User manual

overlaid with a blue threshold overlay. If the blue area in the display is

interfering with setup, press the “Threshold Coloring” button (or ‘T’ on

the keyboard) to remove the threshold overlay. In TRACK.EXE, you can

use keys on either the Display or Host PC to perform all keyboard

shortcut operations while the eye image is displayed.

Threshold bias too low

Properly thresholded

Threshold bias too high

Figure 3-10. Pupil and CR Thresholds and Bias Values

13) A properly thresholded pupil should be solidly blue, with minimal blue

elsewhere in the image. If the threshold is too low, the blue area will be

smaller than the pupil, and the image will show excessive movement. If

the threshold is too high, there will be shadows at the edges and corners

of the eye, especially when the eye is rotated. Therefore, it is important

that the experimenter have the subject look at the corners of the monitor,

and watch for potential pupil image problems. One common problem is

for shadows at the corners of the eye, which can disrupt tracking of the

pupil.

14) In the zoomed camera image, the threshold values for pupil and corneal

reflection are displayed under the camera image. Unlike other versions of

the EyeLink 1000 eye tracker, these threshold values are automatically

updated. The number beside the pupil threshold value is pupil bias – the

extent to which the pupil threshold is modulated (see Figure 3-10). The

user may adjust the bias using the pupil threshold adjustment buttons

or with the UP and DOWN keys. Raising the bias increases pupil

coverage (i.e., increasing the blue area) while lowering the bias decreases

the pupil coverage (i.e., decreasing the blue area). Heuristically, pupil

biases should be in the range of 0.9 to 1.1. A value around 1.05 is

recommended, though this will vary depending on the eye.

An EyeLink 1000 Tutorial: Running an Experiment