Synchrony

Synchrony indicates that two waveforms are coherent (consistent relationship between their peaks and valleys) and in phase (peak and valleys happening at the same time in both waveforms). If that is true when you add the waves together, the peaks are added to the peaks, giving the largest possible positive number, and the valleys are added to the valleys, giving the largest possible negative number. Thus the sum of the two, when we look at the absolute values, removing the plus and minus signs, will be the largest possible values for those two waveforms.

When you subtract the peaks from the peaks, the difference will be the smallest possible number or negative number given the two waveforms, so the absolute value will be smallest.

Since synchrony is distance related, the further apart two sites are, the harder it is for them to respond at the same time. Sites like T3/T4, T5/T6 and F7/F8 are not generally as synchronous as F3/F4, C3/C4, P3/P4, etc. O1/O2 are very close together and pretty easy to get synchronous.

Why would two signals be synchronous?

1. They are coming from two sites communicating with one another in a particular frequency; or

2. Both are coming from the same source.

The reason that most synchrony training is done in slower frequencies (alpha and lower) is that these frequencies are not produced cortically, but are the result of surface neurons synchronizing with sub-cortical rhythm generators like the thalamus or hippocampus. Artifact could be defined as signals that appear in the EEG which do not come from the brain. Hence, if there is artifact, it is likely to appear in both channels and come from the same source in both channels (e.g. an eyeroll, a muscle bracing). That would make it likely to be coherent or even synchronous.

The phase angle, for a synchronous signal, should be close to zero. The TQ8 (after looking at several options) shows the percent of time that the signal was between +/- 30 degrees–very close to zero. The combination of the phase data and the coherence values give us an idea of the level of synchrony between two sites in a frequency.

A Fully Synchronous Brain?

It is unlikely that we will find the entire EEG in all frequencies to be in phase and coherent. It’s tough enough to achieve that with a single band in a naturally synchronous frequency like alpha, which forms nice neat sine waves when rocking along at 10 Hz or thereabouts. But to find two waveforms where ALL the frequencies are synchronous at the same time would be very unusual. So if you take the whole raw EEG and add it together–even if there is synchronous alpha in it, you’ll be adding beta peaks to beta troughs, theta peaks to theta crossing points, delta troughs to delta peaks, etc.–thus you will obscure the phase relationship in alpha.

About Alpha Synchrony

Alpha synchrony relates to the brain’s ability to keep the channels clear and open when the neurons are not active. It’s not just about raising alpha amplitude.

The key is to understand that alpha shouldn’t be dominant in front, but that doesn’t mean that the alpha that exists there shouldn’t be synchronous. Still, in most cases, training it should be done in the parietals/occipitals. You can train in the central strip as well.

It is often true that when someone is producing synchronous alpha, most other frequencies go away, but certainly not always when one is learning to do it will that happen. If you want to pick out and reward those moments when synchrony is starting to happen or happening briefly, work with the alpha band, not the whole EEG.