Maturational change of parallel auditory processing in school-aged children revealed by simultaneous recording of magnetic and electric cortical responses

Objectives: To elucidate the maturational change of cortical auditory processing, we analyzed simultaneously recorded auditory evoked potentials (AEPs) and magnetic fields (AEFs) in school-aged children. Methods: Simultaneous recording of AEP and AEF were performed in 32 healthy children of age ranging from 6 to 14 years and 10 adults. Tone bursts of 1 kHz were presented to the left and right ears alternately with 3 different within-ear stimulus onset asynchronies (SOAs) (1.6, 3.0 and 5.0 s for each ear) under attention-distracted condition. Results: All subjects showed clear N100 and N100m peaks under the longest SOA condition (5.0 s). Under the shortest SOA condition (1.6 s), 4 out of 19 subjects under 12 years (21%) failed to show the N100m component. By contrast, N250 and N250m were observed in the majority of children (29/32: 91%) while those were detected in only 4 out of 10 adults (40%). The spatial distribution of N100 in children under 9 years differed from that in older subjects, whereas the dipole orientation of N100m was constant among age groups, suggesting that radially oriented sources might make additional contribution to the generation of N100 in early childhood. N250 was significantly larger in children than in adults. The strength of N250 was suppressed with longer SOAs, whereas that of N100 was enhanced. The dipole of N250m was located around Heschlʹs gyrus on the superior temporal plane which was significantly medial, anterior and inferior to that of N100m. Conclusions: Dissociation of maturational change between the tangential and radial components of N100 suggests that auditory processing at around 100 ms consists of multiple parallel pathways which mature independently. Furthermore, a negative peak at around 250 ms specifically seen in children has different generators from N100 and might represent a special auditory processing which takes an active part until acquisition of the efficient cortical networks of the adult brain.