lecture 4- eye movements and perception

Question 1

Why study eye movements

Answer 1

Can indicate visual attention.
Inform about complex cognitive processes like reading.
Shows clear link between sensory and action systems.
Can be used as a tool in studying clinical population

Question 2

The eye and retina

Answer 2

Have cones (used in light and high resolution output) and rods (low light conditions)

Lower density of cones as you move away from the fovea which are needed for detail in light conditions. And so lower resolution

Question 3

3 movement axes of the eye

Answer 3

Horizontal vertical and in-plane movements (small and unnoticeable reflex actions

Question 4

Different gaze shifting mechanisms and explain

Answer 4

Smooth pursuit: track object ensuring light from object stays focused on back of eye. Requires continuous feed-back loop and the eye needs to adjust to perceived position of object to have light focused

Saccades: fast movements made 3/4 times a second and show characteristic patterns of acceleration. Both eyes move together almost identically.

Vergence: we move our eyes to focus on objects at different distances away from us- rotating to a different angle so light lands on the fovea

Question 5

Different stabilising eye movements

Answer 5

Vestibulo-ocular reflex: To cancel out the motion of the body and head in reaction to signals from vestibular organs. E.g. when walking/ spinning to stop images from blurring (Moor et al 2001)

Optokinetic reflex: stabilising on the visual scene. When the whole visual field is moving for example on a tra

Question 6

Other minature movements

Answer 6

Micro saccades- small involuntary movement made while fixating on an object thought to help avoiding fading of visual scene

Drift and tremor- drifts are slow meandering motions between micro saccades and tremors are small oscillations on top of drifts

Question 7

Measuring eye movements

Answer 7

Typically eye-trackers
- using small camera focused on the eye and software which tracks where the pupil moves by calibrating the pupil position against fixed points on the screen one can work out where the person is looking.

We can use this to build a picture of where the pupil is looking and we can measure: duration of each fixation, frequency, dwell time (total duration of fixations), order of fixation and reaction times.

Also scan paths: rapid fixate - move (saccade) - fixate pattern. Mark where the eye rested then how we move between

Heat maps show density of fixations through colour

Question 8

stabilisation

Answer 8

It is thought this is why eye movements evolved.
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 9

Adaptation and fading

Answer 9

If an image is fixed on the retina, it starts to fade away in the periphery after a while known as Troxler’s fading 1804 caused by adaptation of the neurons in the retina as they do not fire after a while to the same stimulus.

Question 10

Focusing on the fovea

Answer 10

The retina is not uniformly sensitive a central part called the fovea has high density of photoreceptors. And most of the processing power is given to this area. In order to have a detailed representation of a part of the image the light from that part needs to land in the fovea

Question 11

Passive vs active vision

Answer 11

Passive approach: eye just recieves information from the outside world.
Active approach emphasises dynamic process of the sampling of the visual scenes with reference to the role of the observer in guiding the eye to relevant parts of the scene

Question 12

Combining motor and visual systems

Answer 12

Processes involve strong coupling between visual and motor processes.
Brain areas in eye movements are active both when visual information is being processed and eye movements are being made, these are important cross-modal areas

Question 13

Controlling eye movements

Answer 13

Smooth pursuit involves a continuous feedback loop, the eye position is updated according to where the object is.

Saccades are ballistic and the movements needs to be pre-programmed

Question 14

Reading

Answer 14

Many models try describe the ways we extract meaning from words and eye movements are part of evidence.

We don’t fixated every single word and regressions account for 10% of saccades, we easily move our eyes down to the next line. Suggesting the shape of the word plays a role as we are worse at resolving letters outside of the fovea

When reading we have rightward and a return sweep to the left. Leftwards movement to start of next line.

We don’t fixate on each letter and we typically move about 8 characters with each saccade and do not fixate on many short words at all.

Question 15

Link between eye movements and attention

Answer 15

We are unable to process all visual information at once and are only aware of a small part at any time.

Usually where we are looking is where we are paying attention to but not always

Question is what makes us select something and how to we achieve this selection.

Active vision emphasised the role of eye movements linked with attention in real-life tasks