MAGNETIC-FIELDS ELICITED BY TONES AND VOWEL FORMANTS REVEAL TONOTOPY AND NONLINEAR SUMMATION OF CORTICAL ACTIVATION

A long-latency response component (Nlm) and the sustained field (SF) o f the auditory evoked magnetic field elicited by two composite stimuli (a two-tone combination and a two-formant vowel) and their individual ly presented components (a 600-Hz and a 2100-Hz pure tone and two sing le-vowel formants with formant frequencies matched to the tone frequen cies) were recorded using a 37-channel magnetometer. The response to t he composite stimuli differed from the linear sum of the responses to the respective components in latency, equivalent dipole moment, and eq uivalent dipole location, suggesting an interaction among the processe s elicited by the constituents of composite stimuli. Nlm and SF source locations were more medial for the response to the high tone than to the low tone and more medial for the response to the high vowel forman t than to the low vowel formant. The Nlm formant sources were more lat eral than the Nlm tone sources. These findings suggest that, at the le vel of the auditory cortex, vowels are represented in terms of both th e spectral pitches determined by their most prominent harmonics and, w ithin the latency range of the Nlm, the virtual pitch determined by th e spacing of thr harmonics.