Phonetic invariance in the human auditory cortex.

Neuromagnetic signals evoked by synthesized syllables (/bae/ and /gae/) were recorded over the left auditory cortex of healthy humans. The fundamental frequencies of the syllables varied as if the same speaker had pronounced them at 16 different pitches. Specific mismatch responses to infrequent syllables among frequent syllables of the other type indicated that phonetically invariant information had been extracted at the level of the auditory cortex from the extensive irrelevant pitch variation. Such a detection mechanism is necessary for perceiving speech sounds in natural situations with a great deal of acoustic variation present.