Introducing eye tracking data to your portable EEG study brings many benefits. It combines two powerful modalities to provide a more complete view of cognitive processes in response to visual stimuli.
Eye tracking records external visual behavior and provides additional environmental context to EEG recordings. Combining them gives more accurate timing of brain events with visual information. Also, by integrating these two techniques you have a better understanding of how the brain processes and prioritizes visual information. This can help us better understand the underlying mechanisms of attention, perception, and decision-making.
EEG and eye tracking combined in outdoor studies further enhance their applicability in real-world scenarios. Their non-invasive and portable nature allows you to investigate cognitive processes and visual behavior in natural settings.
If you are looking for a way to synchronize EEG and eye tracking data for your study, we have an easy out-of-the-box solution for your setup.
In this blog, we’ll cover the topic of integrating mbt Smarting PRO line devices with Tobii Pro system. For this, we use transistor-to-transistor logic (TTL) triggering method. All you’ll need is:
1. Smarting PRO line (PRO/PROX) portable EEG device+ recording software (mbtStreamer for Windows or Android application)
2. Tobii Pro Glasses 3 + Tobii Pro Glasses Controller software for Windows. The solution works with both Glasses 2 and Glasses 3.
3. 2.5mm to 3.5mm audio cable for TTL communication between devices
Tobii glasses can send TTL triggers to the smarting PRO amplifier via a cable that connects the two devices. That’s why we refer to these types of triggers as wire/cable/hardware markers. TTL markers will be embedded in both data modalities, marking common points in time and allowing synchronization.
The setup is also great for outdoor studies since both Smarting PRO and Tobii eye tracking systems support data recording directly on device memory. So, there’s no need for a connection with the experiment PC during recording.
EEG cap mounting
For cap mounting, place the cap and amplifier on the participant and connect to mbtStreamer. The next steps depend on which type of cap you have – gel cap or semi-dry cap with sponges. In both cases, make sure to get low and stable impedance values for good signal quality throughout the recording.
We’ll use Smarting PRO device for this example, but the integration steps look the same for Smarting PRO X. Also, we’re using one tablet for both recording software, but it’s possible to use an Android phone as well for setting up EEG data.
Set up eye tracking glasses
Once you have the cap mounted and ready, it’s time to place Tobii Pro glasses next.
Now that you successfully mounted both systems, it’s time to connect them with the TTL cable. Connect TTL cable inputs appropriately, as the EEG cap requires a 2.5mm input while Tobii controller uses a 3.5mm input.
Tobii controller emits a sync signal to the Smarting PRO amplifier once the recording starts. It is initiated by a start sequence in the following format:
• One 500ms long high pulse (3.3 V), followed by a 500ms long zero pulse (0 V), repeated 3 times.
• During recording the sync pulse is emitted once every 10 seconds with a duration of 1000ms, followed by a 9000ms zero pulse.
• If you stop the recording, the sync pulse stops immediately. No stop sequence is transmitted.
• If you pause the recording, the controller immediately stops sending the synchronization signal. When you resume the recording, the controller transmits a resume signal consisting of two 500ms pulses.
Start the recording with Smarting PRO amplifier first, so that the start sequence from Tobii system is recorded in the EEG data as well.
During your recording, you will be able to see TTL markers in mbtStreamer alongside acquired data.
It’s possible to disconnect both Tobii Pro glasses and Smarting PRO from their respective software. The recording will resume normally. After the experiment is done, reconnect both systems and stop the recording.
TTL markers in recorded files
While recording your data with mbt Streamer all your trigger data are stored in XDF or BDF formats together with your EEG data. To open the file recorded with smarting PRO we used EEGLAB (see the figure below), but you can use any analysis software that can open XDF or BDF file format.
The same events are also stored together with the eye-tracking data. You can see those using the Tobii Pro Lab software. Once you open your recording, you’ll be able to see the TTL markers as Sync Events in the lower right panel (see the figure below).
Check out the TTL cable scheme below. It’s necessary to have Tobii controller’s Transmitter end (T) connected to Smarting PRO’s Receiver end (R). The same goes for ground ends (GND) on both 3.5mm and 2.5mm jack.
TTL input of the Smarting PRO/PRO X amplifier is active on zero, meaning that it detects a voltage drop as a trigger. We recommend using a voltage input of 3.3V for PRO and 5V for PRO X device. Since it’s only possible to detect 2 voltage levels (high – 1, low – 0), we refer to these types of triggers as 1-bit TTLs.