Eye tracker

3T setup

For the 3T we have the Cambridge Research Systems LiveTrack monocular system available

Powering up and starting LiveTrack AV (3T)

Replace the standard headcoil mirror with the white mount from the cabinet in the MR room. Tighten the two screws a bit so the mount is connected to the top half of the headcoil, but still allow for sliding movement for fine adjustments. Be careful when disconnecting and connecting the top half, make sure the camera/mirror mount doesn't fall of. Connect the cable to the one coming from the front penetration panel. When plugging it in, make sure ther two red dots are aligned on each side of the connector. It should slide in smoothly and click, don't force it.
Make sure both monitors are connected to the 3t-stim computer and enabled. If one is not enabled (typically the left one), rightclick in the desktop and open NVidia control panel. Find the tab that shows all connected monitors and enable all three. Monitor 1 (the left display) should be set to primary, monitor two (right) and three (BOLD screen) should be clones and secondary. This arrangement is essential to being able to work with the system, other arrangements will not work.
Start the LiveTrack AV software (link is on the desktop) on put it on the secondary screen so it is also visible on the cloned BOLD screen in side the MR room. You should see input from the camera

Participant preparation and placement

Prepare the participant by removing mascara and replacing glasses for MRI glasses. Soft contact lenses can be worn during an eye tracking experiment, but rigid contact lenses may interfere with the corneal reflection (CR) and thus need to be switched for MRI glasses.

When the participant is placed inside the scanner, make sure that the eyes and eyebrows of the participant are not covered by the head coil, as the head coil may cast a shadow over the eyes and that interferes with the experiment. When you slide the participant in the scanner make sure you align on the eyebrows (to make sure every participant is more or less in the same position).

Focus and adjust

After the participant is placed in the bore and the camera is setup correctly, look at the BOLD screen to see if the participant’s eyes are in the centre of the camera FOV. If this is not the case, adjust the camera. Ask the participants if he or she can see all corners of the screen. Under most circumstances, the LiveTrackAV camera and illuminator should be more or less set up correctly and you should see the participant’s eye in camra's FOV. If the eye is not (properly) visible or illuminated, gently move the camera to get the eye in the middel of the camera's FOV. Carefully adjust the focus of the camera so that the size of the CR becomes as small as possible and the aye and eyelashes are in focus. If you are unsure how to do this, please contact the staff of the Spinoza Centre.

Calibrate and validate

WIP

Capture raw video feed

It is possible to capture the raw analog video feed (unprocessed by the LiveTrack AV hardware). A USB video capture device sends the analog (NTSC) video signal to the 3t-physio computer (in the back corner furthest from the scanner). The video can be recorded using honestech TVR 2.5 (shortcut on the desktop). As of yet, no solution is available to record the video feed along with the scanner triggers. Matlab seems an obvious candidate to implement this in.

Shutting down

Quit the LiveTrack AV software on the stimulus computer. Disconnect the LiveTrack AV camera, unscrew the mount from the headcoil and place it back in the cabinet. Put the lens cap back on the camera.

Manual and demo Matlab/Psychtoolbox scripts

LiveTrack User Guide File:071-CRS-0075 R07 LiveTrack User Guide.pdf

LiveTrack Matlab/Psychtoolbox demo scripts

File:LiveTrack AV and FM MATLAB-PTB calibration demo script R02.zip

LiveTrack AV and FM MATLAB-PTB calibration demo script R02

7T setup

For the 7T we have the SR Research Eyelink 1000 Plus system available.

Powering up and starting EyeLink (7T)

Find the top battery, check whether the battery is sufficiently charged (press test button), and bring it into the MR system room. Plug in the two power cables and turn the battery on. The EyeLink software will give an error message during start-up if the battery is not connected and turned on. After placing the battery, remove the lens cap from the EyeLink camera. Don’t forget to turn on the BOLD screen.

Turn on the EyeLink monitor and select DVI as input (by default it is on Display Port, which is for the stimulus computer). Then turn on the EyeLink computer and choose EyeLink mode. You should now see the EyeLink graphical user interface (GUI).

Also turn on the stimulus computer and start up the Track software from the desktop shortcut. Keep the standard EDF filename (SDEMO), and hit enter twice so that the camera image is visible on the BOLD screen. Make sure that you are viewing the overview (zoomed-out) image by pressing the arrow-keys on the stimulus computer.

Important: the EyeLink setup does not return to defaults after shutting down the EyeLink. Make sure that all settings are correct before starting your experiment. For some default settings, see EyeLink Settings

Participant preparation and placement

Prepare the participant by removing mascara and replacing glasses for MRI glasses. Soft contact lenses can be worn during an eye tracking experiment, but rigid contact lenses may interfere with the corneal reflection (CR) and thus need to be switched for MRI glasses.

When the participant is placed inside the scanner, make sure that the eyes and eyebrows of the participant are not covered by the head coil, as the head coil may cast a shadow over the eyes and that interferes with the experiment. When you slide the participant in the scanner make sure you align on the eyebrows (to make sure every participant is more or less in the same position). Don’t forget to exchange the regular mirror (grey backside) for the first surface mirror (mirroring backside). Important: do not touch the first surface mirror with your bare hands, as this irreversibly damages the mirror.

After the participant is placed in the bore and the camera is setup correctly, look past the camera to see if the participant’s eyes are in the centre of the mirror. If this is not the case, adjust the mirror. Ask the participants if he or she can see all corners of the screen. Under normal circumstances, the EyeLink camera and illuminator have already been set up correctly and you should see the participant’s eye in approximately the centre of the overview image, after which you can move on to step 3. If the eye is not (properly) visible or illuminated, first check whether the feet of the BOLD screen tripod are on the marked locations. If they are and the eye is still not in the image, please contact the staff of the Spinoza Centre so that they may realign the EyeLink camera and illuminator.

Focus and adjust

Make sure that the ‘threshold coloring’ option is turned on. Click on the pupil on the EyeLink computer and switch to the zoomed-in view on the stimulus computer by pressing the arrow-keys. Carefully adjust the focus of the camera so that the size of the CR becomes as small as possible. This means that the pupil is in focus. Press A (or auto threshold) on the EyeLink computer to adjust the thresholds, and inspect the values under the image. The entire pupil should be coloured dark blue, and the pupil value (P) should be above 70. The CR value should be as small as possible. If too much light enters the pupil, part of the pupil may not be dark blue anymore, and you will see question marks behind the P value. If this is the case, adjust the lens of the illuminator slightly (see 7.2). A red P or CR value means the software cannot determine the position of the pupil and/or CR, which may often be solved by adjusting the position of the mirror. Another cause may be that the participant is wearing mascara. If neither are the case, the illuminator alignment needs to be adjusted. If you are unsure how to do this, please contact the staff of the Spinoza Centre.

Calibrate and validate

Set the desired calibration and validation settings. It is recommended to perform calibration in randomized order. Instruct the participant to focus on the centre of the calibration points and not to change their gaze until the calibration point disappears. Note that even when you’re not doing an eye tracking experiment, but are only monitoring pupil size, it is still recommended to do at least a three-point calibration.

Calibrate

Start calibration by pressing the C button or clicking the calibrate button. The first calibration point needs to be manually accepted by the researcher by pressing the spacebar. On the screen you will see a small green triangle, which moves if the participant moves their eyes. Wait for the triangle to be completely still and then press the spacebar to start calibration. Whenever the participants gaze reaches a calibration point, a green cross will appear on the EyeLink image. For a 9-point calibration you want these crosses to form a perfect grid. Either accept the calibration by pressing enter or clicking accept, or move on to validation.

Validate

After calibrating, start validation by pressing V or clicking the validate button. The first validation point needs to be manually accepted by the researcher by pressing the spacebar. Every time the participant’s gaze reaches a validation point, a value (degrees of deviation) appears on screen. You want these values to all be below 1; if this is not the case, recalibrate.

Shutting down

Quit the Track software on the stimulus computer by repeatedly hitting the escape button. Quit the EyeLink software by clicking ‘Exit EyeLink’ which returns you to MS-DOS, after which you can turn of the EyeLink computer by pressing the power button.

Put the lens cap back on the EyeLink camera, turn off and unplug the battery, place it under the fully charged battery and plug it in to charge. Switch the EyeLink monitor back to Display Port so that other users can use the monitor for the stimulus computer.

EyeLink Settings

For a full overview of possible EyeLink settings, please refer to the EyeLink 1000 manual, which may be found on the leftmost red shelve. The settings discussed here are settings that should generally not be adjusted.

Camera setup

• Sample rate should only exceed 250 Hz for microsaccade experiments; increasing the sample rate may increase noise;
• Eye should be set to left;
• Image Display: ellipse;
• Crosshairs: on;
• Threshold coloring: on.

Set Options

• Pacing interval is the interval between calibration stimuli. This should be at 1000 (ms). Randomize order and repeat first point should also be turned on;
• Configuration should be set to Long Range Mount.

Realigning the EyeLink camera and illuminator

Alignment of the EyeLink camera and illuminator should only be performed by experienced EyeLink user or the Spinoza Centre staff.

Adjusting the camera

The EyeLink camera and illuminator are attached to a mounting-bar, which is held in position by a bolt under the BOLD screen. Loosen the bolt and adjust the camera so that the eye and pupil of the left eye are in the middle of the image and fasten the bolt tightly; adjusting the camera is the easiest when looking past the camera. Make sure that the bar is horizontal, as this makes it more likely that the illuminator is correctly oriented. Important: be very careful not to move the screen while orienting the camera. When the camera is not aligned, it helps to disable colour thresholding so you can orient on anatomical features.

Adjusting the illuminator

Return to the EyeLink computer and click on the pupil. Make sure that the image on the BOLD screen is the overview image, as this makes it easier to see the position of the illuminator beam. If the ‘threshold colouring’ option is turned on, dark areas should be coloured dark blue and overexposed areas should be coloured cyan. In the ideal situation only the pupil is dark, and all other areas are not. It’s better to overexpose than to underexpose. The exposure can be altered by adjusting the illuminator:

• First adjust in vertical direction by loosening the bolt on the bar;
• Next adjust in horizontal direction by loosening the single bolt closer to the lens. Make sure to support the illuminator, else it might bend down;
• Last adjust the lens so that the beam becomes more focussed. Usually it needs to be extended quite far.

As a check you can ask the participant whether he or she can see all lights in the illuminator.

Hotkeys

• ENTER - show eye on monitor
• Left/Right – switch between zoomed in/zoomed out image
• C - calibration
• SPACE - start calibration
• ENTER - accept calibration
• V - validation
• SPACE - start validation
• ENTER - accept validation
• Within Presentation (stimulus computer):
• F4 ->either back to eye screen, or start of experiment
• ENTER -> accept drift check (when you see fixation point in center of screen)
• F4 -> start experiment
• D - Drift check (correct for eye movement on one central fixation point)
• Enter - accept drift correction

Abbreviations

• CR - Corneal Reflex
• GUI - Graphical User Interface
• P - Pupil value

Clean-up

Remember to take care of the equipment:

1. “Exit EyeLink” -> DOS Mode -> Power button OFF
2. Lens cap -> back on
3. Turn off battery and recharge if needed
4. Turn off EyeLink monitor

Converting EDF files to ASC

The EDF files are not in a readable format. In order to use them, you must convert them to ASC. This must be done from the command line on a computer with EyeLink installed.

The code for this conversion is in the Examples folder of the EDF Access API folder.
C:\Program Files (x86)\SR Research\EyeLink\EDF_Access_API\Example

Start-> RUN -> CMD -> set CD to EDF Acces API folder (copy paste directory), which contains edf2asc.exe.

To get a list of all arguments for this function, just type edf2asc
edf2asc –s locationofedffile\filename.edf (-s gives all samples)
edf2asc –e locationofedffile\filename.edf (-e gives all events)
In between these two command lines make sure to rename the .asc file (eg. filename_s.asc for sample file and nameoffile_e.asc for event file) before converting the next one, otherwise the .asc file will be overwritten.

Programming in Presentation with EyeLink Scenario Files with EyeLink code

Check out the example file “track.sce” for an example of using Presentation scripts with EyeLink.

A copy of this code is in the PresLink user guide.

Most of the code can remain, but you must make sure to specify when you want to send a message to the eyetracker EDF file. For example, at the start and end of a trial.

tracker.send_message(“some string”);

Note: EDF file names cannot exceed 8 characters.