qEEG measurement with the NeXus-32
Although EEG was discovered more than 80 years ago the fastest development in the field has been observed only recently as the technology - both software and hardware, has progressed. In step with these developments, new insights have emerged as techniques of neurofeedback and indeed EEG analysis have been refined in both research and clinical practice.
This article provides a step-by-step review of how to do qEEG measurements with the NeXus-32. The NeXus-32 system from Mind Media is a fantastic tool for qEEG, neurofeedback and biofeedback. It is a “multimodal” system capable of measuring unto 32 physiological signals simultaneously. This includes 24 inputs necessary for qEEG as well as 4 inputs for signals such as EMG, ECG, and EOG. Finally there are 4 auxiliary inputs for peripheral signals such as heart rate, skin conductance, respiration and skin temperature measurement. For research it is often necessary to interface with other hardware and trigger the measurement process to synchronise multiple data sources. All this is possible with the NeXus 32 in a unit that fits into the palm of your hand.
In the early days of EEG analysis we learned to "eyeball" the patterns. We have learned over the decades that the moment to moment functioning of the brain is directly linked to the EEG patterns. This being the case trauma should be reflected in the EEG patterns. However, such analysis cannot be carried out subjectively - hence the development of qEEG and so-called normative databases such as Neuroguide.
A normative database is in essence a significant collection of feature matched data, necessary and sufficient to be able to recognise the "normal" (or otherwise) character of a set of EEG signals. These approaches rely of modern hardware and software that offer improved signal fidelity and capture as well as rapid frequency domain (FFT) approaches. We have written about the FFT approaches elsewhere and we wont repeat this here.
We cant cover all of those possibilities in one short article so here we focus on how to carry out qEEG measurements. We touch upon the required hardware, preparation and measurement steps, provide some background information on artefacts, and how to care for materials.
The following equipment is required to perform qEEG measurements:
- NeXus EEG Cap (available in a Small/Medium/Large sizes)
- EEG Cap Adapter
- Conductive (electro)gel
- Syringe with blunt needle to apply gel to the electrodes
- Cotton swab
- Measuring tape
Performing a qEEG measurement
Before the actual measurement can start, the equipment has to be connected. Detailed information on setting up the NeXus-32 can be found in the NeXus-32 User Manual. Minimal of 50/60 Hz interference will be accomplished if the device is powered by the internal batteries.
Plug the 21 numbered connectors of the NeXus EEG adapter into the corresponding unipolarinputs (labeled 1 to 21) of the NeXus-32. Plug the black ground connector labeled Gnd in to the ground of the NeXus-32, in the upper left corner. Snap the NeXus EEG cap in to NeXus EEG Cap Adapter. The connectors will only fit one way.
Measure the circumference of the head to determine the cap size:
- Small (50-54 cm)
- Medium (54-58 cm)
- Large (58-62 cm)
Place the NeXus EEG Cap on the clients head. Place the ears of the client through the openings on the sides. Make sure Fp1 and Fp2 electrodes are placed correctly on the frontside of the client's head. Do a quick check by using some measuring tape and measuring the correct position of e.g. Cz. Fasten the velcro straps and make sure the cap is not strapped too tightly around the chin.
Start the BioTrace+ Software.
Select Signal Library.
Now select your preferred measurement screen (e.g. 21 EEG with FFT).
Make sure the subject sits comfortably, preferably in a slightly reclined position with feet elevated. A rolled towel can be used to support the neck.
Click the recording button.
The “select a client” dialog box will appear.
Select a client and click Continue... or click Add New for adding a new client.
The “New session recording screen” will appear. Click Start recording to start recording a session.
Click EEG and select the reference used (e.g. Linked Ear Reference).
NOW Click the signal check button.
A screen with an overview of all electrodes will appear.
Now fill the syringe with conductive gel and start filling up the electrodes with gel, startingoff with the ground electrode (GND) and the two reference electrodes (M1 and M2).
Slowly retract the syringe while filling up the electrodes with gel, thereby creating a “column of gel” and ensuring a connection between scalp and electrode. After the ground and reference electrode are filled, the numerical values beneath each checkmark will start to show values which are constantly changing.
The electrode contact values give an indication of signal quality.
- Between -25000 mV and +25000 mV (GOOD - shown with a green tick)
- Between ±25000 mV and ±50000 mV (OK - Needs attantion - shown with orange mark)
- Above ±50000 (Poor - shown with Red Cross)
Click page 2 to go to the second signal check screen.
Press the X on the keyboard to open the Training Signal Selection screen. Choose Training
Signal(s) in the right top and choose the position you want to check (e.g. Cz) for 50/60 Hz and muscle tension (EMG) interference. Click OK.
50/60 Hz and muscle tension (EMG) interference can be checked for the selected position
Click the back button. Visually inspect the EEG to pinpoint and possibly reduce artefacts to avoid having to mark and remove artefacts afterwards. For more details about the following artefacts, see Below for Background information.
Electrical activity of the muscles is apparent in this recording (head, shoulders, tongue, jaws, etc.)
Blinking or looking up or down artefact (e.g. in SCP).
In some cases the electrical signal of the heart (ECG) can show up in the EEG. The pulsation of veins below an electrode can also cause artefacts in the EEG.
After having checked signal quality, the actual measurement can be started.
Markers can define a certain event during a recording. These markers can be addedmanually by pressing the marker symbol in Session controls in the left bottom corner (or by
- A dialog box to enter the text of the session marker will appear.
- In order to stop the recording, click the stop button.
- An alert box will appear.
- Click Yes and save the session and enter a description of the session.
- Confirm by clicking OK, the session is now saved.
Select File > Export Session Data. This will open the Export of Session data screen.
Alternatively, select data in the session overview screen, and choose Export selected data
from the drop down menu.
A variety of output formats can be selected. Click OK and Save session.
Exporting data to NeuroGuide
There are two ways to export data to NeuroGuide.
- Exporting data by selecting data in session overview screen
- Select data in the session overview screen, and choose Export QEEG > Neuroguide from the drop down menu.
Click OK and Save session.
Exporting data from Export of Session data overview screen
Select File > Export Session Data. This will open the Export of Session data screen.
Select the measured data channel(s ) to export (35-55). Change Output Format to EDF+format. Click OK and Save session.
Import data into NeuroGuide
Open NeuroGuide. Select File > Open > NeXus..
Select the session data and click Load.
Select linked ear (LE) reference and click OK.
Fill in the Subject Information. Make sure to fill in at least the age and eye status (Open or Closed) Click OK.
50/60 Hz interference The EEG signal activity has very low voltages, expressed in microvolts. Electrical interference can arise from electrical devices, lighting, etc.Electrical devices and cables transport electrical power at a level of 110-240 Volts AC. This power is alternating 50 or 60 times per second and therefore called"alternating current" or AC. This 50 or 60 Hertz activity can show up in the EEG, especially where the electrode doesn't make good contact, or where there are simply too many cables and electrical devices around. This is a very common art ifact.The quality and design of the hardware also strongly influences the presence of 50/60 Hz.
Electrical activity of the muscles (head, shoulders, jaws, tongue, etc) can i nt e rfere with the EEG. EMG activity can reach amplitudes of more than 100 microvolts. Thus the EMG signal is more powerful than the EEG signal. Too much muscle tension cancompletely contaminate the EEG. Make someone aware of EMG artifacts byinstructing them to clench teeth, chewing, frown, raise eyebrows, move the headside to side, up and down and swallow to produce artefacts while looking at their EEG signals. Pay close attention to the temporal electrodes T3, T4, T5 and T6 (modified combinatorial nomenclature or 10-10 sys tem- T7, T8, P7 and P8) to identify muscle artifact from the masseter muscles. EMG may also be observed quitestrongly in the frontal and occipital electrodes.
Eye movement, both slower (looking up or down) and faster (blinking the eye) can cause artefacts, which are larger than the actual EEG signals. These artifact are very frequently observed . Eye blinks are most visible in the frontal locations (Fp1, Fp2, F3, F4, F7, F8). Make someone aware of eye movement artefacts by instructing them to blink, move
eyes up, down, right and left while looking at their EEG signals.
In some cases the electrical signal of the heart (ECG) can show up in the EEG. Thepulsation of veins below an electrode can also cause short spikes in the EEG. This artifact is relatively rare.
Care of materials
Disconnect the NeXus EEG cap from the EEG Cap Adapter.
The full EEG cap and electrodes of the cap can be rinsed out by flushing with lukewarm water. Use a cotton swab to clean the individual electrodes. Gel left overs in the "cup openings" can be removed by stabbing the backside cotton swab through the cups. Make sure the connecter of the EEG cap doesn't get wet. Also make sure there are no residues of gel as dried up gel might lead to poor signals.
When drying the cap, make sure the connector of the cap doesn't hang beneath the wet cap to prevent the connector getting wet. To speed up drying a hair blower can be used. Make
sure not to hold the hair dryer too close to the cap.