Combining actiCHamp Plus with actiCAP slim/snap and NIRScout requires the integration of different pieces of hardware which are described step-by-step in the paragraphs below.
Montage design and cap setup
In order to combine EEG and fNIRS, we need a cap that is compatible with both types of sensors. The good news is that the actiCAP slim/snap electrodes and NIRScout optodes can be placed adjacent to each other according to the 10-20 system (Jasper et al., 1958) on the same EasyCap. Just make sure you have an EasyCap with enough slits to host both actiCAP snap holders and NIRx grommet bases.
For example, if you plan on using 32 EEG electrodes and an 8×8 fNIRS montage, it is probably best to have 96 or 128 slits. If you aim to have a higher number of EEG or fNIRS channels, it is probably best to go with a cap that has 160 slits. In general, the more slits, the closer EEG and NIRS channels can be to the scalp positions of interest. This might be relevant if, for example, you have a motor task and are using a motor cortex fNIRS montage. In this case, both EEG electrodes and NIRS optodes would compete for central locations (C3, C4, Cz, etc.), but enough slits would ensure the possibility of placing electrodes and optodes as close as possible to the motor cortex. It is up to you, the researcher, to define the desired montage design by deciding whether to prioritize placing an EEG electrode or a NIRS optode on the most relevant channel locations
You can create EEG-fNIRS designs by first defining the fNIRS montage according to the 10-20 system, with NIRSite 2.0 (NIRx montage design software), and then complement it by selecting the EEG montage based on the same 10-20 system, or vice versa.
Having a defined montage design is crucial because fitting the cap (with actiCAP snap holders and NIRx grommets bases) is a fairly time-consuming procedure that should be carried out prior to the arrival of the participant. Populating the cap follows the same procedure of any unimodal measurement. In our experience, it is more practical to first fit the cap with the NIRx grommets bases in the relevant slits on the EasyCap. Then, the actiCAP snap holders can be placed according to the desired 10-20 system locations, and the actiCAP slim electrodes can be “snapped” into the holders (the procedure is described here or you can check this video).
Note: Because of the nature of the fNIRS signal, using a black fabric cap is recommended, in order to decrease unwanted optical reflection and improve signal quality. Except for the colour, this is the same cap, i.e. actiCAP snap, that comes with the actiCAP slim electrodes (the standard colour is white but black caps can be ordered as a customization).