Eye tracker instructions

See eye tracker equipment for brief descriptions of physical layout, specific hardware, and hardware adjustments.

Eye tracker operation outline
System startup/shutdown
Subject positioning
Adjust pupil/CR discrimination
Calibration image, set target points
Calibration (point of gaze)
Acquire eye data

Eye tracker operation outline

These are the basic steps to acquire eye tracking data, with links to relevant eye tracking equipment descriptions:

  1. Switch on power to the eye tracking equipment , connect visual stimuli video/serial cables, start the interface software on the eye tracker interface computer , and upload the control unit software.
  2. Locate or create a calibration image , typically with 9 target points, that roughly matches the dimensions of your visual stimulus.
  3. If the calibration image has changed, set the target points: using the interface software's Calibrate|SetTarget Points menu item, mouse click on each target point of the calibration stimulus.
  4. Position the subject and adjust the eye tracker subject mirror (in general this will be the same positioning as is ideal for MRI acquisition).
  5. Turn on the scan room eye monitor . Check that the eye image is satisfactory, and adjust the  subject position / optical alignment / LRO aperture if necessary. Turn off the scan room eye monitor.
  6. Calibrate eye tracking using the calibration image as stimulus. This takes a minute or two, queuing the subject to fixate at each target point and then checking the resulting accuracy.
  7. Start recording eye tracking data. To synchronize the eye tracking data with stimuli and/or MRI scanning, manually mark the data with a number key stroke, or automatically synchronize with stimulus software that sends "open/start/event/stop" codes to the control unit.
  8. Monitor the pupil/CR discrimination through the eye and scene monitor . Adjust discrimination thresholds as required. 
  9. Stop data acquisition, and close the data file (or don't close and repeat steps 7 and 8). Check your data, and copy data to a different computer/CD.
  10. Switch off eye tracker equipment, particularly the illuminator power. 

System startup/shutdown

The eye tracker control unit (Model ASL 5000) software must be uploaded from the eye tracker computer each time the system is powered up.  

If the eye tracker computer is off, switch on the white power strip under the counter, start the computer, and wait for the system to load up. 

Turn on power to eye tracker system by switching on the white box labeled Control Unit (Model ASL 5000).

Run the desktop shortcut labeled Loadnt.  When it has successfully uploaded software to the control unit, its window will disappear. 

Run the desktop shortcut labeled e5win.  In the e5win program window, check that the 'Online' light is green, indicating that the control unit and eye tracker computer are communicating properly.  If the indicator is red, turn off the control unit, shut down the e5win program, and restart. It is necessary that the Loadnt and e5win programs are executed in this order, not reversed.   

Turn on the illuminator using the switch on the power strip labeled ASL EYE TRACKER POWER TO ILLUMINATOR on the operator's console next to the control unit.

Subject positioning

The head only magnet design requires each subject's eyes to be located at very similar positions: the eye tracker requires very little adjustment between subjects.

The main positioning requirement for the eye tracker is that the subject's pupil (left) is within the LRO's field of view, for all gaze directions.

To check the subject's eye position, turn on and view the scan room eye monitor (the switch is at the upper left corner of the monitor). Adjust the subject position (ask the subject to adjust head position, making sure they can still see all of the projected image), optical alignment (adjust the LRO redirection mirror ) and LRO aperture (to obtain a suitable pupil brightness) if necessary. It is helpful to ask the subject to "look around" the projected image to insure that the pupil is in the LRO field of view for all gaze directions. Remember to turn off the scan room eye monitor before leaving the scan room.

Adjust pupil/CR discrimination

Thresholds for pupil/CR discrimination are set using scroll bars on the e5win application window. The discrimination is determined by horizontal scan line image intensity transitions: the pupil must be brighter than the background and the CR must be brighter than (and within) the pupil. The discrimination thresholds determine the value of these intensity transitions.

Good discrimination is recognized by a white circle circumscribing the pupil, and a small black circle around the CR (overlaid on the eye monitor image). The cross hairs should be at the center of these two circles. (NOTE: The circles and cross hairs are offset several pixels to improve visibility.)

Calibration (point of gaze)

Once the subject is positioned, you must calibrate the pupil positions with the POG (point of gaze). Display a calibration image (using the stimulus computer/projector), using any image display program (e.g. MS Paint in the Eye tracker folder, which is also the default application for the .bmp file type). (HINTS: To show a calibration image full screen, use MS Paint View|View Bitmap menu item or CTRL+F. To switch quickly between showing the subject the calibration image and a stimulus display, use the Alt+Tab on the Stimulus Computer to switch between applications). 

It is generally best to have the subject look at all calibration image target points three times:

The first time, examine the pupil/CR image on the eye monitor to make sure that the eye tracker can discriminate the pupil/CR properly at all gaze directions. 

The second time, set the calibration points.  Select the e5win Calibrate|Eye Calibration menu item, and click on the "Store Data for Current Point" button for each gaze direction. It is best to look at the eye monitor to insure good discrimination at each gaze direction, and to insure the subject does not blink when you capture the calibration point. (HINT: Run through the points quickly -- about one per second -- as it is difficult for the subject to fixate accurately for a longer period of time. This requires the mouse pointer to be on the button without moving while you observe the pupil image on the eye monitor, and communicate with the subject through the intercom system). 

The third time, check that the calibrated gaze direction corresponds with the target points (within several millimeters) on the scene monitor. If they don't correspond (for example if a calibration point was recorded incorrectly) you may need to repeat the calibration.

Acquire eye data

Eye tracker data acquisition is initiated with the e5win interface program. Open an Eyedat file (.eyd) and start/stop recording with the File|Start/Stop Recording menu items. The eye tracking data can be synchronized with stimuli and/or MRI scanning: this can be done manually by "marking" the data with a number key stroke, or automatically with stimulus software that sends "open/start/event/stop" codes to the control unit through the XDAT parallel port.

Normally eye data is recorded to this file at a 60 Hz rate, although faster acquisition rates are possible. The raw data can be previewed with the e5win Misc|View Data File menu item, or viewed and processed (e.g. find fixation points/times) with the Eyenal program. Each data field includes the absolute and relative time, horizontal and vertical POG, pupil diameter, external data (values sent to the XDAT parallel port during acquisition), and "Marks" (number keys pressed during acquisition).