Abstract
The audirory-evoked neuromagnetic field elicited by single vowel formants and two-formant vowels was recorded under active listening conditions using a 37-channel magnetometer. There were three single formants with formant with a formant frequency of 200, 400, and 800 Hz, another single fromant with a fromant frequency of 2600 Hz, and three vowels that were constructed by linear superimposition of the high-onto one of the low-frequency formants. P50m and N100m latency values were inversely correlated with the formant frequency of single location was obtained along the postero-anterior axiz, which is orthogonal to the well-established latero-medial tonotopic gradient. Regardless of whether single formants or first formants of vowels were considered N100m sources were more anterior and sustained field sources wre more posterior for higher-frequency than for lower-frequency formants. The velocity of the apparent posterior-to-anterior movement across cortical sorface of N100m sources first reported by Rogers et al. [Rogers, R.L., Papanicolaou, A.C., Baumann, S.B., Saydjari,C., & Eisenberg, H.M. (1990). Neuromagnetic evidence of a dynamic excitation pattern generating the N100 auditory response. Electroencephalography and Clinical Neurophy-siology,77,237-240] decreased as a function of latency. The amount of deceleration was positively correlated with formant frequency. responses to the vowels were superadditive, indicating that the processes elicited by the constituents of composite stumuli interact at one or more stages of the afferent auditory pathway. Such interaction may account for the absence of a lateral-to-medial tonotopic mapping of first formant frequency. The source topography found may reflect activity in auditory fields adjacent to AI with the strenght of the contribution varying with formant frequency. Altematively, it may reflect sharpness-of-tuning and inhbitory response-area asymmetry gradients along isofrequency stripes within AI. Either alternative may be interpreted in terms of a sprctral blurring mechanism that abstracts spectral envelope information form the details of spectral composition, an important step towards the formation of invariant phonetic percepts.