Combining EEG and eye tracking can open new possibilities for your EEG analysis. If you would like to add eye tracking to your EEG setup but are unsure how to implement this, we have great news for you: Thanks to our new cooperation with Tobii Pro, Brain Products now offers complete out-of-the-box solutions for simultaneous EEG and eye tracking!
This article intoduces how you can combine your EEG measurements with simultaneous eye tracking. We offer a full example workflow for a specific lab-based setup, while pointing to generally important aspects for a successful combination of EEG and eye tracking. For our setup, we are using the software Tobii Pro Lab for experimental control, the Tobii Pro Spectrum for recording eye tracking data, and the actiCHamp Plus to record EEG data in combination with our Photo Sensor. In the workflow, we describe how you can design your experiment while setting up shared event markers, how to perform the combined recordings, and how to merge both data streams in BrainVision Analyzer 2.
In the last decade, the combination of eye tracking with measures of brain activity like EEG or fMRI has increased. But why would we want to take a closer look at the eyes when investigating brain activity?
Eye tracking offers two major sources of information:
The position of a person’s gaze gives us insight into the “open” focus their attention, and this information can be highly valuable for EEG research. With this gaze information, you will be able to tell if participants are focusing their attention on a target, when exactly their focus arrives and for how long it stays until shifting elsewhere. This will let you identify trials in which the participant was not paying attention and discard them from your analyses. Most importantly though, you gain precise timing information for your EEG analysis. Event-related potentials (ERPs) can, for example, be calculated with respect to the fixation on a stimulus (i.e., Fixation-Related Potentials), instead of the mere stimulus appearance on the screen. Research addressing topics like attentional processes, visual search, reading or social perception can highly benefit from gaze information.
Changes in pupil size can inform about cognitive and emotional experiences. The pupil reacts with short dilations (in the second range) to different stimuli. These “pupil responses” are a very sensitive physiological measure, and their magnitude reflects the intensity of the undergoing cognitive/emotional processes. Thereby, stimuli that are more emotionally arousing, or that demand higher cognitive effort cause larger pupil responses. By analyzing them, you may be able to check if your experimental manipulation was successful, or to even follow cognitive or emotional processes dynamically throughout your experiment. In combination with EEG, you could, for example, use the magnitude of pupil responses to weigh or categorize different trials in your experiment.
Adding eye tracking to your EEG setup will hence open a range of new possibilities for your research!
Figure 1. Combined EEG and eye tracking setup in a laboratory setting.
Combining two different measures like EEG and eye tracking can be technically challenging, especially if they should be temporally aligned and analyzed together. The key here is setting up shared event markers/triggers that will appear in both the EEG and the eye tracking data at the same time. Note that not all trigger signals need to be shared. It is enough to have a few (at least two) shared event markers to align the data sets after recording. It is however crucial that an equal number of the shared events appear in both data sets, and that they mark common points in time. Therefore, we need to plan our setup and experiment with these shared trigger events in mind.
Event markers are usually generated by the software used for experimental control (like E-Prime®, Presentation®, Psychtoolbox or Tobii Pro Lab‘s Designer module). There are many ways to pass them on to your EEG and eye tracking recordings. The best setup for you will depend on your experimental software and the properties of the computer, EEG amplifier and eye tracker you are using.
Here, we want to show you one concrete example for setting up simultaneous EEG and eye tracking recordings. We are going to explain how you can record high-quality data in a lab-based setup using:
Figure 2. Example workflow for simultaneous EEG and eye tracking recordings