Analysis of Eye Movement Data Frank M. Marchak, Ph.D. Veridical Research and Design Corporation

Analysis of Eye Movement Data Basic Measurement Units Fixations Saccades Blinks

Basic Measurement Units Fixations Relatively stable eye-in-head position: -Spatial dispersion : < 2° -Minimal duration : 100 – 200 msec -Threshold velocity : < 15 – 100° /msec

Fixations Calculation 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 -Moving window 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

Fixations Calculation 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 * Salvucci & Goldberg, 2000 MethodAccuracySpeedRobustness Impl. Ease Parameters VT *** o 1 DT *** * 2 AOI **** 1 + b Comparison

Basic Measurement Units Saccades Involuntary, abrupt, rapid, small movements or jerks of both eyes simultaneously in changing the point of fixation Duration: 30 – 120 msec Amplitude : °/sec Latency : 100 – 30 msec Refractory period : 100 – 300 msec

Basic Measurement Units Mean Fixation Durations and Saccade Lengths Task Mean Fixation Duration (ms) Saccade Length (degrees) Silent Reading2252 ( ~ 8 letters) Oral Reading (~ 6 letters) Visual search2753 Scene Perception3304 Music Reading3751 Typing4001 (~ 4 letters) Rayner & Castelhano, 2007

Analysis of Eye Movement Data Types of Analyses* Measures of Processing Measures of Search Measures of Scanpaths * Goldberg & Kotval, 1999

Types of Analyses Measures of Processing Number of Fixations Location of Fixations Fixation Duration Cumulative Fixation Time Cluster Analysis (AOI)

Types of Analyses Measures of Search Number of Saccades Saccade Amplitude Saccade Velocity Scanpath Length Scanpath Sequence Scanpath Duration

Types of Analyses Scanpaths 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 evidence that viewers attend to informative details

Scanpaths Quantification – Markov Measures Region eye lands Region eye leaves12345Row totals Column totals0.20 Grand Total: 1.00 Random

Scanpaths Quantification – Markov Measures Region eye lands Region eye leaves12345Row totals Column totals0.20 Grand Total: 1.00 Constrained

Scanpaths Quantification – 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 en.wikipedia.org/wiki/Levenshtein_distance Josephson and Holmes, 2002

Analysis of Eye Movement Data Applications of Measures Reading Visual Search Scene Perception

Measures of Processing Example –Number of fixations Σ x + Σ x + Σ x + Σ x = 11 –Number of regions sampled = 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

Basic Measurement Units Blinks Rapid bilateral eyelid closure and co- occurring eye movement Blink types and initiators* * Smit, 2008

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

Reading Characteristics 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)

Applications of Measures Visual Search Variables - Number of Saccades - Saccade Amplitude - Saccade Velocity - Scanpath Length - Scanpath Sequence - Scanpath Duration Rayner & Castelhano, 2007

Applications of Measures Scene Perception Variables - Number of Fixations - First Return Fixation - Number of Regions - 1 st order Markov - 2 nd order Markov Rayner & Castelhano, 2007

Scene Perception Scene Manipulation 60 Stimuli –15 Addition/15 Subtraction –15 Left/right shift –15 Feature change Detail Color Object