Theta oscillations accompanying concurrent auditory stream segregation

• Oscillatory activity evoked by stimuli promoting concurrent sound segregation was studied
• A short-latency theta response appeared both in the passive and active listening conditions
• A longer-latency theta response was elicited in the active but not in the passive listening condition
• All auditory cues elicited theta activity but their scalp distribution differed from each other
• Theta oscillations may mediate concurrent sound segregation processes

The ability to isolate a single sound source among concurrent sources is crucial for veridical auditory perception. The present study investigated the event-related oscillations evoked by complex tones, which could be perceived as a single sound and tonal complexes with cues promoting the perception of two concurrent sounds by inharmonicity, onset asynchrony, and/or perceived source location difference of the components tones. In separate task conditions, participants performed a visual change detection task (visual control), watched a silent movie (passive listening) or reported for each tone whether they perceived one or two concurrent sounds (active listening). In two time windows, the amplitude of theta oscillation was modulated by the presence vs. absence of the cues: 60–350 ms/6–8 Hz (early) and 350–450 ms/4–8 Hz (late). The early response appeared both in the passive and the active listening conditions; it did not closely match the task performance; and it had a fronto-central scalp distribution. The late response was only elicited in the active listening condition; it closely matched the task performance; and it had a centro-parietal scalp distribution. The neural processes reflected by these responses are probably involved in the processing of concurrent sound segregation cues, in sound categorization, and response preparation and monitoring. The current results are compatible with the notion that theta oscillations mediate some of the processes involved in concurrent sound segregation.