Neurological maturation of the auditory cortex or cortical plasticity refers to structural and functional changes of neural properties which occur on different temporal and spatial scales; the temporal scale extends from seconds to a whole life and the spatial scale extends from the molecular level to changes on topographic (scalp) maps [85]. The changes in the morphology of the ICs offer us a way to evaluate the temporal aspect of the plasticity whilst the changes seen in the topographic maps give us the opportunity to follow the spatial aspect of maturation in children with CIs.
Figure 8.5 shows both the ICs and spatial projections related to the AEP in all the children implanted. Each row corresponds to different subjects whilst columns correspond to time after implantation. A closer analysis of the modification of the spatial projections in accordance with the time of implantation is shown in the rest of the figures in this chapter.
Group : 1 <1y post-CI Group 2: ~1y post-CI Group 3: 1-2y post-CI Group 4: ~2.5y post-CI Group 5: >5 post-CI S1 S2 S3 S4 S5 S6
The changes in the IC waveforms associated with the AEP, as well as their spatial projection in the first year of implantation of three different subjects (S1, S2 and S4), are shown in Figure 8.6. In (a) the subjects have used their CIs for less than one year (5 months on average); (b) shows the recordings of the same subjects at one year after implantation. The changes in the latency of P1 peak of the ICs associated with the AEP between both recordings varied among subjects, 16 ms in S1 and only 3 and 4 ms in S2 and S4, respectively. However, changes of the spatial projections are similar in the three subjects, from parietal to front-central at one year of using their implants.
(a) (b)
Figure 8.6 Changes in the IC waveforms and spatial projections of three different subjects
(S1, S2 and S4) during the first year of use of their CIs; (a) St1: less than one year post- implantation and (b) St2: approximately one year post-implantation.
Figure 8.7 shows the changes in the topographic maps and waveforms of the ICs related to the AEP in two subjects at different times after implantation. In subject S3 the latency of P1 changed from 165 ms to 160 ms whilst in subject S5 the latency changed from 156.3 to 143 ms; the spatial projections of these components changed from central (1-2 years post-CI) to fronto-central contra-lateral to the CI (>5 years post-CI). Both subjects have their CIs on the right side.
(a) (b)
Figure 8.7 Changes in the IC waveforms and spatial projections of two different subjects (S3
and S5); (a) between one and two years post-implantation and (b) more than five years post- implantation.
Figure 8.8 shows the ICs related to the AEPs and their spatial projections of one subject (S5, implanted at 4 y.o. in the right side) at two different times of use of his CI (Study 1: 1 year and 9 months and Study 2: 2 years and 8 months post implantation), compared to a normal hearing child (xal, 4 y.o.). The latencies of P1 and N1 remain similar between study 1 and study 2, there is a difference of one year between these two recordings; however, the spatial projection of the ICs related to the auditory response changes from fronto-central almost symmetric to frontal lateralized in the left side. The topographic map in study 2 is very similar to that of a normal hearing child.
Figure 8.8 Changes in the ICs related to the AEP and their spatial projections of one subject
(S5, implanted at 4 y.o. in the right side), at two different time of use of his implant, compared with a normal hearing child (xal, 4 y.o.).
Figure 8.9 shows the ICs associated with the AEPs and their spatial projections of one subject (S3, implant at 7 y.o. in the right side) at two different times of use of his CI (Study 1: 1 year and Study 2: 1 year 8 months post implantation), compared with a normal hearing child (kc, 7 y. o.). In study 1, only a P1 peak is observed; the latency of P1 shifted from 200ms in study 1 to 163 ms in study 2; moreover the spatial projection is more central and localized and is more similar to the topographic map of a normal child.
Figure 8.9 Changes in the IC of the AEP and its spatial projection of one subject at two different times of use of his CI (S3, implanted at 7 y.o. in the right side), compared with a normal hearing child (kc, 7 y. o.).
Figure 8.10 shows the ICs associated with the AEP and their spatial projection of three different subjects (S3, S5 and S6, implanted in the right side) with more than two years using their CIs (2 year 5 months on average); it is possible to identify both P1 and N1 peaks in all subjects. The latency of P1 is 160.3, 148.3 and 174 ms respectively. The spatial projections have a fronto- to fronto-central distribution lateralized opposite to the CI.
Figure 8.10 ICs associated with the AEPs and their spatial projection of three different
subjects (S3 implanted at 7 y.o., S5 and S6 implanted at 4 y.o.) with more than two years using their CI (implanted in the right side); it is possible to identify both P1 and N1 peaks in
all subjects. The spatial projections have a front to front-central distribution lateralized opposite to the CI.
The latencies of the P1 peak in the ICs related to the AEP among subjects are diverse and it is not possible to identify the complex P1-N1 in all the subjects (S1-St1, S2-St1 and S3-St1). In general the latency of this positive peak is shorter as a function of the use of the CI. It is difficult to draw any conclusion about the auditory system maturation of these children using just this parameter. Nevertheless, the spatial projection of these ICs shows more consistent changes in accordance with the use of the CI, across all the subjects. Although it is necessary to increase the number of subjects and recordings, in order to have more reliable results, it is possible to say that subjects implanted younger present topographic maps more focussed in a specific area contra-lateral to the CI than children implanted at an older age (compare for example Figures 8.8 and 8.9).
In general, the spatial projections of the AEPs‟ ICs are spread out around the head with no focus in any specific area, although predominantly parietal when the children have used their CIs for less than one year. At one year after implantation the spatial projections are characterized by a central to fronto-central distribution. Finally, the spatial projections of the ICs have a distribution fronto- to fronto-central, contra- lateral to the CI implantation at more than two years post-implantation. The spatial projections of the ICs related to the AEP show similarities with normal hearing
children‟s spatial projections, which could be used for an objective assessment of the maturation of the auditory system in children with CIs.