Eye Movement Recording

Frank M. Marchak, Ph.D.Veridical Research and Design Corporation

www.vradc.com

Society for Psychophysiological ResearchSeptember 14 2011

History – First Era

Diefendorf &Dodge, 1908

Huey, 1898

Huey

Dodge

Yarbus

History – Second Era

Buswell

Yarbus, 1967

Buswell, 1935

Eye Movement Recording

Types of Eye Tracking Systems• Scleral search coils• Electro-oculography• Video-oculography• Pupil-corneal reflection

Types of Eye Tracking Systems

Scleral Search Coils

www.chronos-vision.de/scleral-search-coils

Scleral Search Coils

Operating Principles

www.primelec.ch

Scleral Search Coils

Performance Comparison

www.primelec.ch

• Extremely accurate – 5 – 10 arc seconds over 5°

• Difficult to use• Invasive• Measurement relative to head

Scleral Search Coils

Trade-offs

www.virtualworldlets.net/Shop/ProductsDisplay/VRInterface.php?ID=90

www.adinstruments.com/solutions/images/eog_human.jpg

Types of Eye Tracking Systems

Electro-oculography (EOG)

EOG

Operating Principles

• Permanent potential difference between the cornea and the fundus of 0.4 -1.0 mV– Small voltages can be recorded from the region

around the eyes which vary as the eye position varies

http://www.liv.ac.uk/~pcknox/teaching/Eymovs/emeth.htm

EOG

Performance• Accuracy : ± 2°• Maximum rotation: ± 70°

– Linearity decreases progressively for angles > 30°• Signal magnitude range: 5 – 20 µV/°

http://www.bem.fi/book/28/28.htm

EOG

Tradeoffs• Inexpensive• Simple operation• Need for frequent calibration and recalibration

– Corneoretinal potential can vary diurnally– Affected by light and fatigue – Drifting- electrode slipping, change in skin resistance– Noise from other electrical devices, face muscles– Blinking

Types of Eye Tracking Systems

Video-oculography

www.smivision.com/en/gaze-and-eye-tracking-systems/products/3d-vog.html

Video-oculography

Operating Principles

• Iris tracking and high-quality video imaging• Senses 3D linear acceleration and 3D rotational

velocity• Horizontal, vertical and torsional eye movements

www.smivision.com/en/gaze-and-eye-tracking-systems/products/3d-vog.html

Video-oculography

Performance• Resolution

– Horizontal : 0.05°– Vertical: 0.05°– Torsional: 0.1°

• Head motion recording– 3D rotational velocity [°/s]– 3D linear acceleration [m/s2]

www.smivision.com/en/gaze-and-eye-tracking-systems/products/3d-vog.html

Video-oculography

Tradeoffs• Highly accurate torsional measurement• Permits comparison of nystagmus slow phase velocity (SPV)

and head rotation velocity• Useful for VOR research and diagnosis• Not practical for standard point-of-regard research

Types of Eye Tracking Systems

Pupil - Corneal Reflection

drivingtraffic.com/wp-content/uploads/2010/08/eye.png

Pupil-Corneal Reflection

Operating Principles

http://www.ime.usp.br/~hitoshi/framerate/node2.html

www.archimuse.com/mw2010/papers/milekic/milekic.Fig1.jpg

• Tradeoffs- Ambient lighting- Eye color- Eyelashes- Makeup

Bright versus Dark Pupil

• Highly accurate - 400 Hz Bandwidth - 1 Minute of Arc Accuracy - Response time of less than 1 ms - Slew Rate >2000 deg/sec - Less than 1 Minute of Arc Resolution

Pupil-Corneal Reflection

Dual Purkinje Method

www.fourward.com

Pupil-Corneal Reflection

Eye Tracker Configurations*• Head mount• Glasses• Desktop• Chin Rest• Real world

* Not exhaustive sampling of manufacturers and models

Eye Tracker Configurations

Head MountedASL

EyeLink II

Arrington

SMIwww.arringtonresearch.com www.asleyetracking.com

www.sr-research.com www.smivision.com

Eye Tracker Configurations

Glasses MountedSMI

ASL

Tobiiwww.smivision.com

www.tobii.com

www.asleyetracking.com

Eye Tracker Configurations

Desktop

Smart Eye

www.smarteye.se

Tobii

www.tobii.com

LC Technologies

www.eyegaze.com

SMI

www.smivision.com

Eye Tracker Configurations

Chin Rest

Arrington

Cambridge Research Systems

www.arringtonresearch.com

www.crsltd.com

Eye Tracker Configurations

Real World

SMI

www.tobii.com

www.smivision.comwww.seeingmachines.com

www.smarteye.se

Tobii TechnologySmart Eye

Seeing Machines

Eye Tracker Configurations

View Counting

• Counts number of views• 10 meter range/ 12° accuracy• No gaze or pupil information

www.xuuk.com

www.xuuk.com

www.xuuk.com

Xuuk

Pupil-Corneal Reflection

Performance• Accuracy: 0.5° - 2°• Sampling Speed: 30 Hz – 2000 Hz• Head Movement Range: 12° - 40°• Viewing Distance: 60 cm – 365 cm

Pupil- Corneal Reflection

Tradeoffs• Support varying degrees of free head motion• Multiple configuration options• Most provide pupil diameter and point-of-regard• Less spatial resolution than some other options• Often easy-to-use with minimal training• Can be affected by eye color, eye lashes and makeup

Eye Movement Recording

Data Collection Considerations• Definition of terms• Sampling rate• Task• Participant configuration• Stimuli• Calibration

InterdependentConstraints

Data Collection Considerations

Definition of terms*• Accuracy

- Average angular offset (distance) Θi (in degrees of visual angle) between n fixations locations and corresponding locations of fixation targets

- Offset =

• Spatial Precision- Root Mean Square (RMS) of angular distance (in degrees

of visual angle) between successive samples (xi, yi) to (x i+1, Yi+1)

- RMS =

*www.cogain.org/ETaccuracy

Data Collection Considerations

Accuracy versus Precision

www.usercentric.com/blogs/uxnuggets/2011/05/18/most-precise-or-most-accurate-eye-tracker

Data Collection Considerations

Definition of terms* (cont.)• System Latency

- Average end-to-end delay from an actual movement of the tracked eye until the recording computer signals that a movement has taken place

• Temporal Precision- Standard deviation of eye-tracker latency

- High if samples arrive with latency but interval between successive samples remains almost constant

*www.cogain.org/ETaccuracy

Eye Tracking

Definition of terms* (cont.)• Noise

- System-inherent- Best possible precision possible with a given eye-tracker (spatial

resolution)- Oculomotor

- Fixational eye-movements tremor, microsaccades, and drift (jitter)

- Environmental- Variation in gaze position signal caused by external disturbances

in recording environment- Optic Artifacts

- False, i.e., physiologically impossible, high-speed movements, caused by interplay between optical situation and gaze estimation algorithm

*www.cogain.org/ETaccuracy

Data Collection Considerations

Sampling Rate• Wide range available

– 30 Hz – 2000 Hz• Faster not necessarily better

– Depends on experimental purpose– Can constrain participant configuration

• Affects what measures can be calculated– e.g., saccadic peak velocity can be estimated with 60

Hz data, but only for saccades > 10° (Enright, 1998)- Saccades during reading typically < 10°

Sampling Rate

Guidelines?• No established guidelines on what frequency

necessary for what effect size across measures• Some de facto standards

– Oscillating eye movements use Nyquist theorem to sample twice the speed of particular eye movement

– Gaze contingent displays with constrained setups use 1000 Hz – 2000 Hz to maintain control

– Naturalistic tasks requiring free head movement typically operate from 30 Hz – 500 Hz

Data Collection Considerations

Tasks/Participant Configuration/Stimuli• Tasks

- High spatial or temporal resolution- Ambient environment (e.g., automobile, MRI, outdoors)

• Participant Configuration- Free head movement- Ambulatory

• Stimuli- Visual- Auditory- Real world

Data Collection Considerations

Calibration• Gaze determined by changes between center of

pupil and corneal reflection• Mapping of ocular changes to measured

parameters required

Drewes, 2010

Data Collection Considerations

Calibration Considerations• Number of points required function of desired

accuracy• Real world environments require know location

of some objects in scene• May not be required if measuring only pupil

diameter• Overall procedures similar but specifics differ

among eye tracker manufacturers