Neurons and objects: the case of auditory cortex

Sensory Target Detection at Local and Global Timescales Reveals a Hierarchy of Supramodal Dynamics in the Human Cortex

To ensure survival in a dynamic environment, the human neocortex monitors input streams from different sensory organs for important sensory events. Which principles govern whether different senses share common or modality-specific brain networks for sensory target detection? We examined whether complex targets evoke sustained supramodal activity while simple targets rely on modality-specific networks with short-lived supramodal contributions. In a series of hierarchical multisensory target detection studies (n = 77, of either sex) using EEG, we applied a temporal cross-decoding approach to dissociate supramodal and modality-specific cortical dynamics elicited by rule-based global and feature-based local sensory deviations within and between the visual, somatosensory, and auditory modality. Our data show that each sense implements a cortical hierarchy orchestrating supramodal target detection responses, which operate at local and global timescales in successive processing stages. Across different sensory modalities, simple feature-based sensory deviations presented in temporal vicinity to a monotonous input stream triggered a mismatch negativity-like local signal which decayed quickly and early, whereas complex rule-based targets tracked across time evoked a P3b-like global neural response which generalized across a late time window. Converging results from temporal cross-modality decoding analyses across different datasets, we reveal that global neural responses are sustained in a supramodal higher-order network, whereas local neural responses canonically thought to rely on modality-specific regions evolve into short-lived supramodal activity. Together, our findings demonstrate that cortical organization largely follows a gradient in which short-lived modality-specific as well as supramodal processes dominate local responses, whereas higher-order processes encode temporally extended abstract supramodal information fed forward from modality-specific cortices.SIGNIFICANCE STATEMENT Each sense supports a cortical hierarchy of processes tracking deviant sensory events at multiple timescales. Conflicting evidence produced a lively debate around which of these processes are supramodal. Here, we manipulated the temporal complexity of auditory, tactile, and visual targets to determine whether cortical local and global ERP responses to sensory targets share cortical dynamics between the senses. Using temporal cross-decoding, we found that temporally complex targets elicit a supramodal sustained response. Conversely, local responses to temporally confined targets typically considered modality-specific rely on early short-lived supramodal activation. Our finding provides evidence for a supramodal gradient supporting sensory target detection in the cortex, with implications for multiple fields in which these responses are studied (e.g., predictive coding, consciousness, and attention).

Enhanced stability of complex sound representations relative to simple sounds in the auditory cortex

Typical everyday sounds, such as those of speech or running water, are spectrotemporally complex. The ability to recognize complex sounds (CxS) and their associated meaning is presumed to rely on their stable neural representations across time. The auditory cortex is critical for processing of CxS, yet little is known of the degree of stability of auditory cortical representations of CxS across days. Previous studies have shown that the auditory cortex represents CxS identity with a substantial degree of invariance to basic sound attributes such as frequency. We therefore hypothesized that auditory cortical representations of CxS are more stable across days than those of sounds that lack spectrotemporal structure such as pure tones (PTs). To test this hypothesis, we recorded responses of identified L2/3 auditory cortical excitatory neurons to both PTs and CxS across days using two-photon calcium imaging in awake mice. Auditory cortical neurons showed significant daily changes of responses to both types of sounds, yet responses to CxS exhibited significantly lower rates of daily change than those of PTs. Furthermore, daily changes in response profiles to PTs tended to be more stimulus-specific, reflecting changes in sound selectivity, as compared to changes of CxS responses. Lastly, the enhanced stability of responses to CxS was evident across longer time intervals as well. Together, these results suggest that spectrotemporally CxS are more stably represented in the auditory cortex across time than PTs. These findings support a role of the auditory cortex in representing CxS identity across time.Significance statementThe ability to recognize everyday complex sounds such as those of speech or running water is presumed to rely on their stable neural representations. Yet, little is known of the degree of stability of single-neuron sound responses across days. As the auditory cortex is critical for complex sound perception, we hypothesized that the auditory cortical representations of complex sounds are relatively stable across days. To test this, we recorded sound responses of identified auditory cortical neurons across days in awake mice. We found that auditory cortical responses to complex sounds are significantly more stable across days as compared to those of simple pure tones. These findings support a role of the auditory cortex in representing complex sound identity across time.

Categorical Encoding of Vowels in Primary Auditory Cortex

Speech perception involves mapping from a continuous and variable acoustic speech signal to discrete, linguistically meaningful units. However, it is unclear where in the auditory processing stream speech sound representations cease to be veridical (faithfully encoding precise acoustic properties) and become categorical (encoding sounds as linguistic categories). In this study, we used functional magnetic resonance imaging and multivariate pattern analysis to determine whether tonotopic primary auditory cortex (PAC), defined as tonotopic voxels falling within Heschl's gyrus, represents one class of speech sounds-vowels-veridically or categorically. For each of 15 participants, 4 individualized synthetic vowel stimuli were generated such that the vowels were equidistant in acoustic space, yet straddled a categorical boundary (with the first 2 vowels perceived as [i] and the last 2 perceived as [i]). Each participant's 4 vowels were then presented in a block design with an irrelevant but attention-demanding level change detection task. We found that in PAC bilaterally, neural discrimination between pairs of vowels that crossed the categorical boundary was more accurate than neural discrimination between equivalently spaced vowel pairs that fell within a category. These findings suggest that PAC does not represent vowel sounds veridically, but that encoding of vowels is shaped by linguistically relevant phonemic categories.

Functional neuroimaging in hearing research and audiology

The various methods of medical imaging are essential for many diagnostic issues in clinical routine, e.g., for the diagnostics and localisation of tumorous diseases, or for the clarification of other lesions in the central nervous system. In addition to these classical roles both positron emission tomography (PET) and magnetic resonance imaging (MRI) allow for the investigation of functional processes in the human brain, when used in a specific way. The last 25 years have seen great progress, especially with respect to functional MRI, in terms of the available experimental paradigms as well as the data analysis strategies, so that a directed investigation of neurophysiological correlates of psychoacoustic performance is possible. This covers fundamental measures of sound perception like loudness and pitch, specific audiological symptoms like tinnitus, which often accompanies hearing disorders, but it also includes experiments on speech perception or on virtual acoustic environments. One important aspect common to many auditory neuroimaging studies is the central question at what stage in the human auditory pathway the sensory coding of the incoming sound is transformed into a universal and context-dependent perceptual representation, which is the basis for what we hear. This overview summarises findings from the literature as well as a few studies from our lab, to discuss the possibilities and the limits of the adoption of functional neuroimaging methods in audiology. Up to this stage, most auditory neuroimaging studies have investigated basic processes in normal hearing listeners. However, the hitherto existing results suggest that the methods of auditory functional neuroimaging - possibly complemented by electrophysiological methods like EEG and MEG - have a great potential to contribute to a deeper understanding of the processes and the impact of hearing disorders.

Pitch, Timbre and Intensity Interdependently Modulate Neural Responses to Salient Sounds

As we listen to everyday sounds, auditory perception is heavily shaped by interactions between acoustic attributes such as pitch, timbre and intensity; though it is not clear how such interactions affect judgments of acoustic salience in dynamic soundscapes. Salience perception is believed to rely on an internal brain model that tracks the evolution of acoustic characteristics of a scene and flags events that do not fit this model as salient. The current study explores how the interdependency between attributes of dynamic scenes affects the neural representation of this internal model and shapes encoding of salient events. Specifically, the study examines how deviations along combinations of acoustic attributes interact to modulate brain responses, and subsequently guide perception of certain sound events as salient given their context. Human volunteers have their attention focused on a visual task and ignore acoustic melodies playing in the background while their brain activity using electroencephalography is recorded. Ambient sounds consist of musical melodies with probabilistically-varying acoustic attributes. Salient notes embedded in these scenes deviate from the melody's statistical distribution along pitch, timbre and/or intensity. Recordings of brain responses to salient notes reveal that neural power in response to the melodic rhythm as well as cross-trial phase alignment in the theta band are modulated by degree of salience of the notes, estimated across all acoustic attributes given their probabilistic context. These neural nonlinear effects across attributes strongly parallel behavioral nonlinear interactions observed in perceptual judgments of auditory salience using similar dynamic melodies; suggesting a neural underpinning of nonlinear interactions that underlie salience perception.

Noise-Sensitive But More Precise Subcortical Representations Coexist with Robust Cortical Encoding of Natural Vocalizations

Humans and animals maintain accurate sound discrimination in the presence of loud sources of background noise. It is commonly assumed that this ability relies on the robustness of auditory cortex responses. However, only a few attempts have been made to characterize neural discrimination of communication sounds masked by noise at each stage of the auditory system and to quantify the noise effects on the neuronal discrimination in terms of alterations in amplitude modulations. Here, we measured neural discrimination between communication sounds masked by a vocalization-shaped stationary noise from multiunit responses recorded in the cochlear nucleus, inferior colliculus, auditory thalamus, and primary and secondary auditory cortex at several signal-to-noise ratios (SNRs) in anesthetized male or female guinea pigs. Masking noise decreased sound discrimination of neuronal populations in each auditory structure, but collicular and thalamic populations showed better performance than cortical populations at each SNR. In contrast, in each auditory structure, discrimination by neuronal populations was slightly decreased when tone-vocoded vocalizations were tested. These results shed new light on the specific contributions of subcortical structures to robust sound encoding, and suggest that the distortion of slow amplitude modulation cues conveyed by communication sounds is one of the factors constraining the neuronal discrimination in subcortical and cortical levels.SIGNIFICANCE STATEMENT Dissecting how auditory neurons discriminate communication sounds in noise is a major goal in auditory neuroscience. Robust sound coding in noise is often viewed as a specific property of cortical networks, although this remains to be demonstrated. Here, we tested the discrimination performance of neuronal populations at five levels of the auditory system in response to conspecific vocalizations masked by noise. In each acoustic condition, subcortical neurons better discriminated target vocalizations than cortical ones and in each structure, the reduction in discrimination performance was related to the reduction in slow amplitude modulation cues.

Neural representation of interaural correlation in human auditory brainstem: Comparisons between temporal-fine structure and envelope

Central processing of interaural correlation (IAC), which depends on the precise representation of acoustic signals from the two ears, is essential for both localization and recognition of auditory objects. A complex soundwave is initially filtered by the peripheral auditory system into multiple narrowband waves, which are further decomposed into two functionally distinctive components: the quickly-varying temporal-fine structure (TFS) and the slowly-varying envelope. In rats, a narrowband noise can evoke auditory-midbrain frequency-following responses (FFRs) that contain both the TFS component (FFR_TFS) and the envelope component (FFR_Env), which represent the TFS and envelope of the narrowband noise, respectively. These two components are different in sensitivity to the interaural time disparity. In human listeners, the present study investigated whether the FFR_TFS and FFR_Env components of brainstem FFRs to a narrowband noise are different in sensitivity to IAC and whether there are potential brainstem mechanisms underlying the integration of the two components. The results showed that although both the amplitude of FFR_TFS and that of FFR_Env were significantly affected by shifts of IAC between 1 and 0, the stimulus-to-response correlation for FFR_TFS, but not that for FFR_Env, was sensitive to the IAC shifts. Moreover, in addition to the correlation between the binaurally evoked FFR_TFS and FFR_Env, the correlation between the IAC-shift-induced change of FFR_TFS and that of FFR_Env was significant. Thus, the TFS information is more precisely represented in the human auditory brainstem than the envelope information, and the correlation between FFR_TFS and FFR_Env for the same narrowband noise suggest a brainstem binding mechanism underlying the perceptual integration of the TFS and envelope signals.

Assessing Top-Down and Bottom-Up Contributions to Auditory Stream Segregation and Integration With Polyphonic Music

Polyphonic music listening well exemplifies processes typically involved in daily auditory scene analysis situations, relying on an interactive interplay between bottom-up and top-down processes. Most studies investigating scene analysis have used elementary auditory scenes, however real-world scene analysis is far more complex. In particular, music, contrary to most other natural auditory scenes, can be perceived by either integrating or, under attentive control, segregating sound streams, often carried by different instruments. One of the prominent bottom-up cues contributing to multi-instrument music perception is their timbre difference. In this work, we introduce and validate a novel paradigm designed to investigate, within naturalistic musical auditory scenes, attentive modulation as well as its interaction with bottom-up processes. Two psychophysical experiments are described, employing custom-composed two-voice polyphonic music pieces within a framework implementing a behavioral performance metric to validate listener instructions requiring either integration or segregation of scene elements. In Experiment 1, the listeners' locus of attention was switched between individual instruments or the aggregate (i.e., both instruments together), via a task requiring the detection of temporal modulations (i.e., triplets) incorporated within or across instruments. Subjects responded post-stimulus whether triplets were present in the to-be-attended instrument(s). Experiment 2 introduced the bottom-up manipulation by adding a three-level morphing of instrument timbre distance to the attentional framework. The task was designed to be used within neuroimaging paradigms; Experiment 2 was additionally validated behaviorally in the functional Magnetic Resonance Imaging (fMRI) environment. Experiment 1 subjects (N = 29, non-musicians) completed the task at high levels of accuracy, showing no group differences between any experimental conditions. Nineteen listeners also participated in Experiment 2, showing a main effect of instrument timbre distance, even though within attention-condition timbre-distance contrasts did not demonstrate any timbre effect. Correlation of overall scores with morph-distance effects, computed by subtracting the largest from the smallest timbre distance scores, showed an influence of general task difficulty on the timbre distance effect. Comparison of laboratory and fMRI data showed scanner noise had no adverse effect on task performance. These Experimental paradigms enable to study both bottom-up and top-down contributions to auditory stream segregation and integration within psychophysical and neuroimaging experiments.

Cortical Representations of Speech in a Multitalker Auditory Scene

The ability to parse a complex auditory scene into perceptual objects is facilitated by a hierarchical auditory system. Successive stages in the hierarchy transform an auditory scene of multiple overlapping sources, from peripheral tonotopically based representations in the auditory nerve, into perceptually distinct auditory-object-based representations in the auditory cortex. Here, using magnetoencephalography recordings from men and women, we investigate how a complex acoustic scene consisting of multiple speech sources is represented in distinct hierarchical stages of the auditory cortex. Using systems-theoretic methods of stimulus reconstruction, we show that the primary-like areas in the auditory cortex contain dominantly spectrotemporal-based representations of the entire auditory scene. Here, both attended and ignored speech streams are represented with almost equal fidelity, and a global representation of the full auditory scene with all its streams is a better candidate neural representation than that of individual streams being represented separately. We also show that higher-order auditory cortical areas, by contrast, represent the attended stream separately and with significantly higher fidelity than unattended streams. Furthermore, the unattended background streams are more faithfully represented as a single unsegregated background object rather than as separated objects. Together, these findings demonstrate the progression of the representations and processing of a complex acoustic scene up through the hierarchy of the human auditory cortex.SIGNIFICANCE STATEMENT Using magnetoencephalography recordings from human listeners in a simulated cocktail party environment, we investigate how a complex acoustic scene consisting of multiple speech sources is represented in separate hierarchical stages of the auditory cortex. We show that the primary-like areas in the auditory cortex use a dominantly spectrotemporal-based representation of the entire auditory scene, with both attended and unattended speech streams represented with almost equal fidelity. We also show that higher-order auditory cortical areas, by contrast, represent an attended speech stream separately from, and with significantly higher fidelity than, unattended speech streams. Furthermore, the unattended background streams are represented as a single undivided background object rather than as distinct background objects.

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.

Advantages of comparative studies in songbirds to understand the neural basis of sensorimotor integration

Sensorimotor integration is the process through which the nervous system creates a link between motor commands and associated sensory feedback. This process allows for the acquisition and refinement of many behaviors, including learned communication behaviors such as speech and birdsong. Consequently, it is important to understand fundamental mechanisms of sensorimotor integration, and comparative analyses of this process can provide vital insight. Songbirds offer a powerful comparative model system to study how the nervous system links motor and sensory information for learning and control. This is because the acquisition, maintenance, and control of birdsong critically depend on sensory feedback. Furthermore, there is an incredible diversity of song organizations across songbird species, ranging from songs with simple, stereotyped sequences to songs with complex sequencing of vocal gestures, as well as a wide diversity of song repertoire sizes. Despite this diversity, the neural circuitry for song learning, control, and maintenance remains highly similar across species. Here, we highlight the utility of songbirds for the analysis of sensorimotor integration and the insights about mechanisms of sensorimotor integration gained by comparing different songbird species. Key conclusions from this comparative analysis are that variation in song sequence complexity seems to covary with the strength of feedback signals in sensorimotor circuits and that sensorimotor circuits contain distinct representations of elements in the vocal repertoire, possibly enabling evolutionary variation in repertoire sizes. We conclude our review by highlighting important areas of research that could benefit from increased comparative focus, with particular emphasis on the integration of new technologies.

Knowledge About Sounds-Context-Specific Meaning Differently Activates Cortical Hemispheres, Auditory Cortical Fields, and Layers in House Mice

Activation of the auditory cortex (AC) by a given sound pattern is plastic, depending, in largely unknown ways, on the physiological state and the behavioral context of the receiving animal and on the receiver's experience with the sounds. Such plasticity can be inferred when house mouse mothers respond maternally to pup ultrasounds right after parturition and naïve females have to learn to respond. Here we use c-FOS immunocytochemistry to quantify highly activated neurons in the AC fields and layers of seven groups of mothers and naïve females who have different knowledge about and are differently motivated to respond to acoustic models of pup ultrasounds of different behavioral significance. Profiles of FOS-positive cells in the AC primary fields (AI, AAF), the ultrasonic field (UF), the secondary field (AII), and the dorsoposterior field (DP) suggest that activation reflects in AI, AAF, and UF the integration of sound properties with animal state-dependent factors, in the higher-order field AII the news value of a given sound in the behavioral context, and in the higher-order field DP the level of maternal motivation and, by left-hemisphere activation advantage, the recognition of the meaning of sounds in the given context. Anesthesia reduced activation in all fields, especially in cortical layers 2/3. Thus, plasticity in the AC is field-specific preparing different output of AC fields in the process of perception, recognition and responding to communication sounds. Further, the activation profiles of the auditory cortical fields suggest the differentiation between brains hormonally primed to know (mothers) and brains which acquired knowledge via implicit learning (naïve females). In this way, auditory cortical activation discriminates between instinctive (mothers) and learned (naïve females) cognition.

Echoes of the spoken past: how auditory cortex hears context during speech perception

What do we hear when someone speaks and what does auditory cortex (AC) do with that sound? Given how meaningful speech is, it might be hypothesized that AC is most active when other people talk so that their productions get decoded. Here, neuroimaging meta-analyses show the opposite: AC is least active and sometimes deactivated when participants listened to meaningful speech compared to less meaningful sounds. Results are explained by an active hypothesis-and-test mechanism where speech production (SP) regions are neurally re-used to predict auditory objects associated with available context. By this model, more AC activity for less meaningful sounds occurs because predictions are less successful from context, requiring further hypotheses be tested. This also explains the large overlap of AC co-activity for less meaningful sounds with meta-analyses of SP. An experiment showed a similar pattern of results for non-verbal context. Specifically, words produced less activity in AC and SP regions when preceded by co-speech gestures that visually described those words compared to those words without gestures. Results collectively suggest that what we ‘hear’ during real-world speech perception may come more from the brain than our ears and that the function of AC is to confirm or deny internal predictions about the identity of sounds.

Stimulus-specific adaptation and deviance detection in the auditory system: experiments and models

Stimulus-specific adaptation (SSA) is the reduction in the response to a common stimulus that does not generalize, or only partially generalizes, to other, rare stimuli. SSA has been proposed to be a correlate of 'deviance detection', an important computational task of sensory systems. SSA is ubiquitous in the auditory system: It is found both in cortex and in subcortical stations, and it has been demonstrated in many mammalian species as well as in birds. A number of models have been suggested in the literature to account for SSA in the auditory domain. In this review, the experimental literature is critically examined in relationship to these models. While current models can all account for auditory SSA to some degree, none is fully compatible with the available findings.

Differential activation of human core, non-core and auditory-related cortex during speech categorization tasks as revealed by intracranial recordings

Speech perception requires that sounds be transformed into speech-related objects with lexical and semantic meaning. It is unclear at what level in the auditory pathways this transformation emerges. Primary auditory cortex has been implicated in both representation of acoustic sound attributes and sound objects. While non-primary auditory cortex located on the posterolateral superior temporal gyrus (PLST) is clearly involved in acoustic-to-phonetic pre-lexical representations, it is unclear what role this region plays in auditory object formation. Additional data support the importance of prefrontal cortex in the formation of auditory objects, while other data would implicate this region in auditory object selection. To help clarify the respective roles of auditory and auditory-related cortex in the formation and selection of auditory objects, we examined high gamma activity simultaneously recorded directly from Heschl's gyrus (HG), PLST and prefrontal cortex, while subjects performed auditory semantic detection tasks. Subjects were patients undergoing evaluation for treatment of medically intractable epilepsy. We found that activity in posteromedial HG and early activity on PLST was robust to sound stimuli regardless of their context, and minimally modulated by tasks. Later activity on PLST could be strongly modulated by semantic context, but not by behavioral performance. Activity within prefrontal cortex also was related to semantic context, and did co-vary with behavior. We propose that activity in posteromedial HG and early activity on PLST primarily reflect the representation of spectrotemporal sound attributes. Later activity on PLST represents a pre-lexical processing stage and is an intermediate step in the formation of word objects. Activity in prefrontal cortex appears directly involved in word object selection. The roles of other auditory and auditory-related cortical areas in the formation of word objects remain to be explored.

Sexual attractiveness of male chemicals and vocalizations in mice

Male-female interaction is important for finding a suitable mating partner and for ensuring reproductive success. Male sexual signals such as pheromones transmit information and social and sexual status to females, and exert powerful effects on the mate preference and reproductive biology of females. Likewise, male vocalizations are attractive to females and enhance reproductive function in many animals. Interestingly, females' preference for male pheromones and vocalizations is associated with their genetic background, to avoid inbreeding. Moreover, based on acoustic cues, olfactory signals have significant effects on mate choice in mice, suggesting mate choice involves multisensory integration. In this review, we synopsize the effects of both olfactory and auditory cues on female behavior and neuroendocrine functions. We also discuss how these male signals are integrated and processed in the brain to regulate behavior and reproductive function.

Detection of modulated tones in modulated noise by non-human primates

In natural environments, many sounds are amplitude-modulated. Amplitude modulation is thought to be a signal that aids auditory object formation. A previous study of the detection of signals in noise found that when tones or noise were amplitude-modulated, the noise was a less effective masker, and detection thresholds for tones in noise were lowered. These results suggest that the detection of modulated signals in modulated noise would be enhanced. This paper describes the results of experiments investigating how detection is modified when both signal and noise were amplitude-modulated. Two monkeys (Macaca mulatta) were trained to detect amplitude-modulated tones in continuous, amplitude-modulated broadband noise. When the phase difference of otherwise similarly amplitude-modulated tones and noise were varied, detection thresholds were highest when the modulations were in phase and lowest when the modulations were anti-phase. When the depth of the modulation of tones or noise was varied, detection thresholds decreased if the modulations were anti-phase. When the modulations were in phase, increasing the depth of tone modulation caused an increase in tone detection thresholds, but increasing depth of noise modulations did not affect tone detection thresholds. Changing the modulation frequency of tone or noise caused changes in threshold that saturated at modulation frequencies higher than 20 Hz; thresholds decreased when the tone and noise modulations were in phase and decreased when they were anti-phase. The relationship between reaction times and tone level were not modified by manipulations to the nature of temporal variations in the signal or noise. The changes in behavioral threshold were consistent with a model where the brain subtracted noise from signal. These results suggest that the parameters of the modulation of signals and maskers heavily influence detection in very predictable ways. These results are consistent with some results in humans and avians and form the baseline for neurophysiological studies of mechanisms of detection in noise.

Cortical mapping of mismatch negativity with deviance detection property in rat

Mismatch Negativity (MMN) is an N-methyl-d-aspartic acid (NMDA)-mediated, negative deflection in human auditory evoked potentials in response to a cognitively discriminable change. MMN-like responses have been extensively investigated in animal models, but the existence of MMN equivalent is still controversial. In this study, we aimed to investigate how closely the putative MMN (MMNp) in rats exhibited the comparable properties of human MMN. We used a surface microelectrode array with a grid of 10×7 recording sites within an area of 4.5×3.0 mm to densely map evoked potentials in the auditory cortex of anesthetized rats under the oddball paradigm. Firstly, like human MMN, deviant stimuli elicited negative deflections in auditory evoked potentials following the positive middle-latency response, termed P1. Secondly, MMNp exhibited deviance-detecting property, which could not be explained by simple stimulus specific adaptation (SSA). Thirdly, this MMNp occurred focally in the auditory cortex, including both the core and belt regions, while P1 activation focus was obtained in the core region, indicating that both P1 and MMNp are generated in the auditory cortex, yet the sources of these signals do not completely overlap. Fourthly, MMNp significantly decreased after the application of AP5 (D-(-)-2-amino-5-phosphonopentanoic acid), an antagonist at NMDA receptors. In stark contrast, AP5 affected neither P1 amplitude nor SSA of P1. These results provide compelling evidence that the MMNp we have examined in rats is functionally comparable to human MMN. The present work will stimulate translational research into MMN, which may help bridge the gap between electroencephalography (EEG)/magnetoencephalography (MEG) studies in humans and electrophysiological studies in animals.

Perceptual spaces: mathematical structures to neural mechanisms

A central goal of neuroscience is to understand how populations of neurons build and manipulate representations of percepts that provide useful information about the environment. This symposium explores the fundamental properties of these representations and the perceptual spaces in which they are organized. Spanning the domains of color, visual texture, environmental sound, music, tactile quality, and odor, we show how the geometric structures of perceptual spaces can be determined experimentally and how these structures provide insights into the principles of neural coding and the neural mechanisms that generate the codes, and into the neural processing of complex sensory stimuli. The diversity of the neural architecture in these different sensory systems provides an opportunity to compare their different solutions to common problems: the need for dimensionality reduction, strategies for topographic or nontopographic mapping, the utility of the higher-order statistical structure inherent in natural sensory stimuli, and the constraints of neural hardware.

Eccentricity dependent auditory enhancement of visual stimulus detection but not discrimination

Sensory perception is enhanced by the complementary information provided by our different sensory modalities and even apparently task irrelevant stimuli in one modality can facilitate performance in another. While perception in general comprises both, the detection of sensory objects as well as their discrimination and recognition, most studies on audio-visual interactions have focused on either of these aspects. However, previous evidence, neuroanatomical projections between early sensory cortices and computational mechanisms suggest that sounds might differentially affect visual detection and discrimination and differentially at central and peripheral retinal locations. We performed an experiment to directly test this by probing the enhancement of visual detection and discrimination by auxiliary sounds at different visual eccentricities and within the same subjects. Specifically, we quantified the enhancement provided by sounds that reduce the overall uncertainty about the visual stimulus beyond basic multisensory co-stimulation. This revealed a general trend for stronger enhancement at peripheral locations in both tasks, but a statistically significant effect only for detection and only at peripheral locations. Overall this suggests that there are topographic differences in the auditory facilitation of basic visual processes and that these may differentially affect basic aspects of visual recognition.

Stable bottom-up processing during dynamic top-down modulations in monkey auditory cortex

It is unclear whether top-down processing in the auditory cortex (AC) interferes with its bottom-up analysis of sound. Recent studies indicated non-acoustic modulations of AC responses, and that attention changes a neuron's spectrotemporal tuning. As a result, the AC would seem ill-suited to represent a stable acoustic environment, which is deemed crucial for auditory perception. To assess whether top-down signals influence acoustic tuning in tasks without directed attention, we compared monkey single-unit AC responses to dynamic spectrotemporal sounds under different behavioral conditions. Recordings were mostly made from neurons located in primary fields (primary AC and area R of the AC) that were well tuned to pure tones, with short onset latencies. We demonstrated that responses in the AC were substantially modulated during an auditory detection task and that these modulations were systematically related to top-down processes. Importantly, despite these significant modulations, the spectrotemporal receptive fields of all neurons remained remarkably stable. Our results suggest multiplexed encoding of bottom-up acoustic and top-down task-related signals at single AC neurons. This mechanism preserves a stable representation of the acoustic environment despite strong non-acoustic modulations.

Bayesian inference in auditory scenes

The cocktail party problem is a multi-faceted challenge which encompasses various aspects of auditory perception. Its processes underlie the brain's ability to detect, identify and classify sound objects; to robustly represent and maintain speech intelligibility amidst severe distortions; and to guide actions and behaviors in line with complex goals and shifting acoustic soundscapes. Here, we present a perspective that considers the powerful Bayesian inference as a unifying framework to integrate the role of sensory cues as well as stimulus-driven priors and top-down schemas including attention.

Auditory abstraction from spectro-temporal features to coding auditory entities

The auditory system extracts behaviorally relevant information from acoustic stimuli. The average activity in auditory cortex is known to be sensitive to spectro-temporal patterns in sounds. However, it is not known whether the auditory cortex also processes more abstract features of sounds, which may be more behaviorally relevant than spectro-temporal patterns. Using recordings from three stations of the auditory pathway, the inferior colliculus (IC), the ventral division of the medial geniculate body (MGB) of the thalamus, and the primary auditory cortex (A1) of the cat in response to natural sounds, we compared the amount of information that spikes contained about two aspects of the stimuli: spectro-temporal patterns, and abstract entities present in the same stimuli such as a bird chirp, its echoes, and the ambient noise. IC spikes conveyed on average approximately the same amount of information about spectro-temporal patterns as they conveyed about abstract auditory entities, but A1 and the MGB neurons conveyed on average three times more information about abstract auditory entities than about spectro-temporal patterns. Thus, the majority of neurons in auditory thalamus and cortex coded well the presence of abstract entities in the sounds without containing much information about their spectro-temporal structure, suggesting that they are sensitive to abstract features in these sounds.

Emergence of neural encoding of auditory objects while listening to competing speakers

A visual scene is perceived in terms of visual objects. Similar ideas have been proposed for the analogous case of auditory scene analysis, although their hypothesized neural underpinnings have not yet been established. Here, we address this question by recording from subjects selectively listening to one of two competing speakers, either of different or the same sex, using magnetoencephalography. Individual neural representations are seen for the speech of the two speakers, with each being selectively phase locked to the rhythm of the corresponding speech stream and from which can be exclusively reconstructed the temporal envelope of that speech stream. The neural representation of the attended speech dominates responses (with latency near 100 ms) in posterior auditory cortex. Furthermore, when the intensity of the attended and background speakers is separately varied over an 8-dB range, the neural representation of the attended speech adapts only to the intensity of that speaker but not to the intensity of the background speaker, suggesting an object-level intensity gain control. In summary, these results indicate that concurrent auditory objects, even if spectrotemporally overlapping and not resolvable at the auditory periphery, are neurally encoded individually in auditory cortex and emerge as fundamental representational units for top-down attentional modulation and bottom-up neural adaptation.

Neural mechanisms of rhythmic masking release in monkey primary auditory cortex: implications for models of auditory scene analysis

The ability to detect and track relevant acoustic signals embedded in a background of other sounds is crucial for hearing in complex acoustic environments. This ability is exemplified by a perceptual phenomenon known as "rhythmic masking release" (RMR). To demonstrate RMR, a sequence of tones forming a target rhythm is intermingled with physically identical "Distracter" sounds that perceptually mask the rhythm. The rhythm can be "released from masking" by adding "Flanker" tones in adjacent frequency channels that are synchronous with the Distracters. RMR represents a special case of auditory stream segregation, whereby the target rhythm is perceptually segregated from the background of Distracters when they are accompanied by the synchronous Flankers. The neural basis of RMR is unknown. Previous studies suggest the involvement of primary auditory cortex (A1) in the perceptual organization of sound patterns. Here, we recorded neural responses to RMR sequences in A1 of awake monkeys in order to identify neural correlates and potential mechanisms of RMR. We also tested whether two current models of stream segregation, when applied to these responses, could account for the perceptual organization of RMR sequences. Results suggest a key role for suppression of Distracter-evoked responses by the simultaneous Flankers in the perceptual restoration of the target rhythm in RMR. Furthermore, predictions of stream segregation models paralleled the psychoacoustics of RMR in humans. These findings reinforce the view that preattentive or "primitive" aspects of auditory scene analysis may be explained by relatively basic neural mechanisms at the cortical level.

Multisensory integration of natural odors and sounds in the auditory cortex

VIDEO ABSTRACT

Motherhood is associated with different forms of physiological alterations including transient hormonal changes and brain plasticity. The underlying impact of these changes on the emergence of maternal behaviors and sensory processing within the mother's brain are largely unknown. By using in vivo cell-attached recordings in the primary auditory cortex of female mice, we discovered that exposure to pups' body odor reshapes neuronal responses to pure tones and natural auditory stimuli. This olfactory-auditory interaction appeared naturally in lactating mothers shortly after parturition and was long lasting. Naive virgins that had experience with the pups also showed an appearance of olfactory-auditory integration in A1, suggesting that multisensory integration may be experience dependent. Neurons from lactating mothers were more sensitive to sounds as compared to those from experienced mice, independent of the odor effects. These uni- and multisensory cortical changes may facilitate the detection and discrimination of pup distress calls and strengthen the bond between mothers and their neonates.

Developmental neuroplasticity after cochlear implantation

Cortical development is dependent on stimulus-driven learning. The absence of sensory input from birth, as occurs in congenital deafness, affects normal growth and connectivity needed to form a functional sensory system, resulting in deficits in oral language learning. Cochlear implants bypass cochlear damage by directly stimulating the auditory nerve and brain, making it possible to avoid many of the deleterious effects of sensory deprivation. Congenitally deaf animals and children who receive implants provide a platform to examine the characteristics of cortical plasticity in the auditory system. In this review, we discuss the existence of time limits for, and mechanistic constraints on, sensitive periods for cochlear implantation and describe the effects of multimodal and cognitive reorganization that result from long-term auditory deprivation.

Non-isomorphism in efficient coding of complex sound properties

To the extent that sensorineural systems are efficient, stimulus redundancy should be captured in ways that optimize information transmission. Consistent with this principle, neural representations of sounds have been proposed to become "non-isomorphic," increasingly abstract and decreasingly resembling the original (redundant) input. Here, non-isomorphism is tested in perceptual learning using AXB discrimination of novel sounds with two highly correlated complex acoustic properties and a randomly varying third dimension. Discrimination of sounds obeying the correlation became superior to that of sounds violating it despite widely varying physical acoustic properties, suggesting non-isomorphic representation of stimulus redundancy.

Learning-stage-dependent, field-specific, map plasticity in the rat auditory cortex during appetitive operant conditioning

Cortical reorganizations during acquisition of motor skills and experience-dependent recovery after deafferentation consist of several distinct phases, in which expansion of receptive fields is followed by the shrinkage and use-dependent refinement. In perceptual learning, however, such non-monotonic, stage-dependent plasticity remains elusive in the sensory cortex. In the present study, microelectrode mapping characterized plasticity in the rat auditory cortex, including primary, anterior, and ventral/suprarhinal auditory fields (A1, AAF, and VAF/SRAF), at the early and late stages of appetitive operant conditioning. We first demonstrate that most plasticity at the early stage was tentative, and that long-lasting plasticity after extended training was able to be categorized into either early- or late-stage-dominant plasticity. Second, training-induced plasticity occurred both locally and globally with a specific temporal order. Conditioned-stimulus (CS) frequency used in the task tended to be locally over-represented in AAF at the early stage and in VAF/SRAF at the late stage. The behavioral relevance of neural responses suggests that the local plasticity also occurred in A1 at the early stage. In parallel, the tone-responsive area globally shrank at the late stage independently of CS frequency, and this shrinkage was also correlated with the behavioral improvements. Thus, the stage-dependent plasticity may commonly underlie cortical reorganization in the perceptual learning, yet the interactions of local and global plasticity have led to more complicated reorganization than previously thought. Field-specific plasticity has important implications for how each field subserves in the learning; for example, consistent with recent notions, A1 should construct filters to better identify auditory objects at the early stage, while VAF/SRAF contribute to hierarchical computation and storage at the late stage.

Music and the Auditory Brain: Where is the Connection?

Sound processing by the auditory system is understood in unprecedented details, even compared with sensory coding in the visual system. Nevertheless, we do not understand yet the way in which some of the simplest perceptual properties of sounds are coded in neuronal activity. This poses serious difficulties for linking neuronal responses in the auditory system and music processing, since music operates on abstract representations of sounds. Paradoxically, although perceptual representations of sounds most probably occur high in auditory system or even beyond it, neuronal responses are strongly affected by the temporal organization of sound streams even in subcortical stations. Thus, to the extent that music is organized sound, it is the organization, rather than the sound, which is represented first in the auditory brain.

Understanding auditory spectro-temporal receptive fields and their changes with input statistics by efficient coding principles

Spectro-temporal receptive fields (STRFs) have been widely used as linear approximations to the signal transform from sound spectrograms to neural responses along the auditory pathway. Their dependence on statistical attributes of the stimuli, such as sound intensity, is usually explained by nonlinear mechanisms and models. Here, we apply an efficient coding principle which has been successfully used to understand receptive fields in early stages of visual processing, in order to provide a computational understanding of the STRFs. According to this principle, STRFs result from an optimal tradeoff between maximizing the sensory information the brain receives, and minimizing the cost of the neural activities required to represent and transmit this information. Both terms depend on the statistical properties of the sensory inputs and the noise that corrupts them. The STRFs should therefore depend on the input power spectrum and the signal-to-noise ratio, which is assumed to increase with input intensity. We analytically derive the optimal STRFs when signal and noise are approximated as Gaussians. Under the constraint that they should be spectro-temporally local, the STRFs are predicted to adapt from being band-pass to low-pass filters as the input intensity reduces, or the input correlation becomes longer range in sound frequency or time. These predictions qualitatively match physiological observations. Our prediction as to how the STRFs should be determined by the input power spectrum could readily be tested, since this spectrum depends on the stimulus ensemble. The potentials and limitations of the efficient coding principle are discussed.

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.

Auditory cortex encodes the perceptual interpretation of ambiguous sound

The confounding of physical stimulus characteristics and perceptual interpretations of stimuli poses a problem for most neuroscientific studies of perception. In the auditory domain, this pertains to the entanglement of acoustics and percept. Traditionally, most study designs have relied on cognitive subtraction logic, which demands the use of one or more comparisons between stimulus types. This does not allow for a differentiation between effects due to acoustic differences (i.e., sensation) and those due to conscious perception. To overcome this problem, we used functional magnetic resonance imaging (fMRI) in humans and pattern-recognition analysis to identify activation patterns that encode the perceptual interpretation of physically identical, ambiguous sounds. We show that it is possible to retrieve the perceptual interpretation of ambiguous phonemes-information that is fully subjective to the listener-from fMRI measurements of brain activity in auditory areas in the superior temporal cortex, most prominently on the posterior bank of the left Heschl's gyrus and sulcus and in the adjoining left planum temporale. These findings suggest that, beyond the basic acoustic analysis of sounds, constructive perceptual processes take place in these relatively early cortical auditory networks. This disagrees with hierarchical models of auditory processing, which generally conceive of these areas as sets of feature detectors, whose task is restricted to the analysis of physical characteristics and the structure of sounds.

Behind the scenes of auditory perception

'Auditory scenes' often contain contributions from multiple acoustic sources. These are usually heard as separate auditory 'streams', which can be selectively followed over time. How and where these auditory streams are formed in the auditory system is one of the most fascinating questions facing auditory scientists today. Findings published within the past two years indicate that both cortical and subcortical processes contribute to the formation of auditory streams, and they raise important questions concerning the roles of primary and secondary areas of auditory cortex in this phenomenon. In addition, these findings underline the importance of taking into account the relative timing of neural responses, and the influence of selective attention, in the search for neural correlates of the perception of auditory streams.

Cortical mechanisms for the segregation and representation of acoustic textures

Auditory object analysis requires two fundamental perceptual processes: the definition of the boundaries between objects, and the abstraction and maintenance of an object's characteristic features. Although it is intuitive to assume that the detection of the discontinuities at an object's boundaries precedes the subsequent precise representation of the object, the specific underlying cortical mechanisms for segregating and representing auditory objects within the auditory scene are unknown. We investigated the cortical bases of these two processes for one type of auditory object, an "acoustic texture," composed of multiple frequency-modulated ramps. In these stimuli, we independently manipulated the statistical rules governing (1) the frequency-time space within individual textures (comprising ramps with a given spectrotemporal coherence) and (2) the boundaries between textures (adjacent textures with different spectrotemporal coherences). Using functional magnetic resonance imaging, we show mechanisms defining boundaries between textures with different coherences in primary and association auditory cortices, whereas texture coherence is represented only in association cortex. Furthermore, participants' superior detection of boundaries across which texture coherence increased (as opposed to decreased) was reflected in a greater neural response in auditory association cortex at these boundaries. The results suggest a hierarchical mechanism for processing acoustic textures that is relevant to auditory object analysis: boundaries between objects are first detected as a change in statistical rules over frequency-time space, before a representation that corresponds to the characteristics of the perceived object is formed.

Responses in the inferior colliculus of the guinea pig to concurrent harmonic series and the effect of inactivation of descending controls

One of the fundamental questions of auditory research is how sounds are segregated because, in natural environments, multiple sounds tend to occur at the same time. Concurrent sounds, such as two talkers, physically add together and arrive at the ear as a single input sound wave. The auditory system easily segregates this input into a coherent perception of each of the multiple sources. A common feature of speech and communication calls is their harmonic structure and in this report we used two harmonic complexes to study the role of the corticofugal pathway in the processing of concurrent sounds. We demonstrate that, in the inferior colliculus (IC) of the anesthetized guinea pig, deactivation of the auditory cortex altered the temporal and/or the spike response to the concurrent, monaural harmonic complexes. More specifically, deactivating the auditory cortex altered the representation of the relative level of the complexes. This suggests that the auditory cortex modulates the representation of the level of two harmonic complexes in the IC. Since sound level is a cue used in the segregation of auditory input, the corticofugal pathway may play a role in this segregation.

How the human brain recognizes speech in the context of changing speakers

We understand speech from different speakers with ease, whereas artificial speech recognition systems struggle with this task. It is unclear how the human brain solves this problem. The conventional view is that speech message recognition and speaker identification are two separate functions and that message processing takes place predominantly in the left hemisphere, whereas processing of speaker-specific information is located in the right hemisphere. Here, we distinguish the contribution of specific cortical regions, to speech recognition and speaker information processing, by controlled manipulation of task and resynthesized speaker parameters. Two functional magnetic resonance imaging studies provide evidence for a dynamic speech-processing network that questions the conventional view. We found that speech recognition regions in left posterior superior temporal gyrus/superior temporal sulcus (STG/STS) also encode speaker-related vocal tract parameters, which are reflected in the amplitude peaks of the speech spectrum, along with the speech message. Right posterior STG/STS activated specifically more to a speaker-related vocal tract parameter change during a speech recognition task compared with a voice recognition task. Left and right posterior STG/STS were functionally connected. Additionally, we found that speaker-related glottal fold parameters (e.g., pitch), which are not reflected in the amplitude peaks of the speech spectrum, are processed in areas immediately adjacent to primary auditory cortex, i.e., in areas in the auditory hierarchy earlier than STG/STS. Our results point to a network account of speech recognition, in which information about the speech message and the speaker's vocal tract are combined to solve the difficult task of understanding speech from different speakers.

Interdependent encoding of pitch, timbre, and spatial location in auditory cortex

Because we can perceive the pitch, timbre, and spatial location of a sound source independently, it seems natural to suppose that cortical processing of sounds might separate out spatial from nonspatial attributes. Indeed, recent studies support the existence of anatomically segregated "what" and "where" cortical processing streams. However, few attempts have been made to measure the responses of individual neurons in different cortical fields to sounds that vary simultaneously across spatial and nonspatial dimensions. We recorded responses to artificial vowels presented in virtual acoustic space to investigate the representations of pitch, timbre, and sound source azimuth in both core and belt areas of ferret auditory cortex. A variance decomposition technique was used to quantify the way in which altering each parameter changed neural responses. Most units were sensitive to two or more of these stimulus attributes. Although indicating that neural encoding of pitch, location, and timbre cues is distributed across auditory cortex, significant differences in average neuronal sensitivity were observed across cortical areas and depths, which could form the basis for the segregation of spatial and nonspatial cues at higher cortical levels. Some units exhibited significant nonlinear interactions between particular combinations of pitch, timbre, and azimuth. These interactions were most pronounced for pitch and timbre and were less commonly observed between spatial and nonspatial attributes. Such nonlinearities were most prevalent in primary auditory cortex, although they tended to be small compared with stimulus main effects.