by Dr. Alex Kreilinger
Strategic Product Manager (Brain Products)
This first part of the hyperscanning series introduces our current gold standard hardware solution for this application: the BrainAmps. Combining multiple BrainAmps allows using separate ground and reference channels for each participant and provides perfect clock synchronization in a single EEG recording file.
Hyperscanning refers to obtaining simultaneous recordings from more than one participant. It is useful for interactive and social studies and we have observed a growing interest in this topic in terms of support requests and publications in recent months and years. In this article we will focus mainly on how to record EEG, but we will also provide you with the necessary information on how to complete the setup by adding other physiological signals or signals coming from various sensors (for some examples please see our sensor tutorial).
One of the first questions that comes up when planning hyperscanning experiments is how to synchronize the EEG recorded from multiple participants. Another important question is whether there are any compromises in quality that must be considered in order to accommodate recording multiple participants at the same time.
Let’s start with some good news right away
If you choose the BrainAmp hardware-based hyperscanning solution, you don’t have to worry about either of those questions. Due to the modular design of our BrainAmps, it makes no difference whether you use more BrainAmps for an increased channel count or for combining multiple participants; the resulting EEG recording will always be perfectly synchronized. This synchronization is handled by the USB2 adapter, also known as BUA, where each individual BrainAmp is connected via fiberoptic cables. BrainAmps also have their separate ground and reference. This means that you do not have to think about doing some complicated re-referencing offline in order to access the individual signals. You can even look at EEG data from multiple participants online on the same recording computer, all while acquiring perfectly synchronized data, as all of the signals are collected by the BUA and passed on to BrainVision Recorder as one single data stream.
Furthermore, the BrainAmp setup also allows you to use one single trigger source. Triggers that arrive at the BUA are sampled with the EEG data, hence, assuring optimal synchronization. Simply plug your TriggerBox into the BUA and rest assured that your event markers appear exactly when and where they are supposed to.
Another advantage of the BrainAmp hyperscanning setup is that participants can be seated far apart thanks to the modular design and the very long fiberoptic cables (5 or 20 meters).
How many participants can I record?
As you might know, BrainAmps can be stacked up to a channel count of 256. If you want to tap the full potential of this setup, you can either go up to 8 participants using 32 channels each, or up to 4 participants with 64 channels each, or 2 with 128 channels each; otherwise, you can always go with lower numbers of participants and/or channels.
Please look at the schematic overview in Figure 1.
To optimize your hyperscanning experience and make subsequent analysis as smooth as possible, please also consider the following suggestions:
Make sure to use a separate PowerPack for each participant. Although one PowerPack can, in theory, power two BrainAmps, please do this only if both BrainAmps are connected to a single person.
Use one ControlBox per participant. If you want to use active electrodes, you will also have to use one ControlBox per person.
Keep an eye on the correct channel sequence. The channels are ordered depending on how you connect the BrainAmps to the BUA. The BrainAmp connected to the first fiberoptic port on the BUA represents channels 1–32 in the EEG recording, the next one represents 33–64, etc.
Impedance measurement should be done individually or using the ControlBox. BrainVision Recorder uses only the ground and reference from the first BrainAmp for impedance measurements. It’s impossible to view separate impedances for all participants in the impedance mode at once. Instead, you should create an extra workspace with a lower channel count equal to that of one participant. Then, plug in only the BrainAmp(s) of one participant, beginning at fiberoptic connection 1 of the BUA, and look at impedances. Repeat this process for each participant and save it individually to have a complete impedance measurement of all participants. Of course, you can also rely on the impedance monitoring via the LEDs on the active electrodes when using the ControlBox. This way you do not have to unplug anything.
Optionally, add ExG and AUX channels. You can also add non-EEG signals to the setup. For this purpose, you can either use some of the EEG electrodes, for example, to measure EOG or EMG signals, or you use the ExG AUX Box with a BrainAmp ExG. This way, you can add up to 8 bipolar ExG channels (for example, ECG) and up to 8 AUX channels (for example, a GSR sensor) for every participant. Just consider that, by doing so, you may have to swap out a regular BrainAmp for a BrainAmp ExG. Furthermore, it is important to connect the BrainAmp ExGs to the BUA after all the regular BrainAmps. See how this affects the schematic overview in Figure 2.
To make your life easier during analysis, you might want to think ahead and set proper channel names and coordinates already during recording. For example, add identifiers such as “_P1” to the channels to avoid problems with duplicate channel names during analysis.
The same goes for using triggers. You can use up to 16 bits for your triggers, so you can put valuable information in the trigger codes, for example, if some events are relevant only for a subset of all participants.
Loading the recorded files in BrainVision Analyzer 2 or third-party analysis software should be straightforward as everything is in one place and you do not have to run any additional converting steps. That said, please keep your eyes open for upcoming follow-up articles with more tips regarding the analysis.
Further reading and downloads
Take a look at this user research article from Anna Zamm et.al.  in the current Press Release. Here, the authors describe an impressive by-the-book implementation of the BrainAmp hyperscanning setup, which we can recommend without any doubt. Another great example can be seen in this recent publication from Barraza et al. .
Download Figure 1 of this article (.pdf file)
Download Figure 2 of this article (.pdf file)
 Zamm A., Debener S., Konvalinka I., Sebanz N., Knoblich G. (2020)
The sound of silence: an EEG study of how musicians time pauses in individual and joint music performance
Social Cognitive and Affective Neuroscience, Volume 16, Issue 1-2, January-February 2021, Pages 31–42.
 Barraza, P., Pérez, A., & Rodríguez, E. (2020)
Brain-to-Brain Coupling in the Gamma-Band as a Marker of Shared Intentionality
Frontiers in Human Neuroscience, 14, 295