[FieldTrip] excessive channel correlations

[Roy Cox]

hello EEG community,

We have a rather peculiar problem that we're hoping to get some input on.

In reviewing some existing 64-channel recordings, we noticed that some
records show near-identical signals across al channels, with median
channel-by-channel correlations of >0.99. This suggests some kind of
electrical shorting, somewhere in the
cap-cable-amplifier-montage-recording-exporting chain.

The problem appears to be infrequent (most records have more acceptable R
values of 0.6-0.9), but common enough that we'll need to review several
hundred records.

We haven't been able to identify the cause of the problem yet, and it
affects ongoing data collection.

Some details:

- we use Grass AURA amplifiers and TWin software with the Easycap system
- the problem has been observed with different amplifiers, different caps,
different pcs, and at different sites
- the problem is there regardless of reference montage (e.g., forehead and
averaged mastoids)
- gel bridging, while theoretically possible, seems unlikely given that
literally all EEG channels are (almost) identical

We're still investigating but wanted to hear if  anyone ever observed
anything like this? Were they able to solve it?

Thanks,

Roy
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