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. The cover rubbing should go to M2/CSM2 and M1/DRL2.

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. We have also updated our application to have the EEG quality (EQ) that can help to determine the quality of the signal based on multiple metrics. Each of these metrics is important in assessing whether the recording data accurately captures the underlying brain signal. More information about EQ can be found here.

Do you read data from reference sensors? 

We don’t read data from the references. Our electronics uses CMS (left side, P3 or mastoid) as the electrical reference point and DRL (right side) as the noise cancellation electrode. All signals reflect the potential difference between the EEG sensor and the CMS sensor. You can get relative signals between channels by subtraction – the common CMS voltage cancels out. For example, (F3 – CMS) – (F4 – CMS) = F3 – CMS – F4 + CMS = F3 – F4

I am importing an EDF file exported from EmotivPRO to EEGLab. I cannot see any data from reference sensors in EEGlab. How can I finish the references when preprocessing the raw EEG data? 

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. DRL impedance is measured as part of the impedance for each other sensor – it is common to every CQ measurement. 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 mode 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’s quite common to re-reference by subtracting the mean or median voltage calculated across all “good” channels at each time step. This step removes 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.

Does data collected from EMOTIV devices include subtraction of the reference electrodes? The subtraction is done at the hardware level. We use CMS (left side) as the reference level to feed into the amplifiers for each of the other channels – so at that point we measure EEG(i) – CMS electrically. The DRL (right side) is a noise cancellation electrode. We apply a cancellation signal to DRL based on the signal measured at CMS – this is a common mode cancellation circuit which forces the EPOC+ electronics to ride on top of the common mode body signal. We use this kind of circuit because the headset is fully floating – there is no GROUND connection that can be used to reference CMS against.