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performed at the beginning of each trial, or part-way through a trial while data
are being collected. Regardless of when a drift correction is performed, if true
drift correction is enabled, a fixation sample is acquired to a known location
and a corrective offset to the raw eye-position data is computed and applied.
True drift correction can be enabled by changing the
"driftcorrect_cr_disable"
the instructions in the following section. It is important that before performing a
drift correction the participant be instructed to sit still and fixate on the drift
correction target carefully.
3.11.1 Enabling Drift Correction
To enable the drift correction procedure to adjust the calibration rather than
simply checking that the error level is within a certain range, you may go into
the Drift Check screen, and click on the "Apply Correction" button. This will
toggle on the button and change the screen mode to "Drift Correct" instead of
"Drift Check". Alternatively, the following EyeLink command should be placed
in the FINAL.INI or sent across the link. The default setting in the behavioral
laboratory setting is to turn ON the disabling of the drift correction when the
CR is being used on the EyeLink Portable Duo. We are turning this disabling
OFF so that the drift correction adjustment will take place.
driftcorrect_cr_disable = OFF
Another useful parameter to be aware of is the threshold value that is used to
determine acceptable error levels in the target fixation that is used to assess
drift. The parameter is the
degrees). If the fixation is not within this level of error, other fixation samples
can automatically be taken to ensure that the fixation sample wasn't itself in
error (e.g., attempted during a blink or when the participant was not complying
with instructions to fixate the target) or the experimenter can enter into the
Setup mode (ESC on the keyboard) and undertake a new camera setup or
calibration procedure.
3.11.2 Online Drift Correction
While enabling the standard Drift Correction procedure to update the
calibration parameters based on fixating a target between trials can be a useful
solution, it is not always ideal. For instance, some experiment sessions, such as
in a blocked design, do not lend themselves to the normal drift correction
procedure of having the participant frequently look at a target while a sample is
taken. For such situations it may be desirable to perform an Online Drift
Correction while data recording is underway.
With Online Drift Correction, eye movement recording does not have to be
disrupted to introduce an adjustment to the calibration parameters. Some
64
An EyeLink Portable Duo Tutorial: Running an Experiment
© 2016-2017 SR Research Ltd.
command setting in CALIBR.INI file, according to
'drift_correction_rpt_error'
variable (default is 2.0
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