EEE: Using the ELWaitForFixation PackageCall [34501]

This article applies to:

E-Prime Extensions for EyeLink 3.0

Detail

ELWaitForFixation

Depending on your paradigm, the ELWaitForFixation PackageCall might be used instead of ELDriftCorrect. ELWaitForFixation PackageCall is used to wait until the participant fixates on an object on the screen for a set amount of time. It does not correct or check for drift as ELDriftCorrect does.

NOTE: In most experimental paradigms, ELDriftCorrect and ELWaitForFixation are not used together.

When using ELWaitForFixation, it is necessary to display a fixation sub-object (e.g., +) at the beginning of the trial (i.e., Fixation in the attached experiment). The general method employed by ELWaitForFixation is one of active processing: the PackageCall keeps track of the participant’s fixations by actively accessing the eye tracking data stream and determining the location of the participants gaze. Once a fixation is maintained on the Fixation sub-object for the specified amount of time, the PackageCall terminates the object’s execution by simulating a response to the object. Therefore, the object must have an Input Mask, along with specific values for the TimeLimit and EndAction properties that enable the simulated response to terminate the object. For more information on E-Prime Input Masks, see E-STUDIO: Input Devices [22723].

The PackageCall object should always follow the Slide object that is used to present the fixation. Additionally, the sub-object (contained on the Slide) that presents the visual stimulus used as a fixation must be named “Fixation”. The sub-object that presents the fixation needs to be either a SlideText, SlideImage, or SlideButton. ELWaitForFixation has a couple parameters which are discussed below:

The first parameter is “c.” It passes in the current experiment Context.

The second parameter (theSlide) is a required parameter that specifies the name of the Slide object that presents the fixation. For example, if the Slide that presents the fixation is named “SlideStartTrial”, then SlideStartTrial needs to be specified for this parameter.

NOTE: The Slide object presenting the fixation must contain a sub-object named “Fixation” that presents the fixation point. The sub-object must have its BorderWidth property set to a value greater than 0 if visual feedback is used (i.e., the parameter vVisualFeedbackColor).

The third parameter (nMinFixationDuration) is a required parameter that specifies the amount of time in milliseconds that the participant must maintain fixation before theSlide terminates. The default value is set to 500.

The fourth parameter (vVisualFeedbackColor) is optional and specifies the color of the visual feedback presented to the participant when the appropriate sub-object is fixated upon (e.g., “green” which is the default). This changes the BorderColor of the sub-object named “Fixation” when the participant’s gaze is on the fixation.

The fifth parameter (vKey) is also optional. This parameter specifies the key that is used to simulate a response when the fixation has occurred. If not specified, the "1" key is used.

EXAMPLE: This example uses a modified version of ELFixedPositionIA.es3. The ELWaitForFixation PackageCall is added after a Fixation Slide in the TrialProc. The Fixation object has a SlideText sub-object named Fixation that presents the fixation cross (+). The Fixation Slide is configured in the following way to allow the Slide to work in conjunction with the PackageCall:

• Duration = 0
• Data Logging = Standard
• Input Mask = Keyboard
• Allowable keys for the input mask = {ANY}
• TimeLimit for the input mask = (Until Feedback) 

The ELWaitForFixation PackageCall's parameters are configured in the following way:

c, Fixation, 2000

This indicates that the Slide's name is "Fixation" and the time fixation is required by the PackageCall is 2000 milliseconds. Additionally, because they remain unspecified, the values of vVisualFeedbackColor and vKey are "green" and "1" respectively.