Recording Gaze Position; Drift Checking And Drift Correction - SR Research EyeLink Portable Duo User Manual

3.10 Recording Gaze Position

After the system is set up and calibrated, we can monitor gaze position in real
time, and record it for later analysis or viewing. This can be done by pressing
the "Record" button or the 'O' key from the Setup screen.
In this session, we assume the TRACK application is running on the Display
PC. When TRACK senses that the Host PC has entered Record mode, it sets up
a recording session under its own control.
On the Display PC, it displays a page of text or a grid of letters on its own
screen for the participant to read, alternating between recording sessions. The
Host PC screen will display a background image of what the participant sees.
This serves as a reference for the gaze-position cursor displayed by the eye
tracker during recording, allowing the experimenter to see where the participant
is looking and detect problems in eye tracking or participant's inattention.
TRACK displays the gaze position as a red cursor on the participant's display.
The cursor can be toggled on and off by the 'G' key on the Display PC keyboard.
To implement this feedback, TRACK requests that EyeLink send it up to 2000
samples per second of gaze-position via the EyeLink Display Software. This data
is used to move the gaze cursor.
TRACK also sends commands to the Host PC to create a data file (SDEMO.EDF)
on the Host PC's hard disk, which contains samples, fixations, and saccade
data. When the TRACK exits, this file will be automatically transferred from the
Host PC to the Display PC. SDEMO.EDF may be viewed with EyeLink Data Viewer
or processed with other EDF utilities. Information on the EDF file format can
also be found in Chapter 4 of the current document.

3.11 Drift Checking and Drift Correction

The "Drift Correct" screen displays a single target to the participant and then
measures the difference between the computed fixation position during
calibration and position of the current target. Unlike earlier EyeLink I and II eye
trackers, correcting the calibration map based on the drift correction result has
no significant effect on gaze accuracy. Therefore, the default drift correction
behavior of the EyeLink Portable Duo system is to only report the calculated
fixation error from the drift correction procedure and to not actually adjust the
calibration map in any way. Therefore the drift correction procedure is better
viewed as a "Drift Checking" procedure in the EyeLink Portable Duo in its
default configuration.
We strongly recommend that users keep the default "drift checking" behavior,
but true drift correction can be enabled if desired. Drift correction can be
An EyeLink Portable Duo Tutorial: Running an Experiment 63
© 2016-2017 SR Research Ltd.