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Neural distance amplification of lexical tone in human auditory cortex

In tonal languages, like Chinese, lexical tone serves as a key feature to provide contrast in word meaning. Behavior studies suggest that Mandarin Chinese tone is categorically perceived. However, the neural mechanism underlying Mandarin tone perception is still poorly understood. In this study, an Oddball paradigm was designed by selecting two standard-deviant stimulus pairs with same physical distance but different category labels, among the synthesized tones with continuously varying pitch contours. Using electrocorticography (ECoG) recording over human auditory cortex, high temporal and spatial resolution cortical neural signals were used for the first time to investigate the cortical processing of lexical tone. Here, we found different neural responses to the two standard-deviant tone pairs, and the difference increased from low to high level along the hierarchy of human auditory cortex. In the two dimensional neural space, cross-category neural distance of lexical tones is selectively amplified on those high level electrodes. These findings support a hierarchical and categorical model of Mandarin tone perception, and favor the using of high-level electrodes for a better performance of lexical tone discrimination in speech brain computer interface.