Lecture 26: Eye TrackingInf1-Introduction to Cognitive Science
Diego Frassinelli
March 21, 2013
Experiments at the University of Edinburgh
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An Eye is Not a Camera
“Vision is a process that produces from images of the externalworld a description that is useful to the viewer and not clutteredwith irrelevant information.” (David Marr, Vision, 1982)
Back to lecture 4
The eye is not a passive recorder
Vision involves many layers of active interpretation andprocessing
A process that maps one representation to a different one
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Eye Tracking
The Eye-Mind Hypothesis (Just & Carpenter, 1980)
Where participants are looking indicates what they are processing.How long they are looking at indicates how much processing effortis needed.
An eye-tracker makes possible to record the eye-movements ofparticipants while they are performing a cognitive task
Based on a slide by Frank Keller.
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Some History
Louis Emile Javal (1879)
he attached a microphone to the closed eyelid of a personwhen the person was reading (with the other eye) themicrophone was recording the noise produced by the corneacolliding with the microphone
Edmond Delabarre (1898)
he put a plaster cap in his eye (“sufficiently cocainised”)the cap had a hole for the pupilthe cap was wired to a lever which drew horizontal lines on apanel when the eye was moving during reading
Based on a slide by Tobii eye-tracking research.
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Nowadays
More freedom and more natural information
But also more noisy and less accurate data
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How does it work
Find a mapping between the eye position and the image gazed
A camera records the eye movements projecting someinfra-red light against the eye of the subject
The cornea and the pupil reflect infra-red light: easier torecognise and to track their movements (no heavy imagerecognition)
Calibration process: providing some examples of the area fixedand the reflections produced by the cornea and the pupil
An algorithm hew window should get opened when a linkleads out of the current docuas to “superimpose” the fixationsto the image recorded (gaze estimation)
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How does it work (contd.)
The eye tracker records two eye movement events (but notonly!):
Fixations: collection of most of the visual information(200-300 ms)Saccades: a rapid movement from one fixation to the other(30-80 ms). They are the fastest body movements. We areblind during most of them
The experimenter decides what a fixation is
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Different Scenarios
Nowadays, eye-trackers are used in different fields:
Scene Perception (Playing Cards)
Web Design (Ikea Website)
Marketing analyses (Supermarket)
Sport studies (Ronaldo)
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Different Scenarios - Google Glasses
“We created Glass so you can interact with the virtual worldwithout distracting you from the real world. We don’t wanttechnology to get in the way.” Google designer Isabelle Olsson
Still a prototype
It adds another layer toreality
Multitasking does not exist
How does this informationaffect perception?
Google Glasses Project
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Eye-tracking and Cognitive Science
Can you think of other applications of eye-tracking? Come up withcases in which recording eye-movements is useful to study:
Language Processing
Visual Cognition
Memory
Cognitive Impairment
Based on a slide by Frank Keller.
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Designing your own experiment
Clearly formulating the research question of your experimentis the first step for producing a good design
The null hypothesis (H0): no effect is expected betweentwo or more conditions
An experiment is aimed to reject H0 supporting thealternative hypothesis (H1)
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Designing your own experiment - Independent Variables
Independent variables: the conditions manipulated by theexperimenter
increasing the amount of IV requires a higher number ofsubjects:
You have to find themYou have to pay themYou have to spend time collecting the dataThe equipments and the lab are not always available and theycost money
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Designing your own experiment - Dependent Variables
Dependent Variables: the outcome variables notmanipulated by the experimenter:
Number of fixations towards a specific targetReaction times: time required to perform an actionError Rates: number of mistakes occurred
The question is directly related to the technology we use
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