The correlation coefficient and the goodness of fit in source localization of noise recorded by magnetoencephalography

We recorded magnetic field activity to evaluate the specificity of the correlation coefficient and the goodness of fit in source localization. Magnetic noise was recorded (bandpass 1-200 Hz) with a 37-channel magnetometer with and without a subject present in a magnetically shielded room After averaging of the recorded epochs, the data were subjected to a single equivalent current dipole algorithm. The correlation coefficient and the goodnesses of fit were computed for each dipole fit. The number of high correlations and high goodnesses of fit was determined. In recordings without a subject present the averaged original data never achieved a correlation higher than 0.70 and a goodness of fit higher than 0.85. After more restrictive filtering (bandpass 1-20 Hz) there was a significant increase for both correlation and goodness of fit. In recordings with a subject present (but not receiving external stimulation) the correlation and the goodness of fit for the averaged original data reached 0.96 and 0.97; after filtering, both measures reached values greater than or equal to 0.99. The results of this study indicate that both the correlation coefficient and the goodness of fit lack the specificity to distinguish stimulus-related brain activity from background noise.