Lexical tone, neuroplasticity and language evolution

Jackson GANDOUR

Voice pitch is an important information-bearing component of language that is subject to experience-dependent plasticity at both early cortical and subcortical stages of processing. Pitch processing itself is influenced by linguistically-relevant functional properties of the language that an individual is exposed to. Tonal languages provide a unique window for tracing the hierarchical transformation of pitch along the auditory pathway and beyond in the human brain, and as a consequence, affords us an opportunity to study lexical tone, neuroplasticity and language evolution. Human brain responses at both cortical and subcortical levels are differentially weighted depending on specific temporal attributes of pitch. Functional neuroimaging (fMRI) shows that pitch processing recruits the hemispheres differentially as a function of its phonological relevance to the listener. The mismatch negativity (MMN)—a neural index of early, cortical processing—shows that pitch processing is shaped by the relative saliency of tonal features. The frequency following response (FFR)—a neural index of pitch encoding at the level of the auditory brainstem—shows that enhancement of pitch features is sensitive to rapidly-changing sections of tonal contours. The cortical pitch-specific response (CPR)—a series of neural markers sensitive to temporal attributes of dynamic pitch contours (Na, Pb, Nb; Na–Pb, Pb–Nb)—reveals language-dependent sensitivity to separate attributes of pitch during early stages of sensory processing in the auditory cortex. These restrictions to separate pitch attributes suggest that relative weighting of CPR components varies depending on their sensitivity within a particular temporal window. Manipulation of pitch salience and pitch height yields heightened sensitivity in the earlier temporal window (Na-Pb). In contrast, changing rate of pitch acceleration relative to a constant rate evokes heightened sensitivity in the later temporal window (Pb–Nb). Differences in relative changes in magnitude between cortical and brainstem components may implicate transformation in pitch processing at the cortical level, presumably mediated by local sensory and/or extrasensory influence overlaid on the brainstem output. A theoretical framework for a neural network is proposed. It involves coordination between local, feedforward, and feedback components that can account for experience-dependent enhancement of pitch representations at multiple locations of an integrated, distributed pitch processing network. In regard to the theme of this conference, these empirical data on experience-dependent neuroplasticity of pitch processing may add another perspective to include in the story of the evolution of language.