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

[Nathan Sanders]

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