BioSemi.nl

Hello,

My lab is trying to record ABR using the Biosemi ABR cord. The directions that came with the electrodes advs placement as follows: LA on left mastoid, Ref on forehead, and RA on right mastoid, along with CMS/DRL also on the forehead. Our goal is to use a vertical montage with either linked mastoid or link earlobe reference, so I put LA and RA on the left and right earlobes and Ref at CZ. The issue is that Actiview auto saves the feed with LA and RA referenced to Ref (instead of giving an option of referencing to CMS for example), and doesn't give an option to re-reference the data (we were trying to try to manually do this is BESA). Can you please give advice as to how we can accomplish the vertical montage set up? We are trying to use the electrode placement that is most commonly used in ABR research.

Alternatively, I see that speed mode 7 has the same sampling rate as the ABR mode (speed 9). Could I just use flat type electrodes and record data this way? Or are the flat type electrodes not fit for this type of recording?

The ABR cord plus special ABR version of ActiView does indeed generate (referenced) output voltages LA-Ref and RA-Ref. Linked mastoid reference is (LA+RA)/2. You can re-reference against the linked mastoid reference by subtracting the average of the two output voltages from each output voltage:

LA versus Linked Mastoid: (LA-Ref) - {((LA-Ref)+(RA-Ref))/2)} = LA - {(LA+RA)/2}
RA versus Linked Mastoid: (RA-Ref) - {((LA-Ref)+(RA-Ref))/2)} = RA - {(LA+RA)/2}

Note that both the above resulting signals are half the potential difference between RA and LA ({(LA-RA)/2}, one is the inverse of the other). So, when the ABR potentials on both mastoids are the same, the resultant voltages are zero, and when you want to see the difference between the left and right ABR signals, you are better of by just calculating {(LA-Ref)-(RA-Ref)} = (LA-RA).

It is possible to measure ABRs with the standard flat electrodes (speedmode 7 and standard ActiView). However, there will be increased noise and decreased CMRR (see viewtopic.php?f=1&t=845&p=2569) and it will change nothing with respect to the reference issue outlined above.

Best regards, Coen (BioSemi)

Hello,

Thank you for your reply. If I understand correctly, that won't give us what we would like. We would like the electrode at CZ to be the "active" electrode, which is referenced to linked mastoids or earlobes. If I switched the electrodes to have LA at CZ, RA at the right earlobe and Ref at the left earlobe, could I simply subtract RA from LA (i.e., [LA-Ref]-[RA-Ref])?

Put LA and RA on left and right mastoids and put Ref on the Cz position. To obtain Cz versus Linked Mastoids ((LA+RA)/2), add the two output signals and divide by -2:

{(LA-Ref)+(RA-Ref)}/-2 = Ref - {(LA+RA)/2}

Best regards, Coen (BioSemi)

Thank you for your assistance!