14 - Control of Eye Movements Flashcards
What are the types of eye movements?
Conjugate movements where the eyes move in the same direction (yolked together): can be fast or slow.
Vergence movements in which two eyes move in opposite directions: can be convergence or divergence.
What is Saccades? What are the uses?
Horizontal or vertical *conjugate* eye movements that rapidly shift gaze to a new visual target.
It’s fast, voluntary, and reflexive (from visual, auditory, or tactile systems).
Scanning the environment and reading.
What are the eye fields involved in horizontal saccades? What role does each have?
Frontal eye fields: initiate horizontal saccades.
Parietal eye fields: direct attention to things in the peripheral visual field
Supplementary eye fields: involved in multiple saccades
What is the horizontal gaze center?
The paramedian pontine reticular formation (PPRF): cluster of neurons that recieves input from the superior colliculus (visual reflex) and projects to the IPSI abducens nucleus to coordinate it’s activity.
Abducens encodes velocity or eye movement.
Voluntary saccades are initiated in the _______.
Cerebral cortex : frontal eye fields, brodmann’s area 8.
What is the function of the frontal eye fields (FEF)?
“points” the eyes to the opposite size.
Right FEF: eyes look toward the left
Left FEF: eyes look toward the right
What is vertical saccades initiated by?
Diffuse areas of the cerebral cortex.
Neurons project bilaterally to the midbrain via the medial longitudinal fasciculus (MLF)
The rostral interstitial nucleus (of cajal) of the MLF neurons control vertical saccades through CN III and CN IV.
What is Parinaud’s syndrome?
Impaired vertical gaze.
Because the neurons involved in vertical saccades project bilaterally, very few lesions occur.
What is smooth pursuit? What is the speed?
Eye movements that keep the image of a moving target stabilized on the fovea.
Slower than saccades.
What initiates smooth pursuit? What is it modified by?
The parieto-occipito-temporal cortex, which “pulls” the eyes to the same side by activating the ipsilateral PPRF (ie the right side controls smooth eye movements of the eyes to the right).
Modified by the vestibular nuclei and the flocculonodular lobe.
What is the role of the flocculonodular lobe in smooth pursuit eye movements?
It supresses the vestibulo-ocular reflex for large smooth pursuit movements.
Normally (with VOR): head turns right > eye turns toward left
VOR suppressed: head turns right > eyes turn toward right to track an object
What isthe function of optokinetic nystagmus? What type of movements does it involve?
Functions to keep a moving visual field on the retina as long as possible (in a bus watching the telephone poles go by).
Involves alternate smooth pursuit and saccadic movements: nystagmus.
Describe the two types of movements involved in optokinetic nystagmus? What initiates each?
Parieto-occipito-temporal cortex does smooth pursuit phase (ipsilateral)
Frontal eye fields do saccadic phase (contralateral)
What is vergence? What are the types?
Reflexive movements of the eyes in opposite directions to stabilize an image on the fovea.
Convergence and divergence.
What is the near response?
Accommodation, vergence, and pupillary constriction.
What is the result of a cortical lesion in the region of the L frontal eye field?
Since the frontal eye field crosses to the contralateral side, a L sided lesion will result in the inability for the patient to look to their right.
What frontal eye field and corticospinal tract deficits are seen with a cortical lesion on the L side?
FEF > ipsilateral gaze preference (your eyes will be looking towards the side of the lesion)
CST > contralateral hemiparesis (because the corticospinal tract crosses in the medulla)
What eye problems result from a pontine lesion on the L side?
Frontal eye fields project into the pons. Wiping out this pathway at the level of the pons effects the opposite side that swoops through.
Therefore, a L lesion of the pons results in the inability to look left (so your gaze preference is contralateral)
What horizontal gaze and corticospinal tract deficits occur with a pontine lesion on the left side?
Horizontal gaze center > contralateral gaze preference (cannot look towards the side of the legion).
Corticospinal tract > contralateral hemiparesis (because the CST hasn’t crossed yet - it crosses in the medulla).
What is the result of a right abducens nerve palsy?
Right eye cannot abduct because the abducens nerve is not innervating it.
What is the result of a lesion of the right abducens nucleus?
Right lateral gaze palsy: right eye cannot abduct due to loss of abducens nerve action, and left eye cannot adduct due to loss of left MLF.
What results in a lesion of the right PPRF?
Right lateral gaze palsy: right eye cannot abduct due to loss of abducens nerve action, and left eye cannot adduct due to loss of left MLF.
This is because the right PPRF is unable to “talk” to the right abducens nucleus and therefore all of the functions of the abducens nucleus are lost.
What results from a lesion of the left medial longitudinal fasciculus?
Internuclear ophthalmaplegia.
Left medial rectus isn’t working so left eye cannot adduct (cannot look R) . Right eye exhibits nystagmus when looking to the right as if to tell the left eye “come on, lets look this way”.
What results from a left medial longitudinal fasciculus and abducens nucleus lesion?
Called one and a half syndrome.
Cannot look left at all due to knocking out the left abducens nucleus.
Left eye unable to adduct, which causes the right eye to have nystagmus because it can look right but the left eye cannot.
