In the cocktail party situation, people with normal hearing usually follow a single
speaker among multiple concurrent ones. However, there is no agreement in the literature
as to whether the background is segregated into multiple streams/speakers. The current
study varied the number of concurrent speech streams and investigated target detection
and memory for the contents of a target stream as well as the processing of distractors.
A male-voiced target stream was either presented alone (single-speech), together with
one male-voiced distractor (one-distractor), or a male- and a female-voiced distractor
(two-distractor). Behavioral measures of target detection and content tracking performance
as well as target- and distractor detection related event-related brain potentials
(ERPs) were assessed. We found that the N2 amplitude decreased whereas the P3 amplitude
increased from the single-speech to the concurrent speech streams conditions. Importantly,
the behavioral effect of distractors differed between the conditions with one vs.
two distractor speech streams and the non-zero voltages in the N2 time window for
distractor numerals and in the P3 time window for syntactic violations appearing in
the non-target speech stream significantly differed between the one- and two-distractor
conditions for the same (male) speaker. These results support the notion that the
two background speech streams are segregated, as they show that distractors and syntactic
violations appearing in the non-target streams are processed even when two speech
non-target speech streams are delivered together with the target stream.
