The aim of the study is to identify cortical regions that process the meaning of spoken language, irrespective of acoustical realization, by means of functional MRI. Distorted speech material is used along with clean speech to allow for a controlled manipulation of intelligibility, depending on the presentation order. Speech distortion was performed by rotating spectral features. Psychophysical experiments were performed with 230 sentences of a speech intelligibility test in German language, to explore the effect of presentation order of clean and distorted speech. Results from 15 listeners indicate that distorted signals can reliably be presented either in a way well to understand, or completely unintelligible, depending on the exact presentation relative to the other speech material. The results were used to design a functional MRI experiment that allowed us to image brain activation for identical acoustic stimuli, only differing in their intelligibility. The fMRI data indicate that speech stimuli in general cause bilateral activation in the temporal lobes. The contrast between natural and distorted speech only comes up in regions beyond primary auditory areas, mainly in superior temporal gyrus. The contrast between acoustically identical stimuli only differing in their intelligibility reveals several areas, largely in the left hemisphere and clearly separate from the temporal lobes. These regions are interpreted to represent the processing of the meaning of intelligible speech, irrespective of their exact acoustical properties.
Speech intelligibility Language Auditory cortex fMRI
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The authors are grateful for the support of Birger Kollmeier and the Medical Physics group in Oldenburg. This work was funded by Deutsche Forschungsgemeinschaft (Up 10/2-2).
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