1. Society for Psychophysiological Research
Eye Movement Recording Frank M. Marchak, Ph.D. Veridical Research and Design Corporation
Society for Psychophysiological Research
September

2. History – First Era
Huey
Huey, 1898
Dodge
Diefendorf &Dodge, 1908

3. History – Second Era
Buswell
Buswell, 1935
Yarbus
Yarbus, 1967

4. Eye Movement Recording Types of Eye Tracking Systems
Scleral search coils
Electro-oculography
Video-oculography
Pupil-corneal reflection

5. Types of Eye Tracking Systems Scleral Search Coils

6. Scleral Search Coils Operating Principles

7. Scleral Search Coils Performance Comparison

8. Scleral Search Coils Trade-offs
Extremely accurate
5 – 10 arc seconds over 5°
Difficult to use
Invasive
Measurement relative to head

9. Types of Eye Tracking Systems Electro-oculography (EOG)
The third category uses electric potentials measured with electrodes placed around the eyes. The eyes are the origin of a steady electric potential field, which can also be detected in total darkness and if the eyes are closed. It can be modelled to be generated by a dipole with its positive pole at the cornea and its negative pole at the retina. The electric signal that can be derived using two pairs of contact electrodes placed on the skin around one eye is called Electrooculogram (EOG). If the eyes move from the centre position towards the periphery, the retina approaches one electrode while the cornea approaches the opposing one. This change in the orientation of the dipole and consequently the electric potential field results in a change in the measured EOG signal. Inversely, by analysing these changes in eye movement can be tracked. Due to the discretisation given by the common electrode setup two separate movement components – a horizontal and a vertical – can be identified. A third EOG component is the radial EOG channel,[22] which is the average of the EOG channels referenced to some posterior scalp electrode. This radial EOG channel is sensitive to the saccadic spike potentials stemming from the extra-ocular muscles at the onset of saccades, and allows reliable detection of even miniature saccades.[23]
Due to potential drifts and variable relations between the EOG signal amplitudes and the saccade sizes make it challenging to use EOG for measuring slow eye movement and detecting gaze direction. EOG is, however, a very robust technique for measuring saccadic eye movement associated with gaze shifts and detecting blinks. Contrary to video-based eye-trackers, EOG allows recording of eye movements even with eyes closed, and can thus be used in sleep research. It is a very light-weight approach that, in contrast to current video-based eye trackers, only requires very low computational power, works under different lighting conditions and can be implemented as an embedded, self-contained wearable system.[24] It is thus the method of choice for measuring eye movement in mobile daily-life situations and REM phases during sleep.

10. EOG Operating Principles
Permanent potential difference between the cornea and the fundus of mV
Small voltages can be recorded from the region around the eyes which vary as the eye position varies

11. Signal magnitude range: 5 – 20 µV/°
EOG Performance
Accuracy : ± 2°
Maximum rotation: ± 70°
Linearity decreases progressively for angles > 30°
Signal magnitude range: 5 – 20 µV/°

12. Need for frequent calibration and recalibration
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

13. Types of Eye Tracking Systems Video-oculography
The third category uses electric potentials measured with electrodes placed around the eyes. The eyes are the origin of a steady electric potential field, which can also be detected in total darkness and if the eyes are closed. It can be modelled to be generated by a dipole with its positive pole at the cornea and its negative pole at the retina. The electric signal that can be derived using two pairs of contact electrodes placed on the skin around one eye is called Electrooculogram (EOG). If the eyes move from the centre position towards the periphery, the retina approaches one electrode while the cornea approaches the opposing one. This change in the orientation of the dipole and consequently the electric potential field results in a change in the measured EOG signal. Inversely, by analysing these changes in eye movement can be tracked. Due to the discretisation given by the common electrode setup two separate movement components – a horizontal and a vertical – can be identified. A third EOG component is the radial EOG channel,[22] which is the average of the EOG channels referenced to some posterior scalp electrode. This radial EOG channel is sensitive to the saccadic spike potentials stemming from the extra-ocular muscles at the onset of saccades, and allows reliable detection of even miniature saccades.[23]
Due to potential drifts and variable relations between the EOG signal amplitudes and the saccade sizes make it challenging to use EOG for measuring slow eye movement and detecting gaze direction. EOG is, however, a very robust technique for measuring saccadic eye movement associated with gaze shifts and detecting blinks. Contrary to video-based eye-trackers, EOG allows recording of eye movements even with eyes closed, and can thus be used in sleep research. It is a very light-weight approach that, in contrast to current video-based eye trackers, only requires very low computational power, works under different lighting conditions and can be implemented as an embedded, self-contained wearable system.[24] It is thus the method of choice for measuring eye movement in mobile daily-life situations and REM phases during sleep.

14. 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

15. 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]

16. 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

17. Types of Eye Tracking Systems Pupil - Corneal Reflection
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18. Pupil-Corneal Reflection Operating Principles
Bright versus Dark Pupil
Tradeoffs
Ambient lighting
Eye color
Eyelashes
Makeup

19. Pupil-Corneal Reflection Dual Purkinje Method
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

20. Pupil-Corneal Reflection Eye Tracker Configurations*
Head mount
Glasses
Desktop
Chin Rest
Real world
* Not exhaustive sampling of manufacturers and models

21. Eye Tracker Configurations Head Mounted
ASL
Arrington
EyeLink II
SMI

22. Eye Tracker Configurations Glasses Mounted
SMI
ASL
Tobii

23. Eye Tracker Configurations Desktop
SMI
Tobii
Smart Eye
LC Technologies

24. Eye Tracker Configurations Chin Rest
Cambridge Research Systems
Arrington

25. Eye Tracker Configurations Real World
Seeing Machines
SMI
Tobii Technology
Smart Eye

26. Eye Tracker Configurations View Counting
Xuuk
Counts number of views
10 meter range/ 12° accuracy
No gaze or pupil information

27. 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

28. 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

29. Eye Movement Recording Data Collection Considerations
Definition of terms
Sampling rate
Task
Participant configuration
Stimuli
Calibration
Interdependent
Constraints

30. 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 = *

31. Data Collection Considerations Accuracy versus Precision

32. 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 *

33. 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 *

34. 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°

35. 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

36. Data Collection Considerations Tasks/Participant Configuration/Stimuli
High spatial or temporal resolution
Ambient environment (e.g., automobile, MRI, outdoors)
Participant Configuration
Free head movement
Ambulatory
Stimuli
Visual
Auditory
Real world

37. 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

38. 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