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:
- 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
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
Comparison
Method
Accuracy
Speed
Robustness
VT
**
*
o
DT
*
**
AOI
*
*
Impl.
Ease
Parameters
1
**
**
*
*
*
1+b
2
* Salvucci & Goldberg, 2000
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 : 400 - 600°/sec
• Latency : 100 – 30 msec
• Refractory period : 100 – 300 msec
Basic Measurement Units
Mean Fixation Durations and
Saccade Lengths
Mean Fixation
Duration (ms)
Saccade Length
(degrees)
Silent Reading
225
2 ( ~ 8 letters)
Oral Reading
275
1.5 (~ 6 letters)
Visual search
275
3
Scene Perception
330
4
Music Reading
375
1
Typing
400
1 (~ 4 letters)
Task
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
http://eyetracking.com.ua/frmtext/10.jpg
Scanpaths
Quantification – Markov Measures
Random
Region eye lands
Region eye leaves
1
2
3
4
5
1
0.04
0.04
0.04
0.04
0.04
0.20
2
0.04
0.04
0.04
0.04
0.04
0.20
3
0.04
0.04
0.04
0.04
0.04
0.20
4
0.04
0.04
0.04
0.04
0.04
0.20
5
0.04
0.04
0.04
0.04
0.04
0.20
0.20
0.20
0.20
0.20
0.20
Grand Total: 1.00
Column totals
Row totals
Scanpaths
Quantification – Markov Measures
Constrained
Region eye lands
Region eye leaves
1
2
3
4
5
1
0.00
0.20
0.00
0.00
0.00
0.20
2
0.00
0.00
0.20
0.00
0.00
0.20
3
0.00
0.00
0.00
0.20
0.00
0.20
4
0.00
0.00
0.00
0.00
0.20
0.20
5
0.20
0.00
0.00
0.00
0.00
0.20
0.20
0.20
0.20
0.20
0.20
Grand Total: 1.00
Column totals
Row totals
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
Josephson and Holmes, 2002
en.wikipedia.org/wiki/Levenshtein_distance
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
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)
*
xx
x
x xx
x
x
x
x
x
Basic Measurement Units
Blinks
• Rapid bilateral eyelid closure and cooccurring 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
-1st order Markov
-2nd order Markov
Rayner & Castelhano, 2007
Scene Perception
Scene Manipulation
60 Stimuli
– 15 Addition/15 Subtraction
– 15 Left/right shift
– 15 Feature change
% Correct Classifications
• Detail
• Color
• Object
100
90
80
70
60
50
40
30
20
10
0
Rv Mv N
R/Mv N
Rv N
Rv M
Mv R/N
Mv N