User manual

threshold may now be automatically set by pressing the ‘Auto Threshold’ button

or the ‘A’ key when the camera image is selected. The pupil of the eye should be

solidly blue, with no other color in the image when the thresholding is properly

set. If large areas are colored, the subject may have blinked: press Auto

Threshold again.

If the subject wears eyeglasses, reflections may block the pupil in the image. If

the eyeglasses have an anti-reflective coating, image contrast may be poor and

pupil tracking may be noisy. These reflections are automatically reduced as

much as possible by the EyeLink system; however please be advised that not

every subject with glasses will be usable.

The pupil threshold should be checked by looking at the green areas in the

image. Figure 3-16 shows the symptoms to look for. If the threshold is too low,

the blue area will be smaller than the pupil, and the eye tracker data will be

excessively noisy. If the threshold is too high, there will be shadows at the edges

and corners of the eye, especially when the eye is rotated. Adjust the pupil

threshold by using the pupil threshold adjustment buttons or with the ⇑ and ⇓

Keyboard Shortcuts: a mnemonic is to think of the ⇑ key as increasing the blue

area, and the ⇓ key as decreasing the blue area.

Threshold Too High: Noisy

Good Pupil Threshold

Threshold too Low: Shadows

Figure 3-16: Symptoms of Poor Pupil Threshold

The Camera Setup display is updated very rapidly, so noise, shadows, etc. will

be easily detected. You can have the subject look at the corners of the monitor,

and watch the pupil image for problems. One common problem is for shadows

at the corners of the eye, which can capture the pupil (see the panel on the

right). These may be eliminated by decreasing the threshold with the ⇓ key. Be

careful not to raise the threshold too much, as the pupil thresholding may be

poor at other eye positions. The pupil on the Host screen should have a cross-

hair drawn around its center, indicating that it has been detected. If a shadow

captures the pupil, or it is clipped by the side of the camera window (as in

Figure 3-17), the crosshair and green box will disappear and the pupil will be

lost. On the Host PC, a red warning message will appear below the small

camera image for the eye indicating “No Pupil”.

66 An EyeLink 1000 Tutorial: Running an Experiment