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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
• 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 Dynamics
In the Oculomotor System
Rotations about the
Center 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 Behaviors
Gaze Holding:
VOR
OKN
Gaze Shifting:
Saccades
Vergence
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 Experiments
Statics
Dynamics
Oculomotor Neurons
During Static Gaze
Dynamics and Statics
3. VOR
Cupula and otoliths move sensory
receptors
Cristae
Maculae
Angular
Acceleration
Angular
Velocity
Angular
Position
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 During
Slow 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 Arc
Primary 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