[Eeglablist] Weird Patterns in the Data: Bad Power or Ground?

[Malte Anders]

Hi Nathan, sorry for emailing again. I re-thought about your problem and
came up with another solution:
g.Tec, in its g.Recorder Software (g.Recorder or similar), gives you the
option to select "No Common Reference", which makes average users (like me)
think that they are recording reference-free EEGs. Which is technically
impossible.
In reality, when you select "No Common Reference" in g.Tecs Software, you
get heavily bamboozled by them, as the GND electrode is used as reference
electrode (without anybody ever telling you this, only if you email their
support team). Thus, what I think happens in your case is that:
-you selected "No Common Reference" in the recording software and hence
-all your 64 electrodes are referenced against their individual GND
electrode in pairs of 4 (remember: according to the manual, each pair of 4
electrodes on your g.USBamp has their individual GND electrode connector).
As you haven't connected a single one of those, all you get is: measuring
electrode - potential of the GND electrode (which is basically just high
amounts of noise because of common mode rejection failing, and nothing
that's coming from the subject, as the electrode is not connected).

That explains why you have groups of 4 EEG lines looking totally similar,
as the noise on each GND output is slightly different in each case.

On top of what I suggested in my previous email you can:
-rereference after recording to a single electrode in eeglab and see if
that looks better (DON'T use average! You have 64 recording electrodes, and
16 ghost-referencing-electrodes that only consist of line noise and other
garbage. Those ghost electrodes will be imposed into your data again after
average re-referencing, making up 1/5 of your data, resulting in another
re-referenced garbage set)
-define a single reference electrode in g.Tecs recording software before
recording starts
-connect all 16 ground electrodes (we've been there, remember?)

Kind regards from Germany,
Malte Anders

Nathan Sanders <nesander at ncsu.edu> schrieb am Mo., 27. Jan. 2020, 18:49:

> Hey Malte,
>
> Thanks so much for responding! It feels like I have just been thrown a
> lifeline.
>
> First, here are some pictures. This <https://imgur.com/ZGQqano> is 64
> channel data after filtering but before re-referencing to average. Here
> <https://imgur.com/HRW4S0T>'s how it looks after you re-reference it. See
> how the channels are grouped in 4s? Those channels (1-4, 5-8, etc)
> correspond to the A, B, C, and D blocks on each amp. That's what led me to
> suspect a ground issue, like maybe these independent blocks which are
> supposed to be referenced to the same ground potential actually are not
> (Actually I am not quite sure how the electronics are connected under the
> hood). Also, the data turns out to be nonsense. I have also had a
> coincidental experience similar to yours -- I can get good data from a
> single 16-channel amp, but if I bridge the ground connections with a jumper
> cable to another amp which is turned on, then the data goes flat. To answer
> some of your other questions:
>
> The g.USBamp seems to have 4 ground connectors, so with 4 amps you have 16
>> ground connectors... Do you have all of those connected to the subject?
>>
>
> Someone from g.tec told me to just connect the ground connections on the D
> blocks together, and someone else in my lab told me to just connect the A
> blocks together. You're saying maybe I should connect them all? I can try
> that. I guess normally the subject would be grounded via the ground
> electrode that is connected to the master gamma.box which is in turn
> connected to the master amplifier, whose ground connection is connected to
> the other 3 amplifiers via the jumper cable in the D block.
>
> The reason for this: g.Tec hardware gets exremly picky if one electrode
>> fails.
>>
>
> You're saying that this could be a result of a bad electrode?
>
> I will take your advice and read that wiki article. I am going to try
> setting up the system in another room (connected to a different circuit
> breaker) and try your suggestions regarding grounding. Again, thank you so
> much for taking the time to help! I can't tell you how much I appreciate it.
>
> Sincerely,
>
> Nathan
>
> On Mon, Jan 27, 2020 at 11:56 AM Malte Anders <malteanders at gmail.com>
> wrote:
>
>> Hello Nathan,
>>
>> good to see somebody struggling with gTec Equipment and power line issues
>> as well. I have been struggling in 2017 to get our g.HIamp going as we had
>> huge power line artifacts. I have spammed the EEGLAB list with walls of
>> text as well, but then solved the problem by complete coincidence when I
>> actually touched a ground connector while having the EEG mounted to myself.
>> Keep in mind that I could not see your EEG-Data as the EEGLAB list does
>> not allow email attachments, you can however email me the Image directly
>> and also send an imageshack-link or something like that to the list so
>> everybody can see.
>>
>> Here are my 2 cents for your problem: I think your subject is not
>> properly grounded. The g.USBamp seems to have 4 ground connectors, so with
>> 4 amps you have 16 ground connectors... Do you have all of those connected
>> to the subject?
>> Some suggestions:
>>
>> -I know that some people might think this is stupid, but try to connect
>> the subject to the same ground as the amps. You can snitch a ground wire
>> directly from the power outlet (dangerous!) or maybe from some medical
>> equipment that is also connected somewhere in the room. For starters, a
>> bare wire that is connected to a ground connection on the one end and the
>> blank part of the other end is held by the subject in his/her hand is
>> sufficient.
>> Ground connections do look like this:
>> https://www.mikrocontroller.net/attachment/246107/bild1.jpg
>> This is the symbol for protective earth inputs:
>>
>> https://cdn3.iconfinder.com/data/icons/electronic-with-circle/64/ground-512.png
>> In my case, the EEG recording looked like garbage when I tried recording
>> without any further grounding measures. As soon as the subject simply
>> touched any grounding spot in the room, the magic happened and boom, the
>> EEG looked beautiful. Without proper grounding, common mode rejection fails
>> and in this case it would be helpful to see your actual recording images.
>>
>> -Connect only one (!) electrode to each amp, so 4 electrodes in total.
>> Then connect all the corresponding 4 ground wires from the amps. You should
>> now have connected 8 electrodes to the patient, 4 regular ones and 4
>> ground. I don't know if the g.USBamp allows you to record like this but
>> with the HIamp its possible, as the ground wire is then used as reference
>> electrode. If you need to specify another reference electrode, connect 4
>> more electrodes that correspond to the same ground wires you have
>> connected. Either way, try to record with as little electrodes connected to
>> the amps as possible (8 or 12) and look if the data looks better. The
>> reason for this: g.Tec hardware gets exremly picky if one electrode fails.
>> In the g.HIamp, if one electrode is broken, you _seem_ to get good data but
>> the output is completly nonsense. There is no way to know if one electrode
>> has failed other than doing impendance measurements from time to time (all
>> black: please find the on electrode out of 64 that screws up everything,
>> have fun) or having a close look at the amplitude values. With the g.HIamp,
>> amplitude values of 1 µV or less (even nV are plotted) are a good indicator
>> that one of your expensive electrodes went bad.
>>
>> -Does the g.USBamp provide an interface for impendance measurements? If
>> yes, what are the results? Do you get any readings or does impendance
>> measurement fail?
>>
>> Heres some further reading:
>> https://en.wikipedia.org/wiki/Ground_loop_(electricity)
>>
>> Keep me posted, we'll figure this out. I've been through a lot with g.Tec
>> hardware, the only good news is: once its up and running, you'll get really
>> good data.
>>
>> Am Mo., 27. Jan. 2020 um 17:20 Uhr schrieb Nathan Sanders <
>> nesander at ncsu.edu>:
>>
>>> Hey Everyone,
>>>
>>> I am in the middle of trying to diagnose what I think is some sort of
>>> hardware issue and I could use the benefit of your experience. I
>>> apologize
>>> for the wall of text but please bear with me!
>>>
>>> Backstory: originally I had all four of our 16-channel g.USBamps plugged
>>> into a single AC adapter with a four-way splitter. I have been able to
>>> get
>>> really nice data in the past, but recently channels have begun to
>>> flatline
>>> one after another until they were all flat. This intermittent problem has
>>> been getting steadily worse until a few days ago I couldn't get any
>>> signal
>>> at all. So I replaced the sync cables, but that did not solve the
>>> problem.
>>> I then discovered that this flatline problem would *not* occur if I only
>>> tried to collect data from one amp at a time (16 channels).
>>> Interestingly,
>>> the problem would come back if I turned on another amp (even if I was
>>> still
>>> only collecting data from the first amp), but it would go away again if I
>>> disconnected the ground jumper between the two (even if the second amp
>>> remained on). That led me to think that maybe our AC adapter was going
>>> bad
>>> or had a bad ground. We had three spares, so I tried hooking up the
>>> master
>>> amp with the suspected bad adapter, and the other three slave amps with
>>> the
>>> spare adapters. I was able to get a signal in all 64 channels
>>> (progress!).
>>> I recorded 5 minutes of resting state data as a test and this is what it
>>> looked like (see two attached photos)...
>>>
>>> data_filt_noreref.png shows what the data looked like right after
>>> filtering, *but before re-referencing to average*. All the channels look
>>> suspiciously correlated.
>>>
>>> After re-referencing to average, an interesting pattern emerges. You can
>>> see that the correlation pattern appears in groups of four channels.
>>> These
>>> channels correspond to the A, B, C, and D groups in each amplifier.
>>>
>>> I ran ICA and did not see anything that looked like a brain source.
>>>
>>> At first I thought maybe I had the channel locations mapped incorrectly,
>>> but I double checked and they are correct. I now think it has something
>>> to
>>> do with the AC adapter -- maybe it's failing or there is a bad ground or
>>> something and it is somehow contaminating the signals via the ground
>>> jumper. I know that bad power supplies and grounds can cause weird
>>> problems
>>> with computers and ham radio equipment...
>>>
>>> Has anyone ever seen anything like this? Any comments or suggestions
>>> would
>>> be greatly appreciated. Maybe they will be useful to someone in the
>>> future
>>> as well.
>>>
>>> Thanks as always,
>>>
>>> Nathan
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>>
>>
>> --
>> Mit freundlichen Grüßen,
>>
>> Malte Anders
>>
>