NeXus system basics
The three different NeXus systems have different technical specifications. Depending on your needs and whether you are a clinician or a scientific researcher, there is always a suitable NeXus solution. These are all advanced, multimodal devices optimised for biofeedback, neurofeedback and physiological data capture.
All the NeXus systems are capable of measuring a wide range of physiological signals simultaneously. They all come with different types of input slots. These inputs create a perfect combination for use with high quality NeXus sensors. The configuration of the sensors is stored by the software in the so called 'channel sets' so with a little knowledge, these systems can be configured to suit very demanding research or clinical applications.
The NeXus-4 is the smallest and most portable system of the NeXus series. It is our most affordable recording device, capable of measuring 4 channels at once. It is able to record of a wide variety of signals, such as brainwaves (EEG), muscle tension, heart rate, relative blood flow, skin conductance, respiration and temperature. The NeXus-4 satisfies the basic demands of the user and is suitable for biofeedback and neurofeedback applications as well as professional grade monitoring. The solid NeXus-4 provides a lot of freedom in movement during recording. It communicates wirelessly with a computer using Bluetooth technology and because of its size and weight it can fit in one’s pocket of be attached to a belt. Its built-in flash memory of 64 MB can store more than 8 hours of physiological ambulant data.
NeXus 10 MkII
The NeXus-10 MKII is our most popular recording device. It is capable of measuring a wide variety of signals, such as brainwaves (EEG), muscle tension, heart rate, relative blood flow, skin conductance, respiration and temperature, just like the NeXus-4. However, the NeXus-10 MKII is able to record much more channels at once. With 4 single channel inputs, 2 dual channel inputs, 1 oximetry input and 1 digital multichannel input, the NeXus-10 MKII offers 10 up to 25 channels. In addition, the combined oximetry/trigger input allows for oxygen reading and trigger interfacing in e.g. ERP/VEP and other experiments. These features makes the NeXus-10 MKII is really versatile and advanced system, which settles the needs of the more demanding clinician and researcher. It is particularly suitable for biofeedback and neurofeedback applications and scientific physiological research.
The NeXus-32 is our top of the line system for physiological monitoring, QEEG and neurofeedback and biofeedback applications. All in one! It offers up to 24 channels QEEG functionality, 4 channels of electrophysiology (EEG, sEMG, ECG, EOG or any combination), 3 channels of peripheral parameters (like GSR, BVP, temperature and respiration). The NeXus-32 is equipped with an oximetry input as well, that also allows for trigger interfacing.
The signal Inputs
Many of our clients are clinicians and scientists and not necessarily engineers with expertise in signal processing. Of course some knowledge and skills are valuable but the NeXus system designers have done their best to take some of the 'pain away' of working with the types of signals typically of interest in clinical practice and research. When using the BioTrace+ software and the NeXus units you are as close to plug and play as we have seen with any of the systems on the market. Let's take a look at the signal types you can handle.
The NeXus so called 'EXG' inputs measure electrophysiological signals. Typical examples are EEG, EMG, ECG, EOG and slow cortical potentials (SCP). On the NeXus-10 MK II these inputs are labeled A&B and C&D for example. These inputs are bipolar inputs, capable of doing bipolar recording for high fidelity. Electrophysiological signals are all recorded in more or less the same way. They require a single ground electrode and a positive and negative source for each input channel. The signals differ mostly in terms of the expected amplitude and frequency ranges of the signals.
The EXG inputs acquire electrophysiological signals through advanced DC instrumentation amplifiers that are built into the NeXus systems. The DC instrumentation amplifiers amplify the difference between a ‘plus’ and ‘minus’ signal. At the NeXus sensors, this is the difference between the red active electrode and the black reference electrode. The white ground electrode is required to keep the potential of the person measured and the potential of the NeXus system at about the same level. When using the EXG inputs, the ground electrode must always be used. Its location on the body is not critical and depends on the application, although we recommend to place it out of reach of the measuring electrodes. Also, for good measurement quality, it is crucial to make good electrode contact with the ground electrode.
Since the input impedance of the active channels is very high (10 to the power of 12 Ω), the influence of electrode impedance is very small. It should not be necessary to perform electrode impedance measurements. Instead, the NeXus systems offer electrode OFFSET checking, even while recording data. A poor electrode contact results in a high electrode offset. If the offset becomes too high, the NeXus systems will ‘flat line’ the signal. The PC software may give a warning.
All electrode cables are shielded with the average of the ‘plus’ and ‘minus’ electrode signal (active shielding). This ensures a measurement that is virtually free of movement artifacts. Mains (110/220 V) interference (50/60 Hz) is hereby also reduced to a minimum.
After the first amplifier stage the signals go directly to the Analog-to-Digital converter (ADC). No high pass or low pass filters that can cause signal phase shifts or filter overflows are present. All further digital processing takes place on the PC.
'Peripheral' signals are measured by the NeXus AUX inputs. Examples of interest to researchers and clinicians are Blood Volume Pulse, Heart Rate, Respiration, Skin temperature and conductance, Force and Acceleration. On the NeXus-10 MKII these connectors are labelled E, F, G and H.
Each auxiliary input has a 5-pin connector. Signals on this connector are:
- +5V output
- -5V output
- +signal input
- –signal input.
The +5V/-5V/GND pins are used to power an active sensor. The + and – inputs are connected to an instrumentation amplifier with a gain of 1. An unconnected input will automatically be switched off (i.e. will show a zero signal without noise). The output of the amplifier goes to the Analog-to-Digital converters (ADCs) without any filtering.
The NeXus-10 MKII is capable of measuring digital signals. These two inputs are located on the back of the NeXus-10 MKII. We distinguish the following two inputs:
Digital input: Trigger / SpO2 (oximetry) input