Probability dependence and functional separation of the object-related and mismatch negativity event-related potential components

The separation of concurrently presented sounds into distinct auditory objects is accompanied by the elicitation of the object-related negativity (ORN) component of the event-related potential (ERP). In the present study, participants were exposed to tone sequences containing not only concurrent (harmonicity), but also sequential cues (similarity to previous stimuli in the series) for auditory object formation. Although ORN was consistently elicited by two-object relative to one-object sounds, it was modulated by the presentation rate. With increasing probability of sound segregation, ORN amplitude decreased, and its scalp topography shifted from a bilateral towards a unilateral (left-hemispheric) activation pattern. Probability dependence of the ORN matches previous behavioral data. The differential sensitivity of the left- and right-hemispheric response to this manipulation may explain why an ORN probability effect was not obtained in previous electrophysiological studies. The specific functional role of ORN for concurrent sound perception was confirmed by dissociating it from the mismatch negativity (MMN) component as a correlate of processing sequential cues. ORN probability dependence is argued to be functionally relevant for the interaction of concurrent and sequential mechanisms of auditory scene analysis.