Magnetic mismatch fields elicited by vowels and consonants

Auditory-evoked mismatch fields (MMFs) elicited by vowel contrasts and plosive stop consonant place-of-articulation contrasts were recorded over the left hemisphere of neurologically and audiologically normal subjects. Two experiments were conducted: vowels were presented in isolation in experiment 1 and embedded in consonant-vowel syllables in experiment 2. Best-fit equivalent MMF sources were obtained using the model of a single, spatiotemporal current dipole in a sphere. In both experiments, MMF sources activated by place-of-articulation contrasts were later in latency and smaller in dipole moment amplitude than MMF sources excited by vowel contrasts. There was evidence, albeit not unambiguous, for the vowel-contrast MMF sources being located more posteriorly than the consonant-contrast MMF sources in experiment 1 and more laterally in experiment 2. In both experiments, the MMF source excited by the contrast between /da/ and /ga/ was more anterior than the MMF source excited by the contrast between /da/ and /ba/. The effects on latency and dipole moment may be interpreted to mirror differences in perceptual discriminability and auditory memory decay between consonantal place-of-articulation contrasts and vowel contrasts. Similarly, the effects on location may be interpreted to reflect featural specificity of the mismatch response. Interestingly, the dipole source analysis results show a correspondence to the pattern of preservation and loss of the mismatch response to Vowel and consonant place-of-articulation contrasts recently observed in Wernicke's aphasia.