Oblique (CN III - OCULOMOTOR) - rotates anticlockwise
  • Lateral Rectus (CN VI - ABDUCENS)
  • Medial Rectus (CN III)
  • Superior Rectus (CN III)
  • Inferior Rectus (CN III)
• Saccadic and Vergent Eye Movements:

  • Both involved in gaze shifting - to acquire a target
  • Saccadic = conjugate movements

  *shift fovea rapidly on chosen target

  • Vergence = disconjugate movements (controlled by rectus muscles and oculomotor nucleus)

  *shift eyes in opposite directions so images on both fovea

  ! Saccades connect a series of active fixations - can reach 900 degs/s

  ! Can change their direction and amplitude but not speed - velocity affected by the distance it moves

  ! Respond to other sense stimuli
• Fixational Eye Movements:

  • Occur when fixating on a static object - eye moves very tiny movements (INVOLUNTARY)

  1)MICROSACCADES = small rapid movements, only refresh edges

  2)DRIFT = made up of tremors

  3)TREMOR = rapid small changes in position

  • Adaptive optics can use this to detect Parkinson's disease - changes in fixation eye movements on retina
• The Step-Pulse Response:

  • PULSE = during a saccade, the firing rate of extraocular motor neurons increase to move the eye to intended location
  • STEP = extraocular motor neurons change their base-line firing rate to reflect the new position of the eye at the end of the eye movement (slightly higher than what it was initially)
• SC in Saccadic Generation:

  • Superior Colliculus - visuomotor integration (2 layers) neurones at front (foveal edge) hold eye steady

  1)Superficial layers are visual - 1/2 neurones exhibit heightened activity if visual stimulus is target of saccade, 1/2 fire depending on whether a stimulus is present or not

  2)Deep layers are motor - indidvudal movement cells have large overlapping movement fields (fire just before saccades)

  *Output controlled by cerebral cortex
• FEF and SEF in Saccadic Generation:

  • FEF - trigger intentional saccades

  *Activate SC which activates burst neurones in brain stem

  • SEF = organise groups of saccades into sequences

  BURST CELLS:

  • motor circuits lie in brain stem
  • burst cells fire just before and during saccades = PULSE
  • People with brainstem lesions cannot generate the change in baseline, therefore cannot maintain eye movements
• Gaze Holding:

  • Smooth Pursuit = keep image of a moving target on fovea (100 degs/s - much slower than saccades but cane combined with them)

  *generated by calculating how fast target is moving and moving the eyes accordingly

  • Vestibuloocular = keep images still on fovea during brief head movements

  1)Rotational = head rotation

  2)Translational = linear head

  3)Ocular-counter rolling response = head-tilt

  • Optokinetic = hold images still during sustained head movements
• How do VORs and Optokinetic reflexes complement each other?

  • Systems respond identically to head movements and movement of image on retina . Both give input to vestibular nuclei - VORs decay over time whereas Optokinetic builds up over time to compensate

  How can Optokinetic reflex lead to illusions of self-motion?

  • When sitting stationary, movement of visual scene can make person think they are moving = CIRCULAR VECTION

  *person perceives to move in opposite direction then constant speed around a stationary cylinder
• Nystagmus due to Rotational VOR:

  • causes eyes to rotate in opposite direction to any rotational head movement detected by vestibular system
  • sustained rotation does not cause eyes to be drive past orbit but instead makes a rapid resetting movement back across centre of gaze

  *zigzag diagram

  *x axis = time

  *y axis = eye angle