Contextual effects on preattentive processing of sound motion as revealed by spatial MMN

The magnitude of spatial distance between sound stimuli is critically important for their preattentive discrimination, yet the effect of stimulus context on auditory motion processing is not clear. This study investigated the effects of acoustical change and stimulus context on preattentive spatial change detection. Auditory event-related potentials (ERPs) were recorded for stationary midline noises and two patterns of sound motion produced by linear or abrupt changes of interaural time differences. Each of the three types of stimuli was used as standard or deviant in different blocks. Context effects on mismatch negativity (MMN) elicited by stationary and moving sound stimuli were investigated by reversing the role of standard and deviant stimuli, while the acoustical stimulus parameters were kept the same. That is, MMN amplitudes were calculated by subtracting ERPs to identical stimuli presented as standard in one block and deviant in another block. In contrast, effects of acoustical change on MMN amplitudes were calculated by subtracting ERPs of standards and deviants presented within the same block. Preattentive discrimination of moving and stationary sounds indexed by MMN was strongly dependent on the stimulus context. Higher MMNs were produced in oddball configurations where deviance represented increments of the sound velocity, as compared to configurations with velocity decrements. The effect of standard-deviant reversal was more pronounced with the abrupt sound displacement than with gradual sound motion.