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
• System cabling and integration for noise/artifact
minimization
• 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° average accuracy (0.25 - 0.5°
typical)
• 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
Long Range Mount
Camera Head
Infrared Illuminator with Focusing Lens
Camera 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 MRI
MRI Mounting Options – Trio Tray Moun
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
Display PC
Updates Screen
TTL
Pulse
Message
When
Pulse
Received
Time
Message
When
Screen
Updated
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
Optional
Constant
Analog
Out to EEG
Time
TTL
Within
When
1 msec
Screen
Updated
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: support@sr-research.com
•Phone: 1-613-271-8686/ 1-866-821-0731
•Web: http://www.sr-support.com
EyeLink Support