Source localization accuracy and precision

Comparison of V1 activation between fMRI and MEG at the onset of M50 (42.4 ms). The results are shown in the paracalcarine slice best covering V1 for each subjects. Lower row shows the corresponding magnified views. Blue outlines mark indicates the boundary of statistically significant fMRI activations (P < 0.01) and white dashed circles mark the estimate for the targeted area. For MEG yellow and red outlines mark the P < 0.05 and P < 0.001 contours, respectively, and filled red contours denote regions with statistically significant activations within 50% of the peak t value.

Responses in areas V1, V4, V5/MT+, vPOS and dPOS. ROI centres identified in the left hemisphere in response to right hemifield stimulations. The solid gray line on each axial MRI slice together with the X Talairach coordinate in the lower right corner for each area indicate the location of the corresponding sagittal slice. White arrows point to the parieto-occipital sulcus (POS), and calcarine sulcus (CS). Insert in the upper left corner shows the schematic image of the stimuli. Coloured symbols are depicted to indicate the correspondence between the stimulated locations, and identified ROIs; they were not presented to subjects. Triangles and diamonds indicate lower and upper; and cyan and yellow, peripheral and parafoveal visual field stimulations, respectively. The green lines on the sagittal slice showing the localization results for V1 indicate the V1/V2 borders (representation of vertical meridian), which were obtained in a separate fMRI experiment. ROI timecourses for three days (red - Day 1, blue - Day 2, green - Day 3) are superimposed. The black, filled circle in the schematic image of stimuli displayed on the upper left part of each figure indicates the stimulated location of the visual field for the timecourses displayed in that figure. POS, parieto-occipital sulcus; vPOS, ventral POS; dPOS, dorsal POS.

Simulated data.Source dipoles (light blue arrows) together with their MFT/SPM estimates (yellow contours). Sagittal slices of MR images where the dipoles were placed are shown. Yellow contours encompass the regions with a statistical significance of P < 0.000001. Names of the areas are given on top of each MRI. The distance between the dipoles location and the centroid of its estimate is indicated next to each dipole. Latencies of the estimated activations are shown at the bottom of each MRI. POS, parieto-occipital sulcus; vPOS, ventral POS; dPOS, dorsal POS.

Phantom data. (A) The spheroidal phantom placed in the MEG helmet. The total phantom height is 372.8 mm, and the plastic wall thickness 5 mm. It has a plastic spheroid globe of an elliptical vertical cross section with inner dimensions of 130 X 185.72 mm. The smooth inner surface of the phantom is interrupted by two plastic constructions that mimicked the effect of eye sockets in a real head. The inset shows a schematic representation of the inserted physical dipoles. (B) The four dipole locations over-plotted on the phantom's MRI scans. Each location corresponds roughly to one structure of the brain (the supplementary motor area (SMA), the putamen, the hippocampus, and the lateral geniculate nucleus (LGN)) representing the full range of difficulty in localizing generators in the brain. The distance of the dipole from the centre of the spheroid, d, is printed in cm for each case. (C) The localization error (LE in mm) versus the dipole current (in mF) using the multiple local spheres (MLS) conductivity model. Dashed lines show values with no significant t-values for SAM. For MFT, dashed lines represent LE of MFT derived from the average signal (used when MFT analysis of single trial data produced no significant t-values).

Statistical parametric maps (SPMs) showing common activations in the brainstem for the 100 ms period just before saccades. In each case the SPMs were computed for each subject separately by comparing the moduli of the current density in the 100 ms leading to the saccade with the corresponding distributions in the far pre-stimulus period (-900 to -600 ms). The results were then transformed to the Talairach space and common activations across subjects identified for each voxel. The results were then backtransformed to the MRI of one of the subjects for display. (A) Shows the SPMs for horizontal eye movements (move left on left and move right on right) and (B) shows the SPMs for vertical eye movement (move up on left and move down on right). The red area corresponds to voxels of common statistically significant increase (P < 0.05) for all three subjects in the first three directions, but for only two subjects in down, possibly because for down, the subject produced fewer saccades satisfying the selection criteria.