Source analysis of AEPs for the assessment of CI users

Figure 9.8 shows two symmetric fixed coherent ECDs (the position of the dipoles is fixed and only the strength of the dipole vary) for different subjects at different time after implantation. Sources were fitted in a window of approximately 10 ms before and after the AEP P1 peak with a BEM head model, a standard 10-20 system was used to project the 19 electrodes position on the scalp surface of the head model. Subject are organized into four group (based on time after implantation); the confidence ellipsoids were not included for a better visualization of the ECDs. The location of the ECDs in the group at one year after implantation was the inferior lobe

than 5 years post-implantation the locations of the ECDs were the superior temporal lobe (including the Brodmann areas 22, 41 and 42); contra-lateral to the CIs.

Figure 9.8 Changes in the ECDs locations for the P1 peak of the AEP from different subjects

at different time post-implantation. ECDs were fitted using a fixed coherence model and superimposed onto cortex segmentation from averaged MRI.

Although the AEPs for each child at less than one year after implantation (between 3 and 9 months after implantation) was recorded, it was not possible the find the ECDs of P1 peak with low residual values and small confidence ellipsoids, in most of the cases; additionally, the anatomical locations of the dipoles were not necessarily in the temporal area.

Figure 9.9 shows the changes in the localization of the P1 peak of the AEP in accordance with the time of implantation for three different subjects. ECDs were fitted using two symmetric fixed coherence dipoles and superimposed onto cortex segmentation from averaged MRI. The ECDs location changed from inferior or

middle temporal gyrus (Brodmann area 20 and 21) to superior temporal gyrus (Brodmann area 42). 1-2 y using a CI >3 y using a CI S1 S3 S5

Figure 9.9 Changes in the location of the P1 peak of the AEP from three subjects in

accordance with the time of use of their CIs (between 1 and 2 year and more than 5 year after implantation). ECDs were fitted using two symmetric fixed coherence dipoles and BEM head model.

Figures 9.10 shows the global changes in the localization of the P1 peak of the AEP in accordance with the time of implantation in subject S3; the ECD location changes from inferior temporal gyrus (Brodmann area 20) to middle temporal gyrus (Brodmann 38) and finally to the superior temporal gyrus (Brodmann area 22).

S3-St1 1y using a CI S3-St2 1y 8m using a CI S3-St3 5y 5m using a CI

Figure 9.11 shows another example of the changes in the location of ECDs in a child after using his implant for more than 5 years. In this case, the position of the dipoles changed from the middle temporal to the superior temporal gyrus at 2 y and 8 m after implantation. At 5 years 1 month after implantation the position of the dipoles remains in the superior temporal gyrus (Brodmann area 41), but closer to the location of normal hearing children (see Figure 9.7)

S5-St1 1y 9m using a CI S5-St2 2y 8m using a CI S5-St3 5y 1m using a CI

Figure 9.11 Changes in the ECDs location in accordance with the time of implantation, for

subject S5. After 2y 8m after implantation, the positions of the fixed coherent dipoles are in the superior temporal gyrus.

In both normal hearing children and children with CIs, the BEM head model gets better localization (as expected because this head model best fit the temporal lobes and the base of the head), but not necessarily smaller ellipsoids according to the number of ECDs, this could be because of the low number of electrodes used in this dataset.

The lowest Res. Dev. value obtained in the de-noised signals is 13.4% for condition C4; even though the Res. Dev. is no lower than 10%, the anatomical location of these dipoles for the de-noised signal were in the temporal lobe (superior, middle and inferior temporal gyrus). One way to increase the accuracy of the source analysis is by increasing the number of electrodes, however increasing the number of electrodes increases the test time and the complexity of the analysis.

9.7 Summary

In this chapter the basic theory of source analysis and the parameters used in a commercial software package to determine the electrical activity in the brain, were included. The differences in the source analysis accuracy of the P1 peak between a

temporal ICA (TDSEP-ICA) algorithm and a statistically based algorithm (FastICA – default ICA algorithm implemented in Curry) used for spatial filtering of EEG from children with CIs and normal hearing children are shown. The results of the ECDs of the P1 peak for both normal hearing children and children with CIs were shown; in general source analysis was simplest after removing the CI artifact using TDSEP-ICA –as expected.

Moreover, the changes of the location of the dipoles in children with CIs, in accordance to the time of use of their implants, are shown at the end of this chapter. In the first period after implantation, the locations of the ECDs are principally in the inferior temporal lobe (Brodmann area 20); between 1 and 2 years after implantation the sources are located at the middle temporal gyrus (Brodmann area 21 and 38). From 3 year and more than 5 year after implantation the position is the superior temporal lobe (Brodmann areas 22, 41 and 42).

The number of electrodes used in these recordings is limited (19 electrodes plus 2 reference electrodes and 1 ground); the electrodes resolution does not permit us to determine in detail the changes in the tonotopy of the auditory cortex at different stimuli frequencies, but this was not the fundamental aim of this research [88]. The purpose is to determine the global changes in the ECD localization, in accordance with the time of implantation in order to implement an objective procedure to follow the maturation of the auditory system in those children that can be put into clinical practice.