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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