Assessing the background decomposition of a complex auditory scene with event-related brain potentials

A listener who focusses on a sound source of interest must continuously integrate the sounds emitted by the attended source and ignore the sounds emitted by the remaining sources in the auditory scene. Little is known about how the ignored sound sources in the background are mentally represented after the source of interest has formed the perceptual foreground. This is due to a key methodological challenge: the background representation is by definition not overtly reportable. Here we developed a paradigm based on event-related brain potentials (ERPs) to assess the mental representation of background sounds. Participants listened to sequences of three repeatedly presented tones arranged in an ascending order (low, middle, high frequency). They were instructed to detect intensity deviants in one of the tones, creating the perceptual foreground. The remaining two background tones contained timing and location deviants. Those deviants were set up such that mismatch negativity (MMN) components would be elicited in distinct ways if the background was decomposed into two separate sound streams (background segregation) or if it was not further decomposed (background integration). Results provide MMN-based evidence for background segregation and integration in parallel. This suggests that mental representations of background integration and segregation can be concurrently available, and that collecting empirical evidence for only one of these background organization alternatives might lead to erroneous conclusions.