A cross-language fMRI study of sentence-level prosody in Mandarin

Abstract

Speech prosody engages a large-scale spatially distributed network in both hemispheres (Friederici & Alter, 2004; Hickok & Poeppel, 2004). It may be conceptualized as a mosaic of multiple local asymmetries that allows for the possibility that different regions may be differentially weighted in laterality depending on language-, modality-, and task-related features (Scott & Wise, 2004). Cross-language comparisons of speech prosody have demonstrated that hemispheric laterality effects vary depending on the role of specific brain region at different stages of processing (Gandour et al., 2004). Mandarin Chinese is a tone language that offers unique opportunities to address issues of functional asymmetry that involve prosodic contrasts based primarily on variations in pitch. In addition to its four lexical tones, pitch variations are also exploited to signal post-lexical, prosodic phenomena at the sentence level: narrow focus and intonation. Using functional magnetic resonance imaging (fMRI), this study investigates the perception of sentence focus and intonation in Mandarin in an affectively neutral context. Whereas our earlier work focused on word, syllable, and phonemic units of prosody, or comparisons of wordto sentence-level units, this study directly compares two sentence-level units. Sentences were designed with two sentence types (statement, question) in combination with two locations of sentence focus or emphatic stress (initial, final). For example (bold= focused word; superscript numbers=Chinese tones) bi ge cao. ‘Bi mows lawns.’, statement/ initial focus; kou du shu. ‘Kou reads books.’, statement/final focus; song da qiu? ‘Song plays golf?’, question/initial focus; dai hua tu? ‘Dai draws maps?’, question/final focus. Chinese and English listeners were asked to selectively attend to either focus (F) or intonation (I) in paired three-word sentences, and make speeded-response discrimination judgments. Scanning was performed on a 1.5T Signa GE LX Horizon scanner. Functional data were acquired using a gradient-echo EPI pulse sequence. Image analysis was conducted using the AFNI software package (Cox, 1996). Nine anatomically constrained, 6-mm radius spherical regions of interest (ROI) were chosen that have been implicated in previous functional neuroimaging studies of phonological processing, speech perception, semantic processing, attention, and working memory. ROIs were symmetric in nonoverlapping frontal, temporal, and parietal regions of both hemispheres. For each ROI, mean z scores were calculated for each task (F, I) and hemisphere (LH, RH) for every subject. These z scores within each ROI were analyzed using ANOVAs to compare activation between tasks (F, I), hemispheres (LH, RH), and groups (Chinese, English). Results showed that both language groups share right-sided activity in the mid portion of the middle frontal gyrus (BA 46/9) across tasks, and bilateral activity in the dorsal aspect of inferior parietal cortex (BA 40/7) in a direct task comparison (focus > intonation). The Chinese group, on the other hand, exhibited stronger activity than the English group across tasks in the left-sided anterolateral ventral aspect of the supramarginal gyrus (BA 40) and posterior aspect of the middle temporal gyrus (BA 21/20/37). Language-dependent task effects (focus > intonation) were observed in the left posterior middle temporal gyrus (BA 21/20/37) for the Chinese group. The major findings of this study demonstrate that brain activity in response to Mandarin sentence focus and intonation depends on languageand task-related features. All regions in the frontal, temporal, and parietal lobes that are lateralized to the LH in response to one or both tasks are found in the Chinese group only. Conversely, one regions in the frontal lobe that is lateralized to the RH is found in both language groups. No region shows a leftward asymmetry in the English group. Instead of viewing hemispheric roles as singular in nature, we argue that there are multiple levels of specialization that are associated with different stages of processing (e.g., acoustic, linguistic). Such findings support the emerging view that speech prosody perception involves a dynamic interplay among widely distributed regions not only within a single hemisphere but also between the two hemispheres (Friederici & Alter, 2004). Although language experience can shape the internal representation of an external auditory signal, it is not a question of whether any single factor by itself—language experience, stimulus parameter, stimulus function, task demands—is the driving force behind hemispheric laterality of speech prosody. All of the above are relevant but may vary in their individual weight depending on experimental design and, as a consequence, the degree of involvement of the LH. Brain and Language 95 (2005) 54–55