Sequential streaming, binaural cues and lateralization.

Interaural time differences (ITDs) and interaural level differences (ILDs) associated with monaural spectral differences (coloration) enable the localization of sound sources. The influence of these spatial cues as well as their relative importance on obligatory stream segregation were assessed in experiment 1. A temporal discrimination task favored by integration was used to measure obligatory stream segregation for sequences of speech-shaped noises. Binaural and monaural differences associated with different spatial positions increased discrimination thresholds, indicating that spatial cues can induce stream segregation. The results also demonstrated that ITDs and coloration were relatively more important cues compared to ILDs. Experiment 2 questioned whether sound segregation takes place at the level of acoustic cue extraction (ITD per se) or at the level of object formation (perceived azimuth). A difference in ITDs between stimuli was introduced either consistently or inconsistently across frequencies, leading to clearly lateralized sounds or blurred lateralization, respectively. Conditions with ITDs and clearly perceived azimuths induced significantly more segregation than the condition with ITDs but reduced lateralization. The results suggested that segregation was mainly based on a difference in lateralization, although the extraction of ITDs might have also helped segregation up to a ceiling magnitude.