Eye tracker setup
General information on how to setup and calibrate an eye tracker.
Start by making sure the subject is comfortably seated. For most remote systems height and distance are critical, so you will have to make sure the subject is in the right location. The hi-speed eye tracker is mounted on a table that's height adjustable. Make sure the seat is set to a comfortable height first, and then adjust the height of the table. The hi-speed also has an adjustable chin rest.
When the subject is in position you first need to make sure that you are getting a clear picture from the camera. The picture should be in focus and the eye should be in the center of the image and should be well lid without shadows of the eyelids on the eye. Make sure to verify that the picture is good while the subject is looking at all four corners of the screen, not just the middle. If an area of the eye picture causes problems the part of the picture that is used for eye tracking can be reduced. This works just like resizing a window.
It is important to always have a good look at the actual eye image that the camera sees to verify that the pupil is clearly visible and that the eye tracker is working properly. The RED eye tracker will only show a very crude representation of the eyes, just two ellipses, which is only good to see if the subject is positioned properly. Another window, showing the actual eye image, can be opened, and this is highly recommended, even if you think everything is fine. The SMI eye image will show an outline around the area that is interpreted as the pupil, and the area that is seen as the corneal reflection. Both these areas will also have cross-lines through them. This makes it very easy to see if the eye is being tracked properly. Ask your participant to look at all 4 corners of the screen while you inspect the eye image, as problems will ususally only show up with extreme eye angles.
With the high-speed tracker, you should ask your subject to look straight ahead for a few seconds while you click the 'auto adjust' button. This ensures that the amplifiers for the two eyes (or two halves of the image in monocular tracking) are balanced.
Calibration and Validation
Before an experiment can be run you need to do a calibration. This is an automatic procedure that will adjust the main parameters to the specific subject. It requires the subject to fixate on a point that is being moved to a number of locations all over the screen. Calibration shouldn't take more that 20-30 seconds. If the subject leaves the setup, during a break, you will have to calibrate again at the beginning of the next block. Any calibration should be checked with a subsequent validation, which will show a number of point to the subject again and then report the precision with which the eye tracker can measure the gaze location. Calibration and validation should be repeated until the validation shows sufficient precision.
It is also possible to do a quick 'drift correction' during the experiment, where the subject only fixates a central point. This will correct for linear drift.
Monocular versus binocular eye tracking
Some of our eye trackers offer the choice between monocular and binocular eye tracking. There is no real consensus about which is better. All systems only have a single camera, so it will have to be zoomed out to cover both eyes. This will lower the resolution, and with that the precision. It is unclear if the advantage of tracking both eyes will compensate for that. The RED system will always record binocular data, and the MEG and MRI systems will usually record monocular data. The hi-speed system can be switched between the two modes very quickly.
In binocular mode the locations of the left and right eye can be averaged. This can give enhanced precision when both eyes are correctly tracked, but can reduce reliability when one of the eyes has problems. The only way to be sure which option is best is to inspect the raw output for both eyes.
When using an eye tracker in monocular mode, the next question is which eye to use. There are two possibilities: using the same eye for all subjects, or using the dominant eye for each subject. The dominant eye can be found by asking the subject to look through a small hole in a card. People do this with their dominant eye.
The advantage of using the same eye for all subjects is that your data will be more consistent over subjects. This is because of the distortion associated with extreme left or right gaze locations. This will be less of a problem when your experiment only involves central gaze locations.
The advantage of using the dominant eye is that the data will be less noisy. The non-dominant eye can have the tendency to make 'glissades', small eye movements to align itself with the dominant eye at the end of a saccade. These will not be present to the same extent in the dominant eye.