Unless Marker tracking is disabled, the EyeLink 3 provides high-speed (up to 1000 Hz) drift-free six degrees of freedom (6DOF) head-tracking data that is synchronized with the eye movement data. The head tracking data is produced by the eye tracker every 1, 2 or 4 milliseconds and reported in the same eye movement sample. The types of head tracking data available are explained below.

6DOF Head Tracking

The EyeLink 3 provides 6DOF Head Tracking data. With an unrestrained head, it is free to change position through translation – movement along the three axes of the Cartesian coordinate system, as well as by rotation around the same three axes. Thus, any head pose in three-dimensional space can be described with three translational values (X, Y, and Z coordinates) and three corresponding rotational angles, providing 6 data points in total. The rotation angles are typically referred to as pitch (the rotation around the X-axis), yaw (the rotation around the Y-axis), and roll (the rotation around the Z-axis). Please see this blog post (https://www.sr-research.com/eye-tracking-blog/background/head-tracking/ ) for an illustration and the origin of the coordinate systems.

The EyeLink 3 uses an IMU installed on the front enclosure to track head movements. If the IR Marker is used, the head tracking data provided by the IMU is fused with the position data provided by the Marker to provide 6DOF positional and rotational head data with no or little drift. When operating the EyeLink 3 with the Marker removed or disabled, the head tracking data is provided by the IMU only. As a result, absolute head position data is not available and the rotational data is susceptible to drift over time. Therefore, when using the head tracking data users should pay attention to the Marker tracking status message reported in the sample line.

Head tracking data is reported in the sample data together with eye tracking data. For parsed eye events, head tracking data of the starting and ending samples and average values across all samples within an event, are also provided.

Head-in-Space

For the EyeLink 3, reporting of the participant’s head pose can also be done in screen pixel coordinates. For the pixel-based head coordinate (Head-in-Space), picture a virtual rod extending forwards from between the participant’s eyes towards the screen/calibrated plane. The point at which the rod intersects with the screen provides the head coordinate. Please see this blog (https://www.sr-research.com/eye-tracking-blog/background/eyelink-3-screen-pixel-data/ ) for further illustration of the head-in-space coordinate.

The head-in-space data is recorded to the EDF file for each sample. Each parsed eye event in the EDF file also reports the head-in-space data of the starting and ending samples and the averaged data across all samples within that event.