Tonotopic cortical representation of periodic complex sounds

Most of the sounds that are biologically relevant are complex periodic sounds, i.e., they are made up of harmonics, whose frequencies are integer multiples of a fundamental frequency (Fo). The Fo of a complex sound can be varied by modifying its periodicity frequency; these variations are perceived as the pitch of the voice or as the note of a musical instrument. The center frequency (CF) of peaks occurring in the audio spectrum also carries information, which is essential, for instance, in vowel recognition. The aim of the present study was to establish whether the generators underlying the 100 m are tonotopically organized based on the Fo or CF of complex sounds. Auditory evoked neuromagnetic fields were recorded with a whole-head magnetoencephalography (MEG) system while 14 subjects listened to 9 different sounds (3 Fo x 3 CF) presented in random order. Equivalent current dipole (ECD) sources for the 100 m component show an orderly progression along the y-axis for both hemispheres, with higher CFs represented more medially. In the right hemisphere, sources for higher CFs were more posterior, while in the left hemisphere they were more inferior. ECD orientation also varied as a function of the sound CF. These results show that the spectral content CF of the complex sounds employed here predominates, at the latency of the 100 m component, over a concurrent mapping of their periodic frequency Fo. The effect was observed both on dipole placement and dipole orientation.