User manual

76 An EyeLink 1000 Tutorial: Running an Experiment

subject’s inattention. Applications can create similar feedback displays by

sending the display screen image to the tracker PC before recording begins.

TRACK displays the gaze position as a red cursor on the subject display. The

cursor can be toggled on and off by the ‘G’ key on the Display PC keyboard. To

implement this feedback, TRACK requests that EyeLink send it 250, 500, 1000,

or 2000 samples per second of gaze-position via the EyeLink Windows DLL. This

data is used to move the gaze cursor.

TRACK also sends commands to the Host PC to create a data file (DATA.EDF) on

the Host PC’s hard disk, which contains samples, fixations, and saccade data.

When the TRACK exits, this file will be automatically transferred from the Host

PC to the Display PC. DATA.EDF may be viewed with EyeLink Data Viewer or

processed with other EDF utilities. Information on the EDF file format can also

be found in the Chapter 4 of the current document.

3.11 Drift Correction / Drift Checking

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. Unlike earlier EyeLink I and II eye trackers,

we have found that correcting the calibration map based on the drift correction

result has no significant effect in gaze accuracy. Therefore, the default drift

correction behavior of the EyeLink 1000 system when in pupil-CR mode is to

only report the calculated fixation error from the drift correction procedure and

to not actually adjust the calibration map in any way. Therefore the drift

correction procedure is better viewed as a “Drift Checking” procedure in the

EyeLink 1000.

However, the user may opt to perform a drift correction at the beginning of each

trial by computing and applying a corrective offset to the raw eye-position data.

This can be done by changing the

“driftcorrect_cr_disable” command

setting in EL1000.INI file. It is important that before performing a drift

correction the subject be instructed to sit still and fixate on the drift correction

target carefully.

If your experiment paradigm permits, it is also possible to perform an online

drift correction in the middle of trial recording by the experimenter. There are

two ways of performing an online drift correction during recording. If it is very

likely that the subject will look at a particular point across trials, a reference

position for drift correction could be defined at that position. This can be done

by editing the value of

“online_dcorr_refposn” in the CALIBR.INI or FINAL.INI

file under C:\ELCL\EXE directory of the Host PC or, more preferably, by