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

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Fig – 14. Top is coherence (y-axis) vs signal-to-noise ratio (x-axis). Bottom is phase angle on the y-axis and signal-to-noise ratio on the x-axis. Phase locking is minimal or absent when coherence is less than approximately 0.2 or 20%. Figure 14 (from Thatcher et al, 2004) shows increased variability of EEG phase angle or difference as noise is systematically added to the 30 degree shifted sine wave. Note that non-linear dynamical processes are suggested by the fact that the mean = 30 degrees when coherence < 0.2. Chaotic dynamics and reproducible correlations are often embedded in similar time data.
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.
Page 1 and 2: (unpublished manuscript - Version 1
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Gray, C.M., Konig, P., Engle, A.K.
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