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The oculomotor system

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

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The oculomotor system

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  1. The oculomotor system Bijan Pesaran April 29, 2008

  2. 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

  3. 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

  4. Motor neurons command muscle forces • Linear increase for static forces • Pulse for dynamic forces

  5. Optokinetic reflex • Optokinetic nystagmus • Neural pathway convergent with VOR

  6. Saccadic system

  7. 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

  8. 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

  9. 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

  10. Population averaging scheme

  11. Sensory-motor transformations • Deep layers • Auditory-oculomotor • Auditory neurons • Bimodal neurons • Somatosensory-oculomotor • Body maps • Update in response to eye movements

  12. 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

  13. 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

  14. 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

  15. Smooth pursuit system

  16. 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

  17. 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

  18. Vergence • Four sources • Disparity • Accomodation • Tonic • Proximal vergence • Brainstem • Burst and Burst-tonic neurons • Similar to saccadic system

  19. Coordinated vergence/version movements • Vergence starts sooner • Saccade finishes faster • Systems interact • Saccade omnipause inhibits vergence bursters

  20. 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

  21. Clinical diagnosis from eye movements

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