User manual

An EyeLink 1000 Tutorial: Running an Experiment

height; ALT + ⇐ and ⇒ to adjust the width). The position of the search limits

can be adjusted with SHIFT and cursor keys. In a binocular setup, size/position

of the search limits can be adjusted for each of the eyes separately.

3.6 Pupil Tracking Algorithm

The EyeLink 1000 implements two pupil tracking algorithms: Centroid vs.

Ellipse Fitting. The Centroid mode tracks the center of the thresholded pupil

using a center of mass algorithm whereas the Ellipse mode determines the

center of the pupil by fitting an ellipse to the thresholded pupil mass. In the

ellipse mode, the host software draws a green ellipse around the pupil area,

representing the ellipse fitting solution used to determine pupil position.

For most purposes, the centroid algorithm is recommended as it has very low

noise. However, if the pupil may be significantly occluded (for example by the

eyelids) the ellipse fitting algorithm may give a more accurate estimation of

pupil position. The ellipse-fitting mode decreases drift potential and copes well

with pupil occlusion but at the cost of a higher noise level.

The EyeLink Remote exclusively uses the ellipse-fitting pupil model.

3.7 Calibration

The preceding steps set up the EyeLink 1000 camera system to track the

positions of the pupil and CR of the selected eye. Almost all eye-movement

research requires information on the subject'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 subject display. We do this by performing a system

calibration, displaying several targets for the subject 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

different purpose. By default, a nine-point calibration type (“HV9”) is used. This

is good for most of the eye tracking applications. However, if a large calibration

region is used, the “HV13” calibration type should be used for best calibration

accuracy. Press the “Set Options” button from the Camera Setup screen to

display the Set Options screen. Check to ensure that the following options are

selected for practice:

• Calibration type: 9-point grid (the EyeLink Remote has fewer calibration grid

options than some other modes)