Magnetic fields elicited by tones and vowel formants reveal tonotopy and nonlinear summation of cortical activation

Authors


  • This research was supported by the Deutsche Forschungsgemeinschaft. The MEG measurements were conducted in the Biomagnetism Laboratory of the Physikalisch-Technische Bundesanstalt Berlin.

  • We express our appreciation for the support of our work by M. Burghoff. K.-H. Kiehne, W. Müller, H. Scheer, U. Steinhoff, L. Trahms, and R. Zimmermann. Valuable suggestions made by two anonymous reviewers of an earlier version of the manuscript are gratefully acknowledged.

Dr. Eugen Diesch. Institute of Psychology. Technical University of Berlin, Dovestrasse 1–5. 10587 Berlin, Germany. E-mail: dies0230@mailszrz.zrz.tu-berlin.de.

Abstract

A long-latency response component (N1m) and the sustained field (SF) of the auditory evoked magnetic field elicited by two composite stimuli (a two-tone combination and a two-formant vowel) and their individually presented components (a 600-Hz and a 2100-Hz pure tone and two single-vowel formants with formant frequencies matched to the tone frequencies) were recorded using a 37-channel magnetometer. The response to the composite stimuli differed from the linear sum of the responses to the respective components in latency, equivalent dipole moment, and equivalent dipole location, suggesting an interaction among the processes 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 formant than to the low vowel formant. The Nlm formant sources were more lateral than the Nlm tone sources. These findings suggest that, at the level of the auditory cortex, vowels are represented in terms of both the spectral pitches determined by their most prominent harmonics and, within the latency range of the Nlm, the virtual pitch determined by the spacing of the harmonics.

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