An EyeLink 1000 Plus Tutorial: Running an Experiment
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2013-2017 SR Research Ltd.
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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.
For most applications, the Centroid algorithm is recommended 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 and therefore should only be used when Centroid
mode may fail.
Remote tracking exclusively uses the Ellipse-Fitting pupil tracking method.
3.7 Calibration
The preceding steps set up the EyeLink 1000 plus eye tracker to track the
positions of the pupil and CR of the selected eye. 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 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, a nine-point calibration type (“HV9”) is used. This
is good for most eye tracking applications. However, if a large calibration region
is used, the “HV13” calibration type should be used for the best calibration
accuracy. When using the Remote Mode, the 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. infants, or any other
population that has difficulty stably fixating over many points), you might
consider using fewer calibration points.
Open the Set Options screen by pressing the “Set Options” button from the
Camera Setup screen. Check to ensure the following options are selected:
•
Calibration type: 13-point for the Remote Mode, 9-point for all other modes
•
Randomize target order: YES
•
Auto-trigger pacing: 1000 msec
