Mapping hemispheric symmetries, relative asymmetries, and absolute asymmetries underlying the auditory laterality effect

Research Area: Research Year: 2014
Type of Publication: Article Keywords: Dichotic listening; Hemispheric asymmetry; Speech perception; Planum temporale
Authors: Westerhausen, Rene; Kompus, Kristiina; Hugdahl, Kenneth
Functional hemispheric differences for speech and language processing have been traditionally studied by using verbal dichotic-listening paradigms. The commonly observed right-ear preference for the report of dichotically pre- sented syllables is taken to reflect the left hemispheric dominance for speech processing. However, the results of recent functional imaging studies also show that both hemispheres – not only the left – are engaged by dichotic lis- tening, suggesting a more complex relationship between behavioral laterality and functional hemispheric activation asymmetries. In order to more closely examine the hemispheric differences underlying dichotic-listening perfor- mance, we report an analysis of functional magnetic resonance imaging (fMRI) data of 104 right-handed subjects, for the first time combining an interhemispheric difference and conjunction analysis. This approach allowed for a distinction of homotopic brain regions which showed symmetrical (i.e., brain region significantly activated in both hemispheres and no activation difference between the hemispheres), relative asymmetrical (i.e., activated in both hemispheres but significantly stronger in one than the other hemisphere), and absolute asymmetrical acti- vation patterns (i.e., activated only in one hemisphere and this activation is significantly stronger than in the other hemisphere). Symmetrical activation was found in large clusters encompassing temporal, parietal, inferior frontal, and medial superior frontal regions. Relative and absolute left-ward asymmetries were found in the posterior supe- rior temporal gyrus, located adjacent to symmetrically activated areas, and creating a lateral–medial gradient from symmetrical towards absolute asymmetrical activation within the peri-Sylvian region. Absolute leftward asymme- try was also found in the post-central and medial superior frontal gyri, while rightward asymmetries were found in middle temporal and middle frontal gyri. We conclude that dichotic listening engages a bihemispheric cortical net- work, showing a symmetrical and mostly leftward asymmetrical pattern. The here obtained functional (a)symme- try map might serve as a basis for future studies which – by studying the relevance of the here identified regions – clarify the relationship between behavioral laterality measures and hemispheric asymmetry.
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