05 Eye Movements Flashcards
1
Q
What makes the Fovea special?
A
- area of highest visual acuity and colour vision
- densely/entirely packed with cones
2
Q
Why are eye movements important?
A
- essential for active visual perception
- example for motor control
- key indicators of neurological function and dysfunction
3
Q
What are the different types of eye movements and which 2 basic functions do they serve?
A
- shifting gaze: saccade, smooth pursuit, vergence
- stabilizing gaze: vestibulo-ocular reflex, optokinetic reflex
4
Q
Saccade Features
A
- 3-5 every second
- elicited reflexively or voluntarily
- ballistic
- fastest movement of the human body
5
Q
What is Smooth Pursuit?
A
- tracking movement to keep fovea aligned with moving stimulus
- much slower than saccades
- in absence of moving stimulus only after intensive training
6
Q
What is Vergence?
A
- disconjugate eye movement to switch focus between objects that are not equally distant
- far-to-near focus (convergence), near-to-far focus (divergence)
7
Q
vestibulo-ocular & optokinetic reflex
A
- operate together to stabilize gaze relative to external world
- vestibulo-ocular reflex compensates for rapid head movements using vestibular information
- optokinetic reflex compensates for slow, large-scale movement of visual scene using cues for visual field motion
8
Q
What extraocular muscles are there and what are their roles?
A
- lateral vs. medial rectus muscles (horizontal axis)
- superior vs. inferior rectus muscles (vertical axis)
- superior vs. inferior oblique muscles (torsional axis)
- horizontal movements controlled by lateral and medial rectus muscles
- vertical movements require coordination of superior and inferior rectus (elevation/depression) and oblique muscles
- contributions depend on position on horizontal axis
9
Q
adduction vs. abduction
A
- adduction: movement toward nose
- abduction: movement away from nose
10
Q
What cranial nerves innervate which extraocular muscles?
A
- oculomotor nerve (III) innervates ipsilateral medial rectus, superior and inferior rectus, and inferior oblique muscle (as well as levator muscle of eyelid and involved in pupillary constriction)
- trochlear nerve (IV) innervates contralateral superior oblique muscle
- abducens nerve (VI) innervates ipsilateral lateral rectus muscle and portion of contralateral oculomotor nucleus innervating medial rectus muscle (via medial longitudinal fasciculus)
11
Q
How is saccade amplitude encoded?
A
- by duration of neural activity in lower motor neurons of oculomotor nuclei
- baseline level of discharge is correlated with position of eye in orbit
- steady baseline level of firing generates muscle force needed to hold eye in its current position
12
Q
How is saccade direction encoded?
A
- determined by activated combination of eye muscles
- specified by relative activation of 2 gaze centers in reticular formation
- horizontal gaze center: paramedian pontine reticular formation (PPRF)
- vertical gaze center: rostral interstitial nucleus
13
Q
Projections from PPRF
A
- innervates ipsilateral abducens nucleus (activation)
- innervates inhibitory neurons in ipsilateral medullary reticular formation projecting to contralateral abducens nucleus (inhibition)
14
Q
Which brain areas are involved in the initiation and accurate targeting of saccades?
A
- superior colliculus
- frontal eye field (FEF)
- cerebellum
- basal ganglia
- other areas in frontal and parietal lobe
15
Q
How are FEF and superior colliculus involved in the execution of saccades?
A
- both project to contralateral gaze centers
- FEF directly as well as indirectly via projections to superior colliculus
- they provide complementary pathways, but are functionally distinct
