User manual

74 An EyeLink 1000 Tutorial: Running an Experiment

To begin the validation procedure, select the “Validate” button or press the ‘V’

key in the Camera Setup screen. The Host PC display will show the gaze

position as a round colored cursor. Note the movements of the cursors, and the

change in relative horizontal position (vergence) following saccades. Once the

cursor appears stable, and close to the target, press the ↵ (ENTER) key to

accept the first fixation. The remaining points are collected automatically or

manually, as in the calibration process.

As each fixation is collected, a cross is used to mark its computed position

relative to the target. The error (in degrees) is printed next to the cross. Similar

to the calibration procedure, the user can use the “Backspace” key in the

middle of a validation sequence to redo data collection for the last or last few

validation points collected. After the final fixation is collected, the average and

worst errors are displayed at the bottom of the screen, and the accuracy is

scored. Each eye is graded separately, using colored messages similar to the

calibration results:

GOOD (green background): Errors are acceptable.

FAIR (grey background): Errors are moderate, calibration should be improved.

POOR: (red background): Errors are too high for useful eye tracking.

Observe the pattern of the errors for each of the targets. If only one target has a

high error, the subject may simply have mis-fixated that point, and the

validation may be repeated to check this: press ‘ESC’ to return to the Camera

Setup screen, and ‘V’ to repeat the validation. If a regular pattern is seen (i.e. all

fixations on the left side are too low) there was probably a calibration or camera

setup problem. In this case, press ‘ESC’ to return to the Camera Setup screen,

and re-calibrate.

3.9 Improving Calibration Quality

The quality of calibrations determines how useful the data recorded will be and

how accurate the gaze calculation will be. Try some of these simple procedures

to improve data quality and gaze accuracy:

• The threshold pupil area must be inside the pupil box (displayed as a red box

around pupil) when the subject is looking at any area of the display. If a

portion of the pupil exits this box, the pupil will be lost.

• The corneal reflection should never be lost or misidentified when the subject

looks around the calibrated area.