Eye Movements

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Eye Movements
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1. The Plant
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The Oculomotor Plant Consists Of only 6 muscles
in 3 pairs
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This Yields 3 degrees of Mechanical Freedom
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Donders Law/ Listings Law
Neural Constraints Reduce this to 2 degrees of
freedom
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3-D eye movements

• Donders Law
• Relates torsion to eye position
• Listings law
• Torsion results from rotation of eye around
  perpendicular axis
• Listings plane
• Plane orthogonal to line of sight
• Does not apply when head is free

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Kinematics vs Dynamics In the Oculomotor System
Rotations about the Center of Gravity No
Loads No Inertia Force Position
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Oculomotor muscles and nerves

• Oculomotor nerve (III)
• Medial rectus
• Superior/Inferior recti
• Inferior oblique
• Trochlear nerve (IV)
• Superior oblique
• Abducens nerve (VI)
• Lateral rectus
• Medial longitudinal fasciculus

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2. The Behaviors
Gaze Holding VOR OKN Gaze Shifting Saccades
Vergence Smooth Pursuit
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Classes of eye movements

• Reflexive gaze stabilization
• VOR
• Stabilize for head movements
• Optokinetic
• Stabilize for image motion
• Voluntary gaze shifting
• Saccades
• Acquire stationary target
• Smooth pursuit
• Acquire moving target
• Vergence
• Acquire target in depth

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Gaze During Nystagmus
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Saccades
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3-D Gaze Trajectory
Vergence
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2. The Motor Neurons
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Force Patterns
Robinsons Lollipop Experiments Statics Dynamics
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Oculomotor Neurons During Static Gaze
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Dynamics and Statics
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3. VOR
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Cupula and otoliths move sensory receptors
Cristae
Maculae
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Angular Acceleration
Angular Velocity
Angular Position
Cupula Deflection
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Canal afferents code velocity

• Spontaneous activity allows for bidirectional
  signaling
• S-curve is common
• Different cells have different ranges and
  different dynamics
• Population code

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Canal Output During Slow Sinusoidal Rotation
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VOR With and Without Vision
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rVOR gain varies with frequency

• Almost perfect gt 1Hz
• Low gain for low frequencies (0.1Hz)
• Sensory mechanisms can compensate (optokinetic
  reflex)

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Oculomotor muscles and nerves

• Oculomotor nerve (III)
• Medial rectus
• Superior/Inferior recti
• Inferior oblique
• Trochlear nerve (IV)
• Superior oblique
• Abducens nerve (VI)
• Lateral rectus

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The 3-Neuron ArcPrimary Effects of Canals on Eye
Muscles

• Canal Excites
  Inhibits
• Horizontal Ipsi MR, Contra LR Ipsi LR,
  Contra MR
• Anterior Ipsi SR, Contra IO Ipsi
  IR, Contra SO
• Posterior Ipsi SO, Contra IR Ipsi
  IO, Contra SR

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Robinsons Model of the VOR
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Robinson
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4. OKN
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Type I Vestib Neuron
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Bode Plot of OKN
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Bode Plot of VOR
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Bode Plot of OKN
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5. Saccades
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Saccadic system
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OPN Stimulation
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Brainstem saccadic control

• Paramedian pontine reticular formation (PPRF)
• Burst and omnipause neurons
• Aim to reduce horizontal motor error
• Project to directly to lateral rectus motor
  neurons
• Projects indirectly to contralateral medial
  rectus
• Medial longitudinal fasciculus
• Mesencephalic reticular formation
• Also influenced by omnipause neurons
• Vertical motor error
• Projects to superior and inferior rectus motor
  neurons

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Robinsons Model of the VOR
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Lee, Rohrer and Sparks
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Jay and Sparks
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5. Pursuit
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Smooth pursuit

• Track movement on part of retina
• Two theories
• Motor (Robinson)
• Retinal slip only provides velocity
• Does not capture pursuit onset
• Sensory (Lisberger and Krauzlis)
• Position, velocity and acceleration

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Smooth pursuit system
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Smooth pursuit brainstem

• Eye velocity for pursuit medial vestibular
  nucleus and nucleus prepositus hypoglossi
• Project to abducens and oculomotor nuclei
• Input from flocculus of cerebellum encodes
  velocity
• PPRF also encodes velocity
• Input from vermis of cerebellum encodes velocity
• Dorsolateral pontine nucleus
• Relays inputs from cortex to cerebellum and
  oculomotor brainstem

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Smooth pursuit cortex

• Visual motion areas MT and MST
• Active in visual processing for pursuit
• Stimulation influences pursuit speed
• Projects to DLPN and FEF
• Does not initiate pursuit
• Frontal eye fields
• Stimulation initiates pursuit
• Lesions diminish pursuit

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Jergens
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Scudder
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