Abstract
Human listeners can follow the voice of one speaker while several others are talking at the same time. This process requires segregating the speech streams from each other and continuously directing attention to the target stream. We investigated the functional brain networks underlying this ability. Two speech streams were presented simultaneously to participants, who followed one of them and detected targets within it (target stream). The loudness of the distractor speech stream varied on five levels: moderately softer, slightly softer, equal, slightly louder, or moderately louder than the attended. Performance measures showed that the most demanding task was the moderately softer distractors condition, which indicates that a softer distractor speech may receive more covert attention than louder distractors and, therefore, they require more cognitive resources. EEG-based measurement of functional connectivity between various brain regions revealed frequency-band specific networks: (1) energetic masking (comparing the louder distractor conditions with the equal loudness condition) was predominantly associated with stronger connectivity between the frontal and temporal regions at the lower alpha (8–10 Hz) and gamma (30–70 Hz) bands; (2) informational masking (comparing the softer distractor conditions with the equal loudness condition) was associated with a distributed network between parietal, frontal, and temporal regions at the theta (4–8 Hz) and beta (13–30 Hz) bands. These results suggest the presence of distinct cognitive and neural processes for solving the interference from energetic vs. informational masking.
Highlights
When two different sound streams are presented at the same time, the auditory system is able to separate them even though they may be interleaved and/or overlapped in time and frequency
The experiment was conducted in full accordance with the World Medical Association Helsinki Declaration and all applicable national laws and it was approved by the institutional review board, the United Ethical Review Committee for Research in Psychology (EPKEB)
As non-target numerals did not elicit the P3b response separate analysis of variance (ANOVA) were conducted for the P3b amplitude for target numerals with the factor of LOUDNESS, which revealed significant main effect (F4,104 = 5.446; ε = 0.8558; p < 0.001; η2p = 0.1667)
Summary
When two different sound streams are presented at the same time, the auditory system is able to separate them even though they may be interleaved and/or overlapped in time and frequency (auditory scene analysis, see Bregman, 1990). Two forms of masking may occur while listening to two competing speech streams: energetic and informational masking (Freyman et al, 1999; Brungart et al, 2001; Arbogast et al, 2002). While behavioral studies extensively investigated these masking effects on speech perception (Arbogast et al, 2002; Freyman et al, 2004; Ihlefeld and Shinn-Cunningham, 2008) the underlying neural processes are poorly understood. The cognitive and underlying neural mechanism for informational and energetic masking were assessed in dichotic listening conditions by combining the measurements of behavioral psychophysics and cortical electrophysiology (event-related potential [ERP] and functional connectivity [FC] analysis)
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