EyeLink II User Manual version 2.12
(07/02/2006)
©
2002-2006 SR Research Ltd.
48
(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 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.
If a binocular calibration was performed, the system will automatically select
the best eye to record with during the calibration. To accept this selection, press
the ENTER key; to override this selection and record in binocular mode, or to
select the less accurate eye, make your selection using the mouse and then
press the ENTER key to accept your selection. Note, to have the eyetracker
always record in binocular mode and not to have it select the best eye, a change
in the default settings must be made. Open the final.ini file in the
c:\eyelink2\exe directory of the host PC and add the following line:
select_eye_after_validation = NO
Often the computed gaze position shows a systematic shift of all the targets.
The average of this bias can be used to correct future gaze-position calculations,
lowering tracking error. By accepting the validation the system performs a gaze
calculation correction (similar to a drift correction) using this data.
