Promoting the perception of two and three concurrent sound objects: An event-related potential study

The auditory environment typically comprises several simultaneously active sound sources. In contrast to the perceptual segregation of two concurrent sounds, the perception of three simultaneous sound objects has not yet been studied systematically. We conducted two experiments in which participants were presented with complex sounds containing sound segregation cues (mistuning, onset asynchrony, differences in frequency or amplitude modulation or in sound location), which were set up to promote the perceptual organization of the tonal elements into one, two, or three concurrent sounds. In Experiment 1, listeners indicated whether they heard one, two, or three concurrent sounds. In Experiment 2, participants watched a silent subtitled movie while EEG was recorded to extract the object-related negativity (ORN) component of the event-related potential. Listeners predominantly reported hearing two sounds when the segregation promoting manipulations were applied to the same tonal element. When two different tonal elements received manipulations promoting them to be heard as separate auditory objects, participants reported hearing two and three concurrent sounds objects with equal probability. The ORN was elicited in most conditions; sounds that included the amplitude- or the frequency-modulation cue generated the smallest ORN amplitudes. Manipulating two different tonal elements yielded numerically and often significantly smaller ORNs than the sum of the ORNs elicited when the same cues were applied on a single tonal element. These results suggest that ORN reflects the presence of multiple concurrent sounds, but not their number. The ORN results are compatible with the horse-race principle of combining different cues of concurrent sound segregation.