Common eye tracking problems
When ambiant light levels are too high the raw eye picture can become a bit washed-out and the software will have trouble tracking the pupil and corneal reflection. Try to keep your light levels as low as possible and make sure there is no direct light from a strong source falling on the subject's face.
In the MRI and MEG trackers it is also possible that the IR light source for the eye tracker isn't correctly pointed at the subject's eye. Since IR light isn't visible to the naked eye, you'll have to check the raw eye picture while trying to re-adjust the IR light source.
Glasses are normally not a problem. Bifocal or varifocal glasses can be more problematic, but normal glasses are no obstacle for eye tracking. The only problem can be with reflections. If the orientation of the glasses is such that a clear reflection of the infra-red light source is visible eye tracking can be impossible. This can usually be solved by changing the angle of the glasses. Dirty or scratched glasses can be a problem too. Contact lenses are usually no problem at all.
The current MRI-compatible glasses use very small lenses, so it can be difficult to get to a point where the participant can see the entire screen, and the eye tracker can see the entire eye. Sometimes it is possible to get good data despite glasses, but it's probably wise to encourage your participants to wear contact lenses if they have any.
Mascara can make eye tracking impossible, as the software will interpret the black regions in the picture as the pupil, and mascara is very black too. Mascara will always have to be removed, and it is best to ask people not to wear any mascara when they participate in an experiment involving eye tracking.
The same precautions apply to eyeliner, eyeshadow, etc. Makeup remover is available in MRI.
Drooping eye lids can be a problem, as they can partly obscure the pupil. This is more common with older people. The solution is to move the camera to a lower position, if possible, so that the eye is filmed from below. If that doesn't help an eyelash curler can be used. Sadly, the SMI software will output an incorrect gaze position when the pupil is partly covered, instead of not producing a location at all. Keep an eye on the iViewX output window to verify that eye tracking is functioning correctly, and make a note if pupil covering occurs.
Asymmetries in tracking accuracy
There are typically left-right differences in (monocular) data. Eye tracking data will always be less reliable and distorted towards the edges of the screen, and the problems are more serious for the eye on the opposite side. Try to present your stimuli in the center section of the screen, if possible.
Unbalanced brightness for the two eyes / vertical line in monocular tracking
The high-speed tracker seems especially prone to this problem. The simple solution is to ask your subject to look straight ahead for a few second, while you click the 'auto adjust' button in iViewX.
This happens because the eye tracking image is a composite of two fields, which have separate readout amplifiers. At startup, these amplifiers are often not quite matched. The best way to avoid this problem is to auto adjust before each use of the high-speed eye tracker.
Multiple corneal reflexes
By default, the eye tracking software assumes a single corneal reflex. In some subjects you may get more than one, which causes the CR tracking to jump between the reflections. If one reflex is much weaker, you can minimise the problem by adjusting the CR threshold to some very stringent level so that only the stronger reflex is captured. You can also get around this by adjusting the mirror and camera position to get a slightly different recording angle or by reducing the size of the window the eye tracker is active in.
A related problem can occur with soft contact lenses. These sometimes trap small air bubbles which can then distort the reflection, breaking it up in several smaller reflections. The only solution for this problem is to ask the subject to take out the contact lens, rinse it and then re-insert it.
In MRI, you can also get a related problem where there is an entire row of 4 or 5 corneal reflexes. This happens when the mirror has been mounted the wrong way around. Flip it 180 degrees and the problem should disappear.
Pupil Center Shift
With some subject the center of the pupil shifts when the pupil changes diameter. This can actually have a quite strong effect on the eye tracking results, with some people proving almost impossible to track for this reason. There is currently no real solution for the problem, except for the advice to keep luminance levels stable during your experiment to try to keep pupil diameter from changing too much. But since diameter is not only depending on luminance, this will not always work. As ever, it remains important to keep a close eye on the tracker's output and eye picture to verify that tracking is correct.