Pre-attentive and attentive processing of static and motion stimuli in the acoustic free field

The four electrophysiological studies of this thesis investigate: (i) the partly inconsistent hypotheses of contralateral and/or right- and/or left-hemispheric dominance in pre-attentive and attentive spatial processing in the auditory cortex; (ii) the specific relation between auditory evoked cortical responses and the localization blur across azimuthal space; (iii) the representation of azimuthal space in the auditory cortex; and (iv) the differences in auditory processing of static and motion sounds in auditory cortex. (i) The results provide strong evidence for a contralateral dominance in pre-attentive cortical processing of (occasional changes in) static sound source positions. Contrary, respective results for attentive auditory spatial processing of occasional chances in sound source positions indicate a pivotal role of the right auditory cortex. This striking discrepancy (between pre-attentive and attentive cortical space processing) suggests the involvement of a specific, predominantly right-lateralized, attention mechanism which is necessary for attentively listening to and differentiating between spatial positions. Furthermore, for the attentive discrimination of static and motion sounds it is found that the hemispheric lateralization (i. e. left-, right- or contralateral dominance) depends on the attended acoustic attribute (‘static’, ‘motion’) and on the context, in which the task-related stimuli are presented. (ii) The combination of electrophysiological and behavioural findings implicate that during sound source discrimination the central auditory system integrates the spatial separation [in degree] between two sound sources in azimuth even though – at least at threshold levels – respective differences in spatial separations across azimuth are consciously imperceptible. From this it is suggested that the pre-attentive discrimination of sound sources is based on an object-related comparison process which might rely on the integration of all available location cues related to a defined position in space. (iii) For sound source representations in auditory cortex, the results provide evidence for a general contralateral dominance in cortical processing of acoustic space information, which might relate to the functional dominance of contralateral projections of cortical afferents. (iv) Present findings for static and motion stimuli provide electrophysiological evidence supporting the assumption of distinct cortical processing mechanisms at a time period of 200ms and 300-400ms after stimulus offset.