Eye Movement Flashcards
Types of Eye Movements
- smooth pursuit (Tracking; to keep an object on the fovea)
- Saccades (rapid, ballistic; to bring an object onto the fovea)
- Optokinetic nystagmus (OKN); VOR (these are a combo of pursuit and saccades)
- Vergence (moving the fovea to an object closer (convergence) or farther away (divergence).
Where is fovea when you look around?
Put the fovea on whatever obect you are interested in. (“foveating”)
High visual acuity only occurs in the fovea (central 5 degrees).
Smooth pursuit
tracking (capable of tracking something moving at 50 degrees/second)
- completely dependent on visual input
- analyze position, direction of movement, speed in visual cortex. Descending commands to brainstem conjugate movement pattern generators.
Saccades
jumps of eyes (you only make smooth movements if you are tracking something)
- On average 3 saccades per second
- Pattern generator for vertical saccades is near the oculomotor nuclei.
- Pattern generator for horizontal saccades is in the reticular formation near the abducens nucleus (paramedian pontine reticular formation–PPRF)
- Control center for saccades are in the CORTEX and SUPERIOR COLLICULUS.
- Voluntary movements can be driven by frontal lobes. The frontal eye field can activate saccades by two pathways:
- one direct to reticular formation
- one via superior colliculus to reticular formation
If the frontal eye field is damaged: temporary loss of ability to generate saccades.
If sup colliculus is damaged, saccades are less accurate.
(Both damaged: permanent loss of saccades ability)
-retinal input to sup colliculus drives saccadic eye movement (even if blind from lesion in visual cortex)
Optokinetic Nystagmus
Slow tracking, then fast snap back/recovery.
Left beating nystagmus with slow tracking to right and quick recovery to left
Ex: watching telephone poles from a car
Slow phase (tracking), and fast phase.
VOR: keep objects sharply in focus if head is moving fast.
What happens when an image is stabilized on the retina?
It disappears. (Ex. blood vessels in eye aren’t visible all the time.)
The eye moves in “microsaccades” to refresh the visual image several times/sec.
What do tracking movements involve?
Stimulus by moving object, analysis of scene by cells in visual association cortex.
Conjugate movements
Both eyes move in same direction (when we move our eyes in response to head movement or to movement of a visual target).
-Initiated by sensory inputs (visual or vestibular)
Vergence movements
Eyes moving in opposite directions (both eyes turn nasally, for ex, during the near reflex–both medial recti contract, both pupils constrict, ciliary muscles contract to fatten lens)
How are horizontal saccades driven?
Contralaterally.
A saccades to the left is driven by activity in the right frontal eye field.
Saccades: reflexive vs voluntary generation
Complex control of saccadic eye movement
reflexive: Parietal eye field (PEF)
voluntary: frontal eye field (FEF)
FEF and PEF activate brainstem gaze center (BGC) either directly or indirectly thru sup collic.
Dorsolateral prefrontal cortex (DLPC) and supplementary eye fields (SEF) in frontal cortex provide planning inhibiiton and coord.
SEF coord saccades with body movement.
DLPC inhibits reflex saccades and provides advaces planning of saccades.
Basal ganglia too: substantia nigra pars reticulata (SNPR) inhibits sup collic.; Caudate nuc inhibits SNPR
(Ex of how activation of striatum can activate a downstream target (sup collic) by DISinhibition)
Vestibular Ocular Reflex (VOR)
-driven by vestibular input
-rightward head rotation is signaled by horizontal canals.
-Leftward eye rotation produced by excitation of motor neurons to the left lateral rectus in left abducens nucleus.
-In right horiz canal, deflection of cupula results in depol of hair cells (left hyperpol)
-Right excitation trans to right vestibular nuclei, cells there project by way of medial longitudinal fasciculus (MLF) to excite left lateral rectus MN in abducens nucleus.
-there is coactivation of abducens motor neurons and internuclear interneurons whose axons cross over and ascend in the MLF to excite right medial rectus MNs.
(Interneurons cross the midline at level of the abducens nucleus–damage to MLF on right b/t VI and III damages axons coming from the left abducens nucleus)
Conjugate vertical movements
The oculomotor nuclei contain motor neurons for the superior and inferior recti, as well as medial rectus and inferior oblique.
Need to activate superior motor neurons, and inhibit inferior motor neurons for both eyes.
The interneurons that constitute the pattern generator for upward gaze are located near the superior colliculi and posterior commissure and cells are distributed bilaterally.
Timing signals for downward gaze come from deeper in the midbrain, near the dorsomedial edge of the red nucleus, and again are bilaterally distributed.
Internuclear ophtalmoplegia (INO)
MLF damaged
Disconnected coord b/t medial and lateral recti during horiz gaze.
common in MS
(Ex: medial rectus that can’t do horizontal gaze saccades or pursuit, but functions fine in vergence movements)
Nystagmus
We all have normal nystagmus: optokinetic nystagmus.
Vestibular nystagmus: move head right, eyes counter rotate left until have to snap back right.
–>direction of nystagmus defined by direction of saccade.
Inappropriate nystagmus: damage to vestibular system or cerebellum
