In the top left side panel of the task editor, click Edit next to the Physical Signals menu to open it and access the Eyetracking settings of the task.
To enable eyetracking in the task, please check the "Enable eye tracking in this task" checkbox. Before the study begins, participants will be prompted to select which webcam they would like to use, allowing for selection of a connected external webcam versus a built-in system. The main eyetracking calibration will automatically happen before the start of the first task with eyetracking enabled. Furthermore, if you enable eye tracking for this task, the head pose will be checked before each trial.
In the Eyetracking Settings of the task, you can define the number of fixations to show between trials. These between-trial fixations have the following purpose:
- They are used to calculate the accuracy of the eyetracking before each trial.
- If you enable "per trial drift correction," the drift is calculated as the median difference between the shown fixation points and the predicted gaze locations. Every gaze prediction during the following trial will then be automatically corrected by this estimated offset.
The following is the recommended way to record a timeseries of [X,Y] coordinates of gaze locations.
- Create an Event with trigger "Eyetracking", which will automatically execute every time a new gaze location is predicted.
- To record the new gaze location, add the action "Set / Record Variable". In this action, on the left side, create a new recorded variable to store the gaze locations.
- In order to store a pair of x- and y-coordinates, it is best to choose format "Array".
- If you want to record all gaze locations over time, then switch the Record Type to "All changes / time series".
- On the right side of the action in the value select menu, select "Last Coordinate [X, Y] Array" from the "Eye Tracking" submenu (see figure).
The resulting recorded gaze coordinates will be in the design units that were used to position elements on your frames, such that you can easily associate coordinates to your stimulus locations.
There are 4 basic values recorded per gaze data:
- X: x (horizontal) location in frame units
- Y: y (vertical) location in frame units
- T: precise/corrected timestamp for the gaze
- C:confidence of the gaze detection (0 indicate eye closure or no face /eye visible, 1 indicate 100% confidence)
Accuracy of Eyetracking
Please note that the accuracy of the predicted gaze coordinates varies over different subjects and their corresponding environment (i.e. lightning and camera). Therefore, we provide estimates of the prediction errors. These estimated errors can be accessed in the value selection menu (on the right side of a "Set / Record Variable" action). The error values are mean euclidean distances between predicted gaze coordinates and the shown target fixation locations. In addition to these euclidean distances, there are also error estimates for only the horizontal ([X]) or vertical ([Y]) error components. There are two types of error values:
- "Error Calibration ..." values are calculated using a validation set of fixations captured during the initial calibration period.
- "Error Trial ..." values are calculated using the inter-trial fixations.