In a real brain, the brain activity produced by the measured EEG is not generated by a single source, but there are several active simultaneously. These sources are not necessarily of the same magnitude, so in this case the SL is explored when there are several brain sources active simultaneously by evaluating the chosen SLMs in this conditions.
For this simulation the number of active local main sources were four placed in different BAs. The global maximum source was located at a random position in the left hemisphere of the BA 6, just like in previous cases, and had a magnitude of 100 mA. The source with the next strongest magnitude was located at a random position in the left hemisphere of the BA 7 and had a magnitude of 80 mA. The next source was located at a random position in the right hemisphere of the BA 4 with a magnitude of 60 mA. The last source was located at a random position in the right hemisphere of BA 30. The first three areas have a biological significance since they are involved in motor tasks. Areas 4 and 6 are the primary and secondary motor cortices, respectively. The BA 7 is in charge of the visuo-motor coordination. Area 30 was chosen because it is located in the bottom back and center of the brain, while the other three areas are in the top and closer to the front of the brain [1] [4] [6]. Each individual source was modelled independently and their contributions were added to form the simulated activity of four sources in the brain. The simulated dipole distribution can be seen in fig. 3.11 alongside its EEG. By observing the EEG it is easy to see it is very similar to the previous case with only one source. However, the shape seems to be more irregular. The brain model has less marked the location of each focus of activity, but its location is still remarkable.
Figure 3.11: Case four real activity. The global maximum dipole was chosen to be at a random position in the left hemisphere of the BA 6 (left). Three local maximum dipoles were located at random positions of the right hemisphere of BA 4, left hemisphere of BA 7, and right hemisphere of BA 30. The EEG produced by the given dipole configuration (right).
The SL results can be seen in fig. 3.12, being the one on the left by MLE, the middle one by MNE and the one on the right by wMNE. By visually comparing these results with the original simulation in fig. 3.11 it can be noted that the estimated activity has less intensity than the simulated activity. It is also notable that local spotlights are easier to appreciate.
The EEGs shown in fig. A.11, in the Appendix, are so similar that is difficult to notice any difference. The shape and array of the estimated brain activity appears to be very similar to the original brain activity.
The qualitative metrics are displayed in table 3.3. In this case, by observing the first metric it is noted that the wMNE estimated the main source to be in BA 6, but in the other side of the brain, while MLE and MNE estimated the main source in the left hemisphere, but in BA 7. The position metric shows the brain grid position of the estimated global main source. With this metric, by observing the x coordinates, it could be guessed that the farthest source is the estimated with wMNE. The last two metrics show that the estimated main source is diminished with respect to the original, but it is greater than in the previous case. The
Figure 3.12: Case four: EEG-SL using MLE (left), MNE (middle) and wMNE (right).
information of the other three sources is shown in table A.3, in the Appendix.
Qualitative metrics
Real MLE MNE wMNE
Hemisphere and BA:
6 left
7 left 7 left 6 right
Grid position (mm):
[-50 0 50]
[-35 -70 50] [-35 -70 50] [65 -5 30]
Dipole momentum:
[59.11 59.11 59.11]
[23.1 21.24 5.5] [23.1 21.24 5.5] [16.21 6.5 17.35]
Dipole magnitude:
102.38
31.86 31.86 24.6171
Table 3.3: Case four: Qualitative metrics.
The quantitative metrics are displayed in table 3.4. This time, the distance error is a lot higher than in the previous case, with wMNE having the greater distance of all three methods.
The second and third metrics show errors higher than in the last case, but by observing the next two metrics, the errors for intensity and magnitude of all dipoles in the brain, acceptable errors appear. This means that, given that the three methods are distributed, they work better when there is brain activity in all of the brain, and with more than one source in the brain, the brain activity is more distributed and active, which favors the SL. The last metric shows the goodness of fit being nearly zero, which means the estimated EEG and simulated EEG are almost the same.
Quantitative metrics
Metric MLE MNE wMNE
(rmax)[mm] 71.6 71.59 116.83
(drmax)[mA] 103.5 103.5 111.95
(||drmax||)[%] 68.9 68.9 75.97
(dI)[mA] -8.42 -8.42 -14.98
(||d||)[%] 5.46 5.46 6.41
G[mV ] 1.23e-9 0.025 0.005
Table 3.4: Case four: Quantitative metrics.
The distance between the estimated global max dipoles and the other three local maxi-mums is, for MLE, 118.3 mm to the max dipole in BA 4, 10 mm to the max dipole in BA 7, and 73. mm to the max dipole in BA 30. For MNE the distance between the local maximums and the estimated global max is 118.3 mm to the one in BA 4, 10 mm to the one in BA 7, and 73.7 to the one in BA 30. And for wMNE the distances are 5 mm to the one in BA 4, 115.9 mm to the one in BA 7, and 71.8 mm to the one in BA 30. The second greatest con-tributor to the EEG was the dipole located in BA 7, which was the one that MLE and MNE got closer to. The wMNE got closer to the local max dipole located in BA 4.