The oculomotor system
Bijan Pesaran
April 29, 2008
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
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
Motor neurons command muscle
forces
• Linear increase for
static forces
• Pulse for dynamic
forces
Optokinetic reflex
• Optokinetic nystagmus
• Neural pathway
convergent with VOR
Saccadic system
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
Eye movements diagnose
brainstem lesions
• PPRF lesions impede horizontal eye
movements
• MRF lesions impede vertical eye
movements
• MLF lesions impede medial rectus
contraction
– Internuclear opthalmoplegia
– No impact on vergence
Superior colliculus
• 7 layered structure. Mammalian optic tectum.
• Superficial layers (3 layers)
– Visual input from retina and striate cortex
– Modulated by saccades but not attention
• Intermediate (2) and deep (2) layers
– Input from dorsal stream and FEF
– Build-up and burst neurons
• Topographic maps encode motor error
• Fixation zone in rostral SC -> Dorsal raphe
nucleus
• Lesions disrupt saccades temporarily
Population averaging scheme
Sensory-motor
transformations
• Deep layers
• Auditory-oculomotor
– Auditory neurons
– Bimodal neurons
• Somatosensory-oculomotor
– Body maps
• Update in response to eye
movements
Parietal cortex
• Area LIP
– Early stage of movement planning
– Visual responses modulated by attention
• Lesions disrupt sensory-motor processes
– Neglect
– Optic ataxia
– Balint’s syndrome
Frontal cortex
• Frontal eye fields
– Visual, movement and visual-movement neurons
– Project to PPRF and MRF
– Lesions: Temporary paresis, long term memory
deficit
• Supplementary eye fields
– Object-centered saccades
• Dorsolateral prefrontal cortex
– Working memory
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
Vergence
• Four sources
–
–
–
–
Disparity
Accomodation
Tonic
Proximal vergence
• Brainstem
– Burst and Burst-tonic neurons
• Similar to saccadic system
Coordinated vergence/version
movements
• Vergence starts sooner
• Saccade finishes faster
• Systems interact
– Saccade omnipause inhibits
vergence bursters
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
Clinical diagnosis from eye
movements