Locating the initial stages of speech-sound processing in human temporal cortex

It is commonly assumed that, in the cochlea and the brainstem, the auditory system processes speech sounds without differentiating them from any other sounds. At some stage, however, it must treat speech sounds and nonspeech sounds differently, since we perceive them as different. The purpose of this study was to delimit the first location in the auditory pathway that makes this distinction using functional MRI, by identifying regions that are differentially sensitive to the internal structure of speech sounds as opposed to closely matched control sounds. We analyzed data from nine right-handed volunteers who were scanned while listening to natural and synthetic vowels, or to nonspeech stimuli matched to the vowel sounds in terms of their long-term energy and both their spectral and temporal profiles. The vowels produced more activation than nonspeech sounds in a bilateral region of the superior temporal sulcus, lateral and inferior to regions of auditory cortex that were activated by both vowels and nonspeech stimuli. The results suggest that the perception of vowel sounds is compatible with a hierarchical model of primate auditory processing in which early cortical stages of processing respond indiscriminately to speech and nonspeech sounds, and only higher regions, beyond anatomically defined auditory cortex, show selectivity for speech sounds.