The neural basis for lexical tone perception

Abstract

Research on how lexical tone is neuroanatomically represented in the human brain is central to our understanding of cortical regions subserving language. While lexical tones contain acoustic and linguistic information, past research has overwhelming focused on the acoustic processing of lexical tone with auditory stimuli, while our knowledge of semantic processing of lexical tones and how lexical tones contribute to reading remains superficial. Four functional magnetic resonance imaging studies were conducted with healthy, native Mandarin-speaking adults. Resting-state and task-based fMRI data were collected to investigate the neural systems and connectivity patterns for lexical tone perception. In Study 1, we used a novel methodology to investigate the neural substrates underlying auditory lexical tone perception. Prior neuroimaging research on cognitive processing of lexical tones has yielded inconsistent results. Also, experimental materials used in past studies carried minimal lexical semantics, an important dimension since speech tones serve to distinguish lexical meanings. Subjects performed a same-different tone discrimination task with meaningful, disyllabic Chinese words. Results showed that the left inferior frontal gyrus, right middle temporal gyrus and bilateral superior temporal gyri are responsible for the perception of linguistic pitches. We hypothesize that the left superior temporal region is engaged in primary acoustic analysis of the auditory stimuli, while the right middle superior temporal gyrus and the left inferior frontal region are involved in tonal and semantic processing of the language stimuli. In Study 2, we conducted a meta-analysis of neuroimaging studies of auditory lexical tone processing. A resting-state connectivity analysis was also conducted. Our results suggest that both hemispheres play roles in the auditory processing of lexical tone information with the left hemisphere playing a more dominant role. Yet, the resting-state connectivity data revealed an opposite pattern of laterality. We infer that tonal processing laterality is apparent only during active processing. In Study 3, subjects were required to make lexical tone judgments of visually presented Chinese characters. We found that tone perception of printed Chinese characters was mediated by strong brain activation in bilateral frontal regions, left inferior parietal lobule, left posterior middle/medial and inferior temporal region and bilateral visual areas. Surprising, no activation was found in superior temporal regions, brain sites well known for speech tone processing. The findings provided novel insights regarding how the activation in the superior temporal cortex associated with lexical tone perception depends on the input modality. In Study 4, we addressed the neural systems for the processing of tonal information in reading. We found that the extraction of tonal information in reading is lateralized to the right fronto-parietal network. Moreover, we examined the functional connectivity within cortical regions in the right hemisphere. Seed-to-voxel analyses showed that right frontal regions are connected to left frontal parietal and bilateral subcortical areas to support Chinese tone reading. Our connectivity results lend support to the view that both hemispheres participate and interact in processing lexical tones. Together, these results provide significant insights into the neural circuitries involved in tonal processing.