Eye Movements

Question 1

Name 5 types of EYE Movements

Answer 1

Vestibulo-ocular reflex (VOR)
Optokinetic reflex (OKN)
Saccades
Smooth pursuit
Vergence

Question 2

Name Differences in 5 types of EYE movements

Answer 2

VOR and OKN are:

• old
• no fovea necessary
• stabilize eye when head moves

Saccades, Smooth pursuit and Vergence are:

• new
• fovea necessary
• keep fovea on visual target

Question 3

Name 3 complimentary pairs of eye muscles

Answer 3

1. lateral rectus & medial rectus —-> abduction & adduction
2. superior rectus & inferior rectus —-> elevation/intorsion & depression/extorsion
3. superior oblique & inferior oblique —-> depression/intorsion & elevation/extorsion

Question 4

Measuring EYE Movements:

What is an EOG?

Answer 4

An Electro-oculogram is a charge at the back of the retina that moves with the eye.

Question 5

Measuring EYE Movements:

What is a Scleral search coil?

Answer 5

• magnetic fields induce currents in a coil on the eye
• either surgically implanted on the eye (animals) or embedded within a contact lens (human)
• current is proportional to the angle of the coil

Question 6

Measuring EYE Movements:

What is video-based eye tracker?

Answer 6

It’s infrared, iris match

Question 7

Name all 6 EYE vs SOMA MUSCLE properties, part 1.

Answer 7

• All 6 eye muscles participate in all 5 eye movement types
• Smaller fibers with high innervation ratio
  
  • • # nerve terminals : # muscle fibers = 1:10
  • • 8ms twitch time (2-3 x faster than somatomotor fibers)
  • • Fast, strong and very precise

Question 8

Name all 6 EYE vs SOMA MUSCLE properties, part 2.

Answer 8

• Eye muscles never get tired.
• No recurrent inhibition, but a lot of connective tissue
• Each muscle has 2 layers:
  
  1. Global layer – inserts on eyeball
  2. Orbital layer – inserts on connective tissue
• More proprioceptors, especially muscle spindles (detect stretch)
  
  • WHY? Oculomotor muscles never deal with changes in load

Question 9

Name 3 attributes of the Vestibulo-Ocular Reflex (VOR)

Answer 9

1. Functions to stabilize the eye during head motion
2. Fast , stereotyped response (7 - 15 ms latency)
3. Active almost all the time and allows clear vision while moving

Question 10

Talk about the Mechanism of the Vestibulo-Ocular Reflex (VOR)

Answer 10

1. Semicircular canals of the vestibular system detect head rotation
2. The oculomotor system rotates the eyes by the same amount, but in the opposite direction

Question 11

What are the 2 types of Vestibular-Ocular Reflex (VOR)?

Answer 11

1. Rotational VOR (rVOR)
   when the head rotates with a certain speed and direction, the eyes rotate with the same speed but in the opposite direction
2. Translational VOR (tVOR)
   When the head translates, the eyes rotate in the opposite direction by an amount proportional to the distance of the object and is a function of gaze angle

Question 12

BASIC BRAINSTEM CIRCUIT for the rVOR:

The rVOR consists of a 3-neuron arc, name them

Answer 12

1. semicircular canal (SCC) afferent neurons
2. vestibular nucleus (VN) interneurons
3. oculomotor abducens neurons

Question 13

BASIC BRAINSTEM CIRCUIT for the rVOR:

Describe with a rightward head rotation.

Answer 13

1. right SCC afferents excited
2. ipsilateral (right) VN cells excited
3. VN interneurons project across the midline and excite the contralateral (left) abducens nucleus
4. Abducens motoneurons excite left lateral rectus
5. Abducens interneurons project back across the midline to excite right oculomotor neurons, which excite the right medial rectus
6. Opposite inhibitory path from left canal

Question 14

Describe 7 Properties of Nystagmus

Answer 14

1. the VOR generates the slow phases of nystagmus
2. when eye approaches the edge of the oculomotor range, a saccade (fast phase) is generated in the opposite direction
3. normal nystagmus is seen during large head rotations
4. during constant velocity rotation…..habituation of the VOR
5. time constant of habituation = 5 seconds
6. brainstem “velocity storage” extends this to 25 seconds
7. Post-rotatory nystagmus occurs in opposite direciton

Question 15

Describe Saccades

Answer 15

1. rapid (up to 900o/s), ballistic eye movements to change direction of fixation
2. no visual feedback
3. ~3 saccades per second
4. microsaccades during fixation
5. like fast phase of nystagmus
6. used for redirecting gaze, visual search, reaching, REM sleep

Question 16

Saccades:

Describe Saccade Generation

Answer 16

1. both the SC and the Frontal Eye Fields (FEF) can initiate saccades
2. both the SC and FEF use population coding to generate saccades
   • • takes into account the activity of all active neurons and weighs each according to the strength of its firing rate

Question 17

Moving to the study of Optokinetic reflex (OKN):

Describe the OKN

Answer 17

1. Uses visual rather than vestibular input, therefore sluggish, but will maintain its response indefinitely.
2. Complements the VOR.

Gain = signal output / signal input
Gain VOR = eye movement / head movement

Question 18

Describe the 2 pathways of the Optokinetic reflex (OKN)

Answer 18

1. sub-cortical via brainstem accessory optic system
2. cortical via dorsal cortical pathway

BOTH pathways converge on the vestibular nucleus

• explains why OKN produces sensation of motion
• e.g., waiting at traffic light, waterfall illusion, IMAX theatre

Question 19

Saccades:

Describe Saccade Main Sequence.

Answer 19

Velocity of the saccade depends on distance of the target
- one CAN voluntarily change the amplitude and direction of a saccade but one CANNOT change its velocity

Similarly, the Duration of the saccade depends on the distance of the target
- one CANNOT change its duration

Question 20

Saccades:

Describe the Superior Colliculus (SC)

Answer 20

• the SC contains a topographical map of eye movement vectors
• activity in a specific SC region produces an eye movement directed to a distinct spatial location (relative to the fovea)
• SC cells fire just prior to an eye movement, eliciting a “go to” command

Question 21

Smooth pursuit:

Describe Smooth Pursuit

Answer 21

1. smooth pursuit moves the eyes to keep moving targets on the fovea
2. requires a moving stimulus (with some exceptions)
3. maximum velocity = 100o/s (degrees per second)
4. smooth pursuit is a closed loop system that uses visual motion feedback

Question 22

Vergence:

Describe Vergence

Answer 22

1. convergence brings the eyes together for near targets
2. divergence takes the eyes apart for far targets
3. associated with accommodation response of the lens
4. involves primary visual cortex (VI) for detecting visual target, area MT for measuring angle disparity, and brainstem neurons to move the eyes proportional to target distance
5. very slow ~25o/s
6. goal: to maintain or obtain singular binocular vision

Question 23

Final Common Pathway:

Describe the specifics

Answer 23

1. eye velocity signals drive eye to a specific location in space
2. to keep the eye at this location, position signal is necessary
3. Q: How can one get position from velocity?
4. A: Integration
5. The brainstem final common pathway integrates velocity commands and then adds the integral to the command.
6. velocity = pulse
   position = step
7. pulse-step is seen in the spiking rate of oculomotor neurons
8. separate final common pathways for horizontal and vertical/torsional components

Question 24

Which cranial nerves are used in eye movements?

Answer 24

Superior Oblique 4 (trochlear nerve)
Lateral Rectus 6 (abducens nerve)
all the rest of the muscles use 3 (oculomotor nerve)