Lecture 26: Eye Tracking Inf1-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 external world a description that is useful to the viewer and not cluttered with irrelevant information.” (David Marr, Vision, 1982)

Back to lecture 4

The eye is not a passive recorder

Vision involves many layers of active interpretation and processing

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 effort is needed.

An eye-tracker makes possible to record the eye-movements of participants 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 person when the person was reading (with the other eye) the microphone was recording the noise produced by the cornea colliding with the microphone

Edmond Delabarre (1898)

he put a plaster cap in his eye (“sufficiently cocainised”) the cap had a hole for the pupil the cap was wired to a lever which drew horizontal lines on a panel 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 some infra-red light against the eye of the subject

The cornea and the pupil reflect infra-red light: easier to recognise and to track their movements (no heavy image recognition)

Calibration process: providing some examples of the area fixed and the reflections produced by the cornea and the pupil

An algorithm hew window should get opened when a link leads out of the current docuas to “superimpose” the fixations to the image recorded (gaze estimation)

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How does it work (contd.)

The eye tracker records two eye movement events (but not only!):

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 are blind 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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http://www.youtube.com/watch?v=lvypPbWAc7E http://www.youtube.com/watch?v=xKdOMgu0C5Q http://www.youtube.com/watch?v=SQxrsUXqKCM http://www.youtube.com/watch?v=2NcUkvIX6no

Different Scenarios - Google Glasses

“We created Glass so you can interact with the virtual world without distracting you from the real world. We don’t want technology to get in the way.” Google designer Isabelle Olsson

Still a prototype

It adds another layer to reality

Multitasking does not exist

How does this information affect perception?

Google Glasses Project

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http://www.youtube.com/watch?v=JSnB06um5r4

Eye-tracking and Cognitive Science

Can you think of other applications of eye-tracking? Come up with cases 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 experiment is the first step for producing a good design

The null hypothesis (H0): no effect is expected between two or more conditions

An experiment is aimed to reject H0 supporting the alternative hypothesis (H1)

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Designing your own experiment - Independent Variables

Independent variables: the conditions manipulated by the experimenter

increasing the amount of IV requires a higher number of subjects:

You have to find them You have to pay them You have to spend time collecting the data The equipments and the lab are not always available and they cost money

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Designing your own experiment - Dependent Variables

Dependent Variables: the outcome variables not manipulated by the experimenter:

Number of fixations towards a specific target Reaction times: time required to perform an action Error Rates: number of mistakes occurred

The question is directly related to the technology we use

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