Eye Tracking and its Application in MRI and EEG Settings

Marcus Johnson

SR Research Ltd.

Toronto - Ottawa, Canada

Agenda

• Discussion of eye tracking hardware components • Synchronization options • Discussion of Recording/Messaging Options

EyeLink 1000 Overview

• Focal Imaging Technology – Large camera field of view – High resolution eye data • EyeLink CL hardware – EyeLink CL High Speed Infrared Camera – IR illuminator – CameraLink cable – Custom High Speed Image Processing Board

EyeLink 1000

• Fastest Sampling Rate: – 2000 / 1000 / 500 / 250 Hz recording • High Resolution: – 0.01 ° RMS in pupil-CR 1000 Hz tracking mode • Accurate: – Drift free, down to 0.15

° typical) average accuracy (0.25 - 0.5

° • Real-time: – Access eye position data with < 2-ms delay

System Architecture

System Architecture

Host Application

• Controls the eye tracker •Performs image analysis •Performs data recording •Performs eye event parsing •Configures preferences • Provides real-time feedback •Gaze view: Gaze cursor on background image •Plot view: Eye traces over time • Controls other optional setups •Analog output board •Gaze video overlay system

System Architecture

Display PC Software

• Allow full experiment control •Integrates calibration and gaze position collection into one step •Sets any tracker preference •Sends commands to control tracker • Time stamps experiment events with messages • Real-time access to eye sample and eye event data structures • Allows the users to focus on stimulus presentation and data analysis

System Hardware

Desktop Mount

• Useful in EEG Environment • Remote Mode with Sticker on EEG Cap • Chinrest Mode for More Precise Recording – One concern is Chinrest

Camera Lens

Long Range Mount

Camera Head Infrared Illuminator with Focusing Lens Different Lenses (75mm,50mm,35mm) for Different Distances (overall range: 60-150cm) Required for MRI, Potentially Useful with EEG

Typical MRI Configuration

MRI Mounting Options - Tripod

(Picture from MEG setup) Difficult to find tripods with no ferromagnetic metals in them

MRI Mounting Options – Screen Mount

No ferromagnetic metals Flexible positioning

MRI Mounting Options – Screen Mount

No ferromagnetic metals Flexible positioning Works with most MRIs

MRI Mounting Options – Trio Tray Mount

No ferromagnetic Metals Rests on Trio’s bed rails Optional built-in screen Works with Siemens 3T Trio

MRI Head Coil Mirrors

Allows subject to see screen and for eye tracker to see subject Best to use front-surfaced mirrors to prevent ghosting and secondary reflections

Long Range System Cabling

Long Range System Cabling

Camera Base Box on Side of Host PC Fiber Optic Data Cable Power Options Patch Panel Connection Options (DB9 or BNC) Lemo Connectors (to Camera/Illum.) Battery Option

Long Range System Cabling

Long Range System Cabling

No detectable interference with proper cabling (Graph from system use in MEG)

System Synchronization

System Synchronization

•Pre-Experiment activities •Check Tracker Settings (via Set Options Screen and sending commands from Display PC) •Participant setup •Calibration •Validation

MRI/Eye Tracker Synchronization

•Running the experiment •Blocks •Trials •MRI sends synchronization pulse (TTL) to Display PC – Display PC in turn sends Message to Host PC •Pulse triggers Display screen onset – on screen onset Display PC sends Message to Host PC •Pulse typically sent from MRI to Parallel Port or USB Device •Can optionally be split and sent to Host PC Parallel Port •Parallel Port status on Host recorded on every sample •Optional drift correction/drift checking •Usually between Blocks •Can also enable online drift correction with mouse click

MRI/Eye Tracker Synchronization

TTL Pulse Display PC Updates Screen Message When Pulse Received Message When Screen Updated Time

EEG/Eye Tracker Synchronization

•Running the experiment •Blocks •Trials •Display PC draws to screen – at same time sends Message to Host PC and sync pulse (TTL) to EEG •Message/Pulse occur at screen retrace event •Pulse typically sent from Parallel Port or USB Device of Display PC to EEG – can also be sent from Parallel Port of Host PC •Event types can be coded (32 different pulse values) •Optional drift correction/drift checking •Usually between Trials •Can also enable online drift correction with mouse click

Analog Card

•Option for Host PC – usually used for devices like EEG •Converts Digital Data to Analog Voltages •Output to BNC connectors •Voltage range configurable •Three Channels per eye being tracked •Horizontal Position •Vertical Position •Pupil Size •Also allows for extra Digital In/Out (in addition to Parallel Port) •Quick – •Data can be inserted into empty EEG channels •Allows for easy alignment with EEG data •Dirty – •Noise is added by D/A and A/D conversion •Saccade/Fixation information is lost

EEG/Eye Tracker Synchronization

Display PC Updates Screen Message When Screen Updated TTL When Screen Updated Within 1 msec Optional Constant Analog Out to EEG Time

EEG/Eye Tracker Synchronization

Display PC Updates Screen OR About 1 msec Message When Screen Updated TTL from Host PC to EEG Time Optional Constant Analog Out to EEG

Recording and Messaging

Recording and Trials

•Messages are Not Just For Marking Stimulus Events •Data Viewer Format: •Trial Onset Messages TRIALID 1 (TRIALID2, etc.) •Trial Event Messages DISPLAY_1_ON BUTTON_RECEIVED DISPLAY_2_ON •Trial Variable Messages !V TRIAL_VAR trial_condition condition_1 !V TRIAL_VAR reaction_time 2364 •Trial Interest Area Messages !V IAREA RECTANGLE 1 100 200 200 300 square •Target Position Messages (for moving targets) !V TARGET_POS Targ1 (512, 384) 1 0 !V TARGET_POS Targ1 (512, 364) 1 0 •Trial Offset Messages TRIAL_RESULT 0

Recording and Trials

•Messages are Not Just For Marking Stimulus Events •Data Viewer Format: For full description of Data Viewer format see: Data Viewer User Manual (Help -> Contents) “Protocol for EyeLink Data to Viewer Integration”

Recording and Trials

•EyeLink Recording and Analysis Messages are Independent •Scenario 1: •Start Recording •Start Trial Loop •For Each Trial: •Trial Onset Message •Stimulus Event Messages •Mark Every Event of Trials •Trial Condition Messages •Independent Variables •Behavioral (non eye-based) Dependent Measures •Trial Interest Area Messages •Trial Offset Message •End Trial Loop •Stop Recording

Recording and Trials

•EyeLink Recording and Analysis Messages are Independent •Scenario 2 (Like Experiment Builder): •Start Trial Loop •For Each Trial: •Start Recording •Trial Onset Message •Stimulus Event Messages •Mark Every Event of Trials •Stop Recording •Trial Condition Messages •Independent Variables •Behavioral (non eye-based) Dependent Measures •Trial Interest Area Messages •Trial Offset Message •End Trial Loop

Recording and Trials

•EyeLink Recording and Analysis Messages are Independent • Can Use Scenario 1 or 2 for Most Programming Environments e.g., Psychtoolbox (Matlab), E-Prime, Presentation, C, Python • If using Experiment Builder, Must Stick to Scenario 2 •Many Messages Sent Automatically with Experiment Builder

Thank you!

EyeLink Support

• Documents •EyeLink 1000 User Manual •EyeLink 1000 Installation Guide •SR Research Experiment Builder •Windows Programmers Guide •EyeLink Data Viewer • Contact Information •E-mail: [email protected]

•Phone: 1-613-271-8686/ 1-866-821-0731 •Web: http://www.sr-support.com

EyeLink Support