Date Published: July 20, 2017
Publisher: Public Library of Science
Author(s): Christina Siu-Dschu Fan, Xingyu Zhu, Hans Günter Dosch, Christiane von Stutterheim, André Rupp, Christian Friedrich Altmann.
In tonal languages, such as Mandarin Chinese, the pitch contour of vowels discriminates lexical meaning, which is not the case in non-tonal languages such as German. Recent data provide evidence that pitch processing is influenced by language experience. However, there are still many open questions concerning the representation of such phonological and language-related differences at the level of the auditory cortex (AC). Using magnetoencephalography (MEG), we recorded transient and sustained auditory evoked fields (AEF) in native Chinese and German speakers to investigate language related phonological and semantic aspects in the processing of acoustic stimuli. AEF were elicited by spoken meaningful and meaningless syllables, by vowels, and by a French horn tone. Speech sounds were recorded from a native speaker and showed frequency-modulations according to the pitch-contours of Mandarin. The sustained field (SF) evoked by natural speech signals was significantly larger for Chinese than for German listeners. In contrast, the SF elicited by a horn tone was not significantly different between groups. Furthermore, the SF of Chinese subjects was larger when evoked by meaningful syllables compared to meaningless ones, but there was no significant difference regarding whether vowels were part of the Chinese phonological system or not. Moreover, the N100m gave subtle but clear evidence that for Chinese listeners other factors than purely physical properties play a role in processing meaningful signals. These findings show that the N100 and the SF generated in Heschl’s gyrus are influenced by language experience, which suggests that AC activity related to specific pitch contours of vowels is influenced in a top-down fashion by higher, language related areas. Such interactions are in line with anatomical findings and neuroimaging data, as well as with the dual-stream model of language of Hickok and Poeppel that highlights the close and reciprocal interaction between superior temporal gyrus and sulcus.
While in most Indo-European languages such as English and German pitch is used to transmit prosodic information, varying pitch contours are used in tonal languages such as Chinese at a syllable level to discriminate lexical meaning [1–3]. The lexical significance of pitch contours is a special challenge in speech processing for speakers of a non-tonal language. It is therefore an interesting question on which level neural processing differs between these groups.
The aim of our investigation was to isolate language specific effects in cortical responses to acoustical stimuli, in Chinese listeners as compared to German listeners. An important difference between Mandarin Chinese and German is the use of pitch variations in a syllabic context, namely in the function of lexical tone for the discrimination of meaning [2, 3]. Beside this difference in the phonological systems, single syllables typically form words in Chinese, but not in German, and thus carry meaning. The main result of this cross-linguistic MEG study is a language related difference in the sustained response elicited by natural speech sounds. The current data suggest that phonological differences in the subjects’ mother tongue have a major impact upon processing in auditory cortex. The observed SF differences between Chinese and German listeners (Fig 3a–3b) extend previous investigations on sound processing at the subcortical level [41, 28, 42, 43, 30].
Although, as pointed out by Manca and Grimaldi , neither the N1 nor the SF can be used to derive a speech-specific representation, the current experiments show that the SF might be employed to track AC-STG interactions in lateral HG. In order to reveal more generalized findings, future investigations should apply “roving” designs, i.e., investigate language related sustained responses evoked by much more different speakers as carried out by the pioneering work of Mesgarani et al. . Such material should be contrasted with nonnative f0 contours to bridge the gap between investigations which employed either natural or synthetic stimuli. Taken together, the current data provide evidence that the SF is involved in a complex feedforward-feedback loop and reflects language specific properties at higher levels in the temporal lobe. This neurophysiological finding is of special importance since the fMRI BOLD response seems to be dominated by more transient activity .