Locating accuracy of a current source of neuromagnetic responses: simulation study for a single current dipole in a spherical conductor.

The locating accuracy of a current dipole of neuromagnetic responses in a spherical conductor was examined in terms of the residual field which was a difference between the measured and calculated magnetic fields in a least squares fitting. Characteristics of the residual field as a function of the noise field, dipole depth, and number of measurement points were studied by computer simulations. The confidence region of the dipole location was determined by changing a dipole parameter along a coordinate axis until the sum of squares of the residual field increased by a factor given for each axis. As an example for a dipole at 2.4 cm depth, 95% confidence limits extend 2 mm along the transverse axis, 6 mm along the longitudinal axis, and 5 mm along the radial axis when the signal-to-noise ratio is 2.5 and the measurement points are 42. The method was applied to auditory evoked magnetic responses to examine location differences of the dipoles for different stimuli in the auditory cortex.