Local Connectivity
We’ve seen that low frequencies are not produced by cortical neurons but rather by rhythm generators in the area below the cortex. As a result, neurons over large regional or even global areas of the brain can resonate to these frequencies. As they do so, they tend to operate jointly in a manner we can describe with the concepts of phase, coherence and synchrony.
Neurons which are synchronized to one of these generators use very little energy and tend to be able to pass information smoothly. When levels of slow-wave synchrony are low, this may indicate damage to the neurons or their connections, but it can also indicate the presence of unnecessary faster rhythms in the cortex.
Local frequencies
When a pool of neurons is activated to perform a task, it begins producing its own energy and de-synchronizes from subcortical generators. This activity, which we call Beta, is produced by neurons firing 13-21 times per second (13-21 Hz). As we mentioned in the previous section, when 12-15 Hz., the lowest of these frequencies, appears in the sensorimotor cortex, it is called Sensory-Motor Rhythm (SMR) and is produced by a generator in the thalamus. Outside of that area, however, this frequency is known as Low Beta or Beta One. We will call frequencies from 13-21 Hz “functional Beta”. They are used for processing tasks.
One important characteristic of beta frequencies is that they tend to be local and relatively transitory. Ideally when a pool of neurons has a task to perform it activates, complete the task, passes on information and returns to a synchronized resting state.
Phase and coherence
Because of the speed of beta frequencies, they tend not to operate in phase. Because they appear and operate in relatively small areas at any given point in time, they tend not to be coherent. As we mentioned previously, when two sites are working together or passing information in beta frequencies there are temporary coherence relationships. Some trainers specifically work to improve these linkages between particular sites, training at task. This has been reported to be quite effective in working with some learning disabilities. However, in most cases high levels of coherence in beta frequencies is not a desirable finding.
Coherent beta
Tension or movement in muscles near the electrodes can produce fast electrical activity which may be picked up and presented in the EEG. Obviously this is not brain activity. It is called artifact, and we will focus a good deal on eliminating it in later sections. When high levels of Beta coherence appear, one of the first things we need to rule out is the presence of such muscle artifact. Because this artifact signal seen in various areas is all coming from the same source—a muscle—it appears highly coherent.
Another example of highly coherent Beta is a seizure. Fast wave activity beginning at some site in some brains can begin to spread from that center in a process called kindling. Since this Beta is not related to task performance but is simply the result of excessive excitation in the neurons, it tends to be very coherent. The effect is to lock up and shut down an area of the brain. A seizure can result.
Effects of coherent beta
There are two apparently opposite outcomes that can result from highly coherent beta. One is extreme sensitivity. For example, a migraine headache, combined with extreme light sensitivity, can produce or result from highly coherent beta in the occipital lobe at the back of the head where visual signals are processed.
Another is the locking together of pools of neurons so as to block their function.
It is not uncommon in the brains of people who are autistic to see coherent fast-wave activity in the sensory processing areas at the rear of their brains. The result of this can be an extreme sensitivity to sound, touch or visual stimuli.
However, by locking these sites together with coherent beta, the brain can also keep them from functioning, so they literally block out information they are expected to process. By locking together these sensitive neurons, the brain loses its ability to have them process independently, and their function can suffer. On the other hand, by locking them together it can make them a super-responder producing excessive sensitivity.
High fast-wave coherence in the front of the brain often appears as rigidity of thought, getting stuck on ideas, obsessive thinking or compulsive behavior. These are consistent with the blocking of function. However, the result can also be anxiety, consistent with excessive reactivity.
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