Studies of tonotopy based on wave N100 of the auditory evoked field are problematic

There is still dissension as to whether the auditory evoked field (AEF) reflects tonotopy in the auditory cortex. That notwithstanding, particularly the pronounced AEF wave occurring about 100 ms after stimulus onset (N100 m) is increasingly used for the investigation of issues such as cortical reorganization and representation of virtual pitch. Thus, it appears to be time for a critical revaluation of the supposed tonotopic organization of the N100 m generator. In the present magnetoencephalography study, the response to tonebursts of 500 ms duration, monaurally presented 60 dB above threshold, was recorded with a 37-channel axial gradiometer system over the hemisphere contralateral to the side of stimulation. The stimulus frequencies were 250, 500, 1000, and 2000 Hz. About 250 stimuli of each type were presented in random order in four independent sessions at intervals uniformly distributed between 2 and 2.8 s. An analysis of 19 hemispheres in 10 normal-hearing subjects showed a high intraindividual reproducibility, but also a substantial interindividual variability. In most cases, the dipole location either exhibited no significant frequency dependence at all, the dipoles for the four frequencies were not orderly aligned, or the data disagreed with the single-dipole model. In the few cases showing an arrangement of dipoles consistent with the assumption of an orderly tonotopic cortical map, the most relevant coordinate varied from subject to subject. Regarding theses results, it seems crucial to understand wave N100 m on the basis of individual subjects, whereas conclusions relying on mean dipole locations for groups of subjects are problematic.