Everything you need to know about reference sensors of EMOTIV hardware
EPOC+ and EPOC X have two options for positioning CMS (common mode sensor) either at P3 location (slightly left of centre) or on the left mastoid location. DRL (common mode cancellation sensor) can be placed symmetrically on the right side. INSIGHT places both CMS and DRL on the left mastoid location. EPOC Flex (all models) allow completely arbitrary placement of both CMS and DRL reference sensors. EPOC Flex uses the same referencing circuit and strategy as EPOC X. EPOC Flex allows you to position the references anywhere you want them. Ear clips are available for Emotiv EPOC Flex Gel.
Where are the reference sensors? Do you have an impedance check for the hardware? Our electronics use CMS/P3 (left side) as the electrical reference point and DRL/P4 (right side) as the noise cancellation electrode. We use a CMS/DRL common mode cancellation circuit which includes injection of a small high-frequency signal into DRL. We measure the amplitude of that signal at each sensor location to determine the conductivity in real-time, fed back to the user through the contact quality map, with black/red/orange/green indicators at each location. M2/CMS2 and M1/DRL2 are alternative reference sensors.
EPOC 14-channel models are shipped from the factory with non-conductive rubber comfort pads fitted to the mastoid locations. These pads do not conduct signals, they are intended to cushion the mastoid reference arms for comfort when you use P3/P4 locations for CMS and DRL. They should be removed, or fitted to P3/P4 if you wish to use mastoid references at M1 and M2.
We use CQ (Contact Quality) to evaluate the impedance and have CQ display in our software. It’s a visual representation of the current contact quality of the individual headset sensors. You can observe each sensor’s status in real-time to adjust sensors to optimize contact quality. The color-coding is Green (good), Orange (moderate), Red (poor), Black (very poor). You can refer to this for more information. Our software application also measures the quality of the EEG signal for each sensor (EQ), detecting artefacts, flat-line signals, excessive interference, poor CQ and poor wireless connection. Good EQ values indicate the sensor is accurately capturing the underlying brain signal. More information about EQ can be found here.
Does data collected from EMOTIV devices include subtraction of the reference electrodes? The CMS (left side) reference voltage is subtracted from each EEG sensor voltage in hardware, at the input amplifier for each of the other channels – so at that point we measure EEG(i) – CMS electrically for each EEG sensor (i). The DRL (right side reference) is a noise cancellation electrode. We apply a cancellation signal to DRL based on the signal measured at CMS. The CMS/DRL common mode cancellation circuit forces the analog input circuits to ride on top of the common mode body signal. We use this kind of circuit because the headset and user are fully floating – there is no GROUND connection that can be used as a reference for CMS. This arrangement is known as a differential montage.
Do you read data from reference sensors? We don’t read data from the references. EMOTIV products use a differential montage relative to CMS (after common-mode noise cancellation). All signals reflect the potential difference between the EEG sensor and the CMS sensor. A differential montage allows you to derive relative signals between channels by subtraction: the common CMS voltage cancels out. For example, (F3 – CMS) – (F4 – CMS) = F3 – CMS – F4 + CMS = F3 – F4 is the voltage that would be observed at F3, if F4 is used as a reference level.
When I export an EDF file from EmotivPRO and open it with EEGLab, I cannot see any data from the reference sensors. How can I find the references when preprocessing the raw EEG data? Most analysis in EEGLab does not require a reference channel to be designated. EMOTIV devices measure each EEG channel as a differential signal relative to the CMS sensor. CMS represents the background body potential, which is effectively subtracted from each channel, leaving the “local” potential signal. So for example, the voltage measured at position AF4 is actually V(AF4)-V(CMS), while the voltage at T7 is actually V(T7)-V(CMS). Differential montage measurements allow you to re-reference your signals in any way you like, so for example it you decide to use T7 as a reference point, you simply subtract the T7 voltage from each of the other channels. In the above example, AF4(rel to T7) = V(AF4) – V(CMS) – V(T7) + V(CMS) = V(AF4) – V(T7). You don’t need to know the explicit value of CMS because it cancels out. . It is quite common to synthesise a reference level from the active EEG channels after the DC offset level has been removed using a high-pass filter. For example, a reference can be generated from the mean, median, or inter-quartile mean IQM (mean of the middle half) of the “good” EEG sensor voltages at each time step. Subtracting the synthesised reference level from each EEG channel can remove much of the common background signal which is not already cancelled by the CMS/DRL feedback loop and CMS referencing. There are many other re-referencing schemes in the literature, depending on the application