Lecture 18 - Eye Movements: Mechanical & neural control (LA)

Question 1

What are conjugate eye movements?

Answer 1

It refers to motor coordination of the eyes that allows for bilateral fixation on a single object. Several centers in the brainstem are involved.

1) Smooth pursuit - voluntary control, keeps image of moving object on fovea (only used by foveate predators and primates but not afoveate animals like rabbits). Works with saccadic system to get on target if the eyes fall behind.
2) Vestibulo-ocular reflex (ancient reflex aka 3 neuron arc) - stabilize images on the retinas (in yoked vision) during head movement by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field
3) Optokinetic nystagmus (OKN) - reflexive response to large visual field motion that allows us to adequately follow moving objects when we keep our head steady while eyes move.

Question 2

What is disjunctive eye movement?

Answer 2

Vergence - the simultaneous movement of the pupils of the eyes towards or away from one another during focusing. It allows both eyes to maintain fixation on an object when it is close to the viewer It works with accommodation (increases lens power at near) and the pupil (constricts to aid in focusing at near)

Question 3

How many muscles move the eyes?

Answer 3

6 muscles per eye - 4 approximately orthogonal rectus muscles and 2 diagonally running obliques

Question 4

Do eyes face straight ahead when we are looking straight ahead?

Answer 4

No - When the muscles are nicely relaxed in the orbits, the eyes are pointing in different directions. To look straight ahead, the eyes are turned within each orbit

Question 5

Eye movements are ___-referenced? Adduction, abduction, intorsion, extorsion, elevation, depression

Answer 5

Nose-referenced Adduction -> towards the nose Abduction -> away from nose Intorsion -> top of eye rotates towards nose Extorsion -> top of eye rotates away from the nose Elevation -> up Depression -> down

Question 6

2 Horizontal rectus muscles

Answer 6

Lateral Action - abductor Innervation - 6th nerve

Medial Action - adductor Innervation - Inferior division of 3rd nerve

Question 7

2 vertical rectus muscles

Answer 7

Superior Action - elevation Innervation - superior division of 3rd nerve
Inferior Action - Depression Innervation - Inferior division of 3rd nerve

Question 8

2 oblique muscles

Answer 8

Superior Oblique - Intorsion Innervation - 4th nerve Inferior (only muscle that does not originate at the back of the orbit)
Inferior Oblique - Extorsion Innervation - Inferior division of 3rd nerve

Question 9

Saccades are fast BUT the muscles resist them - what is the major hindrance?

Answer 9

Viscosity of the EOMs e.g. the faster you try to move the honey, the more force you need as resistance is high. If you move slowly, there is less resistance To get the muscles to move quickly, a BURST of innervation is needed. Antagonist forces must also be turned off. After the eyes get where they’re going, tonic innervation is needed to keep them in their new location

Question 10

Where is the centre of saccadic action?

Answer 10

SC - excit inputs from retina and various cortical areas. Inhibit inputs from frontal cortex via striatum and sub nigra

Question 11

What cortical areas control these inputs to the superior colliculus: reflexive saccade, reflexive saccade inhibition, intentional saccade?

Answer 11

Reflexive saccade - Posterior parietal cortex (PPC)
Reflexive saccade INHIBITION - Dorsal prefrontal cortex (DLPFC)
Intentional saccade - Frontal eye field (FEF)

Question 12

T/F: Afoveate animals can also do smooth pursuit

Answer 12

F- they cannot since they don’t have a fovea so there’s no point because it wouldn’t be clear

Question 13

The property of the stimulus which elicits a smooth movement is real or perceived (illusory) target motion - the illusion of motion is enough for your pursuit system

Answer 13

The property of the stimulus which elicits a smooth movement is real or perceived (illusory) target motion - the illusion of motion is enough for your pursuit system

Question 14

Why do pursuit and saccades overlap so much?

Answer 14

They both keep an object of interest imaged on the fovea and work together to achieve this. The ability of saccades to step in to track a moving object when pursuit falls behind may be why its rare for someone to complain that they can’t pursue, whereas an inability to make saccades will bring someone into the emergency department ASAP.

Question 15

What motions does the vestibular system sense?

Answer 15

Translational motion - along axes (3 possible) Rotational motion - around axes (3 possible)

Question 16

What type of acceleration do: semicircular canals sense? Utricle and saccule?

Answer 16

Angular Linear (including gravity)

Question 17

Cupula - gelatinous mass which has hair cell cilia embedded within it. The canals are filled with endolymph whose passage is almost blocked by the cupula. Turning your head pushes the endolymph onto the cupula - leading to depol/hyperpol. Each canal comes in a pair. What excites one, inhibits the other. Normally, both are firing so that rotation modulates their firing rate in opposite ways

Answer 17

Cupula - gelatinous mass which has hair cell cilia embedded within it. The canals are filled with endolymph whose passage is almost blocked by the cupula. Turning your head pushes the endolymph onto the cupula - leading to depol/hyperpol. Each canal comes in a pair. What excites one, inhibits the other. Normally, both are firing so that rotation modulates their firing rate in opposite ways

Question 18

What happens when you rotate at a constant velocity (hard to do in nature) - in regards to the endolymph?

Answer 18

It usually lags behind and pushes on the cupula BUT in this case it is able to catch up with the canal and the cupula straightens up and the response sent along the vestibular nerve goes back to its baseline level

Question 19

What nerve innervates the ear?

Answer 19

8th cranial nerve

Question 20

Gain = ? When you fixate something stationary in the environment as your head moves?

Answer 20

1

Question 21

Gain = ? When you fixate something moving with you as your head moves (e.g. looking at your watch as you walk)

Answer 21

0

Question 22

Is gain higher or lower in the dark?

Answer 22

Lower ~0.6

Question 23

What is stimuli for vergence?

Answer 23

Disparity and blur

Question 24

The phi phenomenon

Answer 24

The optical illusion of perceiving a series of still images, when viewed in rapid succession, as continuous motion

Question 25

Beta movement

Answer 25

Whereby a series of static images on a screen creates the illusion of a smoothly flowing scene

Question 26

List 4 areas crucial for smooth pursuit

Answer 26

FEF
LIP (Lateral Intraparietal Area)
MT
PPN (precerebellar pontine nuclei)

Question 27

Direct VOR - 3 neurone-arc

Answer 27

With 3 synapses, not much delay between stimulus (head turn) and response (opposing eye movement)

Vestibular nerve -> vestibular nucleus -> Abducens nucleus -> ocular motor nuclei

Question 28

x fills in when VOR can’t stabilize gaze. x responds nicely to constant velocity rotation where else VOR is more effective at lower/higher frequencies.

Answer 28

OKN fills in when the VOP can’t serve to stabilise gaze

OKN responds nicely to constant velocity rotation

VOR is more effective at higher frequencies

Question 29

T/F - Vergence has independent access to the lateral recti - without going through that nucleus

Answer 29

F- Medial rectus not lateral