Hemispheric lateralization of sentence intonation in left handed subjects with typical and atypical language lateralization: an fMRI study
|Eszter Varga, Zsuzsanna Schnell, Gabor Perlaki, Gergely Orsi, Mihaly Aradi, Tibor Auer, Flora John, Tamas Doczi, Samuel Komoly, Norbert Kovacs, Attila Schwarz, Tamas Tenyi, Robert Herold, Jozsef Janszky and Reka Horvath|
Introduction: Prosody (as the melody of speech) is an important component of human social interactions. More specifically, linguistic prosody conveys meaning of speech through syllable, word, or sentence level stress and intonation. In the modern neuroimaging era the hemispheric representation of sentence intonation is widely investigated. Most of these studies suggest bilateral activations predominantly in the perisylvian language areas and in the subdominant homologues. However, there are some inconsistencies about the hemispheric representation and lateralization of linguistic prosody. These inconsistencies could be due to the lack of attention on the language lateralization of the subjects. Aims: The present study aims to investigate the hemispheric representation and lateralization of linguistic prosody with a sentence intonation task in two groups of left handed subjects with typical and atypical language lateralization. Functional MRI was used to test the assumption that - according to the functional lateralization hypothesis - the representation of sentence intonation is predominantly lateralized within the language dominant hemisphere and the lateralization of sentence intonation is associated with language lateralization in both groups. Methods: Left handers were examined to create two groups of subjects with typical and atypical language lateralization. In all, 32 healthy subjects were evaluated with a standard verbal fluency task with fMRI in order to assess functional hemispheric language lateralization. In our final investigation the atypical group consisted of 8 subjects with right hemispheric language dominance (LI<-0.2) and the typical group also consisted of 8 subjects with left hemispheric language dominance (LI>0.2). Sentence intonation task was utilized to test linguistic prosody skills with fMRI. 49 pairs of sentences (18 pairs of neutral-neutral sentences, 10 pairs of interrogative-interrogative sentences, and 1 pair of interrogative-neutral sentence) were presented with an event-related design. Sentences were matched in terms of syntactic structure, semantic complexity and length and all were affectively neutral. In the fMRI data analysis interrogative pairs were compared to neutral pairs. Results: One of the main findings of our study is that subjects with both typical and atypical language lateralization activated the middle temporal gyrus (MTG) on the right side. The activation of the MTG on the right hemisphere is classically associated with the encoding of prosodic information. Furthermore, both groups recruited the frontal language areas only in the language-dominant hemisphere. Moreover, between-group comparison showed significantly stronger activations in subjects with typical language lateralization only in left sided language areas: pars triangularis of the inferior frontal gyrus, the superior frontal gyrus and the inferior parietal lobule. Conclusion: This finding is in accordance with the functional lateralization hypothesis of prosody, and suggests a correlation between linguistic prosody lateralization and language lateralization.