The use of connectivity in the brain adds an interesting dimension to neurofeedback. Training based on amplitude or power addresses cortical activation or relaxation. Any changes that result in increased/decreased communication in the brain are secondary to activation/inhibition. In contrast, connectivity based NF provides the direct ability to alter how brain regions connect and brain interactivity. There are numerous connectivity measures, however one of the most common measures is coherence.
Coherence reflects the stability of the phase relationship between signals and is independent of the actual amplitudes of the signals. That is, it measures how stable the phase relationship is over time, which is a measure of the amount of information sharing between the two sites selected to be measured. If the two sites have a stable phase relationship, they will have high coherence. It is important to consider that coherence is neither good nor bad in of itself, too much and too little can be equally bad. In the brain there are centres that need to be in communication, and likewise, there are centres that usually have little to no communication. What is important is the clinical relevance of the connectivity. High coherence may not always indicate that two sites are well connected, as another site may be serving as a source of information (and have high coherence) to both the other sites (making them look like they have connectivity even though it is actually through third party. Coherence also does not indicate the direction of information flow.