Preattentive processing of auditory spatial information in humans.

Auditory event-related potentials were recorded from reading subjects to frequent and infrequent tones. Frequent tones presented by a loudspeaker in front of the subject were interspersed with infrequent tones delivered either by one of the symmetrically-placed lateral loudspeakers, or by both lateral loudspeakers simultaneously. This latter sound was perceived as originating from a spacious source in the direction of the central loudspeaker. A sizable mismatch negativity (MMN) and P3a were elicited by all three infrequent stimuli, suggesting that infrequent changes in the direction or perceived spaciousness of the sound source were preattentively detected. In addition, a dissociation between the MMN and P3a amplitudes was found: whereas lateral deviants elicited a larger P3a than the simultaneous left + right deviant, the MMN amplitude was approximately equal for all three deviants.