Global and Regional Connectivity
Let’s begin connecting some dots from material we previously discussed.
Source of signals
When we spoke about frequencies earlier, we said that slow frequencies like Delta, Theta and Alpha are not produced by neurons in the cortex. They are frequencies we see when cortical neurons are not working. These lower frequencies can be thought of as transmissions which are broadcast throughout the brain from structures in its center. When neurons in the cortex are resting, they may tune in to any of these frequencies and begin to resonate to them.
Theta frequencies are produced by two separate sets of nuclei, which operate like the rhythm generators in a drum machine. Slow Theta (4-6 Hz) is produced by generators in the thalamus. Fast theta (6-8 Hz) is produced in the hippocampus. So-called hippocampal Theta comes from the brain’s Memory Center and is seen in specific areas of the brain when information is being placed into or accessed from memory. The slow and fast Alpha frequencies—from 8-10 Hz and 10-12 Hz—are produced by separate rhythm generators in the thalamus.
Sensorimotor rhythm (SMR 12-15 Hz) is found, as we discussed earlier, only in the brain’s sensory motor cortex. It is also a sub-cortically-generated rhythm, and it comes from its own set of rhythm generators in the thalamus.
When the brain’s lowest frequencies—Delta and Theta—are seen in the cortex, they generally appear globally. They represent very low levels of cortical activation. As we will see later, it is possible to find specific areas in the cortex where Delta remains dominant with eyes closed, eyes open or at task. This is an indicator of a white-matter brain injury, a tearing of the communication cables. In most cases though, global Delta indicates a brain which is unconscious and global Theta is seen when we are in a Twilight state, either sliding into or coming out of sleep.
The Alpha frequencies generally appear regionally. They appear especially in the back of the brain, though they can also appear globally in states like meditation.
We mentioned in the article on synchrony that there were two conditions likely to result in coherent or synchronous activity: The first was the relatively transitory situation where two sites are working together on the same task and sharing information. The more common condition is when both sites being measured are receiving signals from a common source. Clearly, since Delta, Theta and Alpha are produced by a single generator, about equidistant from most sites on the cortex, if neurons are tuned to that same generator, we would expect the signal to be synchronous.
What synchronicity tells us
When slow frequency activity is highly synchronous, it suggests that the neurons are generally able to rest between tasks without wasting energy. It also suggests that they are open to sharing information efficiently. When synchronous activity is low, this suggests the brain is excitable, with a tendency to produce sudden bursts of beta without external stimulation. Each such burst breaks up the synchronous connections and wastes energy.
It is also possible that disruptions in phase angle are related to some form of damage to pools of neurons which disrupts the smooth flow of the generated rhythm so a signal arrives later or earlier than to other areas. Either of these disturbs communication as static or noise breaks up a radio broadcast.
The effects of low coherence often include fatigue—especially as the client gets older—and limited ability to perform standard functions. For example, low levels of coherence in the rear of the head—the sensory processing areas—often correlates with limitations in processing visual or auditory stimuli, sensations of touch or awareness of where one is in space (getting lost easily, etc.) Learning difficulties are often related to disturbed connectivity in sensory areas.
In the front of the brain which handles motor functions, physical coordination may be disturbed (e.g. clumsiness).
Expected low coherence
Areas across the top of the head which share a common frontier (e.g. the frontal or parietal lobes) are more directly linked than those on the sides (e.g. temporal lobes), so their ability to share a signal seamlessly tends to be greater. Coherence between separated temporal sites are not expected to be in the same range for that reason.