eye movements

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