Coherence

Coherence is a measure of how effectively two sites are able to link and unlink, to share information. Whether coherence is a good thing or a bad thing depends to some extent on what frequency we are talking about and what sites and what task. Slower frequencies, alpha and lower, are generally produced by rhythm generators in the middle of the brain, below the cortex. When the cortex, the thinking part of the brain, is working, we expect to see its neurons “de-synchronize” from those rhythm generators, just as you might turn off music you were listening to when you had to focus on a task. Faster frequencies, the beta frequencies, are produced in the cortex, generally in local pools of neurons which work together to perform specific functions.

Coherence is not measured in Hertz or microvolts. It is a measure, like a correlation coefficient, which ranges from 0-1 or (in percentage terms) from 0-100. When we look at coherence in alpha or theta in a brain, especially during the eyes closed or eyes-open baseline conditions–not at task–we expect to see it run between about 40-70. All the non-working neurons who are listening to the specific “radio station” (frequency generator) should all be dancing to the same rhythm. When this coherence does not appear, it suggests that the neurons in the cortex are having difficulty letting go of their working state and shifting into a lower-energy resting-ready state. People who are anxious or stressed or depressed may show these low slow coherence levels. When the coherence stays locked in during a task, it suggests that the neurons are not able to shift into productive states.

Coherence is directly related to phase relationships (phase is a measure of the timing delay between what is happening in a particular frequency between the two sites). The further apart the two sites are, the harder it is to see high coherence. We rarely see coherences high between T3 and T4. Ditto F7 and F8 or T5 and T6. Fp1 and Fp2, on the other hand, are so close together that timing relationships tend to be pretty strong and coherences are often high there; same with Cz and Pz.

There are five-six frequencies produced by sub-cortical rhythm generator nuclei in the thalamus, the hippocampus and perhaps the brain stem. These frequencies are described as Global (Delta and Theta), Regional (Alpha and SMR) or Local (beta). Beta is produced by cortical neurons when they are performing a task. When an area of the cortex is resting, it can resonate to one of the sub-cortical rhythms. Since all sites are resonating to the same source, we would expect them to be coherent (consistently related) and in phase (firing at about the same time)–synchronous.

Fast-Wave Coherence

Fast beta frequencies come from the neurons in the cortex. There is no reason for them to be generally coherent, is the case with slower frequencies. Fast wave coherence, especially with eyes closed or open in baseline conditions, should generally be below 40. In a productive, functional brain, working neurons in a given pool link up quickly with others to share information or perform a function, then quickly unlink and perhaps link with others. At task we should see shifting patterns of connections.

When fast wave coherence is high consistently, it suggests that the neuron pools are locked together. High fast wave coherences can relate to brains which are less productive, sometimes rigid, locked-in, so high frontal fast wave coherence would suggest a person who easily got stuck.

Fast frequencies–beta and above–are energy intensive but they only happen in specific areas at a time, pass on the job and rest. They would be expected to be very independent. A high level of coherence in these frequencies–especially in non-task recordings–would suggest that the brain is overly activated, not able to rest. It’s like a basketball team where all five guys try to dribble the ball down the court at the same time.

High coherence in fast frequencies sometimes appears as extreme sensitivity–sometimes as a wall. Fast wave coherence in the occipitals can be related to light sensitivity. Locking the neurons together can either dramatically increase the effect of a stimulus; or it can block the neurons from doing their processing job.

Slow-Wave Coherence

Coherence in slower frequencies like alpha is an indication of how effectively the neurons are going into neutral and (in alpha) remaining in a ready state, present but not processing. Low coherence suggests either that there are differential timing loops between sets of neurons and the rhythm generators in the thalamus or, more likely, that the neurons on the surface of the cortex aren’t really letting go very well. They keep trying to burst up into beta.

Slow wave coherence can be thought of as a measure of the degree to which the areas being measured are capable of “letting go” of beta. In an anxious brain, the brain is constantly bursting into beta–even when there is no work to do–so the neuron pools don’t do a very good job of linking up to the sub-cortical generators. P3 does, but at that moment P4 produces a burst of beta; then P4 synchs up but P3 bursts into beta. The alpha coherence between P3 and P4 remains very low. Those slow-wave coherences are often related to very low-energy resting/ready states, very efficient, like a first-rate athlete coasting between bursts of activity toward the goal. So brains that can’t do that state very well tend to be tired, incapable of resting, always running around in circles and wasting energy.

Slow frequencies should be coherent/synchronous.

Low coherence in slow frequencies suggests an irritated, excited brain, one which does not communicate well within itself, wastes a lot of energy, an engine that doesn’t know how to idle. Problems with low slow-wave coherence can be anxiety, inability to be still and relaxed, difficulty performing sensory integration tasks (in the back of the head) and perhaps some sleep issues.

I really haven’t worried much about coherence values too HIGH in slow frequencies. I have seen it from time to time in delta or alpha, but I’ve never focused training on it.

Coherence at Locations

Coherence is a measure of the brain’s ability to communicate through a clean, accurate channel between two sites. When coherences in slow frequencies between sites like F3 and F4, C3 and C4, P3 and P4, O1 and O2, which connect at the midline are low, that suggests that something is interfering with the connections. That something may be excessive excitement in the neurons (they won’t stop producing bursts of beta, even when there is no task requiring beta) or it may be a physical disruption caused by a head injury or some sort of lesion. In either case, it’s likely that performance will be affected.

Sites like F7 and F8, T3 and T4, T5 and T6 don’t share a boundary at the midline. They are very far apart (in brain terms), and they have to send their communications either through the corpus callosum or the anterior commissure, so the delay is greater. They tend to have very low coherences, especially the temporal sites. However, coherence of zero at any site is definitely an error.

Alpha Coherence

If you understand alpha coherence as a measure of neurons’ ability to “let go and dance” to the rhythm of the thalamus, which is a restful and “present” state, then low coherence can be considered as an indicator that the neurons are a bit up-tight and can’t let go very well. The client may also demonstrate some of this type of behavior as well. If alpha/theta ratios are low (below 1.0 in the front of the head or 1.5 in the back with eyes closed) and coherence is down around .4 (or 40 depending on how it is being reported) or below, then the client may be depressed or anxious, almost always stressed, probably tired, perhaps struggling with staying connected to the environment.

You can train alpha coherence wherever you wish, though I would usually do so behind the central strip. Because it is a relationship between two sites, it must be done two-channel, and it is best done (in my opinion) with eyes closed and using monopolar (ear-referenced) placements. P3/A1 and P4/A2 or O1/A1 and O2/A2 are very good sites to train.

Some people find alpha coherence to be tremendously positive and restful and calming; others don’t get much from it and have a hard time doing it. Try it and see.

Beta Coherence

Remember that high coherence means that the neuron pools are either locked together or are linked to a rhythm generator. There isn’t a rhythm generator for beta, so high levels of coherence suggest that the neurons are locked into a communication loop. With eyes closed, what are they communicating about? Beta coherence, then, should generally be a relatively sporadic, short-lived phenomenon that occurs when two pools of neurons are sharing information or working on the same task. When it appears at high levels, or during eyes-closed or eyes-open baseline tasks, the implication is that those neuron pools are no longer operating independently.

In the back of the head, where we process and integrate sensory information, locked up neurons tend to result in higher levels of sensitivity of sensory inputs, reduced ability to differentiate and deal with multiple types of information. It would result in a very simplified and very LOUD view of the world.

In the front of the head, where the executive centers are supposed to be handling inputs from many different sources and making complex decisions–or planning and producing motor outputs–the result can be a kind of rigidity of thought or getting easily overwhelmed by complexity in processing requirement.

Locking

Locking is excessive fast wave coherence. It is a measure of the brain’s ability to communicate between pools of neurons in the cortex. This ability to connect should be relaxed most of the time in alpha, theta and delta frequencies, independent and flexible in beta states and free of every high “background noise” (high-beta).

The ability to connect in resting states appears in coherences in the range of 40 and 80. Slow frequencies come from single generators in the sub-cortex. All signals in these lower frequencies come from the same place at the same time. If they appear at two different sites at nearly the same time, the sites are well-connected and clear. Low levels of slow-wave coherence suggest an excitable brain, bursting into beta at different locations at rest. Energy is wasted and relaxation can be difficult.

Faster beta signals show areas of the cortex that are excited. For an efficient brain, levels of excitation during waking states should be in the alpha/low beta range with bursts of beta in areas where work is being done. Channels of communication should be open and flexible. Coherences in beta speeds when not performing a task should be below 40. If these are high, communication between the sites has a lot of background noise. There may be constant thinking, anxiety, getting stuck. The effects may differ at different sites. When coherence measures are high, that suggests that either the two sites were communicating with one another or that both were communicating with a third site.

Slow frequency coherences are expected to be fairly high since slow frequencies generally come from a single source, a sub-cortical rhythm generator. Fast alpha pretty much anywhere in the brain comes from a set of nuclei in the thalamus which produce the alpha rhythm and broadcast it up to the cortex. So we expect it to be coherent.

Fast frequencies are produced in the cortex itself when a pool of neurons is working. There isn’t really a “generator” for fast frequency activity, as there are for slower frequencies, so we would expect coherences in those frequencies to be lower unless we happened to catch the cortex performing a task that involved the two sites communicating with one another.

We always have to remember Pete’s First Rule of Neurofeedback. When we see something unexpected or unusual in the EEG, we want to verify first that it wasn’t caused by the client, the electrodes, the software or some other extraneous source. We know that muscle activity tends to cause all frequencies to surge in amplitude at the same time. Hence the activity we might see (in slow, medium or fast frequencies) when one, say, grits his teeth, does in fact all come from the same place. It’s just not coming from brain cells; it’s coming from EMG. And since it’s coming from the same place, we often see coherences very high in all frequencies. So a client who was, for example, tightening muscles in his face or forehead to try to keep from blinking could end up producing high coherences in all frequencies that have nothing whatsoever to do with the brain.

High Coherences in All Locations and Frequencies

If you see this, my first guess would be that there was some muscle tension or electromagnetic artifact, either of which can cause coherence values to rise artificially (since the signal is coming from the same place–the muscle).

This would be so rare in real life–and so easily caused by tension or noise, that, seeing it while you are gathering the data, you should try everything to figure out what is causing it.

Coherence at Fp1 and Fp2

Delta and high coherence at Fp1 and Fp2 don’t necessarily mean much. Chances of eye artifact on the forehead, which appears as delta, are very high. Fp1 and Fp2 are very close together, so coherences are often high, especially in slow frequencies when the signal is affected by artifact. Train the prefrontal with HEG.