Gyroscopic effect on EEG sensors

#1

I am getting a (intermittent) signature pattern. The montage can be viewed here.

The effect works synchronously with a slight head movement. When the head rolling movement stops the signature pattern directly reverts to a more normal pattern and this then immediately continues with the same signature pattern when the same head movement resumes. I have tried this several times and found that it affects different sensors and presents different signatures. However, there is always this same correlation. The best way I can describe this phenomena at the moment is ‘sticky’ capacitance that presents a digital relativity that the headset can pick-up for subtle changes in voltage (presuming that a slight charge can be held to an anchor point such as the electric field of the device itself - possibly it’s battery discharge).

Can anyone shed any light on why the EEG headset is picking up gyroscopic movement? Is this a normal phenomena on other sets?

#2

Isn’t it normal for EEG electrodes or headsets to react to head motion? That experience is universal. Head motion directly changes skin-electrode contact pressure. Which in turn is reflected in microvolt level sensitive amplification. Impedance changes are also involved.

I don’t believe it has anything to do with electric fields, anchor points, ‘sticky’ capacitance, etc.

One of the reasons that some EEG headsets incorporate accelerometers, is so they can be alerted to these motion artifacts. And potentially use artifact correction algorithms.

Is this Emotiv EPOC? That system is saline electrode based, with saline soaked pads contacting the skin. So the varying pressure on these pads will also cause microvolt level changes.

William

#3

Movement of sensors has a very clear and obvious reaction to the sensor feedback. As the sensors are designed to pick up very low frequencies any movement of the headset will result in dramatic pattern changes. This has been tested thoroughly with the headset and the tolerances of movement quite well known and calibrated. The samples collated thus far do not include any interference from movement of the headset at all (and it is securely attached to the cranium). This video in the OP (above) is one such example and as can be observed not all the sensors are affected the same (you would get this with a movement of the headset). I also mentioned in the OP that this was an intermittent effect and I mentioned that the sensor signatures change for a yet unknown reason. For instance, I have noticed a similar relationship in the Occipital sensors as well (in isolation). Here is another file which shows another pattern form from a similar movement test.

Basically, I am confident that the gyroscopic affect has nothing to do with interference caused by loose contact with the skin. I have other tests which show similar feedback and different signatures for other movements where the head remains completely still. Kindly contribute if you have experience of this phenomena or can contribute a hypothesis that relates to polarisation.

#4

“gyroscopic effect” is just the same as “head motion”.

Effects on individual sensors do not require “loose contact with the skin” to manifest, each skin/sensor interface is a unique micro environment and can react differently. With headsets that operate through the scalp hair, there is varying amounts of hair in the way between the electrode (or saline pad) and skin surface. Additionally over time as the saline evaporates, the conditions change, leading to intermittencies.

#5

Can your device measure per electrode skin impedance? That might be further clues as to which electrodes have higher impedance, thus are more sensitive to motion changes.

#6

There is no contact issue in the test results provided, this is a feedback measurement. This PM does not relate to shifting the sensors (by mistake). However, the issue of microenvironments is one that I mentioned as potentially capacitive, which the responses so far seems to agree. I too would expect a small transient field to be generated over the headset. I am not sure how this field will be affected by EMF but is definitely affected by fluctuations in voltage and in particular how the battery drains. This PM relates to the EEG device itself which is not gyroscopic, i.e. no normal response to slight movement, nor any computerised adjustment for movement.

This PM relates to a theory of drawing voltage by polarisation.

#7

“There is no contact issue in the test results provided.”

You definitely need to read out the per electrode skin impedance. This is a feature of most EEG equipment these days. Electrodes with skin impedance above a certain threshold will be subject to many more artifacts. The effect you are seeing is NOT capacitive, but resistive. And there is no “transient field” or “drawing voltage by polarization”.

https://www.google.com/search?q=eeg+electrode+impedance

#8

The impedance is not shown but there is a connection measurement; I can’t say how that is calculated I suspect impedance is part of the feedback loop and the associated circuit quality. Capacitance related to self capacitance and the ability to self charge and affect the polarity of the sensors. I am not taking the conventional view of electronics but one of field generation that you mentioned.

The ‘drawing voltage’ was meant to cover the concept of creating a diode effect in the field.

#9

To add to the impedance discussion, I am thinking of holding states of virtual capacitance using linear resistances drawn onto the conventional flow of micro-currents in bipolar (sensor) networks - a form of superposition that might hold relative sensor positions w.r.t. capacitance superposition creating a micro-field environment that shadows the movement of the headset locking a virtual circuit to a felt sensor position in the variability of reality space.

#10

Topic: EEG electrode impedance effect on signal amplitude?

#11

So, I assume you read the paper by the EGI scientists,

No mention there of micro-field environments. Do you know the input impedance of your EEG unit? The lower that is, the more it is susceptible to various artifacts and sensitivity to motion. The OpenBCI ADS1299 has a 1 gigaohm input impedance. And typical skin impedances are in 5K - 10K region. The paper authors found that with high input impedance amps, skin impedance was not so important. And skin impedance could be relatively high, even 40K, with no significant impact on signal quality.

On the other hand, if your amp has an input impedance in the megohms range, then it will be much more prone to artifacts, if the skin impedance goes above 5K ohms.

William