The resolution-field concept.

The concept of a resolution field provides a means to compare arbitrary estimators of a brain activity of interest (AOI), represented, for example, by the amplitude of a dipole at a certain location of interest with well-defined and known direction. Like the lead field, it represents a vector field with the property that a measure of the impact of a hypothetical dipole at an arbitrary point in the brain is obtained by calculating the scalar product with the respective dipole moment. While in the case of the lead field this measure of impact quantifies the contribution of a hypothetical dipole to the data recorded in a specific measurement channel, in the case of the resolution field it quantifies the contribution of a hypothetical dipole to the estimate of the AOI. The resolution-field concept, which uses elements of the Backus-Gilbert theory and is closely related to the concept of a resolution matrix, is illustrated with examples based on a simulated measurement with a 148-channel magnetometer system.