Stationary EEG-fNIRS – actiCHamp Plus with actiCAP electrodes and NIRx NIRScout

In this article, you will find resources specific to the combination of the actiCHamp Plus with actiCAP electrodes with the NIRScout fNIRS from NIRx. The information covered here, should be used together with the main EEG-fNIRS Cookbook. The section numbers used here correspond to the sections in the EEG-fNIRS Cookbook.

2.1 Defining your EEG-fNIRS montage 

The channel positions for your EEG cap can be downloaded from the Brain Products website (.bvef format). These locations then can be loaded either into BrainVision Recorder when setting up the workspace or into your offline analysis software at a later stage. 

The fNIRS montage can be defined in NIRSite 2.0 (NIRx montage design software) based on the 10-20 locations of your fNIRS optodes (Figure 2).  These resources might be helpful:

• NIRx webinar: Montage design with NIRSite 2.0
• NIRx blog: NIRSite 2.0

The fNIRS montage can be defined in NIRSite 2.0 (NIRx montage design software) based on the 10-20 locations of your fNIRS optodes.

2.2 Select the right cap 

For this combination, the caps that come with either of the systems are extremely comparable. In fact, both the actiCAP snap cap and the NIRx cap are manufactured by our sister company Easycap. Below you can find the full list of what you need for this step: 

• actiCAP cap/NIRx cap (e.g., 128 slits, black fabric) 
• EEG: actiCAP snap holders 
• fNIRS: NIRScout cap holders 
• fNIRS: Layer cap

Figure 3. actiCAP cap with actiCAP snap holders for the EEG electrodes (10-20 montage) and NIRSport 2 Grommet bases for the fNIRS (fronto-central montage). Inserting the holders follows a similar procedure to the one described in Figure 4 of the EEG-fNIRS cookbook. 

2.3 Populate the cap 

Once the holders are on the cap, you will need to add the sensors.  

For actiCAP electrodes, you can check these two resources, which contain important information on how to handle the electrodes and how to optimize their placement on the head for the best signal quality and user experience: 

• actiCAP slim active electrodes walkthrough  
• How to avoid cable spaghetti – Our tips and tricks to nicely setup your actiCAP snap cap 

For the NIRx NIRScout cap, you can check the resources in the support center.  

3 Checking the signals 

Before starting with the data acquisition, we recommend checking the signals and eventually making some adjustments accordingly. 

• For the EEG, you should check the impedance levels in BrainVision Recorder (see this detailed description or check this video).  
• For the fNIRS, you can check the contact signal quality in NIRStar (see this resource for more information) 

4 Triggering, Synchronization, and signal acquisition 

For stationary set-ups, we recommend using shared hardware triggers to tackle the problem of signal synchronization and ensure that the data from both systems contains the same event markers.  

Depending on your paradigm, your event markers might be generated by a presentation software (via USB port) or another hardware source (e.g., an LPT or BNC port from a response box). You can use the TriggerBox Plus to receive event markers from different sources and forward them, with millisecond precision, to target devices. In this case, the target devices are the actiCHamp Plus and the NIRScout. For this reason, we need a way of sending the event markers to both.  

4.2.1 Trigger mirroring

Our recommended solution is to use the trigger mirroring function of actiCHamp Plus, which enables the forwarding of received triggers to a second device. Activating this function is quite straightforward. Just connect the TriggerBox (Plus) to the trigger-in port of the actiCHamp Plus amplifier, connect the DSUB-9 F trigger-out to the trigger-in port of the fNIRS device (using a DSUB-9 to LPT cable), and activate the trigger mirroring in Recorder (Figure 8). Your event markers will be automatically forwarded from the EEG to the fNIRS device, without any significant delay, given the slower sampling rate of the fNIRS.  

Figure 7. Trigger Mirroring with actiCHamp Plus.

Figure 8. Trigger Mirroring implementation in Recorder for actiCHamp Plus.

4.2.3. Separate Trigger Replicator

If your Brain Products amplifier does not have the trigger mirroring function, or if you simply prefer not to use it. You can also connect the TriggerBox (Plus) to a trigger replicator or a Y-cable, via a DSUB25-to-LPT cable.  

Figure 9. Schematic overview of a stationary EEG-fNIRS set up.

Note: The Amp port of the TriggerBox Plus, uses a modified version of the traditional LPT port, which we call DSUB25. As described in the TriggerBox Plus Operating Instructions, the main difference is that the Pin 24 of the Amp port of the TriggerBox supplies 5V. Using a traditional LPT-to-LPT cable or a modified DSUB25-to-SUB25 does not allow communication between the TriggerBox and devices with a traditional LPT port, such as the NIRx Parallel Port Replicator. In order to enable such compatibility, as of November 2019, TriggerBox kits are delivered with a DSUB25-to-LPT cable. If you already possess a TriggerBox, please check with your distributor whether you have the correct cable.