Categorical perception mechanism in the auditory cortex and its plasticity

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Based on PET (positron emission tomography) findings on central mechanisms for spoken language processing in normal hearing subjects, those with inner ear hearing loss and in profoundly deaf cochlear implant (CI) users, it has been suggested that the neuronal networks for categorical perception in the auditory cortex may be located in the mid portion of the superior and middle temporal cortex. Perception of distorted and incomplete speech signals coded by cochlear implant may require excessive activity of speech processing networks, and the mid portion of the middle temporal gyrus, that is strongly activated under such condition, may play an important role in categorical perception. It was reported that the excitatory responses of neurons in the middle temporal gyrus had no close links to certain phonemic or prosodic features, and the degraded and incomplete speech signals may be recognized as speech on the basis of coarse coding, in which a profile of responses from a population of co … More arsely tuned units accurately specify a value. A coarse coding strategy may require extensive recruitment of neurons in the auditory cortex during processing degraded and ambiguous acoustic stimuli, which possibly underlie strong speech activation of the temporal cortex in CI users. Under such condition, the Broca's area and the supplementary motor area (SMA) exhibited strong speech activation. The SMA is a higher-order motor area and is known to be activated by pure internal rehearsal of motor sequences. This area is connected by long tracts with Broca's area, and is commonly activated when establishing programs for motor sequences including speech and daily skills. Broca's area has connections with the auditory association area via the arcuate fascicles. A hypothetical microcircuit consisting of the SMA, Broca's area and the auditory association area is supposed to establish working memory for language decoding and encoding, enabling the brain to maintain an internal copy of the original auditory message. Such an internal playback mechanism might be activated more to accomplish accurate categorical perception of incomplete speech signals, recognizing phonemes or words coded by damaged inner ear or by CI is often difficult, and requires comparison of the heard message several times with the stored versions of similar words. More studies focused on the functional anatomy of each area are necessary to verify this hypothesis. Less