Society for Psychophysiological Research September 14 2011

1. Eye Movement RecordingFrank M. Marchak, Ph.D.Veridical Research and Design Corporationwww.vradc.com Society for Psychophysiological Research September 14 2011

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

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

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

5. Types of Eye Tracking SystemsScleral Search Coils www.chronos-vision.de/scleral-search-coils

6. Scleral Search CoilsOperating Principles www.primelec.ch

7. Scleral Search CoilsPerformance Comparison www.primelec.ch

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

9. Types of Eye Tracking SystemsElectro-oculography (EOG) www.adinstruments.com/solutions/images/eog_human.jpg www.virtualworldlets.net/Shop/ProductsDisplay/VRInterface.php?ID=90

10. EOGOperating 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

11. EOGPerformance • 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

12. EOGTradeoffs • 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 SystemsVideo-oculography www.smivision.com/en/gaze-and-eye-tracking-systems/products/3d-vog.html

14. Video-oculographyOperating 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

15. Video-oculographyPerformance • 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

16. Video-oculographyTradeoffs • 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 SystemsPupil - Corneal Reflection drivingtraffic.com/wp-content/uploads/2010/08/eye.png

18. Pupil-Corneal ReflectionOperating Principles Bright versus Dark Pupil www.archimuse.com/mw2010/papers/milekic/milekic.Fig1.jpg • Tradeoffs • Ambient lighting • Eye color • Eyelashes • Makeup http://www.ime.usp.br/~hitoshi/framerate/node2.html

19. Pupil-Corneal ReflectionDual 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

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

21. Eye Tracker ConfigurationsHead Mounted ASL Arrington www.arringtonresearch.com www.asleyetracking.com EyeLink II SMI www.smivision.com www.sr-research.com

22. Eye Tracker ConfigurationsGlasses Mounted SMI ASL www.smivision.com Tobii www.asleyetracking.com www.tobii.com

23. Eye Tracker ConfigurationsDesktop Tobii SMI www.tobii.com www.smivision.com Smart Eye LC Technologies www.smarteye.se www.eyegaze.com

24. Eye Tracker ConfigurationsChin Rest Cambridge Research Systems Arrington www.arringtonresearch.com www.crsltd.com

25. Eye Tracker ConfigurationsReal World Seeing Machines SMI www.smivision.com www.seeingmachines.com Tobii Technology Smart Eye www.smarteye.se www.tobii.com

26. Eye Tracker ConfigurationsView 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

27. Pupil-Corneal ReflectionPerformance • Accuracy: 0.5° - 2° • Sampling Speed: 30 Hz – 2000 Hz • Head Movement Range: 12° - 40° • Viewing Distance: 60 cm – 365 cm

28. Pupil- Corneal ReflectionTradeoffs • 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 RecordingData Collection Considerations • Definition of terms • Sampling rate • Task • Participant configuration • Stimuli • Calibration Interdependent Constraints

30. Data Collection ConsiderationsDefinition 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

31. Data Collection ConsiderationsAccuracy versus Precision www.usercentric.com/blogs/uxnuggets/2011/05/18/most-precise-or-most-accurate-eye-tracker

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 *www.cogain.org/ETaccuracy

33. Eye TrackingDefinition 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

34. Data Collection ConsiderationsSampling 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 RateGuidelines? • 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 ConsiderationsTasks/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

37. Data Collection ConsiderationsCalibration • Gaze determined by changes between center of pupil and corneal reflection • Mapping of ocular changes to measured parameters required Drewes, 2010

38. Data Collection ConsiderationsCalibration 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