Processing of sound location in human cortex.

This functional magnetic resonance imaging study was focused on the neural substrates underlying human auditory space perception. In order to present natural-like sound locations to the subjects, acoustic stimuli convolved with individual head-related transfer functions were used. Activation foci, as revealed by analyses of contrasts and interactions between sound locations, formed a complex network, including anterior and posterior regions of temporal lobe, posterior parietal cortex, dorsolateral prefrontal cortex and inferior frontal cortex. The distinct topography of this network was the result of different patterns of activation and deactivation, depending on sound location, in the respective voxels. These patterns suggested different levels of complexity in processing of auditory spatial information, starting with simple left/right discrimination in the regions surrounding the primary auditory cortex, while the integration of information on hemispace and eccentricity of sound may take place at later stages. Activations were identified as being located in regions assigned to both the dorsal and ventral auditory cortical streams, that are assumed to be preferably concerned with analysis of spatial and non-spatial sound features, respectively. The finding of activations also in the ventral stream could, on the one hand, reflect the well-known functional duality of auditory spectral analysis, that is, the concurrent extraction of information based on location (due to the spectrotemporal distortions caused by head and pinnae) and spectral characteristics of a sound source. On the other hand, this result may suggest the existence of shared neural networks, performing analyses of auditory 'higher-order' cues for both localization and identification of sound sources.

Human cortical representation of virtual auditory space: differences between sound azimuth and elevation.

Sounds convolved with individual head-related transfer functions and presented through headphones can give very natural percepts of the three-dimensional auditory space. We recorded whole-scalp neuromagnetic responses to such stimuli to compare reactivity of the human auditory cortex to sound azimuth and elevation. The results suggest that the human auditory cortex analyses sound azimuth, based on both binaural and monaural localization cues, mainly in the hemisphere contralateral to the sound, whereas elevation in the anterior space and in the lateral auditory space in general, both strongly relying on monaural spectral cues, are analyzed in more detail in the right auditory cortex. The binaural interaural time and interaural intensity difference cues were processed in the auditory cortex around 100-150 ms and the monaural spectral cues later around 200-250 ms.