Human visual search does not maximize the post saccadic probability of identi...
How eyetrackerswork
1. Association for Business Communication
78th Annual International Convention
New Orleans, 2013
Tharon Howard,
Clemson University UsabilityTesting Facility Director
tharon@clemson.edu
2. Part One: An Introduction to Eye Tracking Foundational
Concepts and Two Approaches to Website Navigation
(Tharon)
Part Two: Webby Awards and an Eye Tracking Study
(Heather)
Part Three: Limitations and Strengths of Eye Tracking
(Brian)
10. Saccades and Fixations
Can be as short as 20 milliseconds or
as long as 200 milliseconds
The fastest movement produced by
the human body
Substantial image blur during a
saccade due to this speed
fixation fixation
saccade
Fixation: Eye-movement pauses to acquire
Content
Saccades: The period of eye-movement from
one fixation to the next
11. AlfredYarbus –TheVisitor (1967)
Saccades are not
arbitrary; they’re
influenced by
parafovea vision
Users’ tasks and
goals play a
significant role in
the gaze plot
13. • “Mechanical” or Search Coil
• “Glint” or Dual Purkinje
• Limbus or PupilTracking
• Dual-PointVideo
Popular EyeTracking
Technologies to Consider
14. Search Coil & Magnetic Field
or “Mechanical”
Uses contact lenses
with search coils and
a magnetic field
Usually combined with reflected light
Extremely accurate
Very costly
Uncomfortable & not portable
Source: http://www.iovs.org/content/44/1/275.long
15. Dual Purkinje Image
or “glint”
Uses infrared light to create reflections from
different parts of eye
Video tracks 2 reflections and software
calculates angles
Good accuracy
Comfortable for users
Allows more head movement
Still costly
Source: http://www.fourward.com/dconcept.htm
Source: http://medical-dictionary.thefreedictionary.com/Purkinje-
Sanson+mirror+images
16. Limbus or PupilTracking
or “Single-PointVideo-Based”
Image processing of video is used to locate pupil
and calculate POG
A Starburst algorithm projects rays from an initial
starting point detecting pixels with high image
gradients
Source: Howard & Sulak
Example of rays projected from initial point
Source: Ryan, Duchowski, & Burchfield
17. “Dual-PointVideo-Based”
Image processing tracks pupil and corneal reflection
of IR light is also tracked
Less expensive
Comfortable
Head movement is limited
Only accurate to 1-2 degrees (1o is about 0.5”
at 27” distance)
Example of rays projected from initial point
Source: Ryan, Duchowski, & Burchfield
Source: Howard & Sulak
18. Some OtherTerms to Know
Accuracy—Measured in degrees and typically ranges from 0.5
to 2 degrees. Keep in mind that 1o is about half an inch of
resolution at the normal 27” viewing distance for computer
monitors.
Precision—Smallest change in eye position that can be
measured. Usually this is also measured in fractions of
degrees.
Sampling Rate—Measured in Hertz, this factor is important
because eye movements are the fastest humans can produce.
Over 1000 Hz is needed to capture many eye movements, but
most systems typically run at 50 to 60 Hz.
19. The “F Pattern” and Golden
Triangle
Source: http://blog.mediative.com/en/2011/08/31/eye-
tracking-google-through-the-years/
Source: http://www.nngroup.com/articles/f-shaped-pattern-reading-web-content/
“Eyetracking visualizations
show that users often read
Web pages in an F-shaped
pattern: two horizontal
stripes followed by a vertical
stripe.” Jakob Nielsen
“We recorded how 232 users
looked at thousands of Web
pages.We found that users'
main reading behavior was
fairly consistent across many
different sites and tasks.”
Found consistent pattern
from 2005 - 2011
20. Rosenblatt and Web Page Genres
Efferent(to carry away) Aesthetic (emotional pleasure)
“In readings that fall somewhere in the efferent half of
the continuum, the reader selects out predominantly
more public or cognitive elements than private.The
aesthetic stance, in contrast, accords selective
attention to predominantly more of the penumbra of
private feelings, attitudes, sensations, and ideas than
to the public aspects.” Louise Rosenblatt
Source: “TheTransactionalTheory:Against Dualisms.” College English,Vol. 55, No. 4. (Apr., 1993), p. 383.
21. Rosenblatt and Gaze Plots
Efferent(to carry away) Aesthetic (emotional pleasure)
Do the GoldenTriangle and F-Pattern
obtain for “aesthetic” sites as well as
for “efferent” sites?
25. EyeTrackers can be expensive.This
usedTobii T120 system is $27,500.
(Image courtesy of eBay, inc.)
SensoMotoric Instruments (SMI) Eye
Tracking Glasses 2.0 are priced on a
case by case basis.This could
potentially be quite costly. (Image
courtesy of SensoMotoric Instruments,
GmbH)
Price
26. Lab costs, as well as the costs for participants can be costly. For example,
Eye Tracking, Inc. charges between $5,000-$250,000 per study.These costs
are determined by the type of study, length of study, number of participants,
etc.
Price
27. The Grinbath Eye Guide is roughly $2,495, with discounts
applied to multiple unit purchases. (Image courtesy of Grinbath,
LLC)
Affordable options are available.
28. Eye Tribe’s USB 3.0
Hardware is only $99, but
comes with a Software
Development Kit (SDK),
which means code must be
written in order for this
device to be used. While the
base price is attractive, the
time and cost of hiring a
software engineer is
something to consider.
But, how affordable is affordable?
29. Equipment Sensitivity
Because of the sensitivity of the equipment, the
participants were uncomfortable. (Image Courtesy of Brian Gaines)
33. Retrospective think aloud is a technique used in usability,
and eye tracking in particular, to gather qualitative
information on the user intents and reasoning during a
test. It's a form of think aloud protocol performed after
the user testing session activities, instead of during them.
Fairly often the retrospective protocol is stimulated by
using a visual reminder such as a video replay.
(Guan, Lee, Cuddihy, and Ramey, 2006)
Stimulated RetrospectiveThink Aloud
34. More than 80% of subjects’ verbalizations of what they
were attending to corresponded with the eye movement
data. We reject the notion of subjects’ fabrication since
only less than 3% of their verbalization failed to match up
with objects identified by their eye movement.
(Guan, Lee, Cuddihy, and Ramey, 2006)
Stimulated RetrospectiveThink Aloud
37. Cons
• Certain eye tracking technologies are
expensive
• Lab/participants costs
• Equipment sensitivity
• Accommodating for differences in
participants
Pros
• As technology advances, prices
will drop
• Stimulated Retrospective Think
Aloud is a valid data collection
method
• Results are intantaneous
Conclusions
