Measurement Technique

Fibre optic cables guide infra-red light from a source located outside the scanner into the head coil and onto the eye of the subject. The light is reflected by the eye along the edge of the iris and is transferred back to infra-red sensitive diodes (limbus reflection). These photodiodes are coupled to an amplifier, which provides independent estimates of the horizontal and vertical positions in one eye. The system works with a daylight suppression technique, to prohibit interference from ambient light.

The fibre optic cables are encased in black malleable plastic for easy handling and their long length prevents the MR-Eyetracker Control Unit from interfering with the magnetic field. The ends of the cables are mounted into a custom eyepiece that is attached directly to the head coil. Just a few centimetres of additional space are required to mount the eyepiece, since it is designed to be lowered into the window of the head coil.

A mirror and mounting plate are also available for the head coil so that the subject can view a stimulus projected on to a screen located near the scanner.

Calibrating the Eye Position

Calibration of the MR-Eyetracker is a simple process that takes less than five minutes.

• The subject looks at a fixation point on the screen while the experimenter centres a red/green bar displayed at the bottom of the pupil and adjusts the zero-offset
• Subsequent measurements are made with a stimulus displayed at known horizontal and vertical angles

The fixation and stimulus can be generated with a visual stimulus generator like the ViSaGe and the vsgEyetrace software, or any other computer graphics system.

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