User manual

2.3.6 Drift Correct/Drift Check Screen
32 EyeLink 1000 Host Application
© 2005-2008 SR Research Ltd.
2.3.6.1 Drift Correct/Drift Check Screen Purpose
Figure 2-7. Drift Correct/Drift Check Screen
The Drift Correct screen displays a single target to the participant and then
measures the difference between the computed fixation position during
calibration and the current target. For the EyeLink 1000, the default
configuration leaves the calibration model unmodified. The purpose therefore, is
to check whether the model has become grossly invalidated. If the drift
correction error is large, the experimenter is prompted to acquire another
sample. If the error remains large (i.e., the prior sampling error was
reproduced), the drift correction will fail and another calibration will be required
(see Section 3.11 for more details).
To perform a drift correction, have the subject look at the first fixation point and
click the “Accept Fixation” button, or press ENTER or the Spacebar, to evaluate
the adequacy of the calibration parameters.
Important: In EyeLink I and II systems, the fixation error calculated during
drift correction was used to shift the calibration map. This linear adjustment
often greatly improved the overall accuracy for upcoming recording. However,
with the EyeLink 1000 we have found that correcting the calibration map based
on the drift correction result has no significant effect and can actually reduce
fixation accuracy during recording. The default drift correction behavior of the
EyeLink 1000 system in pupil-CR mode is to report the calculated fixation error
without altering the calibration map in any way. Therefore the drift correction
procedure is better viewed as a “Drift Checking” procedure in the EyeLink 1000.