Eye Movement Recording
Frank M. Marchak, Ph.D.
Veridical Research and Design Corporation
www.vradc.com
Society for Psychophysiological Research
September 14 2011
History – First Era
Huey
Huey, 1898
Dodge
Diefendorf &Dodge, 1908
History – Second Era
Buswell
Buswell, 1935
Yarbus
Yarbus, 1967
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
Scleral Search Coils
Trade-offs
• Extremely accurate
– 5 – 10 arc seconds over 5°
• Difficult to use
• Invasive
• Measurement relative to head
Types of Eye Tracking Systems
Electro-oculography (EOG)
www.adinstruments.com/solutions/images/eog_human.jpg
www.virtualworldlets.net/Shop/ProductsDisplay/VRInterface.php?ID=90
EOG
Operating Principles
http://www.liv.ac.uk/~pcknox/teaching/Eymovs/emeth.htm
• 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
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
www.smivision.com/en/gaze-and-eye-tracking-systems/products/3d-vog.html
• Iris tracking and high-quality video imaging
• Senses 3D linear acceleration and 3D rotational
velocity
• Horizontal, vertical and torsional eye movements
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
Bright versus Dark Pupil
www.archimuse.com/mw2010/papers/milekic/milekic.Fig1.jpg
• Tradeoffs
http://www.ime.usp.br/~hitoshi/framerate/node2.html
-
Ambient lighting
Eye color
Eyelashes
Makeup
Pupil-Corneal Reflection
Dual Purkinje Method
www.fourward.com
• 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
Eye Tracker Configurations*
•
•
•
•
•
Head mount
Glasses
Desktop
Chin Rest
Real world
* Not exhaustive sampling of manufacturers and models
Eye Tracker Configurations
Head Mounted
Arrington
www.arringtonresearch.com
EyeLink II
www.sr-research.com
ASL
www.asleyetracking.com
SMI
www.smivision.com
Eye Tracker Configurations
Glasses Mounted
SMI
ASL
www.smivision.com
Tobii
www.asleyetracking.com
www.tobii.com
Eye Tracker Configurations
Desktop
SMI
Tobii
www.smivision.com
Smart Eye
www.smarteye.se
www.tobii.com
LC Technologies
www.eyegaze.com
Eye Tracker Configurations
Chin Rest
Cambridge Research Systems
Arrington
www.arringtonresearch.com
www.crsltd.com
Eye Tracker Configurations
Real World
Seeing Machines
SMI
www.smivision.com
www.seeingmachines.com
Smart Eye
Tobii Technology
www.smarteye.se
www.tobii.com
Eye Tracker Configurations
View Counting
Xuuk
www.xuuk.com
www.xuuk.com
• Counts number of views
• 10 meter range/ 12° accuracy
• No gaze or pupil information
www.xuuk.com
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
Interdependent
Constraints
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