LANGUAGE EXPERIENCE SHAPES PROCESSING OF PITCH RELEVANT INFORMATION IN THE HUMAN BRAINSTEM AND AUDITORY CORTEX: ELECTROPHYSIOLOGICAL EVIDENCE.
Pitch is a robust perceptual attribute that plays an important role in speech, language, and music. As such, it provides an analytic window to evaluate how neural activity relevant to pitch undergo transformation from early sensory to later cognitive stages of processing in a well coordinated hierarchical network that is subject to experience dependent plasticity. We review recent evidence of language experience dependent effects in pitch processing based on comparisons of native vs. nonnative speakers of a tonal language from electrophysiological recordings in the auditory brainstem and auditory cortex. We present evidence that shows enhanced representation of linguistically relevant pitch dimensions or features at both the brainstem and cortical levels with a stimulus dependent preferential activation of the right hemisphere in native speakers of a tone language. We argue that neural representation of pitch relevant information in the brainstem and early sensory level processing in the auditory cortex is shaped by the perceptual salience of domain specific features. While both stages of processing are shaped by language experience, neural representations are transformed and fundamentally different at each biological level of abstraction. The representation of pitch relevant information in the brainstem is more fine grained spectrotemporally as it reflects sustained neural phase locking to pitch relevant periodicities contained in the stimulus. In contrast, the cortical pitch relevant neural activity reflects primarily a series of transient temporal neural events synchronized to certain temporal attributes of the pitch contour. We argue that experience dependent enhancement of pitch representation for Chinese listeners most likely reflects an interaction between higher level cognitive processes and early sensory level processing to improve representations of behaviorally relevant features that contribute optimally to perception. It is our view that long term experience shapes this adaptive process wherein the top down connections provide selective gating of inputs to both cortical and subcortical structures to enhance neural responses to specific behaviorally relevant attributes of the stimulus. A theoretical framework for a neural network is proposed involving coordination between local, feedforward, and feedback components that can account for experience dependent enhancement of pitch representations at multiple levels of the auditory pathway. The ability to record brainstem and cortical pitch relevant responses concurrently may provide a new window to evaluate the online interplay between feedback, feedforward, and local intrinsic components in the hierarchical processing of pitch relevant information.
Acoustics Australia