A masker can reduce target intelligibility both by interfering with the target's peripheral representation ("energetic masking") and/or by causing more central interference ("informational masking"). Intelligibility generally improves with increasing spatial separation between two sources, an effect known as spatial release from masking (SRM). Here, SRM was measured using two concurrent sine-vocoded talkers. Target and masker were each composed of eight different narrowbands of speech (with little spectral overlap). The broadband target-to-masker energy ratio (TMR) was varied, and response errors were used to assess the relative importance of energetic and informational masking. Performance improved with increasing TMR. SRM occurred at all TMRs; however, the pattern of errors suggests that spatial separation affected performance differently, depending on the dominant type of masking. Detailed error analysis suggests that informational masking occurred due to failures in either across-time linkage of target segments (streaming) or top-down selection of the target. Specifically, differences in the spatial cues in target and masker improved streaming and target selection. In contrast, level differences helped listeners select the target, but had little influence on streaming. These results demonstrate that at least two mechanisms (differentially affected by spatial and level cues) influence informational masking.

In reverberant environments, acoustic reflections interfere with the direct sound arriving at a listener’s ears, distorting the spatial cues for sound localization. Yet, human listeners have little difficulty localizing sounds in most settings. Because reverberant energy builds up over time, the source location is represented relatively faithfully during the early portion of a sound, but this representation becomes increasingly degraded later in the stimulus. We show that the directional sensitivity of single neurons in the auditory midbrain of anesthetized cats follows a similar time course, although onset dominance in temporal response patterns results in more robust directional sensitivity than expected, suggesting a simple mechanism for improving directional sensitivity in reverberation. In parallel behavioral experiments, we demonstrate that human lateralization judgments are consistent with predictions from a population rate model decoding the observed midbrain responses, suggesting a subcortical origin for robust sound localization in reverberant environments.

If spatial attention acts like a "spotlight," focusing on one location and excluding others, it may be advantageous to have all targets of interest within the same spatial region. This hypothesis was explored using a task where listeners reported keywords from two simultaneous talkers. In Experiment 1, the two talkers were placed symmetrically about the frontal midline with various angular separations. While there was a small performance improvement for moderate separations, the improvement decreased for larger separations. However, the dependency of the relative talker intensities on spatial configuration accounted for these effects. Experiment 2 tested whether spatial separation improved the intelligibility of each source, an effect that could counteract any degradation in performance as sources fell outside the spatial spotlight of attention. In this experiment, intelligibility of individual sources was equalized across configurations by adding masking noise. Under these conditions, the cost of divided listening (the drop in performance when reporting both messages compared to reporting just one) was smaller when the spatial separation was small. These results suggest that spatial separation enhances the intelligibility of individual sources in a competing pair but increases the cost associated with having to process both sources simultaneously, consistent with the attentional spotlight hypothesis.

When listening selectively to one talker in a two-talker environment, performance generally improves with spatial separation of the sources. The current study explores the role of spatial separation in divided listening, when listeners reported both of two simultaneous messages processed to have little spectral overlap (limiting "energetic masking" between the messages). One message was presented at a fixed level, while the other message level varied from equal to 40 dB less than that of the fixed-level message. Results demonstrate that spatial separation of the competing messages improved divided-listening performance. Most errors occurred because listeners failed to report the content of the less-intense talker. Moreover, performance generally improved as the broadband energy ratio of the variable-level to the fixed-level talker increased. The error patterns suggest that spatial separation improves the intelligibility of the less-intense talker by improving the ability to (1) hear portions of the signal that would otherwise be masked, (2) segregate the two talkers properly into separate perceptual streams, and (3) selectively focus attention on the less-intense talker. Spatial configuration did not noticeably affect the ability to report the more-intense talker, suggesting that it was processed differently than the less-intense talker, which was actively attended.

When competing sources come from different directions, a desired target is easier to hear than when the sources are co-located. How much of this improvement is the result of spatial attention rather than improved perceptual segregation of the competing sources is not well understood. Here, listeners' attention was directed to spatial or nonspatial cues when they listened for a target masked by a competing message. A preceding cue signaled the target timbre, location, or both timbre and location. Spatial separation improved performance when the cue indicated the target location, or both the location and timbre, but not when the cue only indicated the target timbre. However, response errors were influenced by spatial configuration in all conditions. Both attention and streaming contributed to spatial effects when listeners actively attended to location. In contrast, when attention was directed to a nonspatial cue, spatial separation primarily appeared to improve the streaming of auditory objects across time. Thus, when attention is focused on location, spatial separation appears to improve both object selection and object formation; when attention is directed to nonspatial cues, separation affects object formation. These results highlight the need to distinguish between these separate mechanisms when considering how observers cope with complex auditory scenes.