56
An EyeLink Portable Duo Tutorial: Running an Experiment
©
2016-2017 SR Research Ltd.
mode determines the center of the pupil by fitting an ellipse based on the
thresholded pupil mass. When pupil occlusion is present, the Ellipse mode
interpolates points that may drop behind the eyelid or eyelashes in an attempt
to better approximate the pupil position. The Host software represents the
ellipse-fitting solution with a green ellipse drawn around the pupil area.
The Centroid algorithm is advantageous in some application scenarios as it has
very low noise. However, if the pupil is significantly occluded the Ellipse Fitting
algorithm may give a more accurate estimate of the eye position. The Ellipse-
Fitting mode decreases drift potential and copes well with pupil occlusion at the
cost of a higher noise level.
EyeLink Portable Duo uses the Centroid algorithm in the head-stabilized mode
by default whereas the remote tracking exclusively uses the Ellipse-Fitting pupil
tracking method.
3.7 Calibration
The preceding steps set up the EyeLink Portable Duo eye tracker to track the
position of the selected eye(s). Almost all eye-movement research requires
information on the participant's point of gaze on a display of visual information,
such as a screen of text. To compute this, we need to determine the
correspondence between pupil - CR position in the camera image and gaze
position on the Display screen. We do this by performing a system calibration,
displaying several targets at fixed locations for the participant to fixate. The
pupil - CR position for each target is recorded, and the set of target and pupil -
CR positions is used to compute gaze positions during recording.
There are several possible calibration types available, each of which serves a
different purpose. By default, in head fixed mode, a nine-point calibration type
(“HV9”) is used. This is good for most eye tracking applications. However, if a
large tracking area is used, the “HV13” calibration type should be used for the
best calibration accuracy. When using the Remote Mode, the default 13-point
calibration type provides the best recording accuracy. If you record eye
movements from a special population that is particularly difficult to calibrate
(i.e. young children, or any other population that has difficulty stably fixating
over many points), you might consider using fewer calibration points.
In the “Calibration Settings” panel of the Setup screen, check to ensure the
following option is selected.
•
Calibration type: 13-point for the Remote Mode, 9-point for the Head-
stabilized Mode
Begin calibration by pressing the ‘Calibrate’ button from the Setup menu, or by
pressing the C key. A calibration target will appear on both the Host PC display
