Control of Eye Movements

Control of Horizontal Eye Movements

*Abducens Nerve Damage* CN VI Palsy ipsilateral eye deficit lateral movement *Abducens Nucleus Damage* lateral gaze palsy both eyes cant look towards the side of lesion *PPRF Damage* lateral gaze palsy both eyes cant look towards the side of lesion *MLF Damage* INO; medial rectus is damaged ipsilateral eye cant look medially feedback to lateral rectus of contralateral eye causes nystagmus *MLF and Abducens Nucleus Damage* 1 1/2 Syndrome both eyes cant look towards side of lesion ipsilateral eye cant look in contralateral direction contralateral eye can look away from lesion but with nystagmus

More Testing and Deficits

*Flocculus* ipsilateral smooth pursuit impairment and inability to hold eccentric eye positions *Oculomotor Vermis and Fastigial Nuclei* saccade dysmetria *Nodulus and Ventral Uvula* periodic alternating nystagmus (PAN) or spontaneous horizontal nystagmus (ipsilateral) *Vestibulocerebellum* disturbances to VOR aspects (velocity signal storage, gain adaption)

Testing and Deficits

*Foveation* hold fovea on target; look for drifts, especially holding eccentric gaze *Saccade* acquisition of new target; 100-700 degrees/sec; sedative sensitive *Smooth Pursuit* follow a moving target *Vergence* direct fovea at targets of different distances; 15 degrees/sec with disconjugate saccade faster *VOR* compensate for initial head movement; look for dizziness/nausea *Optokinetics* compensate for continued head movement *Nystagmus Fast Phase* recenter during continuous VOR or OKR; look for alternating fast and slow phase movements

Central Control System: Supranuclear

*Frontal Eye Fields:* (area 8) input from visual association cortex and thalamus (MD nuclei) to inform regarding target location projections directly to brainstem gaze centers and superior colliculus volitional or memory guided saccades *Parietal Eye Feilds* (posterior IPS) reciprocally connected to Frontal Eye Fields with projections to superior colliculus has indirect influence involved in visual selection (attention)= provides "salience" map reflexive saccades

Gaze Stabilization: Optokinetic System

*a reflex response system to compensate for movement of head* compensates for sustained or slowed head movements unlike VOR, visual input is used to infer direction and speed of head movement; especially for whole field movement of visual scene, *retinal slip* activates wide-field retinal ganglion cells terminate in nucleus of optic tract and accessory optic nuclei project to vestibular nuclei and indirectly to vestibulocerebellum produces slow eye movements to compensate for retinal slip (matches direction and velocity) with rapid repositioning phase= *Optokinetic Nystagmus*

Gaze Stabilization: Vestibulo-ocular Reflex (VOR)

*a reflex response system to compensate for movement of head* reflexes are driven by input from the vestibular apparatus (ampullary cristae) and coordinated by the vestibular nuclei produces eye movements that are equal and opposite to the angular head movement

Vergence

*simultaneous movement of both eyes in opposite directions to obtain or maintain a single binocular field* is required to maintain the fovea of each eye on a single target as its distance changes required along with version in gaze shifts motor error commands are produced by neurons of visual cortex with binocular visual fields in response to visual disparity premotor neurons are found in the supraoculomotor area of the midbrain that drives vergence and accomodation

Central Control System: Brainstem

3 pairs of nuclei that contain neurons that innervate the extraocular muscles: 1. oculomotor nucleus 2. trochlear nucleus 3. abducens nucleus these nuclei are connected by the medial longitudinal fasciculus (MLF) versional movements of eyes require coordinated activation of motor neurons in different nuclei and the eye muscles that they innervate

Central Control System: Superior Colliculus

contains retinotopic map of contralateral visual space used in directing eye movement (deeper layers are visuomotor) 1. provides motor error coordinates 2. translates sensory info (visual, auditory, somatosensory) into motor error signal (desired change in position) 3. input from cortical eye fields and SNpr 4. projections to brainstem gaze centers and frontal cortex via thalamus (MD nuclei) 5. reflex orienting movements *cerebellum vermis* plays a role in callibrating saccades; important for long term adaptation in eye movement control example: adjusts for muscle weakness, for differences in elastic restorative forces btw positions: same amplitude

Shifting Gaze

gaze is rapidly shifted from one fixation point to another when we look around rapid conjugate eye movement= saccade motor neuron activity has both pulse and step components in accordance with velocity signal and integrated position signal saccade initially brings fovea into alignment but the eye velocity must then match target velocity *extrastriate visual cortex* (MT and MST) provides info regarding target motion directly to DLPN and via FEF and Posterior Parietal Cortex *dorsal lateral pontine nucleus* (DLPN) encodes direction and velocity of pursuit: sends projections of vestibulocerebellum which interfaces with brainstem oculomotor system via vestibular nuclei

Antisaccade Task

inhibit reflexive saccade and shift gaze in opposite directions *dorsolateral prefrontal cortex (DLPFC)* is involved in saccade inhibition FEF is involved in triggering antisaccade increase errors in frontal lobe dysfunction due to stroke and dementia difficult to ignore or override parietal cortex saliency signal stroke affecting the FEF may produce a transient deviation of gaze to the side of lesion or difficulty in directing gaze to contralateral side

Eye Movement Mechanics

precise control of eye position and movement is required to stabilize an image during movements of head and to direct gaze to stationary or moving targets eye is controlled via 3 pairs of extraocular muscles: 1. medial and lateral rectus 2. superior and inferior oblique 3. superior and inferior rectus eye moves around 3 axis of rotation: 1. X-axis= vertical movements 2. Y-axis= horizontal movements 3. Z-axis= torsion the soft tissues and muscles of orbit produce a resistance to movement resulting in slow dynamics, therefore mvmt requires: 1. force to overcome viscous drag 2. force to maintain eccentric position as a result, movement from eye position to another requires pulse and step like activity of motor neurons

Vision

retina of eye is specialized so that only a small area, *fovea* is responsible for high visual resolution fovea represents an area subtending about 1 degree of visual angle

Mediators of Central Control System

there are 2 gaze control centers: 1. PPRF= horizontal eye movement 2. riMLF= vertical eye movement gaze centers contain both excitatory and inhibitory burst neurons: code for pulse signals (dynamic phase) *step signals* (tonic position) are provided by: 1. Nucleus Prepositus Hypoglossi= horizontal saccades 2. Interstitial Nucleus of Cajal= vertical saccades *omnipause cells* provide inhibition of burst cells during fixation; silenced to allow release of burst cells