Neural Correlates of Auditory Streaming of Harmonic Complex Sounds With Different Phase Relations in the Songbird Forebrain

Changes in neural readout of response magnitude during auditory streaming do not correlate with behavioral choice in the auditory cortex

A fundamental goal of the auditory system is to group stimuli from the auditory environment into a perceptual unit (i.e., "stream") or segregate the stimuli into multiple different streams. Although previous studies have clarified the psychophysical and neural mechanisms that may underlie this ability, the relationship between these mechanisms remains elusive. Here, we recorded multiunit activity (MUA) from the auditory cortex of monkeys while they participated in an auditory-streaming task consisting of interleaved low- and high-frequency tone bursts. As the streaming stimulus unfolded over time, MUA amplitude habituated; the magnitude of this habituation was correlated with the frequency difference between the tone bursts. An ideal-observer model could classify these time- and frequency-dependent changes into reports of "one stream" or "two streams" in a manner consistent with the behavioral literature. However, because classification was not modulated by the monkeys' behavioral choices, this MUA habituation may not directly reflect perceptual reports.

The neurobiology of vocal communication in marmosets

An increasingly popular animal model for studying the neural basis of social behavior, cognition, and communication is the common marmoset (Callithrix jacchus). Interest in this New World primate across neuroscience is now being driven by their proclivity for prosociality across their repertoire, high volubility, and rapid development, as well as their amenability to naturalistic testing paradigms and freely moving neural recording and imaging technologies. The complement of these characteristics set marmosets up to be a powerful model of the primate social brain in the years to come. Here, we focus on vocal communication because it is the area that has both made the most progress and illustrates the prodigious potential of this species. We review the current state of the field with a focus on the various brain areas and networks involved in vocal perception and production, comparing the findings from marmosets to other animals, including humans.

European starlings (sturnus vulgaris) discriminate rhythms by rate, not temporal patterns

Humans can perceive a regular psychological pulse in music known as the beat. The evolutionary origins and neural mechanisms underlying this ability are hypothetically linked to imitative vocal learning, a rare trait found only in some species of mammals and birds. Beat perception has been demonstrated in vocal learning parrots but not in songbirds. We trained European starlings (Sturnus vulgaris) on two sound discriminations to investigate their perception of the beat and temporal structure in rhythmic patterns. First, we trained birds on a two-choice discrimination between rhythmic patterns of tones that contain or lack a regular beat. Despite receiving extensive feedback, the starlings were unable to distinguish the first two patterns. Next, we probed the temporal cues that starlings use for discriminating rhythms in general. We trained birds to discriminate a baseline set of isochronous and triplet tone sequences. On occasional probe trials, we presented transformations of the baseline patterns. The starlings' responses to the probes suggest they relied on absolute temporal features to sort the sounds into "fast" and "slow" and otherwise ignored patterns that were present. Our results support that starlings attend to local features in rhythms and are less sensitive to the global temporal organization.

Behavioral measurements of auditory streaming and build-up by budgerigars (Melopsittacus undulatus)

The perception of the build-up of auditory streaming has been widely investigated in humans, while it is unknown whether animals experience a similar perception when hearing high (H) and low (L) tonal pattern sequences. The paradigm previously used in European starlings (Sturnus vulgaris) was adopted in two experiments to address the build-up of auditory streaming in budgerigars (Melopsittacus undulatus). In experiment 1, different numbers of repetitions of low-high-low triplets were used in five conditions to study the build-up process. In experiment 2, 5 and 15 repetitions of high-low-high triplets were used to investigate the effects of repetition rate, frequency separation, and frequency range of the two tones on the birds' streaming perception. Similar to humans, budgerigars subjectively experienced the build-up process in auditory streaming; faster repetition rates and larger frequency separations enhanced the streaming perception, and these results were consistent across the two frequency ranges. Response latency analysis indicated that the budgerigars needed a longer amount of time to respond to stimuli that elicited a salient streaming perception. These results indicate, for the first time using a behavioral paradigm, that budgerigars experience a build-up of auditory streaming in a manner similar to humans.

Neuronal Correlates of Auditory Streaming in Monkey Auditory Cortex for Tone Sequences without Spectral Differences

This study finds a neuronal correlate of auditory perceptual streaming in the primary auditory cortex for sequences of tone complexes that have the same amplitude spectrum but a different phase spectrum. Our finding is based on microelectrode recordings of multiunit activity from 270 cortical sites in three awake macaque monkeys. The monkeys were presented with repeated sequences of a tone triplet that consisted of an A tone, a B tone, another A tone and then a pause. The A and B tones were composed of unresolved harmonics formed by adding the harmonics in cosine phase, in alternating phase, or in random phase. A previous psychophysical study on humans revealed that when the A and B tones are similar, humans integrate them into a single auditory stream; when the A and B tones are dissimilar, humans segregate them into separate auditory streams. We found that the similarity of neuronal rate responses to the triplets was highest when all A and B tones had cosine phase. Similarity was intermediate when the A tones had cosine phase and the B tones had alternating phase. Similarity was lowest when the A tones had cosine phase and the B tones had random phase. The present study corroborates and extends previous reports, showing similar correspondences between neuronal activity in the primary auditory cortex and auditory streaming of sound sequences. It also is consistent with Fishman’s population separation model of auditory streaming.

A Crucial Test of the Population Separation Model of Auditory Stream Segregation in Macaque Primary Auditory Cortex

An important aspect of auditory scene analysis is auditory stream segregation-the organization of sound sequences into perceptual streams reflecting different sound sources in the environment. Several models have been proposed to account for stream segregation. According to the "population separation" (PS) model, alternating ABAB tone sequences are perceived as a single stream or as two separate streams when "A" and "B" tones activate the same or distinct frequency-tuned neuronal populations in primary auditory cortex (A1), respectively. A crucial test of the PS model is whether it can account for the observation that A and B tones are generally perceived as a single stream when presented synchronously, rather than in an alternating pattern, even if they are widely separated in frequency. Here, we tested the PS model by recording neural responses to alternating (ALT) and synchronous (SYNC) tone sequences in A1 of male macaques. Consistent with predictions of the PS model, a greater effective tonotopic separation of A and B tone responses was observed under ALT than under SYNC conditions, thus paralleling the perceptual organization of the sequences. While other models of stream segregation, such as temporal coherence, are not excluded by the present findings, we conclude that PS is sufficient to account for the perceptual organization of ALT and SYNC sequences and thus remains a viable model of auditory stream segregation.SIGNIFICANCE STATEMENT According to the population separation (PS) model of auditory stream segregation, sounds that activate the same or separate neural populations in primary auditory cortex (A1) are perceived as one or two streams, respectively. It is unclear, however, whether the PS model can account for the perception of sounds as a single stream when they are presented synchronously. Here, we tested the PS model by recording neural responses to alternating (ALT) and synchronous (SYNC) tone sequences in macaque A1. A greater effective separation of tonotopic activity patterns was observed under ALT than under SYNC conditions, thus paralleling the perceptual organization of the sequences. Based on these findings, we conclude that PS remains a plausible neurophysiological model of auditory stream segregation.

Recent advances in exploring the neural underpinnings of auditory scene perception

Studies of auditory scene analysis have traditionally relied on paradigms using artificial sounds-and conventional behavioral techniques-to elucidate how we perceptually segregate auditory objects or streams from each other. In the past few decades, however, there has been growing interest in uncovering the neural underpinnings of auditory segregation using human and animal neuroscience techniques, as well as computational modeling. This largely reflects the growth in the fields of cognitive neuroscience and computational neuroscience and has led to new theories of how the auditory system segregates sounds in complex arrays. The current review focuses on neural and computational studies of auditory scene perception published in the last few years. Following the progress that has been made in these studies, we describe (1) theoretical advances in our understanding of the most well-studied aspects of auditory scene perception, namely segregation of sequential patterns of sounds and concurrently presented sounds; (2) the diversification of topics and paradigms that have been investigated; and (3) how new neuroscience techniques (including invasive neurophysiology in awake humans, genotyping, and brain stimulation) have been used in this field.

Comparison of perceptual properties of auditory streaming between spectral and amplitude modulation domains

The two-tone sequence (ABA_), which comprises two different sounds (A and B) and a silent gap, has been used to investigate how the auditory system organizes sequential sounds depending on various stimulus conditions or brain states. Auditory streaming can be evoked by differences not only in the tone frequency ("spectral cue": ΔF_TONE, TONE condition) but also in the amplitude modulation rate ("AM cue": ΔF_AM, AM condition). The aim of the present study was to explore the relationship between the perceptual properties of auditory streaming for the TONE and AM conditions. A sequence with a long duration (400 repetitions of ABA_) was used to examine the property of the bistability of streaming. The ratio of feature differences that evoked an equivalent probability of the segregated percept was close to the ratio of the Q-values of the auditory and modulation filters, consistent with a "channeling theory" of auditory streaming. On the other hand, for values of ΔF_AM and ΔF_TONE evoking equal probabilities of the segregated percept, the number of perceptual switches was larger for the TONE condition than for the AM condition, indicating that the mechanism(s) that determine the bistability of auditory streaming are different between or sensitive to the two domains. Nevertheless, the number of switches for individual listeners was positively correlated between the spectral and AM domains. The results suggest a possibility that the neural substrates for spectral and AM processes share a common switching mechanism but differ in location and/or in the properties of neural activity or the strength of internal noise at each level.

Animal models for auditory streaming

Sounds in the natural environment need to be assigned to acoustic sources to evaluate complex auditory scenes. Separating sources will affect the analysis of auditory features of sounds. As the benefits of assigning sounds to specific sources accrue to all species communicating acoustically, the ability for auditory scene analysis is widespread among different animals. Animal studies allow for a deeper insight into the neuronal mechanisms underlying auditory scene analysis. Here, we will review the paradigms applied in the study of auditory scene analysis and streaming of sequential sounds in animal models. We will compare the psychophysical results from the animal studies to the evidence obtained in human psychophysics of auditory streaming, i.e. in a task commonly used for measuring the capability for auditory scene analysis. Furthermore, the neuronal correlates of auditory streaming will be reviewed in different animal models and the observations of the neurons' response measures will be related to perception. The across-species comparison will reveal whether similar demands in the analysis of acoustic scenes have resulted in similar perceptual and neuronal processing mechanisms in the wide range of species being capable of auditory scene analysis.This article is part of the themed issue 'Auditory and visual scene analysis'.

Evaluating auditory stream segregation of SAM tone sequences by subjective and objective psychoacoustical tasks, and brain activity

Auditory stream segregation refers to a segregated percept of signal streams with different acoustic features. Different approaches have been pursued in studies of stream segregation. In psychoacoustics, stream segregation has mostly been investigated with a subjective task asking the subjects to report their percept. Few studies have applied an objective task in which stream segregation is evaluated indirectly by determining thresholds for a percept that depends on whether auditory streams are segregated or not. Furthermore, both perceptual measures and physiological measures of brain activity have been employed but only little is known about their relation. How the results from different tasks and measures are related is evaluated in the present study using examples relying on the ABA- stimulation paradigm that apply the same stimuli. We presented A and B signals that were sinusoidally amplitude modulated (SAM) tones providing purely temporal, spectral or both types of cues to evaluate perceptual stream segregation and its physiological correlate. Which types of cues are most prominent was determined by the choice of carrier and modulation frequencies (f mod) of the signals. In the subjective task subjects reported their percept and in the objective task we measured their sensitivity for detecting time-shifts of B signals in an ABA- sequence. As a further measure of processes underlying stream segregation we employed functional magnetic resonance imaging (fMRI). SAM tone parameters were chosen to evoke an integrated (1-stream), a segregated (2-stream), or an ambiguous percept by adjusting the f mod difference between A and B tones (Δf mod). The results of both psychoacoustical tasks are significantly correlated. BOLD responses in fMRI depend on Δf mod between A and B SAM tones. The effect of Δf mod, however, differs between auditory cortex and frontal regions suggesting differences in representation related to the degree of perceptual ambiguity of the sequences.

Active recognition enhances the representation of behaviorally relevant information in single auditory forebrain neurons

Sensory systems are dynamic. They must process a wide range of natural signals that facilitate adaptive behaviors in a manner that depends on an organism's constantly changing goals. A full understanding of the sensory physiology that underlies adaptive natural behaviors must therefore account for the activity of sensory systems in light of these behavioral goals. Here we present a novel technique that combines in vivo electrophysiological recording from awake, freely moving songbirds with operant conditioning techniques that allow control over birds' recognition of conspecific song, a widespread natural behavior in songbirds. We show that engaging in a vocal recognition task alters the response properties of neurons in the caudal mesopallium (CM), an avian analog of mammalian auditory cortex, in European starlings. Compared with awake, passive listening, active engagement of subjects in an auditory recognition task results in neurons responding to fewer song stimuli and a decrease in the trial-to-trial variability in their driven firing rates. Mean firing rates also change during active recognition, but not uniformly. Relative to nonengaged listening, active recognition causes increases in the driven firing rates in some neurons, decreases in other neurons, and stimulus-specific changes in other neurons. These changes lead to both an increase in stimulus selectivity and an increase in the information conveyed by the neurons about the animals' behavioral task. This study demonstrates the behavioral dependence of neural responses in the avian auditory forebrain and introduces the starling as a model for real-time monitoring of task-related neural processing of complex auditory objects.

Recognition of non-harmonic natural sounds by small mammals using competitive training

Animals recognize biologically relevant sounds, such as the non-harmonic sounds made by some predators, and respond with adaptive behaviors, such as escaping. To clarify which acoustic parameters are used for identifying non-harmonic, noise-like, broadband sounds, guinea pigs were conditioned to a natural target sound by introducing a novel training procedure in which 2 or 3 guinea pigs in a group competed for food. A set of distinct behavioral reactions was reliably induced almost exclusively to the target sound in a 2-week operant training. When fully conditioned, individual animals were separately tested for recognition of a set of target-like sounds that had been modified from the target sound, with spectral ranges eliminated or with fine or coarse temporal structures altered. The results show that guinea pigs are able to identify the noise-like non-harmonic natural sounds by relying on gross spectral compositions and/or fine temporal structures, just as birds are thought to do in the recognition of harmonic birdsongs. These findings are discussed with regard to similarities and dissimilarities to harmonic sound recognition. The results suggest that similar but not identical processing that requires different time scales might be used to recognize harmonic and non-harmonic sounds, at least in small mammals.

Stream segregation in the perception of sinusoidally amplitude-modulated tones

Amplitude modulation can serve as a cue for segregating streams of sounds from different sources. Here we evaluate stream segregation in humans using ABA- sequences of sinusoidally amplitude modulated (SAM) tones. A and B represent SAM tones with the same carrier frequency (1000, 4000 Hz) and modulation depth (30, 100%). The modulation frequency of the A signals (f_modA) was 30, 100 or 300 Hz, respectively. The modulation frequency of the B signals was up to four octaves higher (Δf_mod). Three different ABA- tone patterns varying in tone duration and stimulus onset asynchrony were presented to evaluate the effect of forward suppression. Subjects indicated their 1- or 2-stream percept on a touch screen at the end of each ABA- sequence (presentation time 5 or 15 s). Tone pattern, f_modA, Δf_mod, carrier frequency, modulation depth and presentation time significantly affected the percentage of a 2-stream percept. The human psychophysical results are compared to responses of avian forebrain neurons evoked by different ABA- SAM tone conditions [1] that were broadly overlapping those of the present study. The neurons also showed significant effects of tone pattern and Δf_mod that were comparable to effects observed in the present psychophysical study. Depending on the carrier frequency, modulation frequency, modulation depth and the width of the auditory filters, SAM tones may provide mainly temporal cues (sidebands fall within the range of the filter), spectral cues (sidebands fall outside the range of the filter) or possibly both. A computational model based on excitation pattern differences was used to predict the 50% threshold of 2-stream responses. In conditions for which the model predicts a considerably larger 50% threshold of 2-stream responses (i.e., larger Δf_mod at threshold) than was observed, it is unlikely that spectral cues can provide an explanation of stream segregation by SAM.

Widespread Brain Areas Engaged during a Classical Auditory Streaming Task Revealed by Intracranial EEG

The auditory system must constantly decompose the complex mixture of sound arriving at the ear into perceptually independent streams constituting accurate representations of individual sources in the acoustic environment. How the brain accomplishes this task is not well understood. The present study combined a classic behavioral paradigm with direct cortical recordings from neurosurgical patients with epilepsy in order to further describe the neural correlates of auditory streaming. Participants listened to sequences of pure tones alternating in frequency and indicated whether they heard one or two "streams." The intracranial EEG was simultaneously recorded from sub-dural electrodes placed over temporal, frontal, and parietal cortex. Like healthy subjects, patients heard one stream when the frequency separation between tones was small and two when it was large. Robust evoked-potential correlates of frequency separation were observed over widespread brain areas. Waveform morphology was highly variable across individual electrode sites both within and across gross brain regions. Surprisingly, few evoked-potential correlates of perceptual organization were observed after controlling for physical stimulus differences. The results indicate that the cortical areas engaged during the streaming task are more complex and widespread than has been demonstrated by previous work, and that, by-and-large, correlates of bistability during streaming are probably located on a spatial scale not assessed - or in a brain area not examined - by the present study.