Eye movements and perception Flashcards
key structures of the eye:
4 structures
fovea, retina, rods, cones
possible eye movement axes:
3 types
- horizontal movements
- vertical movements (large notieceable movements)
- in plane movements (torsion - very small)
the types of eye movements
2 types
- gaze shifting (orienting) mechanisms - voluntary
- gaze stabilising mechanisms - involuntary
Gaze shifting mechanisms: smooth persuit
3 points
- 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
Gaze shifting mechanisms: saccades
4 points
- Fast, ballistic movements
- Speeds of up to 700/s
- They show characteristic patterns of acceleration
- Both eyes move together in an almost identical fashion
Gaze shifting mechanisms: vergence
- We move our eyes to focus on objects at different distances away from us
types of stabilising eye movements?
2 types
- vestibulo-ocular reflex
- optokinetic reflex
where does the Vestibulo-ocular reflex occur?
in space
what is a vestibulo-ocular reflex?
3 points
- 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
where does the Optokinetic reflex occur?
on the visual scene
what is an optokinetic reflex?
2 points
- stabilising movements that are made in reaction to a whole visual field moving
- they can be studied by using rotating drums of tumbling rooms
Further types of eye movements (miniature eye movements that occur when fixating):
2 types
- microsaccades
- drifts and tremor
what are microsaccades?
3 points
- small rapid eye movements
- they are thought to help with avoiding fading of the visual scene
- they may be linked with attention
what are drifts and tremors?
- Drifts are slow meandering motions between microsaccades.
- Tremors are very small oscillations on top of drifts
what is the most common way to measure eye movements?
using an eye-tracker
how does an eye-tracker function?
2 points
- 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
what are fixation measures
a kind of technology used to build a picture over time of where a participant is looking
what can scientists measure with fixation measures?
4 things
- dwell time (each fixation)
- frequency of fixation
- total duration of fixations (total looking time)
- order of fixation positions
what is a scanpath?
the path followed by the viewer’s eyes when reading a document or following an object
what is a heatmap?
a heatmap is plotted from the data collected by tracking the movement of an individual’s eyes
why do we have eye movements?
3 points
- stabilisation
- adaptation & fading
- focusing on the fovea
why do we have eye movements?: stabilisation
2 points
- 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
why do we have eye movements: adaptation & fading
3 points
- 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
why do we have eye movements: focusing on the fovea?
3 points
- 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
why don’t we notice eye movements?
why don’t we notice eye movements?
In order to build up a detailed representations of scenes we are constantly moving our eyes, yet we do not notice this motion
why don’t we notice eye movements?
what is saccadic supression?
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
what are the problems with eye movements?
3 problems
- 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
when do eye movements go wrong?
Some illusions are caused by our inability to cancel out the motion of the eyes
passive vs active vision
what does a lot of work on vision ignore?
the role of eye movement
passive vs active vision
what does the passive approach see?
it sees the eye as just receiving information from the outside world
passive vs active vision
what does the active approach emphasise?
it emphasises the observer’s eye movements guide the sampling of visual scenes in a dynamic process.
combining motor and visual systems:
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
controlling eye movements:
2
- 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
what is reading?
a complex cognitive process
reading: existing models
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
regression accounts for what?
10% of saccades
what does the window technique show?
that we see information from up to 15 letters away in English (less for more complex characters such as Chinese ones)
what does the window techniqe suggest?
that the shape of a word plays a role as we are worse at resolving letters outside the fovea
fixation on words findings:
3 findings
- 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
the link between eye movements and attention
2 links
- 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)
