Cortical thickness of Planum Temporale in native language tone processing

Abstract

INTRODUCTION: Non-native linguistic tone learning ability has been found to be influenced by the anatomy of primary auditory regions in the left hemisphere (Wong et al., 2008). In native speakers, however, where phonemic categories are firmly established, variations in the macrostructure of more secondary auditory processing areas might be expected to affect performance. The present study investigated the relationship between cortical thickness of the Planum Temporale (PT) and the processing of tones in natural language stimuli by Swedish native speakers. PT has previously been found to be involved in processing Thai (Xu et al., 2006) and Swedish tones (Roll et al., 2015), and in phonological processing generally (Graves et al., 2008). In Swedish, tones on word stems function as predictive cues to upcoming morphosyntactic structure, due to associations between stem tone patterns and specific grammatical suffixes. The degree to which listeners evidence anticipatory behaviour in their responses to such cued suffixes thus can be assumed to be mediated by individual variation in processing preceding tonal cues. METHODS: Magnetic resonance imaging scans were collected from native speakers of Swedish. Participants listened to sentences in which the inflectional suffix on the target word was either validly or invalidly cued by the preceding tone. The time it took to decide on suffix meaning (singular/plural) after suffix onset was measured. Cortical reconstruction and volumetric segmentation was performed using the FreeSurfer image analysis suite (http://surfer.nmr.mgh.harvard.edu/). Each processed subject was transformed into common space (fsaverage) and the average cortical thickness of PT was extracted in FreeSurfer after transformation of estimation of PT as defined in the Harvard-Oxford cortical and subcortical structural atlases in FMRIB Software Library (FSL) into common space. RESULTS: Individual participants’ response time advantage for valid over invalidly cued suffixes positively correlated with cortical thickness in the left PT, but not in the right PT. Thus, thicker left PT cortex was associated with relatively greater disruption in speech comprehension caused by an invalid tonal cue. Subgroup comparison also showed generally faster response times for participants with relatively thicker left PT cortex compared to individuals with thinner cortex in the same area. CONCLUSION: The results suggest that the cortical anatomy of the left PT affects native linguistic tone processing. The PT has been proposed to constitute a computational interface between incoming sound patterns, analysed into components, and stored auditory objects (Griffiths & Warren, 2002). From this perspective, the present results might indicate that the cortical thickness of the left PT plays a facilitating role in the extraction of linguistic tone patterns from stimuli and their matching with stored representations in memory