Eye Movements

Question 1

What makes the Fovea special?

Answer 1

• area of highest visual acuity and colour vision
• densely/entirely packed with cones

Question 2

Why are eye movements important?

Answer 2

• essential for active visual perception
• example for motor control
• key indicators of neurological function and dysfunction

Question 3

What are the different types of eye movements and which 2 basic functions do they serve?

Answer 3

• shifting gaze: saccade, smooth pursuit, vergence
• stabilizing gaze: vestibulo-ocular reflex, optokinetic reflex

Question 4

Saccade Features

Answer 4

• 3-5 every second
• elicited reflexively or voluntarily
• ballistic
• fastest movement of the human body

Question 5

What is Smooth Pursuit?

Answer 5

• tracking movement to keep fovea aligned with moving stimulus
• much slower than saccades
• in absence of moving stimulus only after intensive training

Question 6

What is Vergence?

Answer 6

• disconjugate (= not paired / opposite of conjugate) eye movement to switch focus between objects that are not equally distant
• far-to-near focus (convergence), near-to-far focus (divergence)

Question 7

vestibulo-ocular & optokinetic reflex

Answer 7

• 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

Question 8

What extraocular muscles are there and what are their roles?

Answer 8

• 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

Question 9

adduction vs. abduction

Answer 9

• adduction: movement toward nose
• abduction: movement away from nose

Question 10

What cranial nerves innervate which extraocular muscles?

Answer 10

• 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)

Question 11

How is saccade amplitude encoded?

Answer 11

• 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

Question 12

How is saccade direction encoded?

Answer 12

• 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

Question 13

Projections from PPRF

Answer 13

• innervates ipsilateral abducens nucleus (activation)
• innervates inhibitory neurons in ipsilateral medullary reticular formation projecting to contralateral abducens nucleus (inhibition)

Question 14

Which brain areas are involved in the initiation and accurate targeting of saccades?

Answer 14

• superior colliculus
• frontal eye field (FEF)
• cerebellum
• basal ganglia
• other areas in frontal and parietal lobe

Question 15

How are FEF and superior colliculus involved in the execution of saccades?

Answer 15

• 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

Question 16

How are saccades encoded in FEF and superior colliculus and how does sensorimotor integration come about?

Answer 16

• both contain topographical map of eye movement vectors: activation of particular site elicits saccade with specific amplitude and direction
• superior colliculus also contains sensory maps (visual, auditory, somatosensory): stimulus in space corresponds to particular site in SC
• sensory maps are in register with motor maps: stimulus in space activates region that is generating a saccade to the stimulus

Question 17

Are saccades encoded in retinotopic coordinates?

Answer 17

• no, encoded in movement coordinates
• if another saccade is induced before the intended one, monkeys are still able to reach target location
• if saccades were encoded in retinotopic coordinates, they would systematically miss target by amount of induced saccade