External Email - Use Caution
External Email - Use Caution
Hey Philip,
Sadly I have a system with the reference being plugged with an external
strand and located on FCz.
I have trouble understanding the difference between an "internal" reference
and FCz itself. I do not know but to me it seems that at the end the voltage
recorded at every electrode is just the difference with FCz. What is the
difference with it being internal?
That's a very good question -- I haven't found anything in the Brain
Products documentation to make it clear what exactly their internal
reference is. If you have an explicitly marked FCz reference electrode
though and are using the standard workspaces, then it's the reference
channel and it's safe to add it back in as a bunch of zeros.
I really prefer the way it is working with Biosemi since CMS is not located
on a position of the standard1020.
It actually doesn't matter too much where your online reference is (as
long as you have electrodes at where your final reference will be),
because of the math of re-referencing. Voltage is inherently a
difference, so if your online reference is FCz then the voltage at every
channel is:
v_chan = efield_chan - efield_fcz
(and by definition, v_fcz is 0).
Then to re-reference, you just subtract out the voltage at the desired ref
v_chan_newref = v_chan - v_newref = efield_chan - efield_fcz -
(efield_newref - efield_fcz) = efield_chan - efield_newref
And then
v_fcz_newref = 0 - v_newref
v_newref = v_newref - v_newref = 0
So if you have electrodes on the mastoids and an online FCz reference,
then you can just re-reference to the mastoids offline.
Anyway I disgress but just for maybe
further acquisition do you think it would be ok if I just plug the REF (so
the FCz electrode supposedly) to a mastoid (since I wont use the mastoid for
any analysis).
That's fine -- in PyCorder and BrainVision Recorder, you can even
relabel the channel to reflect your actual recording setup.
Phillip