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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