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