When specific neurons are activated or de-activated, there is an external electrical current produced. These neurons are sources of current. And simply put, the more activity there is in a given region, the higher the current-source density.

What is important in this regard is that the cortical layers are complex, and allow one region to do more than one thing — either in sequence, or at once. This is the basis of the EEG frequencies seen in different bands — delta, theta, alpha, beta, or gamma. By knowing which are the dominant bands in a given region, we know what the region is doing. This is especially important for understanding networks, which bind at a characteristic frequency, such as alpha for the default mode network (DMN).

The dominant activity in a region comes in the form of relative power. This basic metric — used for decades in QEEG — takes on particular importance in LORETA-based methods. Because the volume conduction smears and combines information from many regions into one scalp electrode location, we expect a mixture of frequencies. But with LORETA methods, we are calculating what is happening in one spot in the brain. Any given area of the cortex has limited freedom to do different things at once, and thus will tend to be busy with one thing a time, as the brain goes through its microstates and processing sequences. Here we see an example connection between two areas of the cortex, one sensory, the other motor. Harnessing complex interconnections, these regions communicate with each other using various pathways and frequencies which range from slow delta, all the way through fast gamma, and everything in between. But what is important to know is what any region is doing at the moment.