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