Hemispheric specialization during discrimination of sound sources reflected by MMN.

The present study reports the hemispheric specificity of spatial auditory processing in 15 healthy subjects by measuring location mismatch negativity (MMN) under free field stimulation. The aim was to decide between the partly inconsistent hypotheses of contralateral and/or right-hemispheric dominance in spatial processing in the auditory cortex. The laterality of deviant-standard positions were systematically varied covering the whole of the frontal hemifield from 90 degrees left to 90 degrees right, while the spatial separation of deviant and standard were fixed at 17 degrees. This enabled the evaluation of the specific location-MMNs relating to distinct cortical processing of acoustic space. The inter-hemispheric comparison of the amplitudes of MMNs showed that spatial deviation towards the periphery at -/+17 degrees (relative to 0 degrees -standard) and at -/+90 degrees (relative to -/+73 degrees -standard) elicited a salient contralateral activation. In contrast, positional changes towards front at -/+56 degrees -positions (relative to -/+73 degrees -standard) resulted equal bilateral MMNs. Further, MMN latencies became longer with increasing laterality of respective deviant-standard pairs. Thus, the present study suggests a contralateral pre-attentive cortical processing of acoustic space information in the free field. The direction of positional changes ('towards periphery' vs. 'towards front') seems to augment or reduce this contralateral effect. The sound source discrimination performance across space is mirrored by the location-MMN latency.