EEG and Brain Connectivity: A Tutorial - Bio-Medical Instruments, Inc.

Figure 15 – The x-axis are different ranges of coherence (x100). The y-axis is the
standard deviation of coherence (blue circles) and phase angles (pink squares). The
dashed vertical line shows the level of coherence (20% or 0.2) when the variance of the
phase angle becomes very high. High variance of the phase angle means that there is
minimal or no phase locking.
Figure 15 (from Thatcher et al, 2004) shows that EEG coherence
linearly decreases as a function of the signal-to-noise ratio. It can be seen
that phase angles even with 248 degrees of freedom are instable and poorly
estimated as coherence decreases. EEG coherence at 0.2 or less is used as a
cut-off for accepting phase as a valid and stable linear measure. The
instability of a non-linear system may be present because the mean phase
angle = 30 degrees when coherence is less than 0.2, see Figure 14.
The test signals were computed using the NeuroGuide signal
generation program and by systematically increasing the amount of white
“noise” added to one of the channels used to compute coherence and phase
angle. In general, as the value of coherence decreases below approximately
0.2 or 20% (i.e., coherence x100) then phase angles are extremely variable
and unstable even using 248 degrees of freedom.