Top-down effects can modify the initially stimulus-driven auditory organization

Development of auditory scene analysis: a mini-review

Most auditory environments contain multiple sound waves that are mixed before reaching the ears. In such situations, listeners must disentangle individual sounds from the mixture, performing the auditory scene analysis. Analyzing complex auditory scenes relies on listeners ability to segregate acoustic events into different streams, and to selectively attend to the stream of interest. Both segregation and selective attention are known to be challenging for adults with normal hearing, and seem to be even more difficult for children. Here, we review the recent literature on the development of auditory scene analysis, presenting behavioral and neurophysiological results. In short, cognitive and neural mechanisms supporting stream segregation are functional from birth but keep developing until adolescence. Similarly, from 6 months of age, infants can orient their attention toward a target in the presence of distractors. However, selective auditory attention in the presence of interfering streams only reaches maturity in late childhood at the earliest. Methodological limitations are discussed, and a new paradigm is proposed to clarify the relationship between auditory scene analysis and speech perception in noise throughout development.

Why are listeners hindered by talker variability?

Though listeners readily recognize speech from a variety of talkers, accommodating talker variability comes at a cost: Myriad studies have shown that listeners are slower to recognize a spoken word when there is talker variability compared with when talker is held constant. This review focuses on two possible theoretical mechanisms for the emergence of these processing penalties. One view is that multitalker processing costs arise through a resource-demanding talker accommodation process, wherein listeners compare sensory representations against hypothesized perceptual candidates and error signals are used to adjust the acoustic-to-phonetic mapping (an active control process known as contextual tuning). An alternative proposal is that these processing costs arise because talker changes involve salient stimulus-level discontinuities that disrupt auditory attention. Some recent data suggest that multitalker processing costs may be driven by both mechanisms operating over different time scales. Fully evaluating this claim requires a foundational understanding of both talker accommodation and auditory streaming; this article provides a primer on each literature and also reviews several studies that have observed multitalker processing costs. The review closes by underscoring a need for comprehensive theories of speech perception that better integrate auditory attention and by highlighting important considerations for future research in this area.

Contra assertions, feedback improves word recognition: How feedback and lateral inhibition sharpen signals over noise

Whether top-down feedback modulates perception has deep implications for cognitive theories. Debate has been vigorous in the domain of spoken word recognition, where competing computational models and agreement on at least one diagnostic experimental paradigm suggest that the debate may eventually be resolvable. Norris and Cutler (2021) revisit arguments against lexical feedback in spoken word recognition models. They also incorrectly claim that recent computational demonstrations that feedback promotes accuracy and speed under noise (Magnuson et al., 2018) were due to the use of the Luce choice rule rather than adding noise to inputs (noise was in fact added directly to inputs). They also claim that feedback cannot improve word recognition because feedback cannot distinguish signal from noise. We have two goals in this paper. First, we correct the record about the simulations of Magnuson et al. (2018). Second, we explain how interactive activation models selectively sharpen signals via joint effects of feedback and lateral inhibition that boost lexically-coherent sublexical patterns over noise. We also review a growing body of behavioral and neural results consistent with feedback and inconsistent with autonomous (non-feedback) architectures, and conclude that parsimony supports feedback. We close by discussing the potential for synergy between autonomous and interactive approaches.

Sound category habituation requires task-relevant attention

Introduction

Processing the wealth of sensory information from the surrounding environment is a vital human function with the potential to develop learning, advance social interactions, and promote safety and well-being.

Methods

To elucidate underlying processes governing these activities we measured neurophysiological responses to patterned stimulus sequences during a sound categorization task to evaluate attention effects on implicit learning, sound categorization, and speech perception. Using a unique experimental design, we uncoupled conceptual categorical effects from stimulus-specific effects by presenting categorical stimulus tokens that did not physically repeat.

Results

We found effects of implicit learning, categorical habituation, and a speech perception bias when the sounds were attended, and the listeners performed a categorization task (task-relevant). In contrast, there was no evidence of a speech perception bias, implicit learning of the structured sound sequence, or repetition suppression to repeated within-category sounds (no categorical habituation) when participants passively listened to the sounds and watched a silent closed-captioned video (task-irrelevant). No indication of category perception was demonstrated in the scalp-recorded brain components when participants were watching a movie and had no task with the sounds.

Discussion

These results demonstrate that attention is required to maintain category identification and expectations induced by a structured sequence when the conceptual information must be extracted from stimuli that are acoustically distinct. Taken together, these striking attention effects support the theoretical view that top-down control is required to initiate expectations for higher level cognitive processing.

The impact of spatial variance on precision estimates in an auditory oddball paradigm

Predictive processing theories suggest that a principal function of the brain is to reduce the surprise of incoming sensory information by creating accurate and precise models of the environment. These models are commonly explored by looking at the prediction errors elicited when experience departs from predictions. One such prediction error is the mismatch negativity (MMN). Using this component, it is possible to examine the effect of external noise on the precision of the developed model. Recent studies have shown that the brain may not update its model every time there is a change in the environment, rather it will only update it when doing so will increase precision and or accuracy of the model. The current study examined this process using oddball sound sequences with high and low spatial variability and examining how this affected the elicited MMN to a duration deviant sound. The results showed a strong null effect of spatial variance both at a local and sequence levels. These results indicate that variability in the sound sequence will not invariably affect model precision estimates and thus the amplitude of the MMN component.

Altered evoked responses for motor-related words in children with upper limb motor impairments

OBJECTIVE

Obstetric brachial plexus palsy (OBPP) and amyoplasia, the classical type of arthrogryposis multiplex congenita, manifest themselves as highly limited mobility of the upper limb. At the same time, according to the embodiment cognition theories, the motor impairments might lead to the alteration of cognitive functions in OBPP/amyoplasia patients. In the current study, we examined whether OBPP/amyoplasia children exhibit altered processing of motor-related verbs.

METHODS

We conducted a case-control study using clinical population and control children. Oddball series were used to elicit mismatch negativity (MMN) EEG responses. The series consisted of limb-related verbs (deviant stimuli) and matched pseudowords (standard stimuli). 27 patients and 32 control children were included in the analysis.

RESULTS

We showed that MMN waveforms differed between OBPP/amyoplasia children and their control peers in the frontal and temporal electrodes when the stimuli contained hand-related verbs. In particular, the MMN peak latency in the OBPP/amyoplasia children was significantly delayed as compared with the healthy controls. At the same time, neither series with leg-related verbs nor series of pseudowords resulted in statistically significant differences.

CONCLUSIONS

Our findings suggest altered processing of hand-related verbs in OBPP/amyoplasia children with hand-related disabilities.

SIGNIFICANCE

Our results contribute to the growing evidence in support of the theory of embodied cognition, which proposes that various domains of cognition are shaped by bodily interactions with the environment.

Processing of an Audiobook in the Human Brain Is Shaped by Cultural Family Background

Perception of the same narrative can vary between individuals depending on a listener’s previous experiences. We studied whether and how cultural family background may shape the processing of an audiobook in the human brain. During functional magnetic resonance imaging (fMRI), 48 healthy volunteers from two different cultural family backgrounds listened to an audiobook depicting the intercultural social life of young adults with the respective cultural backgrounds. Shared cultural family background increased inter-subject correlation of hemodynamic activity in the left-hemispheric Heschl’s gyrus, insula, superior temporal gyrus, lingual gyrus and middle temporal gyrus, in the right-hemispheric lateral occipital and posterior cingulate cortices as well as in the bilateral middle temporal gyrus, middle occipital gyrus and precuneus. Thus, cultural family background is reflected in multiple areas of speech processing in the brain and may also modulate visual imagery. After neuroimaging, the participants listened to the narrative again and, after each passage, produced a list of words that had been on their minds when they heard the audiobook during neuroimaging. Cultural family background was reflected as semantic differences in these word lists as quantified by a word2vec-generated semantic model. Our findings may depict enhanced mutual understanding between persons who share similar cultural family backgrounds.

Evidence for Top-down Meter Perception in Infancy as Shown by Primed Neural Responses to an Ambiguous Rhythm

From auditory rhythm patterns, listeners extract the underlying steady beat, and perceptually group beats to form meters. While previous studies show infants discriminate different auditory meters, it remains unknown whether they can maintain (imagine) a metrical interpretation of an ambiguous rhythm through top-down processes. We investigated this via electroencephalographic mismatch responses. We primed 6-month-old infants (N = 24) to hear a 6-beat ambiguous rhythm either in duple meter (n = 13), or in triple meter (n = 11) through loudness accents either on every second or every third beat. Periods of priming were inserted before sequences of the ambiguous unaccented rhythm. To elicit mismatch responses, occasional pitch deviants occurred on either beat 4 (strong beat in triple meter; weak in duple) or beat 5 (strong in duple; weak in triple) of the unaccented trials. At frontal left sites, we found a significant interaction between beat and priming group in the predicted direction. Post-hoc analyses showed mismatch response amplitudes were significantly larger for beat 5 in the duple- than triple-primed group (p = .047) and were non-significantly larger for beat 4 in the triple- than duple-primed group. Further, amplitudes were generally larger in infants with musically experienced parents. At frontal right sites, mismatch responses were generally larger for those in the duple compared to triple group, which may reflect a processing advantage for duple meter. These results indicate infants can impose a top-down, internally generated meter on ambiguous auditory rhythms, an ability that would aid early language and music learning.

Implicit auditory perception of local and global irregularities in passive listening condition

The auditory system detects differences in sounds at an implicit level, but data on this difference might not be sufficient to make explicit discrimination. The biomarkers of implicit auditory memory of ambiguous stimuli could shed light on unconscious auditory processing and implicit auditory learning. Mismatch negativity (MMN) and P3a, components of event-related potentials (ERPs) reflecting stimuli discrimination without direct attention, were previously detected in response to the local (short-term) irregularity in the auditory sequence even in an unconscious state. At the same time, P3b was elicited only in case of direct attention in response to the global (long-term) irregularity. In this study, we applied the local-global auditory paradigm to obtain possible electrophysiological signatures of implicit detection of hardly distinguishable auditory stimuli. ERPs were recorded from 20 healthy volunteers during active discrimination of deviant sounds in the old-ball sequence and passive listening of the same sounds in the sequence with local-global irregularity. The discrimination task consisted of two blocks with different deviant sounds targeted to respond. The sound discrimination accuracy was at an average of 40%, implying the difficulty of explicit sound recognition. Comparing ERPs to standard and deviant sounds, we found posterior negativity in ERP around 450-600 ms in response to targeted deviant sounds. MMN was significant only in response to non-target deviants. In the passive local-global paradigm, we observed an anterior positivity (284-412 ms), compatible with P3a, in response to a violation of local regularity. Violation of global regularity elicited an anterior negative response (228-586 ms), resembling the N400 component of ERPs. Importantly, the other indexes of auditory discrimination, such as MMN and P3b, were insignificant in ERPs to both regularity violations. The observed P3a and N400 components of ERPs may reflect prediction error signals in the implicit perception of sound patterns even if behavioral recognition was poor.

The role of attention and explicit knowledge in perceiving bistable auditory input

The auditory system frequently encounters ambiguous sound input that can be perceived in multiple ways. The current study investigated the role of explicit knowledge in modulating how sounds are represented in auditory memory for a bistable sound sequence that could be perceived equally as integrated or segregated. We hypothesized that the dominant percept of the bistable sequence would suppress representation of the alternative perceptual organization as a function of how much top-down knowledge the listener had about the structure of the sequence. Performance measures and event-related brain potentials were compared when participants had explicit knowledge about one perceptual organization in the first half of the experiment to when they had explicit knowledge of both in the second half. We hypothesized that knowledge would modify the brain response to the alternative percept of the bistable sequence. However, that did not occur. When participants were performing one task, with no explicit knowledge of the bistable structure of the sequence, both integrated and segregated percepts were represented in auditory working memory. This demonstrates that explicit knowledge about the sounds is not a necessary factor for deriving and maintaining representations of multiple sound organizations within a complex sound environment. Passive attention operates in parallel with active or selective attention to maintain consistent representations of the environment, representations that may or may not be useful for task performance. It suggests a highly adaptive system useful in everyday listening situations where the listener has no prior knowledge about how the sound environment is structured.

Limited Evidence for Sensory Prediction Error Responses in Visual Cortex of Macaques and Humans

A recent formulation of predictive coding theory proposes that a subset of neurons in each cortical area encodes sensory prediction errors, the difference between predictions relayed from higher cortex and the sensory input. Here, we test for evidence of prediction error responses in spiking responses and local field potentials (LFP) recorded in primary visual cortex and area V4 of macaque monkeys, and in complementary electroencephalographic (EEG) scalp recordings in human participants. We presented a fixed sequence of visual stimuli on most trials, and violated the expected ordering on a small subset of trials. Under predictive coding theory, pattern-violating stimuli should trigger robust prediction errors, but we found that spiking, LFP and EEG responses to expected and pattern-violating stimuli were nearly identical. Our results challenge the assertion that a fundamental computational motif in sensory cortex is to signal prediction errors, at least those based on predictions derived from temporal patterns of visual stimulation.

Distractor-induced deafness: The effect of multiple auditory distractors on conscious target processing

Conscious access to a target stimulus embedded in a rapid serial visual presentation can be impaired by the preceding presentation of multiple task-irrelevant distractors. While this phenomenon - labeled distractor-induced blindness (DIB) - is established in vision, it is unknown whether a similar effect can be observed in the auditory modality. Considering the differences in the processing of visual and auditory stimuli, modality-specific effects in the inhibitory mechanisms triggered by distractors can be expected. First, we aimed to find evidence for a distractor-induced deafness (DID) for auditory targets in a behavioral experiment. The target was defined by a transient increase in amplitude in a continuous sinusoidal tone, which was to be detected if accompanied or preceded by a deviant tone (cue). Both events were embedded in separate streams in a binaural rapid serial auditory presentation. Distractors preceded the cue and shared the target's features. As previously observed for DIB, a failure to detect the auditory target critically relied on the presentation of multiple distractor episodes. This DID effect was followed up in a subsequent event-related brain potentials (ERP) study to identify the signature of target detection. In contrast to missed targets, hits were characterized by a larger frontal negativity and by a more pronounced centro-parietal P3b wave. Whereas the latter process was also observed in the visual domain, indicating a post-perceptual updating process, the frontal negativity was exclusively observed for auditory DID. This modality-specific process might signal that early attentional control processes support conscious access to relevant auditory events.

Making Sense of Mismatch Negativity

Evoked potentials provide valuable insight into brain processes that are integral to our ability to interact effectively and efficiently in the world. The mismatch negativity (MMN) component of the evoked potential has proven highly informative on the ways in which sensitivity to regularity contributes to perception and cognition. This review offers a compendium of research on MMN with a view to scaffolding an appreciation for its use as a tool to explore the way regularities contribute to predictions about the sensory environment over many timescales. In compiling this work, interest in MMN as an index of sensory encoding and memory are addressed, as well as attention. Perspectives on the possible underlying computational processes are reviewed as well as recent observations that invite consideration of how MMN relates to how we learn, what we learn, and why.

Beta power encodes contextual estimates of temporal event probability in the human brain

To prepare for an impending event of unknown temporal distribution, humans internally increase the perceived probability of event onset as time elapses. This effect is termed the hazard rate of events. We tested how the neural encoding of hazard rate changes by providing human participants with prior information on temporal event probability. We recorded behavioral and electroencephalographic (EEG) data while participants listened to continuously repeating five-tone sequences, composed of four standard tones followed by a non-target deviant tone, delivered at slow (1.6 Hz) or fast (4 Hz) rates. The task was to detect a rare target tone, which equiprobably appeared at either position two, three or four of the repeating sequence. In this design, potential target position acts as a proxy for elapsed time. For participants uninformed about the target's distribution, elapsed time to uncertain target onset increased response speed, displaying a significant hazard rate effect at both slow and fast stimulus rates. However, only in fast sequences did prior information about the target's temporal distribution interact with elapsed time, suppressing the hazard rate. Importantly, in the fast, uninformed condition pre-stimulus power synchronization in the beta band (Beta 1, 15-19 Hz) predicted the hazard rate of response times. Prior information suppressed pre-stimulus power synchronization in the same band, while still significantly predicting response times. We conclude that Beta 1 power does not simply encode the hazard rate, but-more generally-internal estimates of temporal event probability based upon contextual information.

THE EFFECTS OF HYPNOSIS AND HYPNOTIC SUGGESTIONS ON THE MISMATCH NEGATIVITY IN HIGHLY HYPNOTIZABLE SUBJECTS

The neural mechanisms associated with hypnosis were investigated in a group of 9 high hypnotizable subjects by measuring the mismatch negativity (MMN) component of the auditory event-related potential (ERP). ERPs were recorded using a passive oddball paradigm to sinusoidal standard and deviant tone stimuli of 500 and 520 Hz, respectively, in four conditions: prehypnosis, neutral hypnosis, hypnotic suggestion for altering the tone perception, and posthypnotic conditions. Earlier studies have indicated that hypnosis and hypnotic suggestions might have an effect on MMN, but the results of our study contradict these results: No statistically significant differences were found between the conditions in the MMN amplitudes.

A division of labor between power and phase coherence in encoding attention to stimulus streams

Both time-based (when) and feature-based (what) aspects of attention facilitate behavior, so it is natural to hypothesize additive effects. We tested this conjecture by recording response behavior and electroencephalographic (EEG) data to auditory pitch changes, embedded at different time lags in a continuous sound stream. Participants reacted more rapidly to larger rather than smaller feature change magnitudes (deviancy), as well as to changes appearing after longer rather than shorter waiting times (hazard rate of response times). However, the feature and time dimensions of attention separately contributed to response speed, with no significant interaction. Notably, phase coherence at low frequencies (delta and theta bands, 1-7 Hz) predominantly reflected attention capture by feature changes, while oscillatory power at higher frequency bands, alpha (8-12 Hz) and beta (13-25 Hz) reflected the orienting of attention in time. Power and phase coherence predicted different portions of response speed variance, suggesting a division of labor in encoding sensory attention in complex auditory scenes.

A Model for Statistical Regularity Extraction from Dynamic Sounds

To understand our surroundings, we effortlessly parse our sound environment into sound sources, extracting invariant information-or regularities-over time to build an internal representation of the world around us. Previous experimental work has shown the brain is sensitive to many types of regularities in sound, but theoretical models that capture underlying principles of regularity tracking across diverse sequence structures have been few and far between. Existing efforts often focus on sound patterns rather the stochastic nature of sequences. In the current study, we employ a perceptual model for regularity extraction based on a Bayesian framework that posits the brain collects statistical information over time. We show this model can be used to simulate various results from the literature with stimuli exhibiting a wide range of predictability. This model can provide a useful tool for both interpreting existing experimental results under a unified model and providing predictions for new ones using more complex stimuli.

Cascade and no-repetition rules are comparable controls for the auditory frequency mismatch negativity in oddball tasks

The mismatch negativity (MMN) has been widely studied with oddball tasks to index processing of unexpected auditory change. The MMN is computed as the difference of deviant minus standard and is used to capture the pattern violation by the deviant. However, this oddball MMN is confounded because the deviant differs physically from the standard and is presented less often. To improve measurement, the same tone as the deviant is presented in a separate condition. This control tone is equiprobable with other tones and is used to compute a corrected MMN (deviant minus control). Typically, the tones are in random order except that consecutive tones are not identical (no-repetition rule). In contrast, a recent study on frequency MMN presented tones in a regular up-and-down sequence (cascade rule). If the cascade rule is detected more easily than the no-repetition rule, there should be a lower risk of a confounding MMN within the cascade condition. However, in previous research, the cascade and no-repetition conditions differed not only in the regularity of the tone sequence but also in number of tones, frequency range, and proportion of tones. We controlled for these differences to isolate effects of regularity in the tone sequence. Results of our preregistered analyses provided moderate evidence (BF₀₁ >6) that the corrected MMN did not differ between cascade and no-repetition conditions. These findings imply that no-repetition and cascade rules are processed similarly and that the no-repetition condition provides an adequate control in frequency MMN.

Visual mismatch negativity is unaffected by top-down prediction of the timing of deviant events

Visual mismatch negativity (VMMN) is an event-related brain potential component that is automatically elicited by infrequent (deviant) stimuli that are inserted among frequent (standard) stimuli (i.e., an oddball sequence). Although the elicitation of VMMN is basically determined in a stimulus-driven manner, it can be modulated by top-down control. In a previous study using a "patterned" oddball sequence, where deviant (D) stimuli were regularly inserted among standard (S) stimuli (i.e., repetitions of an SSSSD pattern), VMMN was largely reduced when participants noticed the SSSSD pattern and actively predicted both the identity and timing of the deviant stimuli compared to when they did not notice the SSSSD pattern and did not form such active prediction. The present study further investigated whether or not active prediction of only the timing of deviant stimuli is sufficient for the reduction of VMMN. With the patterned oddball sequence with one deviant (here, deviant stimuli were fixed throughout the block), VMMN was reduced when the participants noticed the SSSSD pattern and actively predicted both the identity and timing of deviant stimuli (i.e., replication of the previous finding). In contrast, with the patterned oddball sequence with two deviants (deviant stimuli were randomly varied between two possibilities), VMMN was not significantly reduced when the participants noticed the SSSSD pattern and actively predicted only the timing of deviant stimuli. These results suggest that active prediction of only the timing of deviant stimuli is not sufficient to reduce VMMN.

Neural Decoding of Bistable Sounds Reveals an Effect of Intention on Perceptual Organization

Auditory signals arrive at the ear as a mixture that the brain must decompose into distinct sources based to a large extent on acoustic properties of the sounds. An important question concerns whether listeners have voluntary control over how many sources they perceive. This has been studied using pure high (H) and low (L) tones presented in the repeating pattern HLH-HLH-, which can form a bistable percept heard either as an integrated whole (HLH-) or as segregated into high (H-H-) and low (-L-) sequences. Although instructing listeners to try to integrate or segregate sounds affects reports of what they hear, this could reflect a response bias rather than a perceptual effect. We had human listeners (15 males, 12 females) continuously report their perception of such sequences and recorded neural activity using MEG. During neutral listening, a classifier trained on patterns of neural activity distinguished between periods of integrated and segregated perception. In other conditions, participants tried to influence their perception by allocating attention either to the whole sequence or to a subset of the sounds. They reported hearing the desired percept for a greater proportion of time than when listening neutrally. Critically, neural activity supported these reports; stimulus-locked brain responses in auditory cortex were more likely to resemble the signature of segregation when participants tried to hear segregation than when attempting to perceive integration. These results indicate that listeners can influence how many sound sources they perceive, as reflected in neural responses that track both the input and its perceptual organization.

SIGNIFICANCE STATEMENT Can we consciously influence our perception of the external world? We address this question using sound sequences that can be heard either as coming from a single source or as two distinct auditory streams. Listeners reported spontaneous changes in their perception between these two interpretations while we recorded neural activity to identify signatures of such integration and segregation. They also indicated that they could, to some extent, choose between these alternatives. This claim was supported by corresponding changes in responses in auditory cortex. By linking neural and behavioral correlates of perception, we demonstrate that the number of objects that we perceive can depend not only on the physical attributes of our environment, but also on how we intend to experience it.

Predictive coding in auditory perception: challenges and unresolved questions

Predictive coding is arguably the currently dominant theoretical framework for the study of perception. It has been employed to explain important auditory perceptual phenomena, and it has inspired theoretical, experimental and computational modelling efforts aimed at describing how the auditory system parses the complex sound input into meaningful units (auditory scene analysis). These efforts have uncovered some vital questions, addressing which could help to further specify predictive coding and clarify some of its basic assumptions. The goal of the current review is to motivate these questions and show how unresolved issues in explaining some auditory phenomena lead to general questions of the theoretical framework. We focus on experimental and computational modelling issues related to sequential grouping in auditory scene analysis (auditory pattern detection and bistable perception), as we believe that this is the research topic where predictive coding has the highest potential for advancing our understanding. In addition to specific questions, our analysis led us to identify three more general questions that require further clarification: (1) What exactly is meant by prediction in predictive coding? (2) What governs which generative models make the predictions? and (3) What (if it exists) is the correlate of perceptual experience within the predictive coding framework?

Attentional Resources Are Needed for Auditory Stream Segregation in Aging

The ability to select sound streams from background noise becomes challenging with age, even with normal peripheral auditory functioning. Reduced stream segregation ability has been reported in older compared to younger adults. However, the reason why there is a difference is still unknown. The current study investigated the hypothesis that automatic sound processing is impaired with aging, which then contributes to difficulty actively selecting subsets of sounds in noisy environments. We presented a simple intensity oddball sequence in various conditions with irrelevant background sounds while recording EEG. The ability to detect the oddball tones was dependent on the ability to automatically or actively segregate the sounds to frequency streams. Listeners were able to actively segregate sounds to perform the loudness detection task, but there was no indication of automatic segregation of background sounds while watching a movie. Thus, our results indicate impaired automatic processes in aging that may explain more effortful listening, and that tax attentional systems when selecting sound streams in noisy environments.

Statistical Learning of Melodic Patterns Influences the Brain's Response to Wrong Notes

The theory of statistical learning has been influential in providing a framework for how humans learn to segment patterns of regularities from continuous sensory inputs, such as speech and music. This form of learning is based on statistical cues and is thought to underlie the ability to learn to segment patterns of regularities from continuous sensory inputs, such as the transition probabilities in speech and music. However, the connection between statistical learning and brain measurements is not well understood. Here we focus on ERPs in the context of tone sequences that contain statistically cohesive melodic patterns. We hypothesized that implicit learning of statistical regularities would influence what was held in auditory working memory. We predicted that a wrong note occurring within a cohesive pattern (within-pattern deviant) would lead to a significantly larger brain signal than a wrong note occurring between cohesive patterns (between-pattern deviant), even though both deviant types were equally likely to occur with respect to the global tone sequence. We discuss this prediction within a simple Markov model framework that learns the transition probability regularities within the tone sequence. Results show that signal strength was stronger when cohesive patterns were violated and demonstrate that the transitional probability of the sequence influences the memory basis for melodic patterns. Our results thus characterize how informational units are stored in auditory memory trace for deviance detection and provide new evidence about how the brain organizes sequential sound input that is useful for perception.

Auditory Scene Analysis: An Attention Perspective

PURPOSE

This review article provides a new perspective on the role of attention in auditory scene analysis.

METHOD

A framework for understanding how attention interacts with stimulus-driven processes to facilitate task goals is presented. Previously reported data obtained through behavioral and electrophysiological measures in adults with normal hearing are summarized to demonstrate attention effects on auditory perception-from passive processes that organize unattended input to attention effects that act at different levels of the system. Data will show that attention can sharpen stream organization toward behavioral goals, identify auditory events obscured by noise, and limit passive processing capacity.

CONCLUSIONS

A model of attention is provided that illustrates how the auditory system performs multilevel analyses that involve interactions between stimulus-driven input and top-down processes. Overall, these studies show that (a) stream segregation occurs automatically and sets the basis for auditory event formation; (b) attention interacts with automatic processing to facilitate task goals; and (c) information about unattended sounds is not lost when selecting one organization over another. Our results support a neural model that allows multiple sound organizations to be held in memory and accessed simultaneously through a balance of automatic and task-specific processes, allowing flexibility for navigating noisy environments with competing sound sources.

PRESENTATION VIDEO

http://cred.pubs.asha.org/article.aspx?articleid=2601618.

Auditory sequential accumulation of spectral information

In many listening situations, information about the spectral content of a target sound may be distributed over time, and estimating the target spectrum requires efficient sequential processing. Listeners' ability to estimate the spectrum of a random-frequency, six-tone complex was investigated and the spectral content of the complex was revealed using a sequence of bursts. Whether each of the six tones was presented within each burst was determined at random according to a presentation probability. In separate conditions, the presentation probabilities (p) ranged from 0.2 to 1, the total number of bursts varied from 1 to 16, and the inter-burst interval was either 0 or 200 ms. To evaluate the information acquired by the listener, the burst sequence was followed, after a 500-ms silent interval, by the six-tone complex acting as an informational masker and the listener was required to detect a pure-tone target presented simultaneously with the masker. Greater performance in this task indicates more accurate estimation of the spectrum of the complex by the listener. Evidence for integration of information across bursts was observed, and the integration process did not significantly depend on inter-burst interval.

Transitional Probabilities Are Prioritized over Stimulus/Pattern Probabilities in Auditory Deviance Detection: Memory Basis for Predictive Sound Processing

Representations encoding the probabilities of auditory events do not directly support predictive processing. In contrast, information about the probability with which a given sound follows another (transitional probability) allows predictions of upcoming sounds. We tested whether behavioral and cortical auditory deviance detection (the latter indexed by the mismatch negativity event-related potential) relies on probabilities of sound patterns or on transitional probabilities. We presented healthy adult volunteers with three types of rare tone-triplets among frequent standard triplets of high-low-high (H-L-H) or L-H-L pitch structure: proximity deviant (H-H-H/L-L-L), reversal deviant (L-H-L/H-L-H), and first-tone deviant (L-L-H/H-H-L). If deviance detection was based on pattern probability, reversal and first-tone deviants should be detected with similar latency because both differ from the standard at the first pattern position. If deviance detection was based on transitional probabilities, then reversal deviants should be the most difficult to detect because, unlike the other two deviants, they contain no low-probability pitch transitions. The data clearly showed that both behavioral and cortical auditory deviance detection uses transitional probabilities. Thus, the memory traces underlying cortical deviance detection may provide a link between stimulus probability-based change/novelty detectors operating at lower levels of the auditory system and higher auditory cognitive functions that involve predictive processing.

SIGNIFICANCE STATEMENT

Our research presents the first definite evidence for the auditory system prioritizing transitional probabilities over probabilities of individual sensory events. Forming representations for transitional probabilities paves the way for predictions of upcoming sounds. Several recent theories suggest that predictive processing provides the general basis of human perception, including important auditory functions, such as auditory scene analysis. Our results demonstrate that the memory traces underlying cortical deviance detection form a link between stimulus probability-based change/novelty detectors operating at lower levels of the auditory system and higher auditory cognitive functions that involve predictive processing.

Dynamic Correlations between Intrinsic Connectivity and Extrinsic Connectivity of the Auditory Cortex in Humans

The arrival of sound signals in the auditory cortex (AC) triggers both local and inter-regional signal propagations over time up to hundreds of milliseconds and builds up both intrinsic functional connectivity (iFC) and extrinsic functional connectivity (eFC) of the AC. However, interactions between iFC and eFC are largely unknown. Using intracranial stereo-electroencephalographic recordings in people with drug-refractory epilepsy, this study mainly investigated the temporal dynamic of the relationships between iFC and eFC of the AC. The results showed that a Gaussian wideband-noise burst markedly elicited potentials in both the AC and numerous higher-order cortical regions outside the AC (non-auditory cortices). Granger causality analyses revealed that in the earlier time window, iFC of the AC was positively correlated with both eFC from the AC to the inferior temporal gyrus and that to the inferior parietal lobule. While in later periods, the iFC of the AC was positively correlated with eFC from the precentral gyrus to the AC and that from the insula to the AC. In conclusion, dual-directional interactions occur between iFC and eFC of the AC at different time windows following the sound stimulation and may form the foundation underlying various central auditory processes, including auditory sensory memory, object formation, integrations between sensory, perceptional, attentional, motor, emotional, and executive processes.

Selective entrainment of brain oscillations drives auditory perceptual organization

Perceptual sound organization supports our ability to make sense of the complex acoustic environment, to understand speech and to enjoy music. However, the neuronal mechanisms underlying the subjective experience of perceiving univocal auditory patterns that can be listened to, despite hearing all sounds in a scene, are poorly understood. We hereby investigated the manner in which competing sound organizations are simultaneously represented by specific brain activity patterns and the way attention and task demands prime the internal model generating the current percept. Using a selective attention task on ambiguous auditory stimulation coupled with EEG recordings, we found that the phase of low-frequency oscillatory activity dynamically tracks multiple sound organizations concurrently. However, whereas the representation of ignored sound patterns is circumscribed to auditory regions, large-scale oscillatory entrainment in auditory, sensory-motor and executive-control network areas reflects the active perceptual organization, thereby giving rise to the subjective experience of a unitary percept.

Implicit expectations influence target detection in children and adults

When a sound occurs at a predictable time, it gets processed more efficiently. Predictability of the temporal structure of acoustic inflow has been found to influence the P3b of event-related potentials in young adults, such that highly predictable compared to less predictable input leads to earlier P3b peak latencies. In our study, we wanted to investigate the influence of predictability on target processing indexed by the P3b in children (10-12 years old) and young adults. To do that, we used an oddball paradigm with two conditions of predictability (high and low). In the High-predictability condition, a high-pitched target tone occurred most of the time in the fifth position of a five-tone pattern (after four low-pitched non-target sounds), whereas in the Low-predictability condition, no such rule was implemented. The target tone occurred randomly following 2, 3, 4, 5, or 6 non-target tones. In both age groups, reaction time to predictable targets was faster than to non-predictable targets. Remarkably, this effect was largest in children. Consistent with the behavioral responses, the onset latency of the P3b response elicited by targets in both groups was earlier in the predictable than the unpredictable conditions. However, only the children had significantly earlier peak latency responses for predictable targets. Our results demonstrate that target stimulus predictability increases processing speed in children and adults even when predictability was only implicitly derived by the stimulus statistics. Children did have larger effects of predictability, seeming to benefit more from predictability for target detection.

Sequential search asymmetry: Behavioral and psychophysiological evidence from a dual oddball task

We conducted five experiments in order to explore the generalizability of a new type of search asymmetry, which we have termed sequential search asymmetry, across sensory modalities, and to better understand its origin. In all five experiments rare oddballs occurred randomly within longer sequences of more frequent standards. Oddballs and standards all consisted of rapidly-presented runs of five pure tones (Experiments 1 and 5) or five colored annuli (Experiments 2 through 4) somewhat analogous to simultaneously-presented feature-present and feature-absent stimuli in typical visual search tasks. In easy tasks feature-present reaction times and P300 latencies were shorter than feature-absent ones, similar to findings in search tasks with simultaneously-presented stimuli. Moreover the P3a subcomponent of the P300 ERP was strongly apparent only in the feature-present condition. In more difficult tasks requiring focused attention, however, RT and P300 latency differences disappeared but the P300 amplitude difference was significant. Importantly in all five experiments d’ for feature-present targets was larger than that for feature-absent targets. These results imply that sequential search asymmetry arises from discriminability differences between feature-present and feature-absent targets. Response time and P300 latency differences can be attributed to the use of different attention strategies in search for feature-present and feature-absent targets, indexed by the presence of a dominant P3a subcomponent in the feature-present target-evoked P300s that is lacking in the P300s to the feature-absent targets.

Distinguishing Neural Adaptation and Predictive Coding Hypotheses in Auditory Change Detection

The auditory mismatch negativity (MMN) component of event-related potentials (ERPs) has served as a neural index of auditory change detection. MMN is elicited by presentation of infrequent (deviant) sounds randomly interspersed among frequent (standard) sounds. Deviants elicit a larger negative deflection in the ERP waveform compared to the standard. There is considerable debate as to whether the neural mechanism of this change detection response is due to release from neural adaptation (neural adaptation hypothesis) or from a prediction error signal (predictive coding hypothesis). Previous studies have not been able to distinguish between these explanations because paradigms typically confound the two. The current study disambiguated effects of stimulus-specific adaptation from expectation violation using a unique stimulus design that compared expectation violation responses that did and did not involve stimulus change. The expectation violation response without the stimulus change differed in timing, scalp distribution, and attentional modulation from the more typical MMN response. There is insufficient evidence from the current study to suggest that the negative deflection elicited by the expectation violation alone includes the MMN. Thus, we offer a novel hypothesis that the expectation violation response reflects a fundamentally different neural substrate than that attributed to the canonical MMN.

Aging Affects Neural Synchronization to Speech-Related Acoustic Modulations

As people age, speech perception problems become highly prevalent, especially in noisy situations. In addition to peripheral hearing and cognition, temporal processing plays a key role in speech perception. Temporal processing of speech features is mediated by synchronized activity of neural oscillations in the central auditory system. Previous studies indicate that both the degree and hemispheric lateralization of synchronized neural activity relate to speech perception performance. Based on these results, we hypothesize that impaired speech perception in older persons may, in part, originate from deviances in neural synchronization. In this study, auditory steady-state responses that reflect synchronized activity of theta, beta, low and high gamma oscillations (i.e., 4, 20, 40, and 80 Hz ASSR, respectively) were recorded in young, middle-aged, and older persons. As all participants had normal audiometric thresholds and were screened for (mild) cognitive impairment, differences in synchronized neural activity across the three age groups were likely to be attributed to age. Our data yield novel findings regarding theta and high gamma oscillations in the aging auditory system. At an older age, synchronized activity of theta oscillations is increased, whereas high gamma synchronization is decreased. In contrast to young persons who exhibit a right hemispheric dominance for processing of high gamma range modulations, older adults show a symmetrical processing pattern. These age-related changes in neural synchronization may very well underlie the speech perception problems in aging persons.

Hierarchy of prediction errors for auditory events in human temporal and frontal cortex

Predictive coding theories posit that neural networks learn statistical regularities in the environment for comparison with actual outcomes, signaling a prediction error (PE) when sensory deviation occurs. PE studies in audition have capitalized on low-frequency event-related potentials (LF-ERPs), such as the mismatch negativity. However, local cortical activity is well-indexed by higher-frequency bands [high-γ band (Hγ): 80-150 Hz]. We compared patterns of human Hγ and LF-ERPs in deviance detection using electrocorticographic recordings from subdural electrodes over frontal and temporal cortices. Patients listened to trains of task-irrelevant tones in two conditions differing in the predictability of a deviation from repetitive background stimuli (fully predictable vs. unpredictable deviants). We found deviance-related responses in both frequency bands over lateral temporal and inferior frontal cortex, with an earlier latency for Hγ than for LF-ERPs. Critically, frontal Hγ activity but not LF-ERPs discriminated between fully predictable and unpredictable changes, with frontal cortex sensitive to unpredictable events. The results highlight the role of frontal cortex and Hγ activity in deviance detection and PE generation.

Levels of attention and task difficulty in the modulation of interval duration mismatch negativity

Time perception has been described as a fundamental skill needed to engage in a number of higher level cognitive processes essential to successfully navigate everyday life (e.g., planning, sequencing, etc.) Temporal processing is often thought of as a basic neural process that impacts a variety of other cognitive processes. Others, however, have argued that timing in the brain can be affected by a number of variables such as attention and motivation. In an effort to better understand timing in the brain at a basic level with minimal attentional demands, researchers have often employed use of the mismatch negativity (MMN). MMN, specifically duration MMN (dMMN) and interval MMN (iMMN) have been popular methods for studying temporal processing in populations for which attention or motivation may be an issue (e.g., clinical populations, early developmental studies). There are, however, select studies which suggest that attention may in fact modify both temporal processing in general and the MMN event-related potential. It is unclear the degree to which attention affects MMN or whether the effects differ depending on the complexity or difficulty of the MMN paradigm. The iMMN indexes temporal processing and is elicited by introducing a deviant interval duration amid a series of standards. A greater degree of difference in the deviant from the standard elicits a heightened iMMN. Unlike past studies, in which attention was intentionally directed toward a closed-captioned move, the current study had participants partake in tasks involving varying degrees of attention (passive, low, and high) with varying degrees of deviants (small, medium, and large) to better understand the role of attention on the iMMN and to assess whether level of attention paired with changes in task difficulty differentially influence the iMMN electrophysiological responses. Data from 19 subjects were recorded in an iMMN paradigm. The amplitude of the iMMN waveform showed an increase with attention, particularly for intervals that were the most distinct from a standard interval (p < 0.02). Results suggest that the role of attention on the iMMN is complex. Both the degree of attention paid as well as the level of difficulty of the MMN task likely influence the neuronal response within a timing network. These results suggest that electrophysiological perception of time is modified by attention and that the design of the iMMN study is critical to minimize the possible confounding effects of attention. In addition, the implications of these results for future studies assessing interval duration-based MMN in clinical populations is also addressed.

Effects of explicit knowledge and predictability on auditory distraction and target performance

This study tested effects of task requirements and knowledge on auditory distraction effects. This was done by comparing the response to a pitch change (an irrelevant, distracting tone feature) that occurred predictably in a tone sequence (every 5th tone) under different task conditions. The same regular sound sequence was presented with task conditions varying in what information the participant was given about the predictability of the pitch change, and when this information was relevant for the task to be performed. In all conditions, participants performed a tone duration judgment task. Behavioral and event-related brain potential (ERP) measures were obtained to measure distraction effects and deviance detection. Predictable deviants produced behavioral distraction effects in all conditions. However, the P3a amplitude evoked by the predictable pitch change was largest when participants were uninformed about the regular structure of the sound sequence, showing an effect of knowledge on involuntary orienting of attention. In contrast, the mismatch negativity (MMN) component was only modulated when the regularity was relevant for the task and not by stimulus predictability itself. P3a and behavioral indices of distraction were not fully concordant. Overall, our results show differential effects of knowledge and predictability on auditory distraction effects indexed by neurophysiological (P3a) and behavioral measures.

Auditory perceptual objects as generative models: Setting the stage for communication by sound

Communication by sounds requires that the communication channels (i.e. speech/speakers and other sound sources) had been established. This allows to separate concurrently active sound sources, to track their identity, to assess the type of message arriving from them, and to decide whether and when to react (e.g., reply to the message). We propose that these functions rely on a common generative model of the auditory environment. This model predicts upcoming sounds on the basis of representations describing temporal/sequential regularities. Predictions help to identify the continuation of the previously discovered sound sources to detect the emergence of new sources as well as changes in the behavior of the known ones. It produces auditory event representations which provide a full sensory description of the sounds, including their relation to the auditory context and the current goals of the organism. Event representations can be consciously perceived and serve as objects in various cognitive operations.

Analyzing the auditory scene: neurophysiologic evidence of a dissociation between detection of regularity and detection of change

Detecting regularity and change in the environment is crucial for survival, as it enables making predictions about the world and informing goal-directed behavior. In the auditory modality, the detection of regularity involves segregating incoming sounds into distinct perceptual objects (stream segregation). The detection of change from this within-stream regularity is associated with the mismatch negativity, a component of auditory event-related brain potentials (ERPs). A central unanswered question is how the detection of regularity and the detection of change are interrelated, and whether attention affects the former, the latter, or both. Here we show that the detection of regularity and the detection of change can be empirically dissociated, and that attention modulates the detection of change without precluding the detection of regularity, and the perceptual organization of the auditory background into distinct streams. By applying frequency spectra analysis on the EEG of subjects engaged in a selective listening task, we found distinct peaks of ERP synchronization, corresponding to the rhythm of the frequency streams, independently of whether the stream was attended or ignored. Our results provide direct neurophysiological evidence of regularity detection in the auditory background, and show that it can occur independently of change detection and in the absence of attention.

Statistical context shapes stimulus-specific adaptation in human auditory cortex

Stimulus-specific adaptation is the phenomenon whereby neural response magnitude decreases with repeated stimulation. Inconsistencies between recent nonhuman animal recordings and computational modeling suggest dynamic influences on stimulus-specific adaptation. The present human electroencephalography (EEG) study investigates the potential role of statistical context in dynamically modulating stimulus-specific adaptation by examining the auditory cortex-generated N1 and P2 components. As in previous studies of stimulus-specific adaptation, listeners were presented with oddball sequences in which the presentation of a repeated tone was infrequently interrupted by rare spectral changes taking on three different magnitudes. Critically, the statistical context varied with respect to the probability of small versus large spectral changes within oddball sequences (half of the time a small change was most probable; in the other half a large change was most probable). We observed larger N1 and P2 amplitudes (i.e., release from adaptation) for all spectral changes in the small-change compared with the large-change statistical context. The increase in response magnitude also held for responses to tones presented with high probability, indicating that statistical adaptation can overrule stimulus probability per se in its influence on neural responses. Computational modeling showed that the degree of coadaptation in auditory cortex changed depending on the statistical context, which in turn affected stimulus-specific adaptation. Thus the present data demonstrate that stimulus-specific adaptation in human auditory cortex critically depends on statistical context. Finally, the present results challenge the implicit assumption of stationarity of neural response magnitudes that governs the practice of isolating established deviant-detection responses such as the mismatch negativity.

How does the extraction of local and global auditory regularities vary with context?

How does the human brain extract regularities from its environment? There is evidence that short range or ‘local’ regularities (within seconds) are automatically detected by the brain while long range or ‘global’ regularities (over tens of seconds or more) require conscious awareness. In the present experiment, we asked whether participants' attention was needed to acquire such auditory regularities, to detect their violation or both. We designed a paradigm in which participants listened to predictable sounds. Subjects could be distracted by a visual task at two moments: when they were first exposed to a regularity or when they detected violations of this regularity. MEG recordings revealed that early brain responses (100–130 ms) to violations of short range regularities were unaffected by visual distraction and driven essentially by local transitional probabilities. Based on global workspace theory and prior results, we expected that visual distraction would eliminate the long range global effect, but unexpectedly, we found the contrary, i.e. late brain responses (300–600 ms) to violations of long range regularities on audio-visual trials but not on auditory only trials. Further analyses showed that, in fact, visual distraction was incomplete and that auditory and visual stimuli interfered in both directions. Our results show that conscious, attentive subjects can learn the long range dependencies present in auditory stimuli even while performing a visual task on synchronous visual stimuli. Furthermore, they acquire a complex regularity and end up making different predictions for the very same stimulus depending on the context (i.e. absence or presence of visual stimuli). These results suggest that while short-range regularity detection is driven by local transitional probabilities between stimuli, the human brain detects and stores long-range regularities in a highly flexible, context dependent manner.

How learning to abstract shapes neural sound representations

The transformation of acoustic signals into abstract perceptual representations is the essence of the efficient and goal-directed neural processing of sounds in complex natural environments. While the human and animal auditory system is perfectly equipped to process the spectrotemporal sound features, adequate sound identification and categorization require neural sound representations that are invariant to irrelevant stimulus parameters. Crucially, what is relevant and irrelevant is not necessarily intrinsic to the physical stimulus structure but needs to be learned over time, often through integration of information from other senses. This review discusses the main principles underlying categorical sound perception with a special focus on the role of learning and neural plasticity. We examine the role of different neural structures along the auditory processing pathway in the formation of abstract sound representations with respect to hierarchical as well as dynamic and distributed processing models. Whereas most fMRI studies on categorical sound processing employed speech sounds, the emphasis of the current review lies on the contribution of empirical studies using natural or artificial sounds that enable separating acoustic and perceptual processing levels and avoid interference with existing category representations. Finally, we discuss the opportunities of modern analyses techniques such as multivariate pattern analysis (MVPA) in studying categorical sound representations. With their increased sensitivity to distributed activation changes—even in absence of changes in overall signal level—these analyses techniques provide a promising tool to reveal the neural underpinnings of perceptually invariant sound representations.

Knowledge of sequence structure prevents auditory distraction: an ERP study

Infrequent, salient stimuli often capture attention despite their task-irrelevancy, and disrupt on-going goal-directed behavior. A number of studies show that presenting cues signaling forthcoming deviants reduces distraction, which may be a "by-product" of cue-processing interference or the result of direct preparatory processes for the forthcoming distracter. In the present study, instead of "bursts" of cue information, information on the temporal structure of the stimulus sequence was provided. Young adults performed a spatial discrimination task where complex tones moving left or right were presented. In the predictable condition, every 7th tone was a pitch-deviant, while in the random condition the position of deviants was random with a probability of 1/7. Whereas the early event-related potential correlates of deviance-processing (N1 and MMN) were unaffected by predictability, P3a amplitude was significantly reduced in the predictable condition, indicating that prevention of distraction was based on the knowledge about the temporal structure of the stimulus sequence.

The five myths of MMN: redefining how to use MMN in basic and clinical research

The goal of this review article is to redefine what the mismatch negativity (MMN) component of event-related potentials reflects in auditory scene analysis, and to provide an overview of how the MMN serves as a valuable tool in Cognitive Neuroscience research. In doing so, some of the old beliefs (five common 'myths') about MMN will be dispelled, such as the notion that MMN is a simple feature discriminator and that attention itself modulates MMN elicitation. A revised description of what MMN truly reflects will be provided, which includes a principal focus onto the highly context-dependent nature of MMN elicitation and new terminology to discuss MMN and attention. This revised framework will help clarify what has been a long line of seemingly contradictory results from studies in which behavioral ability to hear differences between sounds and passive elicitation of MMN have been inconsistent. Understanding what MMN is will also benefit clinical research efforts by providing a new picture of how to design appropriate paradigms suited to various clinical populations.

Perceptual organization of auditory streaming-task relies on neural entrainment of the stimulus-presentation rate: MEG evidence

Background

Humans are able to extract regularities from complex auditory scenes in order to form perceptually meaningful elements. It has been shown previously that this process depends critically on both the temporal integration of the sensory input over time and the degree of frequency separation between concurrent sound sources. Our goal was to examine the relationship between these two aspects by means of magnetoencephalography (MEG). To achieve this aim, we combined time-frequency analysis on a sensor space level with source analysis. Our paradigm consisted of asymmetric ABA-tone triplets wherein the B-tones were presented temporally closer to the first A-tones, providing different tempi within the same sequence. Participants attended to the slowest B-rhythm whilst the frequency separation between tones was manipulated (0-, 2-, 4- and 10-semitones).

Results

The results revealed that the asymmetric ABA-triplets spontaneously elicited periodic-sustained responses corresponding to the temporal distribution of the A-B and B-A tone intervals in all conditions. Moreover, when attending to the B-tones, the neural representations of the A- and B-streams were both detectable in the scenarios which allow perceptual streaming (2-, 4- and 10-semitones). Alongside this, the steady-state responses tuned to the presentation of the B-tones enhanced significantly with increase of the frequency separation between tones. However, the strength of the B-tones related steady-state responses dominated the strength of the A-tones responses in the 10-semitones condition. Conversely, the representation of the A-tones dominated the B-tones in the cases of 2- and 4-semitones conditions, in which a greater effort was required for completing the task. Additionally, the P1 evoked fields’ component following the B-tones increased in magnitude with the increase of inter-tonal frequency difference.

Conclusions

The enhancement of the evoked fields in the source space, along with the B-tones related activity of the time-frequency results, likely reflect the selective enhancement of the attended B-stream. The results also suggested a dissimilar efficiency of the temporal integration of separate streams depending on the degree of frequency separation between the sounds. Overall, the present findings suggest that the neural effects of auditory streaming could be directly captured in the time-frequency spectrum at the sensor-space level.