Single-Dipole Analyses of the N100m Are Not Suitable for Characterizing the Cortical Representation of Pitch

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Abstract
In several previous studies, the deflection N100m of the auditory evoked field (AEF), occurring about 100 ms after the onset of a sound, has been used to investigate whether complex tones are represented in the auditory cortex by their spectrum or their periodicity pitch. However, the results partially contradict each other. Moreover, the basic assumption that N100m originates from a single cortical source has been questioned. To gain clarity, the AEF elicited by a complex tone with a periodicity of 4 ms (pitch 250 Hz) and a spectrum between 1000 and 2000 Hz was compared with the AEF in response to pure tones of 250 and 1000 Hz, respectively (on 12 subjects). While a previous analysis [Audiol Neurootol 2001;6: 263–278] was exclusively devoted to the N100m latency, this article presents source analyses with two variants of a single-dipole model (moving dipole and fixed dipole). Earlier reports, suggesting that N100m arises from a cortical source representing the pitch rather than the spectrum of the sound, could not be reproduced, independent of the method used. It appears that an investigation of the functional organization of the human auditory cortex based on N100m source modeling requires a much higher degree of sophistication than a single-dipole model can provide.