Erol J. Ozmeral

Object continuity enhances selective auditory attention

In complex scenes, the identity of an auditory object can build up across seconds. Given that attention operates on perceptual objects, this perceptual buildup may alter the efficacy of selective auditory attention over time. Here, we measured identification of a sequence of spoken target digits presented with distracter digits from other directions to investigate the dynamics of selective attention. Performance was better when the target location was fixed rather than changing between digits, even when listeners were cued as much as 1 s in advance about the position of each subsequent digit. Spatial continuity not only avoided well known costs associated with switching the focus of spatial attention, but also produced refinements in the spatial selectivity of attention across time. Continuity of target voice further enhanced this buildup of selective attention. Results suggest that when attention is sustained on one auditory object within a complex scene, attentional selectivity improves over time. Similar effects may come into play when attention is sustained on an object in a complex visual scene, especially in cases where visual object formation requires sustained attention.

Visually-guided Attention Enhances Target Identification in a Complex Auditory Scene

In auditory scenes containing many similar sound sources, sorting of acoustic information into streams becomes difficult, which can lead to disruptions in the identification of behaviorally relevant targets. This study investigated the benefit of providing simple visual cues for when and/or where a target would occur in a complex acoustic mixture. Importantly, the visual cues provided no information about the target content. In separate experiments, human subjects either identified learned birdsongs in the presence of a chorus of unlearned songs or recalled strings of spoken digits in the presence of speech maskers. A visual cue indicating which loudspeaker (from an array of five) would contain the target improved accuracy for both kinds of stimuli. A cue indicating which time segment (out of a possible five) would contain the target also improved accuracy, but much more for birdsong than for speech. These results suggest that in real world situations, information about where a target of interest is located can enhance its identification, while information about when to listen can also be helpful when targets are unfamiliar or extremely similar to their competitors.

Cortical interference effects in the cocktail party problem

Humans and animals must often discriminate between complex natural sounds in the presence of competing sounds (maskers). Although the auditory cortex is thought to be important in this task, the impact of maskers on cortical discrimination remains poorly understood. We examined neural responses in zebra finch (Taeniopygia guttata) field L (homologous to primary auditory cortex) to target birdsongs that were embedded in three different maskers (broadband noise, modulated noise and birdsong chorus). We found two distinct forms of interference in the neural responses: the addition of spurious spikes occurring primarily during the silent gaps between song syllables and the suppression of informative spikes occurring primarily during the syllables. Both effects systematically degraded neural discrimination as the target intensity decreased relative to that of the masker. The behavioral performance of songbirds degraded in a parallel manner. Our results identify neural interference that could explain the perceptual interference at the heart of the cocktail party problem.

Spatial unmasking of birdsong in human listeners: Energetic and informational factors

Spatial unmasking describes the improvement in the detection or identification of a target sound afforded by separating it spatially from simultaneous masking sounds. This effect has been studied extensively for speech intelligibility in the presence of interfering sounds. In the current study, listeners identified zebra finch song, which shares many acoustic properties with speech but lacks semantic and linguistic content. Three maskers with the same long-term spectral content but different short-term statistics were used: (1) chorus (combinations of unfamiliar zebra finch songs), (2) song-shaped noise (broadband noise with the average spectrum of chorus), and (3) chorus-modulated noise (song-shaped noise multiplied by the broadband envelope from a chorus masker). The amount of masking and spatial unmasking depended on the masker and there was evidence of release from both energetic and informational masking. Spatial unmasking was greatest for the statistically similar chorus masker. For the two noise maskers, there was less spatial unmasking and it was wholly accounted for by the relative target and masker levels at the acoustically better ear. The results share many features with analogous results using speech targets, suggesting that spatial separation aids in the segregation of complex natural sounds through mechanisms that are not specific to speech.

Spatial Unmasking of Birdsong in Zebra Finches (Taeniopygia guttata) and Budgerigars (Melopsittacus undulatus)

Budgerigars and zebra finches were tested, using operant conditioning techniques, on their ability to identify a zebra finch song in the presence of a background masker emitted from either the same or a different location as the signal. Identification thresholds were obtained for three masker types differing in their spectrotemporal characteristics (noise, modulated noise, and a song chorus). Both bird species exhibited similar amounts of spatial unmasking across the three masker types. The amount of unmasking was greater when the masker was played continuously compared to when the target and masker were presented simultaneously. These results suggest that spatial factors are important for birds in the identification of natural signals in noisy environments.

Exploring the benefit of auditory spatial continuity

Continuity of spatial location was recently shown to improve the ability to identify and recall a sequence of target digits presented in a mixture of confusable maskers [Best et al. (2008). Proc. Natl. Acad. Sci. U.S.A. 105, 13174-13178]. Three follow-up experiments were conducted to explore the basis of this improvement. The results suggest that the benefits of spatial continuity cannot be attributed to (a) the ability to plan where to direct attention in advance; (b) freedom from having to redirect attention across large distances; or (c) the challenge of filtering out signals that are confusable with the target.

How Visual Cues for when to Listen Aid Selective Auditory Attention

Visual cues are known to aid auditory processing when they provide direct information about signal content, as in lip reading. However, some studies hint that visual cues also aid auditory perception by guiding attention to the target in a mixture of similar sounds. The current study directly tests this idea for complex, nonspeech auditory signals, using a visual cue providing only timing information about the target. Listeners were asked to identify a target zebra finch bird song played at a random time within a longer, competing masker. Two different maskers were used: noise and a chorus of competing bird songs. On half of all trials, a visual cue indicated the timing of the target within the masker. For the noise masker, the visual cue did not affect performance when target and masker were from the same location, but improved performance when target and masker were in different locations. In contrast, for the chorus masker, visual cues improved performance only when target and masker were perceived as coming from the same direction. These results suggest that simple visual cues for when to listen improve target identification by enhancing sounds near the threshold of audibility when the target is energetically masked and by enhancing segregation when it is difficult to direct selective attention to the target. Visual cues help little when target and masker already differ in attributes that enable listeners to engage selective auditory attention effectively, including differences in spectrotemporal structure and in perceived location.

Behavioral and Neural Identification of Birdsong under Several Masking Conditions

Many animals are adept at identifying communication calls in the presence of competing sounds, from human listeners communicating in a cocktail party to penguins locating their kin amongst the thousands of conspecifics in their colony. The kind of perceptual interference in such settings differs from the interference arising when targets and maskers have dissimilar spectrotemporal structure (e.g., a speech target in broadband noise). In the latter case, performance is well modeled by accounting for the target-masker spectrotemporal overlap and any low-level binaural processing benefits that may occur for spatially separated sources (Zurek 1993). However, when the target and maskers are similar (e.g., a target talker in competing speech), a fundamentally different form of perceptual interference arises. In such cases, interference is reduced when target and masker are dissimilar (e.g., in timbre, pitch, perceived location, etc.), presumably by enabling a listener to focus attention on target attributes that differentiate it from the masker (Darwin and Hukin 2000; Freyman et al. 2001). We investigated the interference caused by different maskers when identifying bird songs. Using identical stimuli, three studies compare (a) human performance, (b) avian performance, and (c) neural coding in the avian auditory forebrain. Results show that the interference caused by maskers with spectrotemporal structure similar to the target differs from that caused by dissimilar maskers.

Age-related cortical changes in spatial auditory attention

Along with established effects of age on hearing sensitivity, there is a growing body of evidence that the aging auditory system suffers from reduced temporal resolution as well. This, in combination with changes in attentional-resource allocation, could have profound effects on the ability for older listeners to selectively attend to spatial locations—a key component to successful listening and communication in challenging auditory environments. Because behavioral tasks rarely have an unattended comparison and electrophysiological tasks rarely have an attended comparison, it is difficult to ascertain the extent to which selective attention mediates or sharpens spatial tuning. To address this shortcoming, we measured cortical responses using electroencephalography for moving stimuli in the free field during both passive and active conditions. Active conditions required listeners to respond to the onset of a stimulus when it occurred at a specific location (either 30° to the left or right of center). Both ...

Human electrophysiology of endogenous auditory spatial attention

Whereas perceiving the location of a stimulus in space relies on unique bottom-up processing, focusing attention to that location requires the availability and use of separate higher-level processes. It is unclear from previous work how selective attention might mediate and possibly sharpen spatial tuning, as behavioral tasks often lack an unattended comparison, and electrophysiological tasks often lack an attended comparison. We measured evoked responses to changes in the location of continuous noise in the free field when young, normal-hearing listeners were either passively listening or explicitly attending to one speaker location. Stimuli were presented from 1 of 5 frontal loudspeaker locations in the horizontal plane for 1.6 s before switching to another loudspeaker without pause. To ensure active attention for certain blocks, listeners were directed to press a button at the onset of a sound presented from the target location. Results show evidence of both facilitation for attended locations and supp...