Electromagnetic recordings reveal latency differences in speech and tone processing in humans

Human auditory electromagnetic brain responses to sinusoidal (tone) and spe ech stimuli (Finnish vowel /a/ with two different glottal excitations) were studied with whole-head magnetoencephalogram (MEG) and electrodes placed o n the subject's scalp. The frequency and intensity of the sinusoidal stimul us were optimally adjusted to match the spectra of the speech stimuli. Both tone and speech sounds elicited a prominent electric N1-P2 and magnetic N1 m-P2m response complex. N1 and P2 amplitudes were larger to speech sounds t han those to the tone. This amplitude enhancement was not as evident in the N1m and P2m obtained in MEG. Both the N1(m) and P2(m) latency always peake d earlier for the tone than for the vowels. The source origin of Nlm for bo th the tone and speech stimuli was in the auditory cortex, there being no s ignificant location differences as a function of stimulus type. Nlm in the right hemisphere was anterior to that on the left, and P2m was anterior to Nlm in both hemispheres. Varying the perceptual quality of the vowels by ch anging their glottal excitations (from "soft" /a/ to "pressed'' /A/) had no effect on the response amplitudes or latencies. Thus, the present results show that only the latency behavior of Nl(m) and P2(m) reliably dissociates speech and tone processing in humans. The findings are discussed in relati on to previous observations on cortical processing of sinusoidal and vowel sounds and with regard to the glottal excitation in speech processing. (C) 1999 Published by Elsevier Science B.V. All rights reserved.