For example, in the case of a common reference, an increase in coherence can result both from an increase or a decrease in activity at the reference. The former arises when the reference activity is dominant at all electrodes. The latter when reduced interference from the reference signal reveals the true coherence between the active electrodes due to the dominance of the remaining activity and its spread to both channel locations. The third problem is that it is not easy to determine the significance of a particular coherence value as confidence limits are wide. A larger number of epochs may be used to reduce these limits, but at the risk of the signal not being sufficiently stationary for the results to be reliable.
Coherence measures the degree of synchronisation between signals.
With the above reservation, it indicates the spread of fields through underlying tissue. This can be by electrical conduction, as well as through functional connectivity. It says nothing about the residual activity localised at each electrode of the pair used in the coherence measure which may itself have functional significance. However, one also needs to be cautious about partial coherence, as it does not simply decrease interference from neighbouring regions, but could in fact remove the very signal one wishes to study. Partial coherence does not distinguish between signals generated at, or spread to, a given location.