more on coherence

[Guest guest]

Deborah~

Another old post.

~Last question first. Coherence is a measure of the relationship between two signals that is supposed to be independent of amplitude or phase. In a sense it is a measure of how well you could predict the signal at one point if you knew it at the other. It ranges from zero (you'd have no chance of accurately predicting) to one--or one-hundred, depending on whether the data are presented as a decimal or a percent--(you'd have a hundred percent chance of accurately predicting).If two people are jumping up and down, one jumping as high as possible and the other barely leaving the ground, and both are jumping once every second, although the height of their jumps would be quite different, once you knew the relationship between their jumping, you could predict when one would jump by knowing when the other was jumping. They'd be coherent. If one reached the peak of his jump when the other was most fully on the ground, their jumping would be 180 degrees out of phase, but you could still predict where one would be in the jump by knowing where the other was. They'd be coherent.If they happened to be jumping at the same rate AND leaving the ground and landing at exactly the same time (they were coherent AND in phase), they would be synchronous. Synchrony is coherence in phase.On to the first question: Although I would generally look for alpha coherence levels between about .4 and .7 (or 40 and 70) with eyes closed, it's not unusual to see them above 70, especially if the client has been meditating. High alpha coherence is not necessarily a problem. What is more important is alpha amplitude (which will tend to be higher, the more coherent the alpha is), and its relationship to theta in the area of the brain where you are looking and the brain's ability to block the amplitude when eyes are opened or the brain is performing a challenge task. Coherence staying up when eyes open or the brain is at task doesn't necessarily mean the alpha is too high. It just means the alpha that IS there is still coming from a common source.So the third most important question for you and your friend is: Does the alpha amplitude drop 30-50% when eyes are opened and stay down or drop further at task? Second most important question is, does the alpha/theta ratio drop to around .7 or below in the front, to below 1.0 in the back of the brain when eyes are opened or at task? And the most important question is, what desired change on the client's list are you relating to the alpha and alpha coherence levels? From my point of view (as you have heard me say ad nauseum), there's nothing wrong with any of the above if it doesn't affect the client's happiness and performance.Very high alpha levels can be related with anxiety, sometimes pain, sometimes an inability to perform processing tasks effectively. If your friend happened to be a B2 bomber pilot of the sort Sterman studied, he would have worked very hard to increase his alpha and alpha coherence levels.Hope this helps.Pete