This article applies to:
E-Prime Extensions for EyeLink 3.0
In EyeLink experiments, online drift correction mode is disabled by default. However, it can be enabled by setting some parameters on the Host PC. It is disabled by default as a precaution since it can add "noise" to ELCameraSetup if not used carefully. ELDriftCorrect in EyeLink-enabled experiments can only be performed whenever the eye tracker is not recording. In an fMRI environment, typically recording happens over an entire block and it is not possible to stop recording to go into the drift correct mode. Therefore, it can be useful to use online drift correction which can occur during recording.
To enable online drift correcting to a fixed location, the Host PC needs to be configured manually by adding some commands to the FINAL.INI file. For more information on configuring FINAL.INI, please consult SR Research. Additionally, it can be configured through E-Prime by sending the following commands across the link:
- drift_correct_cr_disable = OFF
- online_dcorr_refposn 512,384
- online_dcorr_button = ON
After enabling drift correction, these commands set the reference position to which the drift correction occurs. Next the “Drift Corr” button is enabled on the Recording Screen. Drift correction can be executed on the Host PC by clicking the Drift Correction button.
The drift correction can also be executed when an assigned key is pressed. The following command defines the F9 key to be used to trigger the drift correction action.
- key_function F9 “online_dcorr_trigger”
The variable online_dcorr_maxangle specifies the maximum distance in degrees of visual angle that is an acceptable drift correction. This can prevent adjustments greater than the size. The default value is 5.0 degrees of visual angle:
- online_dcorr_maxangle = 5.0
The attached experiment shows how this is completed in E-Prime. The "PrepareOnlineDriftCorrect" InLine object contains the properties and commands discussed above. The User Script contains the sub routine "PerformDriftCorrect" which is used to send the key to take the drift correction measurement and display it on the Host PC. Lastly, the Fixation object is configured with a Fixation.Keyboard(1).Correct Task Event that fires the sub routine whenever the response to Fixation is "1" (i.e., the vKey specified in ELWaitForFixation).