Near Infrared Spectroscopy (NIRS) is a functional imaging technique, that employs low-energy optical radiation (mostly in 2-3 different wavelengths) to assess absorption changes in the underlying brain tissue. In fact, these absorption changes reflect the changes in local concentration of oxy- and deoxy-hemoglobin, which in turn are related to and triggered by the alternation in neuronal activity. That is, NIRS is a non-invasive imaging tool and utilizes endogenous chromophores to assess brain’s functional activity.
EEG and NIRS are sensitive to different cascade of events that are yet linked to the very same neuronal activities. In addition, these both modalities possess complementary temporal and spatial features. The combination of EEG and fNIRS offers therefore the possibility to examine brain’s functional activity more comprehensively. Even though fMRI is superior to fNIRS in terms of spatial resolution, spatial coverage and being able to address deeper brain areas related questions, it has one big disadvantage: immobility. That is, the feature of NIRS, that it is much more compact even mobile, makes it a good companion for the EEG.
Technically, the combination of the Brain Products EEG equipment and a NIRS system is quite easily done: The majority of the commercially available NIRS systems does not interfere with the EEG signal or induce artefacts. That is, all of our amplifier system can be used for simultaneous EEG-fNIRS measurements. Yet, in combining these both techniques two issues need to be considered: sensor/electrode placement and data stream synchronization.
EEG holders –such as our actiCAP or EasyCap standard holders – offer a good platform to accommodate NIRS optodes and EEG electrodes in the same cap. Hence usually these EEG caps are used for the combined measurements. In addition, these caps come with non-built-in electrodes; that is this setup can be used to realize several electrode-optode-configurations it is recommended to use caps made of black fabric in order to decrease the unwanted optical reflection.
The NIRS and EEG sensor scan either be placed jointly – same position holds both type of sensor – or alternately. The former one is certainly more challenging, and used more frequently in case the area of interest is limited, for instance during baby measurements. The joint placement is possible only if ring EEG electrodes and transparent gel is used and the NIRS optode is small enough to fit into the slit of the electrode.
Data Stream Synchronization
During the combined measurement, the EEG and NIRS systems are working independently, therefore the same time point has to be marked in both data streams. This is typically done by the triggers: the same marker signal has to be sent to the EEG and to the NIRS system. The hardware based TTL triggers offer the highest possible precision. These shared markers make it possible to identify and analyze the recordings acquired exactly in the same time.