Drift Correction

A drift correct needs to appear before each trial. The drift correct is similar to the dots you use for calibration, and the participant needs to fixate it before the trial begins. The tracker can use this dot to correct for small drifts in the calculation of gaze position.  A drift correction therefore can be used for two purposes:
  1. It can be a test to confirm whether the calibration is still valid – you cannot run the trial without completing a drift correct successfully, it will not trigger if the participants’ eye is not on the drift correct.
  2. It can function as a “fixation dot” (more commonly seen in non-eye tracking research as a cross) for the participant to look at so that they always start the trial in the same place (the centre).
One issue with the DRIFT_CORRECT action is that it can’t be used in a SEQUENCE where you are recorded eye moments.

We also cannot place it just before the SEQUENCE after the INSTRUCTION_SCREEN because it would then only occur once at the beginning of the experiment!

In order to remedy this, we need to create another SEQUENCE that will contain the DRIFT_CORRECT so that we can have a DRIFT_CORRECT before every trial. 

So, similar to when we created the SEQUENCE previously we will need to create a new SEQUENCE and cut and paste the original SEQUENCE inside it (do NOT copy and paste as it will cause confusion be duplicating variables). For simplicity we will rename the SEQUENCE that contain the recorded trials to the “RECORDING_SEQUENCE”.

We will now go through this process step by step.

Step 1: Rename Recording Sequence

Step 2: Cut and Paste the Recording Sequence

We then need to follow these steps:
  • Create a new SEQUENCE and delete the links connected to the RECORDING_SEQUENCE
  • Click on the new SEQUENCE and Paste the RECORDING_SEQUENCE within in
  • Join it to the START

Step 3: Link up new Sequence

Next, we go back to the top Experiment layer and link up the new SEQUENCE to nodes on that layer

Step 4: Add Prepare Sequence

The next thing is that we need to decide what needs to go in this new SEQUENCE layer – what do we want to happen before every trial? We decide that we need the following:
  • We want to prepare the image in memory ready to be shown in the trials. As before, to do this we use a PREPARE_SEQUENCE action.
  • We want a DRIFT_CORRECT to occur for the participant to fixate before seeing each trial.
Let’s begin by adding the PREPARE_SEQUENCE in the right place. Go to the RECORDING_SEQUENCE and move the PREPARE_SEQUENCE up into the new SEQUENCE layer.

Step 5: Add Drift Correct

Drag a DRIFT_CORRECT into the SEQUENCE layer and insert it before the RECORDING_SEQUENCE so that it occurs before each trial, as shown below.

We can change the location of the DRIFT_CORRECT to occur anywhere we wish from the Properties panel, as shown below. We can make similar changes to those seen in the CAMERA_SETUP.
2014-04-06 16_25_20-Developing Experiments using Experiment Builder_EYETRACKING - Microsoft PowerPoi