Divided listening in the free field becomes asymmetric when acoustic cues are limited

Abstract

[EN] Verbal communication in social environments often requires dividing attention between two or more simultaneous talkers. The ability to do this, however, may be diminished when the listener has limited access to acoustic cues or those cues are degraded, as is the case for hearing-impaired listeners or users of cochlear implants or hearing aids. The aim of the present study was to investigate the ability of normal-hearing (NH) listeners to divide their attention and recognize speech from two simultaneous talkers in simulated free-field listening conditions, with and without reduced acoustic cues. Participants (N = 11 or 12 depending on the experiment) were asked to recognize and repeat as many words as possible from two simultaneous, time-centered sentences uttered by a male and a female talker. In Experiment 1, the female and male talkers were located at –15° and +15°, –45° and +45°, or –90° and +90° azimuth, respectively. Speech was natural or processed through a noise vocoder and was presented at a comfortable loudness level (∼65 dB SPL). In Experiment 2, the female and male talkers were located at –45° and +45° azimuth, respectively. Speech was natural but was presented at a lower level (35 dB SPL) to reduce audibility. In Experiment 3, speech was vocoded and presented at a comfortable loudness level (∼65 dB SPL), but the location of the talkers was switched relative to Experiment 1 (i.e., the male and female talkers were at –45° and +45°, respectively) to reveal possible interactions of talker sex and location. Listeners recognized overall more natural words at a comfortable loudness level (76%) than vocoded words at a similar level (39%) or natural words at a lower level (43%). This indicates that recognition was more difficult for the two latter stimuli. On the other hand, listeners recognized roughly the same proportion of words (76%) from the two talkers when speech was natural and comfortable in loudness, but a greater proportion of words from the male than from the female talker when speech was vocoded (50% vs 27%, respectively) or was natural but lower in level (55% vs 32%, respectively). This asymmetry occurred and was similar for the three spatial configurations. These results suggest that divided listening becomes asymmetric when speech cues are reduced. They also suggest that listeners preferentially recognized the male talker, located on the right side of the head. Switching the talker’s location produced similar recognition for the two talkers for vocoded speech, suggesting an interaction between talkers’ location and their speech characteristics. For natural speech at comfortable loudness level, listeners can divide their attention almost equally between two simultaneous talkers. When speech cues are limited (as is the case for vocoded speech or for speech at low sensation level), by contrast, the ability to divide attention equally between talkers is diminished and listeners favor one of the talkers based on their location, sex, and/or speech characteristics. Findings are discussed in the context of limited cognitive capacity affecting dividing listening in difficult listening situations.