THE NEUROTOPOGRAPHY OF VOWELS AS MIRRORED BY EVOKED MAGNETIC-FIELD MEASUREMENTS

The auditory evoked neuromagnetic field elicited by synthetic specimen s of the vowels [a], [ae], [u], and [i] was recorded over the left and the right hemisphere of 11 subjects. The N100m and the SF deflection of the recorded signal was submitted to equivalent current source anal ysis using the model of a single dipole in a spherical volume conducto r. Vowel processing was hypothesized to occur in a multistage process rendering a sequence of representations of the acoustic input. Vowel r epresentations were considered to differ among each other in the featu res they make salient, thus, in the kind of dissimilarity relationship they establish, and, by implica tion, in terms of the vowel space def ined by the respective set of dissimilarities. It was investigated whe ther a mapping exists between at least one of a number of hypothetical vowel spaces and the cortical response space spanned by the spatial d istribution of vowel evoked equivalent current dipoles. Although the s patial con figuration of vowel evoked sources proved to be highly vari able across subjects, the ordering of distances between N100m and SF e quivalent current dipole locations turned out to correspond to the ord ering of distances between the corners of a vowel trapezium. There wer e some, albeit weak, indications of hemispheric differences in vowel p rocessing. The results suggest that the spatial distribution of the eq uivalent current dipole sources of both the N100m and the SF deflectio n of the neuromagnetic field elicited by vowels reflect a processing s tage transitional between auditory and phonetic representation. (C) 19 96 Academic Press, Inc.