Auditory Grouping Research Articles

Effect of auditory stream segregation cues on binaural pitch fusion

Binaural pitch fusion, the fusion of dichotically presented tones that evoke different pitches across ears, can be influenced by temporal envelope cues such as coherent amplitude modulation (Oh and Reiss, ASA 2017). This suggests that binaural pitch fusion may be governed at least in part by top-down processes involved in auditory grouping. The current study was designed to investigate how temporal streaming cues either prior or posterior to a fused dichotic tone pair can influence binaural pitch fusion. Six normal-hearing (NH) listeners and six bilateral hearing-aid (HA) users were tested in a modified auditory streaming paradigm (Steiger and Bregman, 1982). A pair of simultaneous, dichotic reference and comparison stimuli was placed in rapid alternation with a third binaural (diotic) capture stimulus, for a total of 10 alternations. The binaural fusion ranges, the frequency ranges over which binaural pitch fusion occurred for dichotic stimuli, were smaller when measured with the streaming paradigm than ...

Binaural Pitch Fusion: Effects of Amplitude Modulation.

Hearing-impaired adults, including both cochlear implant and bilateral hearing aid (HA) users, often exhibit broad binaural pitch fusion, meaning that they fuse dichotically presented tones with large pitch differences between ears. The current study was designed to investigate how binaural pitch fusion can be influenced by amplitude modulation (AM) of the stimuli and whether effects differ with hearing loss. Fusion ranges, the frequency ranges over which binaural pitch fusion occurs, were measured in both normal-hearing (NH) listeners and HA users with various coherent AM rates (2, 4, and 8 Hz); AM depths (20%, 40%, 60%, 80%, and 100%); and interaural AM phase and AM rate differences. The averaged results show that coherent AM increased binaural pitch fusion ranges to about 2 to 4 times wider than those in the unmodulated condition in both NH and bilateral HA subjects. Even shallow temporal envelope fluctuations (20% AM depth) significantly increased fusion ranges in all three coherent AM rate conditions. Incoherent AM introduced through interaural differences in AM phase or AM rate led to smaller increases in binaural pitch fusion range compared with those observed with coherent AM. Significant differences between groups were observed only in the coherent AM conditions. The influence of AM cues on binaural pitch fusion shows that binaural fusion is mediated in part by central processes involved in auditory grouping.

Selective auditory grouping by zebra finches: testing the iambic\u2013trochaic law

Humans have a strong tendency to spontaneously group visual or auditory stimuli together in larger patterns. One of these perceptual grouping biases is formulated as the iambic/trochaic law, where humans group successive tones alternating in pitch and intensity as trochees (high–low and loud–soft) and alternating in duration as iambs (short–long). The grouping of alternations in pitch and intensity into trochees is a human universal and is also present in one non-human animal species, rats. The perceptual grouping of sounds alternating in duration seems to be affected by native language in humans and has so far not been found among animals. In the current study, we explore to which extent these perceptual biases are present in a songbird, the zebra finch. Zebra finches were trained to discriminate between short strings of pure tones organized as iambs and as trochees. One group received tones that alternated in pitch, a second group heard tones alternating in duration, and for a third group, tones alternated in intensity. Those zebra finches that showed sustained correct discrimination were next tested with longer, ambiguous strings of alternating sounds. The zebra finches in the pitch condition categorized ambiguous strings of alternating tones as trochees, similar to humans. However, most of the zebra finches in the duration and intensity condition did not learn to discriminate between training stimuli organized as iambs and trochees. This study shows that the perceptual bias to group tones alternating in pitch as trochees is not specific to humans and rats, but may be more widespread among animals.

Auditory Rhythms Influence Judged Time to Contact of an Occluded Moving Object

We studied the expected moment of reappearance of a moving object after it disappeared from sight. In particular, we investigated whether auditory rhythms influence time to contact (TTC) judgments. Using displays in which a moving disk disappears behind an occluder, we examined whether an accompanying auditory rhythm influences the expected TTC of an occluded moving object. We manipulated a baseline auditory rhythm — consisting of equal sound and pause durations — in two ways: either the pause durations or the sound durations were increased to create slower rhythms. Participants had to press a button at the moment they expected the disk to reappear. Variations in pause duration (Experiments 1 and 2) affected expected TTC, in contrast to variations in sound duration (Experiment 3). These results show that auditory rhythms affect expected reappearance of an occluded moving object. Second, these results suggest that temporal auditory grouping is an important factor in TTC.

Speech-on-speech recognition in children and adults: Effect of whispered speech

The harmonic structure of speech appears to provide adults with a robust auditory grouping cue, improving speech-on-speech recognition performance. This study evaluated the effect of harmonicity on speech-on-speech recognition in normal-hearing children (5-7 yrs) and adults (19-35 yrs). Masked word recognition was evaluated using a 3AFC procedure with a picture-pointing response. Targets were one-syllable words that lent themselves to illustration, and maskers were samples of two-talker speech. All stimuli were recorded by female talkers in two styles: whispered and natural. Thresholds corresponding to 70.7% correct word recognition were estimated adaptively by adjusting SNR while holding the overall level of the signal-plus-masker at 60 dB SPL throughout testing. Performance was evaluated for whispered target words presented in a whispered two-talker masker and for naturally produced target words presented in a naturally produced two-talker masker. Consistent with previous reports, adults required a more...

Auditory Scene Analysis and the Perception of Sound Mass in Ligeti’s Continuum

Sound mass has been an influential trend in music since the 1950’s and yet many questions about its perception remain unanswered. Approaching sound mass from the perspective of auditory scene analysis, we define it as a type of auditory grouping that retains an impression of multiplicity even as it is perceived as a perceptual unit. Sound mass requires all markers of the individual identities of sounds to be deemphasized to prevent them from splitting off into separate streams. Seeking to determine how consistent listeners are in their perception of sound mass, and whether it is possible to determine sound parameters and threshold values that predict sound mass perception, we conducted two perceptual studies on Ligeti’s Continuum. This piece consists of an extremely rapid, steady stream of eighth-note dyads with no tempo changes. We addressed the claim by Ligeti and others that the fusion into a continuous texture or sound mass occurs at ca. 20 attacks/s, hypothesizing that other factors such as pitch organization, emergent rhythm, timbre, and register would affect this value. A variety of factors were found to affect sound mass perception, suggesting that the threshold value is not absolute but varies according to principles of auditory scene analysis.

Neural realignment of spatially separated sound components.

Natural auditory scenes often consist of several sound sources overlapping in time, but separated in space. Yet, location is not fully exploited in auditory grouping: spatially separated sounds can get perceptually fused into a single auditory object and this leads to difficulties in the identification and localization of concurrent sounds. Here, the brain mechanisms responsible for grouping across spatial locations were explored in magnetoencephalography (MEG) recordings. The results show that the cortical representation of a vowel spatially separated into two locations reflects the perceived location of the speech sound rather than the physical locations of the individual components. In other words, the auditory scene is neurally rearranged to bring components into spatial alignment when they were deemed to belong to the same object. This renders the original spatial information unavailable at the level of the auditory cortex and may contribute to difficulties in concurrent sound segregation.

A function for binaural integration in auditory grouping and segregation in the inferior colliculus.

Responses of neurons to binaural, harmonic complex stimuli in urethane-anesthetized guinea pig inferior colliculus (IC) are reported. To assess the binaural integration of harmonicity cues for sound segregation and grouping, responses were measured to harmonic complexes with different fundamental frequencies presented to each ear. Simultaneously gated harmonic stimuli with fundamental frequencies of 125 Hz and 145 Hz were presented to the left and right ears, respectively, and recordings made from 96 neurons with characteristic frequencies >2 kHz in the central nucleus of the IC. Of these units, 70 responded continuously throughout the stimulus and were excited by the stimulus at the contralateral ear. The stimulus at the ipsilateral ear excited (EE: 14%; 10/70), inhibited (EI: 33%; 23/70), or had no significant effect (EO: 53%; 37/70), defined by the effect on firing rate. The neurons phase locked to the temporal envelope at each ear to varying degrees depending on signal level. Many of the cells (predominantly EO) were dominated by the response to the contralateral stimulus. Another group (predominantly EI) synchronized to the contralateral stimulus and were suppressed by the ipsilateral stimulus in a phasic manner. A third group synchronized to the stimuli at both ears (predominantly EE). Finally, a group only responded when the waveform peaks from each ear coincided. We conclude that these groups of neurons represent different "streams" of information but exhibit modifications of the response rather than encoding a feature of the stimulus, like pitch.

Identification and multiplicity of double vowels in cochlear implant users.

The present study examined cochlear implant (CI) users' perception of vowels presented concurrently (i.e., double vowels) to further our understanding of auditory grouping in electric hearing. Identification of double vowels and single vowels was measured with 10 CI subjects. Fundamental frequencies (F0s) of vowels were either 100 + 100 Hz or 100 + 300 Hz. Vowels were presented either synchronously or with a time delay. In "Double" sessions, subjects were given only double vowels. In "Double + Single" sessions, while double and single vowels were presented, subjects reported the number and identity of the vowel(s). In addition to clinical settings, stimuli were delivered via an experimental method that interleaved pulse streams of two vowels. Although the time delay between vowels had a large effect on identification, the effect of change in fundamental frequency (ΔF0) was modest. Enumeration was poor in general, and identification of synchronous vowels was above chance in only the Double sessions with a priori knowledge about presentation. Interleaved presentation of vowel streams provided no benefit for identification and a marginal benefit for enumeration. The results demonstrate the importance of episodic context for CI users. Unreliable perception of multiplicity observed in the present results suggests that auditory grouping in CIs may be driven by a schema-based process.

The Amount of Language Exposure Determines Nonlinguistic Tone Grouping Biases in Infants From a Bilingual Environment

Duration‐based auditory grouping preferences are presumably shaped by language experience in adults and infants, unlike intensity‐based grouping that is governed by a universal bias of a loud‐soft preference. It has been proposed that duration‐based rhythmic grouping preferences develop as a function of native language phrasal prosody. Additionally, it has been suggested that phrasal prosody supports syntax acquisition (e.g., prosodic bootstrapping of word order within phrases). Using a looking preference procedure, in the current study, 9‐to‐10‐month‐old Spanish‐dominant and Basque‐dominant bilingual infants’ rhythmic preferences in response to nonlinguistic tones alternating in duration or intensity were assessed. In the intensity‐based condition no effects of language experience was present. In the duration‐based condition, however, infants exhibited grouping patterns as predicted by the phrasal prosody of their dominant input. Considering the proposed link between syntactic bootstrapping and perceptual tone grouping, our overall results suggest that syntax acquisition (e.g., learning the rules of word order) is supported by different auditory perceptual mechanisms for the dominant syntax than for the less dominant syntax in the infant's dual language input.

Temporal and spatial coherence as cues for across-frequency grouping in treefrogs

Humans exploit environmental regularities in sounds to perceptually bind acoustic energy occurring simultaneously at different frequencies. Such abilities influence vowel perception in speech and timbre perception in music. Other animals solve similar binding problems in the recognition of species-specific acoustic signals. Moreover, they commonly do so using auditory systems that differ in notable ways from that of mammals. This study of two treefrog species investigated temporal and spatial coherence as cues that promote grouping of two spectral bands emphasized in their acoustic signals. In two-alternative choice tests, females preferred temporally and spatially coherent calls over alternatives in which the onsets/offsets of the two bands were time-shifted by more than 25 ms or in which the two bands were spatially separated by 7.5° or more. These results, which suggest temporal coherence and spatial coherence promote across-frequency auditory grouping, are notable given differences in how the two spectral bands are processed by the anuran auditory system. Sound energy in the high- and low-frequency bands primarily enters the auditory system via different pathways (tympanum and body wall, respectively) and is encoded primarily by different papillae in the inner ear (basilar papilla and amphibian papilla, respectively).

Treefrogs as animal models for research on auditory scene analysis and the cocktail party problem

The perceptual analysis of acoustic scenes involves binding together sounds from the same source and separating them from other sounds in the environment. In large social groups, listeners experience increased difficulty performing these tasks due to high noise levels and interference from the concurrent signals of multiple individuals. While a substantial body of literature on these issues pertains to human hearing and speech communication, few studies have investigated how nonhuman animals may be evolutionarily adapted to solve biologically analogous communication problems. Here, I review recent and ongoing work aimed at testing hypotheses about perceptual mechanisms that enable treefrogs in the genus Hyla to communicate vocally in noisy, multi-source social environments. After briefly introducing the genus and the methods used to study hearing in frogs, I outline several functional constraints on communication posed by the acoustic environment of breeding “choruses”. Then, I review studies of sound source perception aimed at uncovering how treefrog listeners may be adapted to cope with these constraints. Specifically, this review covers research on the acoustic cues used in sequential and simultaneous auditory grouping, spatial release from masking, and dip listening. Throughout the paper, I attempt to illustrate how broad-scale, comparative studies of carefully considered animal models may ultimately reveal an evolutionary diversity of underlying mechanisms for solving cocktail-party-like problems in communication.

Foreground‐background discrimination indicated by event‐related brain potentials in a new auditory multistability paradigm

For studying multistable auditory perception, we propose a paradigm that evokes integrated or segregated perception of a sound sequence, and permits decomposition of the segregated grouping into foreground and background sounds. The paradigm combines 3-tone pitch patterns with alternating timbres, resulting in a repeating 6-tone structure that can be perceived as rising based on temporal proximity, or as falling based on timbre similarity. Listeners continuously report their percept while EEG is recorded. Results show an ERP modulation starting at ∼70 ms after sound onset that can be explained by whether a sound belongs to perceived foreground or background, with no additional effect of integrated versus segregated grouping. Auditory grouping as indexed by the mismatch negativity did not correspond with reported sound grouping. The paradigm offers a new possibility for investigating effects of conscious perceptual organization on sound processing.

Musical Elaborations in Sound and Sight in the Work of Charles Seeger and Hans Richter

Musical Elaborations in Sound and Sight in the Work of Charles Seeger and Hans Richter

Do frog-eating bats perceptually bind the complex components of frog calls?

The mating calls of male túngara frogs, Physalaemus pustulosus, attract intended (conspecific females) and unintended (eavesdropping predators and parasites) receivers. The calls are complex, having two components: a frequency-modulated "whine" followed by 0-7 harmonic bursts or "chucks". The whine is necessary and sufficient to elicit phonotaxis from females and the chuck enhances call attractiveness when it follows a whine. Although chucks are never made alone, females perceptually bind the whine and chuck when they are spatially separated. We tested whether an unintended receiver with independent evolution of phonotaxis, the frog-eating bat, Trachops cirrhosus, has converged with frogs in its auditory grouping of the call components. In contrast to frogs, bats approached chucks broadcast alone; when the chuck was spatially separated from the whine the bats preferentially approached the whine, and bats were sensitive to whine-chuck temporal sequence. This contrast suggests that although disparate taxa may be selected to respond to the same signals, different evolutionary histories, selective regimes, and neural and cognitive architectures may result in different weighting and grouping of signal components between generalist predators and conspecific mates.

Release From Perceptual Masking for Children and Adults

The purpose of this study was to test the hypothesis that a carrier phrase can improve word recognition performance for both children and adults by providing an auditory grouping cue. It was hypothesized that the carrier phrase would benefit listeners under conditions in which they have difficulty in perceptually separating the target word from the competing background. To test this hypothesis, word recognition was examined for maskers that were believed to vary in their ability to create perceptual masking. In addition to determining the conditions under which a carrier-phrase benefit is obtained, age-related differences in both susceptibility to masking and carrier-phrase benefit were examined. Two experiments were conducted to characterize developmental effects in the ability to benefit from a carrier phrase (i.e., "say the word") before the target word. Using an open-set task, word recognition performance was measured for three listener age groups: 5- to 7-year-old children, 8- to 10-year-old children, and adults (18-30 years). For all experiments, target words were presented in each of two carrier-phrase conditions: (1) carrier-present and (2) carrier-absent. Across experiments, word recognition performance was assessed in the presence of multi-talker babble (Experiment 1), two-talker speech (Experiment 2), or speech-shaped noise (Experiment 2). Children's word recognition performance was generally poorer than that of adults for all three masker conditions. Differences between the two age groups of children were seen for both speech-shaped noise and multi-talker babble, with 5- to 7-year-olds performing more poorly than 8- to 10-year-olds. However, 5- to 7-year-olds and 8- to 10-year-olds performed similarly for the two-talker masker. Despite developmental effects in susceptibility to masking, both groups of children and adults showed a carrier-phrase benefit in multi-talker babble (Experiment 1) and in the two-talker masker (Experiment 2). The magnitude of the carrier-phrase benefit was similar for a given masker type across age groups, but the carrier-phrase benefit was greater in the presence of the two-talker masker than in multi-talker babble. Specifically, the children's average carrier-phrase benefit was 7.1% for multi-talker and 16.8% for the two-talker masker condition. No carrier-phrase benefit was observed for any age group in the presence of speech-shaped noise. Effects of auditory masking on word recognition performance were greater for children than for adults. The time course of development for susceptibility to masking seems to be more prolonged for a two-talker speech masker than for multi-talker babble or speech-shaped noise. Unique to the present study, this work suggests that a carrier phrase can provide an effective auditory grouping cue for both children and adults under conditions expected to produce substantial perceptual masking.

Comodulation masking release: Effects of training and experimental design on use of within- and across-channel cues

The effects of training and experimental design on comodulation masking release (CMR) were assessed. The study of Dau et al. [(2009), J. Acoust. Soc. Am. 125, 2182-2188], which used auditory-grouping manipulations to distinguish the use of within- and across-channel cues to CMR, was replicated in Experiment One but using naive subjects and an experimental design that minimized familiarization with the cues. Subjects made effective use of within- but not across-channel cues. Experiment Two examined training effects over more testing sessions, across four experimental designs (to minimize or maximize repeated exposure to the cues) and using an auditory grouping manipulation ("postcursors") to distinguish the use of within- and across-channel cues. Naive subjects were tested with either two or four flanking bands (FBs), to determine if training effects varied with the amount of FB information. Within-channel cues could be used from the outset, but effective use of across-channel cues required training when they were less salient. Increased repeated exposure enhanced the effects of training. Experiment Three tested naive subjects using two FBs, but with noise presented continuously and a different auditory grouping manipulation, after Grose et al. [(2009), J. Acoust. Soc. Am. 125, 282-293]. CMR was large from the outset.

Why do fish school?

Abstract Synchronized movements (schooling) emit complex and overlapping sound and pressure curves that might confuse the inner ear and lateral line organ (LLO) of a predator. Moreover, prey-fish moving close to each other may blur the electro-sensory perception of predators. The aim of this review is to explore mechanisms associated with synchronous swimming that may have contributed to increased adaptation and as a consequence may have influenced the evolution of schooling. The evolutionary development of the inner ear and the LLO increased the capacity to detect potential prey, possibly leading to an increased potential for cannibalism in the shoal, but also helped small fish to avoid joining larger fish, resulting in size homogeneity and, accordingly, an increased capacity for moving in synchrony. Water-movements and incidental sound produced as by-product of locomotion (ISOL) may provide fish with potentially useful information during swimming, such as neighbour body-size, speed, and location. When many fish move close to one another ISOL will be energetic and complex. Quiet intervals will be few. Fish moving in synchrony will have the capacity to discontinue movements simultaneously, providing relatively quiet intervals to allow the reception of potentially critical environmental signals. Besides, synchronized movements may facilitate auditory grouping of ISOL. Turning preference bias, well-functioning sense organs, good health, and skillful motor performance might be important to achieving an appropriate distance to school neighbors and aid the individual fish in reducing time spent in the comparatively less safe school periphery. Turning preferences in ancestral fish shoals might have helped fish to maintain groups and stay in formation, reinforcing aforementioned predator confusion mechanisms, which possibly played a role in the lateralization of the vertebrate brain.

Mechanisms of Auditory Grouping in Cope’s Gray Treefrog

Event Abstract Back to Event Mechanisms of Auditory Grouping in Cope’s Gray Treefrog Katrina Schrode1* and Mark A. Bee1, 2 1 University of Minnesota, Graduate Program in Neuroscience, United States 2 University of Minnesota, Department of Ecology, Evolution, and Behavior, United States Many animals communicate acoustically in large, social aggregations, where the overlapping signals of conspecifics make it difficult to follow a particular signal. Part of the solution to this problem requires listeners to integrate or group sound elements by their sources. Humans use spectral proximity and other acoustic properties as auditory grouping cues, but less is known about the mechanisms other animals may use. Here, we tested the hypothesis that females of Cope’s gray treefrog (Hyla chrysoscelis) use spectral proximity to group sequential elements of communication signals. Males of this species produce a pulsed advertisement call, to which females exhibit robust approach behavior (phonotaxis). We manipulated the spectral proximity of pulses within an advertisement call by raising or lowering the frequency of some pulses from the baseline carrier frequency. Females were given a choice between this manipulated call and a shorter call with no spectral incoherence. When presented with two calls, females generally discriminate strongly against the shorter of the two, and against calls that are missing pulses. We expected if females were able to use spectral proximity to group pulses, at small ΔF they should perceive the longer signal as a coherent, attractive call, and prefer it to the shorter call. Conversely, when ΔF was large, pulses of the long call were less likely to be grouped, making short calls more attractive. Natural calls have a bimodal frequency spectrum, with peaks at 1.3 kHz and 2.6 kHz. Each of these frequencies is primarily encoded by a distinct sensory organ (papilla) in the frog’s inner ear. We tested females with calls that had only one of the natural frequencies as its carrier frequency. This design ensured that most stimuli were primarily encoded by one of the sensory papillae, but at high stimulus amplitudes, dual stimulation of the papillae was likely. To further elucidate the roles of the sensory papillae, we performed replicates of the experiment at levels of 85 dB, 73 dB, and 61 dB. The lower levels reduced the likelihood of dual activation and better isolate the roles of the individual papillae. Female responses indicated that they are able to group acoustic signals using frequency cues, but that the mechanism responsible seems to differ between the sensory papilla. Responses to low frequency stimuli suggested that females were grouping based on frequency proximity, as humans do, while responses to high frequency stimuli were indicative of a mechanism based on perceived level differences. Our results suggest that frogs, like humans, use frequency cues for auditory grouping, but that their use of frequency cues may be supported by two different neural mechanisms, one similar to that of humans, and one that differs. [Supported by NIDCD 5R01DC009582.] Keywords: auditory grouping, Hyla chrysoscelis, sound source segregation Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Poster Presentation (see alternatives below as well) Topic: Sensory: Audition Citation: Schrode K and Bee MA (2012). Mechanisms of Auditory Grouping in Cope’s Gray Treefrog. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00183 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 28 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Ms. Katrina Schrode, University of Minnesota, Graduate Program in Neuroscience, St. Paul, United States, schro641@umn.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. 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Auditory grouping mechanisms reflect a sound's relative position in a sequence

The human brain uses acoustic cues to decompose complex auditory scenes into its components. For instance to improve communication, a listener can select an individual “stream,” such as a talker in a crowded room, based on cues such as pitch or location. Despite numerous investigations into auditory streaming, few have demonstrated clear correlates of perception; instead, in many studies perception covaries with changes in physical stimulus properties (e.g., frequency separation). In the current report, we employ a classic ABA streaming paradigm and human electroencephalography (EEG) to disentangle the individual contributions of stimulus properties from changes in auditory perception. We find that changes in perceptual state—that is the perception of one versus two auditory streams with physically identical stimuli—and changes in physical stimulus properties are reflected independently in the event-related potential (ERP) during overlapping time windows. These findings emphasize the necessity of controlling for stimulus properties when studying perceptual effects of streaming. Furthermore, the independence of the perceptual effect from stimulus properties suggests the neural correlates of streaming reflect a tone's relative position within a larger sequence (1st, 2nd, 3rd) rather than its acoustics. By clarifying the role of stimulus attributes along with perceptual changes, this study helps explain precisely how the brain is able to distinguish a sound source of interest in an auditory scene.