CONFIDENCE-INTERVAL OF SINGLE DIPOLE LOCATIONS BASED ON EEG DATA

Noise in EEG and MEG measurements leads to inaccurate localizations of the sources. A confidence volume is used to describe the amount of lo calization error. Previous methods to estimate the confidence volume p roved insufficient. Thus a new procedure was introduced and compared w ith previous ones. As one procedure, Monte Carlo simulations (MCS) wer e performed. The confidence volume was also estimated using two method s with different assumptions about a linear transfer function between source location and the distribution of the potential. One method used variable (LVM) and the other fixed dipole orientations (LFM). Finally , the confidence volume was estimated through a procedure in which the re was no linearization of the transfer function. This procedure scans the confidence volume by varying the dipole location in multiple dire ctions. Confidence volumes were calculated for simulated distributions of the electrical potential and for experimental data including somat osensory evoked responses to stimulation of lower lip, thumb, and litt le finger. Results from simulated data indicated that confidence volum es calculated with the MCS method were largest, and those calculated w ith the LFM method were smallest. For dipole locations close to the br ain surface, the confidence volume was smaller than for a central deep er source. An increase in electrode density resulted in smaller confid ence volumes. When the noise was correlated, only the method using the MCS produced acceptable results. Since the noise in experimental data is highly correlated, only the MCS method would appear to be useful i n estimating the size of the confidence volume of the dipole locations . Thus, using real data with the MCS method, we easily distinguished s eparate and distinct representations of the thumb, little finger, and lower lip in the somatosensory cortex (SI). It was concluded that adeq uate estimation of confidence volumes is useful for localizing neural activity. On a practical level, this information can be used prior to an experiment for determining the conditions necessary to distinguish between different dipole sources, including the required signal to noi se ratio and the minimum electrode density.