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Eyetracking in MRI

BIAC currently has eyetracking capabilities available on its 3T MRI scanner.
The eyetracking system involves 3 components:

  • Eyetracking camera – Mounted inside scanner with view of pupil
  • Viewpoint software – Calculates eye position and pupil diameter
  • Data collection and synchronization with behavioral paradigm software

The eyetracking camera is an MR compatible miniature analog video camera manufactured by Resonance Technology Inc.. The Viewpoint eyetracker software is a product of Arrington Research Inc. Although the camera was originally designed to be used with the video goggles, we have built a custom-made bracket so that it can now also be used with the video projector in the 3T scanner. The eyetracker camera adds about 1 cm of height to the goggles, so for subjects with large heads the goggles+camera might not fit inside the head coil. Using the projector system, head size is not a problem because the camera is mounted outside the head coil. This also makes set-up and adjustment somewhat easier with the projector than with the goggles.

Typical applications of the eyetracking camera in fMRI are:

  • Using the camera to monitor subject alertness (no tracking)
  • Eyetracking to verify correct eye position (e.g. fixation)
  • Eyetracking to follow eye position within a scene

Note: The eyetracking hardware available inside the scanner has limited spatial and temporal resolution compared to high-precision devices that are not MR-compatible (specifications.)

Connecting the camera

Set up of the eyetracker hardware inside the magnet room will be done entirely by the MR technician.

The magnet room eyetracker hardware (diagram) consists of:

  • Power supply
  • Connector box
  • Video cable from penetration panel to connector box
  • Camera with cable to connector box
  • Camera mounting hardware (different mounts for goggles and projector viewing)

Set-up steps:
1) Plug the power supply into the wall outlet at the back of the magnet room, nearest the video projector. (The power supply is usually found on the floor at the back of the magnet room, near the penetration panel.)
2) Connect the power supply to the connector box (see Figure).
3) Make sure the video cable is connected between the connector box and BNC port J64 on the penetration panel.
4) Connect the camera cable to the connector box.
5) Turn ON the eyetracker switch on the connector box.

Note: The scanner magnetic field can affect the power supply; to minimize problems, keep the power supply as far away from the scanner as possible, and unplug the power supply after you are finished using it.

Starting the eyetracker software

You must start the “Viewpoint” eyetracker program (on Donders at the 3T) in order to activate the video camera. During the setup process, the Viewpoint display should be visible to the subject. Use the video switch to send the Viewpoint image to the goggles, if the subject is wearing the goggles. Use the switch to send the Viewpoint display to the projector; that way the MR tech can see the camera image as well.

In the Viewpoint window you should be able to see the live camera image. If you don't, use the menu option View→Video Reset to reconnect the camera. If that does not work, check all the cable connections and the On/Off switch inside the magnet room to make sure everything is properly connected. You should also check the View→Mode menu option to make sure the camera is in Setup display mode.

Once you have a video image, use the brightness and contrast sliders in the middle at the top of the Viewpoint display to adjust the camera image until you get a reasonable picture.

Mounting the camera for use with the video projector

To use the head-coil camera mount, the camera is attached to a plastic rod (via a single nylon screw), which is inserted into a mounting bracket that clamps onto the outside of the 8-channel head coil above the subject's right eye (BIAC projector mount.)
Take care to rotate the camera up outside of the head coil before attempting to slide the coil over the subject's head.

Camera position can be adjusted by sliding the bracket along the arm of the coil, and by tilting and rotating the plastic rod to get an optimal view of the eye. Once in position, lock the rod in place using the nylon screw.

Calibrating the eyetracker software

Once the eye is centered and the brightness and contrast have been adjusted to lock onto the iris, the eyetracker needs to be calibrated. To do so, warn the subject to pay attention and focus on the boxes and dot that will appear on the screen. Then select the Calibration→Auto-calibrate option. A series of flickers will play across the screen that the subject should follow with their eyes. Once the calibration is finished, the ‘eyespace’ window should have a reasonably rectilinear grid showing a mapping from pupil space to screen space. If this map looks bad, or points are overlapping, repeat the calibration.

Integrating eyetracking into a behavioral paradigm

Integration with Presentation

Using Presentation, behavioral paradigms can send commands to the Viewpoint program to help synchronize the timing of data files generated by Viewpoint.

Controlling the eyetracker from Presentation

Integration with CIGAL

The Showplay fMRI software in CIGAL can use the eyetracker in two different ways. The easiest is to initialize it to read streaming eye-position data from the ViewPoint eyetracking program in real-time; in this mode, synchronization and generation of eye-position output files is performed automatically for any Showplay paradigm. CIGAL can also be used to send commands to the Viewpoint program to help synchronize the timing of data files generated by Viewpoint

Using the eyetracker with CIGAL

Analyzing eyetracker output

Typical uses of the eyetracker:

  • Watch subject's eye in real-time to monitor alertness
  • Plot eye position to check for errors in fixation
  • Plot eye position as gaze path on stimulus images
  • Correlate eye position with stimulus timing
  • Use eye position as regressor variables in fMRI analysis
biac/experimentalcontrol/eyetracker.txt · Last modified: 2023/02/23 18:43 (external edit)