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Eye Movements. 1. The Plant. The Oculomotor Plant Consists Of only 6 muscles in 3 pairs. This Yields 3 degrees of Mechanical Freedom. Donder’s Law/ Listing’s Law. Neural Constraints Reduce this to 2 degrees of freedom. 3-D eye movements. Donder’s Law Relates torsion to eye position - PowerPoint PPT Presentation
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Eye Movements
1. The Plant
The Oculomotor Plant ConsistsOf only 6 muscles in 3 pairs
This Yields 3 degrees ofMechanical Freedom
Neural ConstraintsReduce this to2 degrees of freedom
Donder’s Law/ Listing’s Law
3-D eye movements• Donder’s Law
– Relates torsion to eye position
• Listing’s law– Torsion results from rotation of
eye around perpendicular axis
• Listing’s plane– Plane orthogonal to line of sight
• Does not apply when head is free
Kinematics vs DynamicsIn the Oculomotor System
Rotations about theCenter of Gravity
No Loads
No Inertia
Force = Position
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
2. The BehaviorsGaze Holding:
VOROKN
Gaze Shifting:SaccadesVergence
Smooth Pursuit
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
Gaze During Nystagmus
Saccades
3-D Gaze Trajectory
Vergence
2. The Motor Neurons
Force Patterns
Robinson’s Lollipop ExperimentsStaticsDynamics
Oculomotor NeuronsDuring Static Gaze
Dynamics and Statics
3. VOR
Cupula and otoliths move sensory receptors
Cristae Maculae
Angular Position
Angular Acceleration
Angular Velocity
Cupula Deflection
Canal afferents code velocity
• Spontaneous activity allows for bidirectional signaling
• S-curve is common• Different cells have
different ranges and different dynamics
• Population code
Canal Output DuringSlow Sinusoidal Rotation
VOR With and Without Vision
rVOR gain varies with frequency
• Almost perfect > 1Hz• Low gain for low
frequencies (0.1Hz)• Sensory mechanisms
can compensate (optokinetic reflex)
Oculomotor muscles and nerves• Oculomotor nerve (III)
– Medial rectus– Superior/Inferior recti– Inferior oblique
• Trochlear nerve (IV)– Superior oblique
• Abducens nerve (VI)– Lateral rectus
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
Robinson’s Model of the VOR
Robinson
4. OKN
Type I Vestib Neuron
Bode Plot of OKN
Bode Plot of VOR
Bode Plot of OKN
5. Saccades
Saccadic system
OPN Stimulation
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
Robinson’s Model of the VOR
Lee, Rohrer and Sparks
Jay and Sparks
5. Pursuit
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
Smooth pursuit system
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
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
Jergens
Scudder