Eye Movements andVisual Attention

An Introduction toEye-Movement Research

Overview:

• Types of Eye Movement• How to Measure Eye Movements• Why Eye-Movement Research?• Examples of Eye-Movement Studies

and Paradigms

Eye Movements

Eye Muscles

Types of Eye Movement

Fixations:

• The eye is almost motionless, for example, while reading a single, short word.

• The information from the scene is almost entirely acquired during fixation.

• Duration varies from 100-1000 ms, typically between 200-600 ms.

• Typical fixation frequency is about 3 Hz.• Fixations are interspersed with saccades.

Types of Eye MovementSaccades:

• Quick “jumps” that connect fixations• Duration is typically between 30 and 120 ms • Very fast (up to 700 degrees/second) • Saccades are ballistic, i.e., the target of a

saccade cannot be changed during the movement.

• Vision is suppressed during saccades to allow stable perception of surroundings.

• Saccades are used to move the fovea to the next object/region of interest.

Types of Eye Movement

Smooth Pursuit Eye Movements:

• Smooth movement of the eyes for visually tracking a moving object

• Cannot be performed in static scenes (fixation/saccade behavior instead)

Types of Eye Movement

Torsional Eye Movements:

• Rotation of the eye around the viewing axis

• Stabilization of visual scene by compensating body rotation (up to about 15 degrees)

Types of Eye Movement

Vergence Eye Movements:

• Slow, smooth movements changing the vergence angle (the angle between the two viewing axes)

• Used for changing gaze from a near to a far object or vice versa

• Can take up to one second• Execution is often interrupted if no

thorough inspection of the object is required.

Types of Eye Movement

Tremor:

• Fast, low-amplitude (seconds of arc) eye-movement “jitter”

• Improves the perception of high spatial frequencies

• Prevents the fading of static images during fixations

How to Measure Eye Movements

Mirror on Eyeball

• Used in first eye tracking experiments (Yarbus in 1960’s)

• Suction cup attaches mirror to eyeball

• Light beam is directed at mirror and reflected onto photo sensitive paper

• Good spatial resolution but no temporal information

• Unpleasant for the subject

How to Measure Eye Movements

Electrooculogram (EOG)

• Skin electrodes around the eyes measure potential differences

• Wide range -- poor accuracy

• Better for relative than absolute eye movements

• Mainly used in neurological diagnosis

How to Measure Eye Movements

Eye Coils• Subject wears contact

lens with wire coil• Homogeneous magnetic

field around subject allows measurement of gaze angle

• Very high temporal and spatial precision

• Special coil also allows measurement of torsional eye movements

• Very uncomfortable

How to Measure Eye Movements

Limbus Tracker

• Photo diodes track the boundary between sclera and iris.

• High temporal resolution

• Poor spatial precision; only for horizontal eye movements

• Inexpensive

How to Measure Eye Movements

Purkinje Eye Tracker

• Laser is aimed at the eye.

• Laser light is reflected by cornea and lens

• Pattern of reflected light is received by an array of light-sensitive elements.

• Very precise• Also measures lens

accomodation• No head movements

How to Measure Eye Movements

Video-Based Systems

• Infrared camera directed at eye• Image processing hardware determines pupil

position and size (and possibly corneal reflection)

• Good spatial precision (0.5 degrees) for head-mounted systems

• Good temporal resolution (up to 500 Hz) possible

How to Measure Eye Movements

EyeLink II System:• Binocular• Head-movement

compensation (head camera looking at IR markers at monitor)

• Temporal resolution 500 Hz

• Spatial precision about 0.5 to 1 degree

• Gaze-position data available in real-time

How to Measure Eye Movements

EyeLink II System Configuration

How to Measure Eye Movements

Measuring vergence eye movements in anaglyphs with EyeLink II (poor subject!)

Why Eye-Movement Research?

In most eye movement studies, the only types of eye movement that are considered are fixations and saccades.

Why?

These eye movements indicate a person’s gaze trajectory while performing a certain task.Moreover, they yield information about a person’s visual attention.

Visual Attention•Visual attention is the selective allocation of visual processing resources.

•For example, we can focus our attention on a particular object of interest in the visual field.

•Visual processing of that object is enhanced while being rather shallow for other objects.

•Also, we can respond more quickly and accurately to changes in an attended region.

•This prioritization is necessary due to our limited processing resources.

Visual Attention•Example: Posner task: Subjects are instructed to fixate on a central marker.

•One of two boxes (left/right) flashes to capture the subject’s attention (an automatic, involuntary response).

•After a short delay (stimulus onset asynchrony - SOA) an asterisk appears in one of the boxes.

•The subject has to report as quickly as possible in which box the asterisk appeared.

The Posner Attention Task

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The Posner Attention Task

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The Posner Attention Task

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The Posner Attention Task

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The Posner Attention Task

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The Posner Attention Task

•For short SOAs (< 200 ms), subjects respond faster if flash and asterisk appear on the same side than when they appear on different sides.

Cueing of attention to relevant location allows faster response.

•For longer SOAs (up to 1000 ms), subjects respond more slowly if flash and asterisk appear on the same side.

Inhibition-of-Return mechanism makes attention less likely to return to the side of the flash until the asterisk appears.

Why Eye-Movement Research?

About eye movements and visual attention:• Usually, saccades follow shifts of

attention to provide high acuity at the attended position.

• It is possible to look at an object without paying attention to it (staring).

• It is possible to shift attention without eye movement (covert shifts of attention).

• It is impossible to perform a saccade while not shifting attention.

• During specific, natural tasks it is reasonable to assume that saccades follow shifts of attention.

Why Eye-Movement Research?

The investigation of visual attention, in turn, is at the core of cognitive science.

• Studying visual attention yields insight into

general attentional mechanisms.• It can provide information on a person’s stream of conscious and unconscious processing while solving a task.• Attention is closely linked to the concept of

consciousness.• Attentional mechanisms could improve artificial vision systems.

Eye-Movement Studies

Eye movements while watching a girl’s face (early study by Yarbus, 1967)

Eye-Movement StudiesEye movements as indicators of cognitiveprocesses (Yarbus):• trace 1: examine at will• trace 2: estimate wealth• trace 3: estimate ages• trace 4: guess previous

activity• trace 5: remember

clothing• trace 6: remember

position• trace 7: time since last

visit

Eye-Movement Studies

Visual scan paths on instruments/dashboards – studies for the improvement of human-computer interfaces

Eye-Movement Studies

Gaze trajectory measurement for the optimization of web page layout

Eye-Movement Studies

Improving advertisements with eye-movement studies

Selectivity in Complex Scenes

Face Recognition

Gaze-contingent window deteriorates face recognition, allows to identify relevant visual

information.