After years of research, neuroscientists have discovered a new pathway in the human brain that processes the sounds of language. The findings suggest that auditory and speech processing occur in parallel, contradicting a long-held theory that the brain processed acoustic information then transformed it into linguistic information.

Sounds of language, upon reaching the ears, are converted into electrical signals by the cochlea and sent to a brain region called the auditory cortex on the temporal lobe. For decades, scientists have thought that speech processing in the auditory cortex followed a serial pathway, similar to an assembly line in a factory. It was thought that first, the primary auditory cortex processes the simple acoustic information, such as frequencies of sounds. Then, an adjacent region, called the superior temporal gyrus (STG), extracts features more important to speech, like consonants and vowels, transforming sounds into meaningful words.

But direct evidence for this theory has been lacking as it requires very detailed neurophysiological recordings from the entire auditory cortex with extremely high spatiotemporal resolution. This is challenging, because the primary auditory cortex is located deep in the cleft that separates the frontal and temporal lobes of the human brain.

«So, we went into this study, hoping to find evidence for that — the transformation of the low-level representation of sounds into the high-level representation of words,» says neuroscientist and neurosurgeon Edward Chang at the University of California, San Francisco.

Over the course of seven years, Chang and his team have studied nine participants who had to undergo brain surgeries for medical reasons, such as to remove a tumor or locate a seizure focus. For these procedures, arrays of small electrodes were placed to cover their entire auditory cortex to collect neural signals for language and seizure mapping. The participants also volunteered to have the recordings analyzed to understand how the auditory cortex processes speech sounds.

«This study is the first time that we could cover all of these areas simultaneously directly from the brain surface and study the transformation of sounds to words,» Chang says. Previous attempts to study the region’s activities largely involved inserting a wire into the area, which could only reveal the signals at a limited number of spots.

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