eye movement recording frank m. marchak, ph.d. veridical research and design corporation society...
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Eye Movement Recording
Frank M. Marchak, Ph.D.Veridical Research and Design Corporation
www.vradc.com
Society for Psychophysiological ResearchSeptember 14 2011
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History – First Era
Diefendorf &Dodge, 1908
Huey, 1898
Huey
Dodge
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Yarbus
History – Second Era
Buswell
Yarbus, 1967
Buswell, 1935
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Eye Movement Recording
Types of Eye Tracking Systems• Scleral search coils• Electro-oculography• Video-oculography• Pupil-corneal reflection
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Types of Eye Tracking Systems
Scleral Search Coils
www.chronos-vision.de/scleral-search-coils
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Scleral Search Coils
Operating Principles
www.primelec.ch
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Scleral Search Coils
Performance Comparison
www.primelec.ch
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• Extremely accurate – 5 – 10 arc seconds over 5°
• Difficult to use• Invasive• Measurement relative to head
Scleral Search Coils
Trade-offs
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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)
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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
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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
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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
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Types of Eye Tracking Systems
Video-oculography
www.smivision.com/en/gaze-and-eye-tracking-systems/products/3d-vog.html
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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
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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
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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
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Types of Eye Tracking Systems
Pupil - Corneal Reflection
drivingtraffic.com/wp-content/uploads/2010/08/eye.png
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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
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• 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
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Pupil-Corneal Reflection
Eye Tracker Configurations*• Head mount• Glasses• Desktop• Chin Rest• Real world
* Not exhaustive sampling of manufacturers and models
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Eye Tracker Configurations
Head MountedASL
EyeLink II
Arrington
SMIwww.arringtonresearch.com www.asleyetracking.com
www.sr-research.com www.smivision.com
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Eye Tracker Configurations
Glasses MountedSMI
ASL
Tobiiwww.smivision.com
www.tobii.com
www.asleyetracking.com
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Eye Tracker Configurations
Desktop
Smart Eye
www.smarteye.se
Tobii
www.tobii.com
LC Technologies
www.eyegaze.com
SMI
www.smivision.com
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Eye Tracker Configurations
Chin Rest
Arrington
Cambridge Research Systems
www.arringtonresearch.com
www.crsltd.com
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Eye Tracker Configurations
Real World
SMI
www.tobii.com
www.smivision.comwww.seeingmachines.com
www.smarteye.se
Tobii TechnologySmart Eye
Seeing Machines
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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
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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
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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
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Eye Movement Recording
Data Collection Considerations• Definition of terms• Sampling rate• Task• Participant configuration• Stimuli• Calibration
InterdependentConstraints
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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
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Data Collection Considerations
Accuracy versus Precision
www.usercentric.com/blogs/uxnuggets/2011/05/18/most-precise-or-most-accurate-eye-tracker
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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
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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
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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°
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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
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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
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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
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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