Eye movements and perception

Question 1

key structures of the eye:

4 structures

Answer 1

fovea, retina, rods, cones

Question 2

possible eye movement axes:

3 types

Answer 2

• horizontal movements
• vertical movements (large notieceable movements)
• in plane movements (torsion - very small)

Question 3

the types of eye movements

2 types

Answer 3

• gaze shifting (orienting) mechanisms - voluntary
• gaze stabilising mechanisms - involuntary

Question 4

Gaze shifting mechanisms: smooth persuit

3 points

Answer 4

• We use this to track a moving object
• It ensures that the light from the object stays focused on the back of the eye
• it requires a continuous feedback loop

Question 5

Gaze shifting mechanisms: saccades

4 points

Answer 5

• Fast, ballistic movements
• Speeds of up to 700/s
• They show characteristic patterns of acceleration
• Both eyes move together in an almost identical fashion

Question 6

Gaze shifting mechanisms: vergence

Answer 6

• We move our eyes to focus on objects at different distances away from us

Question 7

types of stabilising eye movements?

2 types

Answer 7

• vestibulo-ocular reflex
• optokinetic reflex

Question 8

where does the Vestibulo-ocular reflex occur?

Answer 8

in space

Question 9

what is a vestibulo-ocular reflex?

3 points

Answer 9

• they are movements that cancel out the motion of the body and the head
• they are in reaction to the signals from the vestibular organs of the inner ear (balance)
• they happen for example when we are walking or spinning

Question 10

where does the Optokinetic reflex occur?

Answer 10

on the visual scene

Question 11

what is an optokinetic reflex?

2 points

Answer 11

• stabilising movements that are made in reaction to a whole visual field moving
• they can be studied by using rotating drums of tumbling rooms

Question 12

Further types of eye movements (miniature eye movements that occur when fixating):

2 types

Answer 12

• microsaccades
• drifts and tremor

Question 13

what are microsaccades?

3 points

Answer 13

• small rapid eye movements
• they are thought to help with avoiding fading of the visual scene
• they may be linked with attention

Question 14

what are drifts and tremors?

Answer 14

• Drifts are slow meandering motions between microsaccades.
• Tremors are very small oscillations on top of drifts

Question 15

what is the most common way to measure eye movements?

Answer 15

using an eye-tracker

Question 16

how does an eye-tracker function?

2 points

Answer 16

• it involves a small camera focused on the eye and software that tracks where the pupil moves
• By calibrating the pupil position against fixed points on the screen, one can work out where the participant is looking

Question 17

what are fixation measures

Answer 17

a kind of technology used to build a picture over time of where a participant is looking

Question 18

what can scientists measure with fixation measures?

4 things

Answer 18

• dwell time (each fixation)
• frequency of fixation
• total duration of fixations (total looking time)
• order of fixation positions

Question 19

what is a scanpath?

Answer 19

the path followed by the viewer’s eyes when reading a document or following an object

Question 20

what is a heatmap?

Answer 20

a heatmap is plotted from the data collected by tracking the movement of an individual’s eyes

Question 21

why do we have eye movements?

3 points

Answer 21

• stabilisation
• adaptation & fading
• focusing on the fovea

Question 22

why do we have eye movements?: stabilisation

2 points

Answer 22

• To build up a visual representation the image on the back of the eye needs to be kept still
• Stabilisation occurs through optokinetic and vestibulo-ocular reflex

Question 23

why do we have eye movements: adaptation & fading

3 points

Answer 23

• If an image is kept fixed on the retina it starts to fade away in the periphery after a while
• This is called Troxler’s fading (1804)
• It is caused by adaptation of the neurons in the retina, they do not fire after a while to the same stimulus

Question 24

why do we have eye movements: focusing on the fovea?

3 points

Answer 24

• The retina is not uniformly sensitive
• the fovea has a high density of photoreceptors which is given most processing power
• In order to have a detailed representation of a part of the image the light from the part needs to land in the fovea

Question 25

# why don't we notice eye movements? why don't we notice eye movements?

Answer 25

In order to build up a detailed representations of scenes we are constantly moving our eyes, yet we do not notice this motion

Question 26

# why don't we notice eye movements? what is saccadic supression?

Answer 26

it means that stimuli are more difficult to detect during the movement of the eyes * In particular it appears that the motion in the visual scene is ignored * This is still a much investigated area - the question of perceptual stability across consecutive saccades

Question 27

what are the problems with eye movements? | 3 problems

Answer 27

- The brain needs to know which part of the motion on the retina is caused by eye movements and which part is caused by something moving in the scene - This can be done by cancelling out the signal using an “efference copy” from eye muscles - Also, cues from the scene, such as whole image motion can be cancelled out

Question 28

when do eye movements go wrong?

Answer 28

Some illusions are caused by our inability to cancel out the motion of the eyes

Question 29

# passive vs active vision what does a lot of work on vision ignore?

Answer 29

the role of eye movement

Question 30

# passive vs active vision what does the passive approach see?

Answer 30

it sees the eye as just receiving information from the outside world

Question 31

# passive vs active vision what does the active approach emphasise?

Answer 31

it emphasises the observer's eye movements guide the sampling of visual scenes in a dynamic process.

Question 32

combining motor and visual systems:

Answer 32

Brain areas involved in eye movements are active both when visual information is being processed and when eye movements are being made. These are important “cross-modal” areas

Question 33

controlling eye movements: | 2

Answer 33

- Smooth pursuit involves feedback loop - the eye positioning is updated according to where the object is - Saccades are ballistic - the movement needs to be pre-programmed, once initiated the trajectory cannot be changed

Question 34

what is reading?

Answer 34

a complex cognitive process

Question 35

reading: existing models

Answer 35

many models that still exist try and describe the ways in which we extract meaning from written words. However eye movements can be used as evidence to test these theories

Question 36

regression accounts for what?

Answer 36

10% of saccades

Question 37

what does the window technique show?

Answer 37

that we see information from up to 15 letters away in English (less for more complex characters such as Chinese ones)

Question 38

what does the window techniqe suggest?

Answer 38

that the shape of a word plays a role as we are worse at resolving letters outside the fovea

Question 39

fixation on words findings: | 3 findings

Answer 39

- we do not fixate on each letter - we move about 8 characters with each saccade - readers fixate 80% of 'content' words but only 20% of 'function' words

Question 40

the link between eye movements and attention | 2 links

Answer 40

- We are not able to process all visual information at once - Usually where we are looking is where we are paying attention to (although not always)