A Practical Guide for Combined fNIRS & EEG Recordings
Combined fNIRS and EEG recordings become more popular because their complementary approach extract much more information from the brain than one single technique. But in order to conduct a successful measurement, some key factors have to be considered. Here is a practical guide based on more than 20 years of experience, so that you – as a researcher or medical professional – learn what’s important to perfectly record brain activity and to save time, money and nerves 🙂 Let’s get to it!
What’s the difference between fNIRS vs. EEG?
The fNIRS (functional near infrared spectroscopy) measures the oxygenation level and hemodynamic of the brain non-invasively. The oxygenation level changes as brain areas become more active. Researchers can identify brain activity in real-time based on changes in blood oxygenation and other factors. fNIRS is measured with called optodes, using LEDs or lasers to emit light into the brain and the reflected light is measured with a detector. By looking at a certain wavelength of the light the absorption of the haemoglobin can be measured and the blood flow in certain brain regions can be measured. These optodes have normally a distance of 20-30 mm from each other and the fNIRS signal measures the blood flow in between these optodes.
In contrast, the EEG (electroencephalogram) measures the electrical activity of the neurons right underneath the electrodes itself and provides high temporal resolution, unlike fNIRS.
However, a simultaneous recording of both fNIRS and EEG signals in one single device is highly beneficial. EEG signals indicate e.g. short motor imagery while fNIRS signals show long lasting changes like mental counting or pain. In other words, researchers can capture what might be missed when using only EEG or fNIRS.
How can I combine fNIRS and EEG in one single recording?
To measure both the EEG and the fNIRS at the same time, it makes sense to put the EEG electrodes in between the optodes, because EEG electrodes are smaller and easier to place. Then the EEG electrode picks up the electrical activity at the same spot as the fNIRS picks it up.
An important factor is the electrode cap. The EEG cap must be able to keep the fNIRS optodes and EEG electrodes in a fixed position to get artifact free data. The dark electrode cap material ensures that no distortions are produced by ambient light. Furthermore, it’s important that both the EEG electrodes and fNIRS optodes are lightweight, to reduce movement artifacts in the recordings. Besides that, active EEG electrodes help researchers to get top-quality EEG recordings from 64/32/16/8 g.SCARABEO EEG channels and 8 fNIRS channels within a few minutes. This approach is becoming more and more popular because combining two recording types extracts way more information than using one single method.
The wearable g.Nautilus EEG amplifiers can be upgraded with an g.SENSOR fNIRS that enables simultaneous recordings of EEG and fNIRS (functional near-infrared spectroscopy) signals from one device only.
What equipment is suitable for combined fNIRS EEG recordings?
g.tec has the ideal equipment: You can choose between several EEG and biosignal amplifiers, active wet or hybrid dry EEG electrodes, difference sizes of EEG caps and a software solution to quickly setup your neuroscience experiment. Use a g.Nautilus wearable EEG, g.USBamp and g.HIamp biosignal amplifier to measure high-quality EEG signals together with the g.SENSOR fNIRS. The EEG electrodes and the g.SENSOR fNIRS are assembled in the EEG cap to have well defined positions and a proper contact to the scalp.
g.tec’s EEG amplifiers can stream the data in real-time into a computer for real-time analysis in order to run BCI or closed-loop experiments. In this case, it is important that both the EEG and the fNIRS are sampled at the same time. The EEG amplifier has normally the higher sampling frequency and is therefore acting as the “master device”. The g.HIsys Professional software environment has many examples for running motor imagery BCI experiments or to perform mental performance tests such as the STROOP word test.
Pick a Biosignal Amplifier
Choose EEG electrodes
Add a g.SENSOR fNIRS to it
Get the right software solution
For what applications fields are fNIRS+EEG recordings useful?
The combination of fNIRS and EEG is very useful for neuroscience experiments to understand how the brain is processing information. Furthermore, the combination is useful to boost BCI performance by using both the EEG and fNIRS features as inputs. The EEG shows a much higher temporal resolution than the fNIRS. For that reasons the EEG is able to record a short lasting event like a single finger movement, but with the fNIRS it is easy to capture long lasting activity that might by missed with the EEG.
How long is the preparation of fNIRS and EEG?
The preparation time for 10 fNIRS and 32 EEG channels is about 10 minutes with g.tec’s active technology and this is much faster than with passive EEG Electrodes.