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International Journal of Bioelectromagnetism Vol. 4, No. 2, pp. 155-156, 2002. |
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www.ijbem.org |
AUDITORY M100 IN CHILDREN: DEVELOPMENTAL CHANGES REFLECTED IN THE TANGENTIAL NEURAL SOURCEE. W. Pang1, A. Hunjan1,
R. Sharma1, S. Holowka2, H. Otsubo1 Abstract: Although the auditory M100 and its electrical correlate, the vertex N1, have been extensively examined in adults, they are not as well understood in children. Scalp EEG studies suggest that the vertex N1 is seen as a broad positivity in children and the adult morphology does not appear until after age 9yrs. In younger children, the predominent auditory response is located over temporal cortex, presumably generated by radially-oriented neural sources. Since the MEG is preferentially sensitive to tangentially-oriented generators, the question arises as to how the relative contributions of the radial and tangential generators summate on MEG. It may be that the orientation of the tangential dipole is distorted due to the pre-eminence of the radial generator in childhood. 6 adults and 6 children listened to monaurally-presented tones in a 151-channel MEG. Dipole source analysis localized the M100 response into auditory cortex for both the adult and children data. However, the declination and azimuth of the dipoles were significantly different between the two groups. This finding concurs with scalp EEG reports that the polarity of the vertex N1 is inverted in children and suggests that the MEG response reflects the summated response of the tangential and radial generators. INTRODUCTIONThe auditory N1 from electrical recordings, and its magnetic correlate, the M100, have been extensively examined in adults and its neural generators are well understood. The vertex component of the N1 is known to be generated by tangentially-oriented dipoles on the posterior superior temporal plane of the auditory cortices while the temporal components of the N1 are generated by radially-oriented dipoles on the superior temporal gyri [review, see 1]. It has been demonstrated that the components underlying the electrical N1 have distinct and protracted maturational timelines. The vertex N1 is observed as a positivity in childhood and does not demonstrate the adult morphology until at least age 9 yrs [2-4]. Furthermore, it has been demonstrated that the most prominent scalp recorded response in children is at temporal electrodes and this response is much larger than the vertex N1 [5]. MEG studies have reported evoked fields in children but note that these are quite dissimilar to adult responses [6,7]. Given that the MEG is preferentially sensitive to tangentially-oriented generators and that the temporal component is most prominent in childhood, the question arises as to whether the observed dissimilarities are due to differing relative contributions of the radial and tangential generators as they summate on MEG. A comparison of dipole parameters between adults and children will examine this issue. METHODSSubjects6 children (5f,1m; mean=7.8yrs; range=5.3–8.9) and 6 adults (2f,4m; mean=37.9yrs; range=29.1-49.3) were tested. Subjects had normal hearing and no audiologic problems. Stimulus Presentation100 tones (1kHz, 36ms duration, 1 sec ISI, 80dB SPL at the ear, 50 dB contralateral white-noise) were presented monaurally via plastic ear inserts attached to plastic tubes. MEG recordingAll subjects were tested supine. Fiducial coils were attached to nasion and both pre-auricular points. Data were recorded continuously (625Hz A-to-D, DC-100Hz bandpass, third-order spatial gradient) using a CTF Omega 151-channel whole-head system. Data were processed off-line (800 ms trial with 200ms baseline; 1-30Hz filter) and averaged. MEG analysis and Dipole FittingIn the adults, the largest peak was selected as the M100. In the children, the M100 was selected as the peak closest in latency to 100msec and showing a magnetic field distribution with one clear dipolar source in the hemisphere contralateral to the stimulated ear. Individual spherical head models were created. A single dipole was manually seeded in contralateral temporal cortex and then fit to the M100. Independent t-tests were used. Dipoles were overlaid on a 3-D T1-weighted MRI. RESULTSFig. 1a shows a typical adult M100. Fig. 1b shows the corresponding dipole fit; this is consistent with the literature [8]. Fig. 2a shows a typical auditory response in a child. Since we were interested in a comparison with the adult M100, the peak closest to 100ms was selected in the children. Although not the largest peak, the magnetic field map suggested a single dipole in the temporal area; thus implying that this is the first cortical response. Fig. 2b shows the dipole fit for the child M100. Fig. 3 shows the child’s dipoles overlaid on MRI and these localize to auditory cortex. Mean values (left and right ear) for head diameter, M100 latency, fit error, dipole moments, declination and azimuth, for both groups are contained in Table 1. T-tests show significant differences between adults and children for dipole declination and azimuth but not for moment. (a) (b) Figure 1. (a) Adult M100 waveform. (b) Dipole fit for adult M100. Note the orientation of the dipoles in the coronal view.
(a) (b) Figure 2. (a) Typical first cortical response (approximately 100 ms latency) in a child. (b) Dipole fit for above response. DISCUSSION There are striking similarities and differences between the two age groups. The adult data (morphologies, latencies and dipole orientations) are all consistent with the literature. By fitting adult and child dipoles at similar latencies, we were able to consistently place dipoles into auditory cortex. Given the differences in head sizes between the adults and children, the locations of the dipoles seem comparable. Our most salient finding is the significantly different dipole orientations between the groups. The inversion of dipole orientation (best seen on coronal views) brings to the mind the polarity inversion seen at the vertex N1 in children, although from a physiological perspective, it is difficult to imagine that there is a physical inversion of the neurons with development. A more likely explanation for the dipole inversion is cortical growth. In adults, the generator for the temporal N1 component is located on both gyral crest and lateral aspects of the superior temporal gyrus. It may be that, due to the smaller size of the young brain, these neural generators are located more inferiorly on the superior temporal gyrus, thus resulting in a tangential orientation but of opposite direction. As the brain grows, these generators are pushed more laterally, eventually resulting in the adult radial orientation. Because the temporal N1 response is so prominent in children, this response summates with the true tangential response (possibly obscuring it) resulting in very different morphologies for the adult and children MEG recordings. Figure 3. Sagittal and coronal MRI slices in a child showing localization of dipoles to auditory cortex. TABLE I
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