09-07-2020, 04:56 AM
The Drift Correction and Drift Check procedures are very similar in their initial steps. In both routines, the participant must fixate on a single target. Once the participant is accurately fixated on that target either the participant or, more typically, the experimenter must press a key or button to confirm fixation. Upon confirmation the EyeLink system calculates the deviation between the participant's recorded gaze position and the presented target. The key difference between the two lies in how the system uses this calculated information.
- Drift Check: The system simply verifies if the participant's gaze position is less than 2° of visual angle from the presented target. If the gaze is within this tolerance, the experiment can proceed. If not, the participant must attempt the procedure again. No changes are made to the calibration model.
- Drift Correction: The system applies a linear correction to the existing calibration model to compensate for the detected deviation.
- EyeLink I and II Systems: These systems apply a Drift Correction by default. The deviation between the participant's recorded gaze position and the presented target is used to shift the calibration map. This linear adjustment often improves overall accuracy for subsequent recordings, as drift is commonly caused by slippage of the headband.
- EyeLink 1000, 1000 Plus, and Portable Duo Systems: For these systems, performing a full Drift Correction and shifting the calibration map generally does not improve gaze accuracy. In fact, it can even reduce tracking accuracy if the correction is applied when the participant is not fixating properly. Therefore, the default behavior in Pupil-CR mode for these systems is to perform a Drift Check which simply reports the calculated fixation error without altering the calibration map. If there is significant deviation then you will want to recalibrate the system.