Eye Movements and Sensory Integration :(

Question 1

What is Foveation?

Answer 1

Directing the fovea to new objects of interest

Question 2

What are saccades?

Answer 2

A rapid movement of the eye between fixation points.

Quick ballistic eye movements

Little-no visual perception occurs during a saccade

Question 3

What do the results of Yarbus show?

Answer 3

Yarbus used contact lenses with small mirrors on them to document the pattern of eye movements made while individuals examined a variety of objects and scenes

Vision= active process + eye movements shift the view several times each second to direct the fovea towards interesting/informative patrs of the scene

The selection of areas of interest as targets of the saccades shows that non-foveal areas of the retina have sufficient resolution to guide the foveae toward these areas for closer examination

Thus, eye movements allow us to scan the visual field, pausing to focus attention on the portions of the scene that convey the most significant information e.g. eyes, nose, mouth on a face

Question 4

What three antagonistic pairs of muscles control eye movements?

Answer 4

The lateral and medial rectus muscles
The superior and inferior rectus muscles
The superior and inferior oblique muscles

Question 5

What does visual perception depend on?

Answer 5

Frequent changes in a scene

Normally, our view of the world is changed by saccades, and tiny saccades that continue to move the eyes abruptly over a fraction of a degree of visual arc occur even whenfixating on an object of interest.

Moreover, continual drift of the eyes during fixation progressively shifts the image onto a nearby but different set of photoreceptors.

As a consequenceof these several sorts of eye movements, our point of view changesmore or less continually.

Question 6

What demonstrates the importance of a continually changing visual scene?

Answer 6

When the retinal image becomes stabilised

If a small mirror is attached to the eye by a contact lens and an image is reflected off the mirror onto a screen

the individual sees the same thing, whatever the position of the eye

every time the eye moves, the projected image moves by exactly the same amount.

Under these circumstances, the stabilized image actually disappears from perception within a few seconds

Question 7

What innverates extraocular muscles?

Answer 7

Lower motor neurons whos axons form cranial nerves: the abducens, trochlear and the oculomotor

Question 8

Where does the abducens exit the brain and what extraocular muscle does it innervate?

Answer 8

Abducens exits the brainstem at the pons-medullar junction

It innverates the lateral rectus muscle

Question 9

Where does the trochlea nerve exit the brainstem and what extraocular muscle does it innervate?

Answer 9

The trochlear nerve exits from the dorsal surface, the caudal midbrain and crosses the midline to innervate the superior oblique muscle on the contralateral side.

Question 10

Where does the oculomotor nerve exit the brainstem and what extraocular muscles does it innvervate?

Answer 10

The oculomotor nerve exits from the rostral midbrain, medial to the cerebral peduncle

It supplies the rest of the extraocular muscles
It also innervates the levator muscles of the eyelid

Question 11

What are the different types of eye movements?

Answer 11

There are 5 basic types of eye movement that can be grouped into two functional categories: those that serve to SHIFT the direction of gaze and those that serve to STABILISE GAZE

Shift the direction of gaze:
*Saccades
*Smooth persuit movements
*Vergence movements

Stablilise gaze:
*Vestibulo-ocular movements
*Optokinetic movements

Question 12

What do stabilising eye movements do?

Answer 12

Maintain foveation

Induced when the head moves and when there are large-scale movements in the visual field

Question 13

What do shifting eye movements do?

Answer 13

Foveate new targets as they move in visual space

Question 14

What is the function of saccades?

Answer 14

Abruptly change the direction of fixation

They range in amplitude from small movements (e.g. made while reading) to larger movements (e.g. made while gazing around a room.)

Can be elicited voluntarily, but they occur reflexively whenever the eyes are open, even when they are fixated on a target.

Question 15

What are smooth pursuit movements?

Answer 15

Slower tracking movements of the eyes designed to keep a moving stimulus on the fovea once foveation is achieved.

Under voluntary control as the observer can choose whether or not to track a moving stimulus

Question 16

What are vergence movements?

Answer 16

They align the fovea of each eye with targets located at different distances from the observer.

Vergence movements are required to track a visual target that may be moving closer or farther away
More commonly employed when abruptly shifting the direction of gaze, e.g. from a near object to one that is more distant.

They are DISCONJUGATE (or disjunctive) ->they involve either a convergence or divergence of the lines of sight of each eye to foveate an object that is nearer or farther away.

Question 17

What are the three components of the ‘near reflex triad’ that are invovled in vergence movements?

Answer 17

CONVERGENCE -> To look at an object closer by, the eyes rotate towards each other (convergence)

ACCOMMODATION of the lens -> by increasing the curvature of the lens brings the close object into focus

PUPILLARY CONSTRICTION -> by reducing spherical aberration increases the depth of field and sharpens the image on the retina

These three reflexive visual responses are elicited together to shift gaze from a distant to a near object.

Question 18

What are vestibulo-ocular movements and optokinetic eye movements?

Answer 18

They operate together to move the eyes and stabilize gaze relative to the external world, thus compensating for head movements.

Prevent visual images from “slipping” on the surface of the retina as head position varies

Question 19

What do vestibulo-ocular movements do when the head is moved from side-to-side?

Answer 19

-> eyes automatically compensate for the head movement by moving the same distance and at the same velocity but in the opposite direction, thus keeping the image of the object at more or less the same place on the retina.

The vestibular system detects brief, transient changes in head position and produces rapid, corrective eye movement

Question 20

What do optokinetic movements do?

Answer 20

Optokinetic nystagmus - reflexive response to large-scale movements of the visual scene

Its a slow eye movement in the direction of a moving object and a rapid return of eye position in the opposite direction.

Response to movement of whole retinal image
Eyes follow the surround movement so that the image stays stable on the retina

E.g. observing individual telephone poles on the side of the road as we travel by them in a car

Question 21

What are the issues with moving the eyes to fixate a new target in space?

Answer 21

1. Controlling the amplitude of the movement (how far)
2. Controlling the direction of the movement (which way)

Question 22

What is the basic circuitry of the vestibulo-ocular reflex (VOR)?

Answer 22

Vestibular nerve fibres originate in the left horizontal semi-circular canal (which senses head movement) and project to the medial and superior vestibular nuclei (first port of call for sensory information in this reflex)
Excitatory fibres from the medial vestibular nucleus cross to the contralateral abducens nucleus which has 2 outputs (the abducens nucleus contains the neurons that project to the eye muscles that move your eyes horizontally)
One of these is a motor pathway that causes the lateral rectus of the right eye to contract, the other is an excitatory projection that crosses the midline and ascends via the medial longitudinal fasciculus to the left oculomotor nucleus, where it activates neurons that cause the medial rectus of the left eye to contract
Finally, inhibitory neurons project from the medial vestibular nucleus to the left abducens nucleus,
Directly causing the motor drive on the lateral rectus of the left eye to decrease and also indirectly causing the right medial rectus to relax

Question 23

Is feedforward control important to the VOR?

Answer 23

Known for feedforward control

The output of VOR is eye movement but the input to VOR is head movement

So the eye movement doesn’t affect the head at all, the output does not reduce the distrubance that produced it, in contrast to the optokinetic reflex

So it is feedforward control

Question 24

What is the classic 3-neuron reflex arc in the VOR?

Answer 24

1. Sensory neurons signal head movement
2. These then synapse with interneurons in the vestibular nuclei

3.These then synapse onto ocular motoneurones that drive the eye muscles

Question 25

What is ocular motorneuon firing (phase 3 of the 3-neuron reflex arrc)?

Answer 25

The ocular motoneurones send an eye-movement command to the eye muscles

The eye muscles act on the eyeball and associated tissue to produce the actual eye-movement

Question 26

What is the basic circuitry of the optokinetic reflex (OKR)?

Answer 26

Similar to VOR except the input comes from the retina and pretectum and it signals large-field movement of the whole retinal image (‘retinal slip’)

Output of the reflex affects the retinal slip
Retinal slip is the input to the reflex

The output directly affects the input, its FEEDBACK CONTROL

Question 27

Why do we have 2 stabilising eye reflexes?

Answer 27

The OKR is inadequate on its own, it does not work for rapidly changing (high frequency) movements
OKR works well for low-frequency movements but there are feedback delays in high-frequency movements in the retina -> there is a delay in the feedback loop, the retinal processing of image slip takes 50-100ms so we can get runaway instability and higher frequencies

VOR compliments the OKR
VOR is good for high frequency head movements (e.g. walking or running) with a very fast response time of 14ms
But its poor for low frequency constant movement because of the mechanics of semi-circular canals, they eventually stop responding to steady constant head movement

Together they cover the range of movements nicely and theres even a range in the middle where they work together

Question 28

How does the VOR know just how big an eye-movement command to send to the eye muscles?

Answer 28

Possibly genetically wired in, but have to factor growth ->the semi-circular canals get bigger, eye muscels get stronger, aging means the vestibular system isn’t as good as it was, and injury

-> system needs constant maintenance - how? Learning -> type of learning is VOR adaptation and its typical of lots of motor learning that we do (unconsciously) and because VOR is a simple circuit with 3 synapse and a simple motor skill we often use it to assess learning

e.g exps wearing goggles to magnify or reduce the image, we alter what the VOR has to do to adjust the gain, so we induce learning here

The structure used for this =cerebellum
A copy of the eye movement command is sent to the cerebellum and that is used to adjust the gain of the reflex, its a side path that can adjust it accurately and it uses retinal slip as its error signal-
->if the VOR was working well, you dont get retinal slip but if you do then its an error signal and the cerebellum uses this signal to adjust the gain of the reflex

Requires constant calibration and maintenance, carried out in the cerebellum using retinal slip as an error signal