Eye Movements 1 The Plant The Oculomotor Plant

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

r. VOR gain varies with frequency • Almost perfect > 1 Hz • Low gain for low frequencies (0. 1 Hz) • 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