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