jochen triesch, uc san diego, triesch 1 eye movements and eye tracking why move the eyes? see the...

Jochen Triesch, UC San Diego, http://cogsci.ucsd.edu/~triesch 1

Eye Movements and Eye TrackingEye Movements and Eye Tracking

Why move the eyes?• see the same thing better (stabilize image, but not too well!)• see something else (overcome low peripheral resolution)

S. Anstis

• only small part of visual field is sampled at full resolution


Cortical Magnification in V1Cortical Magnification in V1

Eric Schwartz

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Types of eye movementsTypes of eye movements

Major Distinctions:• conjugate vs. disjunctive• abrupt vs. slow

Types:• vestibuocular reflex: counteract head rotation• optokinetic reflex: counteract large field retinal motion• smooth pursuit: counteract object motion (<30 deg/s)• saccades: rapidly shift gaze direction (<600 deg/s)• vergence movements: adjust vergence angle• accomodation: adjust focus• microsaccaes: counteract drift during fixation (1-2 Hz, <5’)• microtremor: .5’, 30-100Hz


SaccadesSaccades

Yarbus (1950s)

• alternations of fixations and saccades, 3 per second


Yarbus, 1967:• examine the picture at

will• estimate the economic

level of the people• estimate the people’s

ages• guess what the people

were doing before the arrival of the visitor

• remember the people’s clothing

• remember the people’s (and objects’) position in the room

• estimate the time since the guest’s last visit


Saccade MetricsSaccade Metrics

from Becker, 1991

approximately linearrelationship betweensaccade amplitude A andsaccade duration D:

dADD 0

20 ms < D0 < 30 ms2 ms/° < d < 3 ms/°

Example:for 20° saccade displayneeds to be changed~75ms after saccade onset


Anthropomorphic Robot HeadAnthropomorphic Robot Head


System overviewSystem overview

Gigabit Router

Desktop A Desktop B Desktop C


Motor SystemMotor System

• 9 DoF (hobby grade servo motors)• 2 Mini SSC II interface boards• Four bar linkage connection• Eye/neck system is redundant


Vision SystemVision System

• 2 Point Grey Research Firefly cameras (Firewire)• 640x480 resolution up to 30 fps• 2, 4, 6, and 8mm focal length lenses


Saccade Performance 1Saccade Performance 1


Saccade Performance 2Saccade Performance 2


Demo videoDemo video


Why Eye trackingWhy Eye tracking

Basic Neuroscience:- measuring brain output- understanding neural control architecture- psychophysics: how do we use gaze during natural tasks

Applications:- user interface design, human factors: e.g. eye tracking for driver of a car- advertising: do people look where I want them to look (in my web page, my newspaper ad, etc.)

Note 1: now several conferences solely devoted to thisNote 2: can also be done in fMRINote 3: possible in the real world: “portable eye trackers”


Eye tracking techniquesEye tracking techniques(Review in Duchowski&Vertegaal, 2000)

Contact lenses: with mirrors or induction loops- fast, accurate, uncomfortable (often used with bite bars)

Video based:- temporal accuracy limited to camera frame rate (60Hz)- less accurate (~1 deg)- typically with infrared illumination of the eye (dark pupil vs. bright pupil)- can be done remotely or camera can be attached to head

Electro-oculogram:- exploits electric dipole property of eye ball- fast but imprecise

Limbus tracking:- predecessor of the video based tracker- imprecise- small operating range


Electro-oculogramElectro-oculogram

•eye is electric dipole•electric field of moving dipole induces measurable voltages•provides analog voltage output that can be digitized and processed extremely fast, but technique is not accurate

electrode


Limbus TrackingLimbus Tracking

utilize difference in reflective properties of iris vs. sclera

infrared LED

photo diode

sclera

limbus

iris


Search CoilSearch Coil

gold standard for speed and accuracy (5-10’’), but quite uncomfortable and head movement restrained


Video based trackingVideo based tracking

most widely used today, good accuracy and speed, easy-to-use

dual purkinje trackerbright pupil image


inside of head mounted displayinside of head mounted display

U. of Rochester


Gaze contingent display changesGaze contingent display changes

Idea:decide what is being displayed contingent on where observer is looking

Saccade contingent updating (SCU):a special case of this: make display changes while a saccade is progressing(pioneering work by McConkie and colleagues)

• powerful technique for studying visual perception• frequently used in e.g. change blindness studies


Combination of video and limbus tracker Combination of video and limbus tracker inside HMD for gaze contingent displayinside HMD for gaze contingent display

analog limbustracker’ssensor with infrared LED,photo diodes(horizontal)

video based pupil tracker’s sensor

LCD displaysinside the HMD

• video based tracker for good accuracy• limbus tracker for low latency saccade detection


System OverviewSystem Overview

• 4 processor high-end graphics computer as backbone• images rendered in V8 helmet (Virtual Research)

Three sensors:• magnetic tracking device (Polhemus Fastrak)• limbus tracker (ASL 210), sampled at 1.25 kHz• pupil tracker (ASL 501), sampled at 60 Hz

Sensors send data via separate serial lines


Latency ComparisonLatency Comparison

in this example:limbus tracker has26 ms advantageover video tracker

on average: limbus tracker has 37±13 ms advantage (15 trials)


chess pieces (dis-)appear contingent on

saccade (~25-30°) with 50% probability

jochen triesch, uc san diego, triesch 1 eye movements and eye tracking why move the eyes? see the...

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