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Analysis of Eye Movement Data Basic Measurement Units

Analysis of Eye Movement Data Frank M. Marchak, Ph.D. Veridical Research and Design Corporation www.vradc.com. Analysis of Eye Movement Data Basic Measurement Units. Fixations Saccades Blinks. Basic Measurement Units Fixations. Relatively stable eye-in-head position:

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Analysis of Eye Movement Data Basic Measurement Units

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  1. Analysis of Eye Movement Data Frank M. Marchak, Ph.D.Veridical Research and Design Corporationwww.vradc.com

  2. Analysis of Eye Movement DataBasic Measurement Units • Fixations • Saccades • Blinks

  3. Basic Measurement UnitsFixations • Relatively stable eye-in-head position: • Spatial dispersion : < 2° • Minimal duration : 100 – 200 msec • Threshold velocity : < 15 – 100° /msec www.uxmatters.com/mt/archives/2006/01/images/4_flickr_cumulative.jpg

  4. FixationsCalculation Methods* • Velocity-based • Based on point-to-point velocities- (threshold : 20°/sec) • Fixations (< 100° /sec) • Saccades (> 300° /sec) • Velocity Threshold Identification • Approximated from angular velocity threshold distance from stimulus is known (20°/sec – Sen & Megaw, 1984) • Dispersion-based • Based on groups of consecutive points within a maximum separation • Movingwindow spans minimum number of points based on duration threshold and sampling frequency • Checks dispersion of points in window by summing max and min x and y values * Salvucci & Goldberg, 2000

  5. FixationsCalculation Methods* (cont.) • Area-of-Interest (AOI) • Identifies only fixations that occur only within specific target areas • Uses duration threshold to distinguish fixations from passing saccades Comparison * Salvucci & Goldberg, 2000

  6. Basic Measurement UnitsSaccades • Involuntary, abrupt, rapid, small movements or jerks of both eyes simultaneously in changing the point of fixation • Duration: 30 – 120 msec • Amplitude : 400 - 600°/sec • Latency : 100 – 30 msec • Refractory period : 100 – 300 msec

  7. Basic Measurement UnitsMean Fixation Durations and Saccade Lengths Rayner & Castelhano, 2007

  8. Analysis of Eye Movement DataTypes of Analyses* • Measures of Processing • Measures of Search • Measures of Scanpaths * Goldberg & Kotval, 1999

  9. Types of AnalysesMeasures of Processing • Number of Fixations • Location of Fixations • Fixation Duration • Cumulative Fixation Time • Cluster Analysis (AOI)

  10. Types of AnalysesMeasures of Search • Number of Saccades • Saccade Amplitude • Saccade Velocity • Scanpath Length • Scanpath Sequence • Scanpath Duration

  11. Types of AnalysesScanpaths • When a particular visual pattern is viewed, a particular sequence of eye movements is executed, and furthermore that this sequence is very important in accessing the visual memory for this pattern • Noton and Stark, 1971 • Theory disputed, but evidencethat viewers attend to informativedetails http://eyetracking.com.ua/frmtext/10.jpg

  12. ScanpathsQuantification – Markov Measures Random

  13. ScanpathsQuantification – Markov Measures Constrained

  14. ScanpathsQuantification – Levenshtein Distance • Metric for measuring the amount of difference between two sequences • Defined as the minimum number of edits needed to transform one string into the other, with the allowable edit operations being insertion, deletion, or substitution of a single character Josephson and Holmes, 2002 en.wikipedia.org/wiki/Levenshtein_distance

  15. Analysis of Eye Movement DataApplications of Measures • Reading • Visual Search • Scene Perception

  16. Measures of ProcessingExample • Number of fixations • Σx + Σ x + Σx + Σx = 11 • Number of regions sampled • 1 + 1 + 1 + 1 = 4 • First return fixation • * • Proportion of fixations to left side of image • 6/11 = .55 • First-order Markov measures • p(x x) • Second-order Markov measures • p(x x x) * x x x x x x x x x x x

  17. Basic Measurement UnitsBlinks • Rapid bilateral eyelid closure and co-occurring eye movement • Blink types and initiators* * Smit, 2008

  18. Applications of MeasuresReading • Variables • First Fixation Duration • Single Fixation Duration • Gaze Duration • Total Fixation Duration • Go-Past Time • Skipping • Regressions (in and out) Rayner & Castelhano, 2007

  19. ReadingCharacteristics • Character spaces used rather than visual angle • Global measures influenced by text properties • Saccade size • Fixation duration • Regression to previously read material(10-15%) • Local effects on processing target words • First fixation duration • Single fixation duration • Gaze duration (sum of all fixations on word)

  20. Applications of MeasuresVisual Search • Variables • Number of Saccades • Saccade Amplitude • Saccade Velocity • Scanpath Length • Scanpath Sequence • Scanpath Duration Rayner & Castelhano, 2007

  21. Applications of MeasuresScene Perception • Variables • Number of Fixations • First Return Fixation • Number of Regions • 1st order Markov • 2nd order Markov Rayner & Castelhano, 2007

  22. Scene PerceptionScene Manipulation • 60 Stimuli • 15 Addition/15 Subtraction • 15 Left/right shift • 15 Feature change • Detail • Color • Object

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