Below are some concepts in understanding montages. For more information in understanding how to ready the montages in your Whole-Brain Training plan, please see our videos.

Just to repeat one of the conventions we use, when a montage is listed, it should be listed as Active/Reference.

If we follow that convention, then if someone writes C4/P4, it means that C4 is active and P4 is reference.

A two-channel montage would be C4/A2/g/P4/A2.

References

There are three kinds of 2-channel (or multi-channel) references:

Independent references, where each active electrode is measured in relation to its own reference. A classic example of this is C3/A1/g/C4/A2. C3 links to A1 and C4 links to A2.

Common references are where both actives link to the SAME reference. For example, C3/A1/g/P3/A1. Both sites are using A1 as their reference. By using a jumper between the reference plugs in the amplifier, you can plug a single electrode into either end of the jumper, and it will serve for both channels.

Linked references, where the two references are linked within the software (BT2) or hardware (Q-wiz) or plugged into the two inputs on a jumper (GP8e), so they are averaged to give the same signal for both references.

There’s a difference between common and linked references. If you are doing, for example, F3/A1, you are actually measuring from the cortex between the two sites – more specifically, primarily the neurons that are parallel to the line between the two electrodes. So if you use a common reference in the assessment, say at A1, then you are measuring from C3 to A1 and from C4 to A1. Generally the greater the distance between the two sites being measured, the greater the amplitude will be (and the lower the coherence). So a common reference may give you an inaccurate comparison of C3 and C4. If you use linked ears, comparing C3 to A1 and C4 to A2, the distances are about the same and you are measuring the same are on the opposite sides of the head.

In assessments and in coherence training and in symmetry training, linking is critical. Most two-channel protocols, though, don’t need linked ears. For example the T3/A1/g/T4/A2 montage should have linked ears when assessing, but you don’t need them (though you could link if you wished) for training.

The reason the linked ears are important is that they remove any effect of differential activation patterns in the temporal lobes. Since the earlobe or mastoid references, which are largely but not completely inert, do pick up fields from the temporals, if the temporal lobes are quite different, that difference will appear throughout the EEG.