Electrophysiological Correlates of Speaker Segregation and Foreground-Background Selection in Ambiguous Listening Situations

In everyday listening environments, a main task for our auditory system is to follow one out of multiple speakers talking simultaneously. The present study was designed to find electrophysiological indicators of two central processes involved - segregating the speech mixture into distinct speech sequences corresponding to the two speakers, and then attending to one of the speech sequences. We generated multistable speech stimuli that were set up to create ambiguity as to whether only one or two speakers are talking. Thereby we were able to investigate three perceptual alternatives (no segregation, segregated - speaker A in the foreground, segregated - speaker B in the foreground) without any confounding stimulus changes. Participants listened to a continuously repeating sequence of syllables, which were uttered alternately by two human speakers, and indicated whether they perceived the sequence as an inseparable mixture or as originating from two separate speakers. In the latter case, they distinguished which speaker was in their attentional foreground. Our data show a long-lasting event-related potential (ERP) modulation starting at 130 ms after stimulus onset, which can be explained by the perceptual organization of the two speech sequences into attended foreground and ignored background streams. Our paradigm extends previous work with pure-tone sequences toward speech stimuli and adds the possibility to obtain neural correlates of the difficulty to segregate a speech mixture into distinct streams.