SYMMETRY

Coherence is a measure of the correlation between two EEG locations as a function of frequency and provides information about functional connectivity between cortical regions. It is the mathematical measure of the degree of similarity of the EEG recorded at two locations and ranges from 0 to 1. If the phase—rising and falling—of the two signals are more similar over time, then it suggests functional connectivity--that those two areas of the brain are working together.

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When EEG presents prominent alpha waves (moderate frequency of 8-12 Hz in a  relaxed, calm, not thinking, reflection, resting state for the brain), sensory inputs tend to be minimized and your mind is generally clear of unwanted thoughts. When your brain shifts gears to focus on a specific thought—in either a positive or negative way—alpha waves tend to disappear and higher frequency beta waves begin running the show. 

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Differences in the alpha waves between the left and ride sides of the brain, recorded from an EEG at the F3 and F4 electrode locations, can give information about emotional states. Studies have found large differences in alpha waves between the left-front and right-front of the brain have been associated with anxiety and depression, often found in PTSD.