Mapping Binaural Responses in the Brain: an fMRI Study
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The neural mechanisms involved in determining the location of sound stimuli are a controversial subject with various theories offering different explanations. One theory speculates that binaural field maps may be responsible for processing the interaural level differences (ILD) as part of these mechanisms. In this study functional magnetic resonance imaging (fMRI) data on five subjects were analysed to identify potential binaural field maps, i.e., areas of the cortical surface tuned to ILD. Eight regions of interest (ROI) were identified in and outside the auditory cortex. These eight ROIs are the auditory cortex (AC), angular gyrus (AG), supramarginal gyrus (SMG), postcentral gyrus (PCG), posterior and anterior regions of Broca’s area (BP and BA respectively), middle frontal gyrus (MFG), and anterior cingulate cortex (ACC). Three models using either ILD or (a combination of) contralateral or ipsilateral loudness level to predict neuronal responses were fit to the gathered data and compared. The model using ILD achieved significantly higher goodness of fit inside the defined regions, indicating that these regions are themselves tuned to ILD. A significant correlation between the preferred ILD of voxels inside the ROIs and their tuning widths was found, with tuning widths increasing as the preferred ILD moves further away from 0. This matches the observation that humans are more sensitive to changes in sound stimuli coming from directly in front of them. The neuronal responses inside the ROIs show repeatable spatial variations in ILD preferences, although the structure of these variations was not identified. These results provide evidence for the existence of binaural field maps and serve as the basis for further research into the existence of population receptive field maps for interaural time difference or the head-related transfer function.