Social experience affects neuronal responses to male calls in adult female zebra finches

Role of the ventral portion of intermediate arcopallium in stability of female Bengalese finch song preferences

The process of decision making is a complex procedure influenced by both external and internal conditions. Songbirds provide an excellent model to investigate the neural mechanisms of decision making, because females rely on acoustic signals called songs as important stimuli in directing their mate choice. Previous experiments by our group and others have implicated secondary auditory brain sites in female evaluation of song quality, including the caudal portions of the nidopallium (NC) and mesopallium (CM). Recent pathway tracing experiments reveal a convergence of those sites onto a third region, the ventral portion of the intermediate arcopallium (AIV), suggesting that AIV may also play an important role in song evaluation and mate choice. Here we combined behavioral testing with lesion inactivation to investigate the role of AIV in song preference of female Bengalese finches (Lonchura striata domestica). Inactivation of AIV was associated with destabilization of rank ordering of song preferences. These data suggest a model in which the convergence of auditory activity in AIV plays an important role in female perception of song quality and production of courtship behaviors. Together with previous results that also demonstrate a role for the auditory areas that converge onto AIV, these findings extend the experimental tractability of this emerging animal model of sensory perception and decision making.

Varied impacts of social relationships on neuroendocrine state

Social relationships, affiliative social attachments, are important for many species. The best studied types of relationships are monogamous pair bonds. However, it remains unclear how generalizable models of pair bonding are across types of social attachments. Zebra finches are a fascinating system to explore the neurobiology of social relationships because they form various adult bonds with both same- and opposite-sex partners. To test whether different bonds are supported by a single brain network, we quantified individuals' neuroendocrine state after either 24 h or 2 weeks of co-housing with a novel same- or opposite-sex partner. We defined neuroendocrine state by the expression of 22 genes related to 4 major signaling pathways (dopamine, steroid, nonapeptide, and opioid) in six brain regions associated with affiliation or communication [nucleus accumbens (NAc), nucleus taeniae of the amygdala (TnA), medial preoptic area (POM), and periaqueductal gray (PAG), ventral tegmental area, and auditory cortex]. Overall, we found dissociable effects of social contexts (same- or opposite-sex partnerships) and duration of co-housing. Social bonding impacted the neuroendocrine state of four regions in males (NAc, TnA, POM, and PAG) and three regions in females (NAc, TnA, and POM). Monogamous pair bonding specifically appeared to impact male NAc. However, the patterns of gene expression in zebra finches were different than has previously been reported in mammals. Together, our results support the view that there are numerous mechanisms regulating social relationships and highlight the need to further our understanding of how social interactions shape social bonds.

Predictive Mouse Ultrasonic Vocalization Sequences: Uncovering Behavioral Significance, Auditory Cortex Neuronal Preferences, and Social-Experience-Driven Plasticity

Mouse ultrasonic vocalizations (USVs) contain predictable sequential structures like bird songs and speech. Neural representation of USVs in the mouse primary auditory cortex (Au1) and its plasticity with experience has been largely studied with single-syllables or dyads, without using the predictability in USV sequences. Studies using playback of USV sequences have used randomly selected sequences from numerous possibilities. The current study uses mutual information to obtain context-specific natural sequences (NSeqs) of USV syllables capturing the observed predictability in male USVs in different contexts of social interaction with females. Behavioral and physiological significance of NSeqs over random sequences (RSeqs) lacking predictability were examined. Female mice, never having the social experience of being exposed to males, showed higher selectivity for NSeqs behaviorally and at cellular levels probed by expression of immediate early gene c-fos in Au1. The Au1 supragranular single units also showed higher selectivity to NSeqs over RSeqs. Social-experience-driven plasticity in encoding NSeqs and RSeqs in adult females was probed by examining neural selectivities to the same sequences before and after the above social experience. Single units showed enhanced selectivity for NSeqs over RSeqs after the social experience. Further, using two-photon Ca2+ imaging, we observed social experience-dependent changes in the selectivity of sequences of excitatory and somatostatin-positive inhibitory neurons but not parvalbumin-positive inhibitory neurons of Au1. Using optogenetics, somatostatin-positive neurons were identified as a possible mediator of the observed social-experience-driven plasticity. Our study uncovers the importance of predictive sequences and introduces mouse USVs as a promising model to study context-dependent speech like communications.SIGNIFICANCE STATEMENT Humans need to detect patterns in the sensory world. For instance, speech is meaningful sequences of acoustic tokens easily differentiated from random ordered tokens. The structure derives from the predictability of the tokens. Similarly, mouse vocalization sequences have predictability and undergo context-dependent modulation. Our work investigated whether mice differentiate such informative predictable sequences (NSeqs) of communicative significance from RSeqs at the behavioral, molecular, and neuronal levels. Following a social experience in which NSeqs occur as a crucial component, mouse auditory cortical neurons become more sensitive to differences between NSeqs and RSeqs, although preference for individual tokens is unchanged. Thus, speech-like communication and its dysfunction may be studied in circuit, cellular, and molecular levels in mice.

Auditory processing neurons influence song evaluation and strength of mate preference in female songbirds

Animals use a variety of complex signaling mechanisms to convey an array of information that can be detected by conspecifics and heterospecifics. Receivers of those signals perceive that information and use it to direct their subsequent actions. Thus, communication such as that which occurs between senders and receivers of vocal communication signals can be a powerful model in which to investigate the neural basis of sensory perception and action initiation that underlie decision-making. In this study, we investigated how female songbirds perceive the quality of acoustic signals (songs) performed by males and use that information to express preference for one song among many possible alternatives. We use behavioral measurement of song preference before and after lesion-induced alteration of activity in an auditory processing area (caudal nidopallium, NC) for which we have previously described its interconnections with other auditory areas and downstream reward pathways. Our findings reveal that inactivating NC does not change a female's ability or willingness to perform behavioral indicators of mate choice, nor does it change their ability to identify the songs of individual males. However, lesioning NC does induce a decrease in the strength of song preference for specific males more than others. That decrease does not result in a complete elimination of preference, as female preferences for specific males are still evident but not as strongly expressed after lesioning of NC. Taken together, these data indicate that NC plays a role in a female's strength of preference in song evaluation and mate choice, and activity in NC is an important facet of mate choice.

Exploring links from sensory perception to movement and behavioral motivation in the caudal nidopallium of female songbirds

Decision making resides at the interface between sensory perception and movement production. Female songbirds in the context of mate choice are an excellent system to define neural circuits through which sensory perception influences production of courtship behaviors. Previous experiments by our group and others have implicated secondary auditory brain sites, including the caudal nidopallium (NC), in mediating behavioral indicators of mate choice. Here, we used anterograde tracer molecules to define projections that emerge from NC in female songbirds, identifying pathways through which NC influences downstream sites implicated in signal processing and decision making. Our results reveal that NC sends projections into the arcopallium, including the ventral intermediate arcopallium (AIV). Previous work revealed that AIV also receives input from another auditory area implicated in song preference and mate choice (caudal mesopallium, CM), suggesting that convergent input from multiple auditory areas may play important roles in initiating mate choice behaviors. In the present results, NC projects to an area implicated in postural and locomotory control (dorsal arcopallium, Ad), suggesting that NC may play a role in directing those forms of copulatory behavior. NC projections also systematically avoid a vocal motor region of the arcopallium that is innervated by CM (robust nucleus of the arcopallium). These results suggest a model in which both NC and CM project to arcopallial pathways implicated in behavioral motivation. These brain regions may exert different influences on pathways through which auditory information can direct different facets of behavioral responses to information detected in those auditory signals.

Reciprocal processes of sensory perception and social bonding: an integrated social-sensory framework of social behavior

Organisms filter the complexity of natural stimuli through their individual sensory and perceptual systems. Such perceptual filtering is particularly important for social stimuli. A shared "social umwelt" allows individuals to respond appropriately to the expected diversity of cues and signals during social interactions. In this way, the behavioral and neurobiological mechanisms of sociality and social bonding cannot be disentangled from perceptual mechanisms and sensory processing. While a degree of embeddedness between social and sensory processes is clear, our dominant theoretical frameworks favor treating the social and sensory processes as distinct. An integrated social-sensory framework has the potential to greatly expand our understanding of the mechanisms underlying individual variation in social bonding and sociality more broadly. Here we leverage what is known about sensory processing and pair bonding in two common study systems with significant species differences in their umwelt (rodent chemosensation and avian acoustic communication). We primarily highlight that (1) communication is essential for pair bond formation and maintenance, (2) the neural circuits underlying perception, communication and social bonding are integrated, and (3) candidate neuromodulatory mechanisms that regulate pair bonding also impact communication and perception. Finally, we propose approaches and frameworks that more fully integrate sensory processing, communication, and social bonding across levels of analysis: behavioral, neurobiological, and genomic. This perspective raises two key questions: (1) how is social bonding shaped by differences in sensory processing?, and (2) to what extent is sensory processing and the saliency of signals shaped by social interactions and emerging relationships?

Distinct timescales for the neuronal encoding of vocal signals in a high-order auditory area

The ability of the auditory system to selectively recognize natural sound categories while maintaining a certain degree of tolerance towards variations within these categories, which may have functional roles, is thought to be crucial for vocal communication. To date, it is still largely unknown how the balance between tolerance and sensitivity to variations in acoustic signals is coded at a neuronal level. Here, we investigate whether neurons in a high-order auditory area in zebra finches, a songbird species, are sensitive to natural variations in vocal signals by recording their responses to repeated exposures to identical and variant sound sequences. We used the songs of male birds which tend to be highly repetitive with only subtle variations between renditions. When playing these songs to both anesthetized and awake birds, we found that variations between songs did not affect the neuron firing rate but the temporal reliability of responses. This suggests that auditory processing operates on a range of distinct timescales, namely a short one to detect variations in vocal signals, and longer ones that allow the birds to tolerate variations in vocal signal structure and to encode the global context.

High responsiveness of auditory neurons to specific combination of acoustic features in female songbirds

Zebra finch (Taeniopygia guttata) is a songbird species in which males sing their unique songs to attract females who then select their preferred male. Acoustic features in the songs of individual males are important features for female auditory perception. While the male of this species is a classic model of vocal production, it has been little known about auditory processing in female. In the higher auditory brain regions, the caudomedial mesopallium (CMM) and nidopallium (NCM) contribute to female's sound recognition, we, therefore, extracted acoustic features that induce neural activities with high detection power on both regions in female finches. A multiple linear regression analysis revealed that neurons were sensitive to mean frequency and Wiener entropy. In addition, we performed an experiment with modified artificial songs and harmonic songs to directly investigate neural responsiveness for deriving further evidence for the contribution of these two acoustic features. Finally, we illustrated a specific ratio combining these two acoustic features that showed highest sensitivity to neural responsiveness, and we found that properties of sensitivity are different between CMM and NCM. Our results indicate that the mixture of the two acoustic features with the specific ratio is important in the higher auditory regions of female songbirds, and these two regions have differences in encoding for sensitivity to these acoustic features.

Auditory processing in the zebra finch midbrain: single unit responses and effect of rearing experience

In birds the auditory system plays a key role in providing the sensory input used to discriminate between conspecific and heterospecific vocal signals. In those species that are known to learn their vocalizations, for example, songbirds, it is generally considered that this ability arises and is manifest in the forebrain, although there is no a priori reason why brainstem components of the auditory system could not also play an important part. To test this assumption, we used groups of normal reared and cross-fostered zebra finches that had previously been shown in behavioural experiments to reduce their preference for conspecific songs subsequent to cross fostering experience with Bengalese finches, a related species with a distinctly different song. The question we asked, therefore, is whether this experiential change also changes the bias in favour of conspecific song displayed by auditory midbrain units of normally raised zebra finches. By recording the responses of single units in MLd to a variety of zebra finch and Bengalese finch songs in both normally reared and cross-fostered zebra finches, we provide a positive answer to this question. That is, the difference in response to conspecific and heterospecific songs seen in normal reared zebra finches is reduced following cross-fostering. In birds the virtual absence of mammalian-like cortical projections upon auditory brainstem nuclei argues against the interpretation that MLd units change, as observed in the present experiments, as a result of top-down influences on sensory processing. Instead, it appears that MLd units can be influenced significantly by sensory inputs arising directly from a change in auditory experience during development.

Statistical learning of transition patterns in the songbird auditory forebrain

Statistical learning of transition patterns between sounds—a striking capability of the auditory system—plays an essential role in animals’ survival (e.g., detect deviant sounds that signal danger). However, the neural mechanisms underlying this capability are still not fully understood. We recorded extracellular multi-unit and single-unit activity in the auditory forebrain of awake male zebra finches while presenting rare repetitions of a single sound in a long sequence of sounds (canary and zebra finch song syllables) patterned in either an alternating or random order at different inter-stimulus intervals (ISI). When preceding stimuli were regularly alternating (alternating condition), a repeated stimulus violated the preceding transition pattern and was a deviant. When preceding stimuli were in random order (control condition), a repeated stimulus did not violate any regularities and was not a deviant. At all ISIs tested (1 s, 3 s, or jittered at 0.8–1.2 s), deviant repetition enhanced neural responses in the alternating condition in a secondary auditory area (caudomedial nidopallium, NCM) but not in the primary auditory area (Field L2); in contrast, repetition suppressed responses in the control condition in both Field L2 and NCM. When stimuli were presented in the classical oddball paradigm at jittered ISI (0.8–1.2 s), neural responses in both NCM and Field L2 were stronger when a stimulus occurred as deviant with low probability than when the same stimulus occurred as standard with high probability. Together, these results demonstrate: (1) classical oddball effect exists even when ISI is jittered and the onset of a stimulus is not fully predictable; (2) neurons in NCM can learn transition patterns between sounds at multiple ISIs and detect violation of these transition patterns; (3) sensitivity to deviant sounds increases from Field L2 to NCM in the songbird auditory forebrain. Further studies using the current paradigms may help us understand the neural substrate of statistical learning and even speech comprehension.

Neuronal Encoding in a High-Level Auditory Area: From Sequential Order of Elements to Grammatical Structure

Sensitivity to the sequential structure of communication sounds is fundamental not only for language comprehension in humans but also for song recognition in songbirds. By quantifying single-unit responses, we first assessed whether the sequential order of song elements, called syllables, in conspecific songs is encoded in a secondary auditory cortex-like region of the zebra finch brain. Based on a habituation/dishabituation paradigm, we show that, after multiple repetitions of the same conspecific song, rearranging syllable order reinstated strong responses. A large proportion of neurons showed sensitivity to song context in which syllables occurred providing support for the nonlinear processing of syllable sequences. Sensitivity to the temporal order of items within a sequence should enable learning its underlying structure, an ability considered a core mechanism of the human language faculty. We show that repetitions of songs that were ordered according to a specific grammatical structure (i.e., ABAB or AABB structures; A and B denoting song syllables) led to different responses in both anesthetized and awake birds. Once responses were decreased due to song repetitions, the transition from one structure to the other could affect the firing rates and/or the spike patterns. Our results suggest that detection was based on local differences rather than encoding of the global song structure as a whole. Our study demonstrates that a high-level auditory region provides neuronal mechanisms to help discriminate stimuli that differ in their sequential structure.SIGNIFICANCE STATEMENT Sequence processing has been proposed as a potential precursor of language syntax. As a sequencing operation, the encoding of the temporal order of items within a sequence may help in recognition of relationships between adjacent items and in learning the underlying structure. Taking advantage of the stimulus-specific adaptation phenomenon observed in a high-level auditory region of the zebra finch brain, we addressed this question at the neuronal level. Reordering elements within conspecific songs reinstated robust responses. Neurons also detected changes in the structure of artificial songs, and this detection depended on local transitions between adjacent or nonadjacent syllables. These findings establish the songbird as a model system for deciphering the mechanisms underlying sequence processing at the single-cell level.

Neural Correlate of Transition Violation and Deviance Detection in the Songbird Auditory Forebrain

Deviants are stimuli that violate one's prediction about the incoming stimuli. Studying deviance detection helps us understand how nervous system learns temporal patterns between stimuli and forms prediction about the future. Detecting deviant stimuli is also critical for animals' survival in the natural environment filled with complex sounds and patterns. Using natural songbird vocalizations as stimuli, we recorded multi-unit and single-unit activity from the zebra finch auditory forebrain while presenting rare repeated stimuli after regular alternating stimuli (alternating oddball experiment) or rare deviant among multiple different common stimuli (context oddball experiment). The alternating oddball experiment showed that neurons were sensitive to rare repetitions in regular alternations. In the absence of expectation, repetition suppresses neural responses to the 2nd stimulus in the repetition. When repetition violates expectation, neural responses to the 2nd stimulus in the repetition were stronger than expected. The context oddball experiment showed that a stimulus elicits stronger neural responses when it is presented infrequently as a deviant among multiple common stimuli. As the acoustic differences between deviant and common stimuli increase, the response enhancement also increases. These results together showed that neural encoding of a stimulus depends not only on the acoustic features of the stimulus but also on the preceding stimuli and the transition patterns between them. These results also imply that the classical oddball effect may result from a combination of repetition suppression and deviance enhancement. Classification analyses showed that the difficulties in decoding the stimulus responsible for the neural responses differed for deviants in different experimental conditions. These findings suggest that learning transition patterns and detecting deviants in natural sequences may depend on a hierarchy of neural mechanisms, which may be involved in more complex forms of auditory processing that depend on the transition patterns between stimuli, such as speech processing.

Caudal mesopallial neurons in female songbirds bridge sensory and motor brain regions

Female songbirds use male song as an indicator of fitness and use that information to select their mate. Investigations of the female auditory system have provided evidence that the neurons within the caudal mesopallium (CM) are involved in the processing of songs that a female finds attractive, however, it is not clear how CM may exert its influence on behavioral indicators of mate choice. In the present study, anterograde tracing revealed the efferent connections of the female songbird CM. The results demonstrate connections to other auditory regions previously described in males, as well as novel connections to brain regions implicated in motor control. As in males, CM neurons in females project robustly to the lateral and medial extents of the caudal nidopallium, and to the ventral intermediate arcopallium. In a novel finding that is not present in males, CM neurons also project to the robust nucleus of the arcopallium and to the caudal striatum. Calling behavior and the expression of copulation solicitation displays are key indicators of female mate choice, and the projections found here bridge critical gaps necessary to understand how auditory perception can influence circuits related to the expression of those affiliative behaviors in female songbirds.

Individual recognition of opposite sex vocalizations in the zebra finch

Individual vocal recognition plays an important role in the social lives of many vocally active species. In group-living songbirds the most common vocalizations during communal interactions are low-intensity, soft, unlearned calls. Being able to tell individuals apart solely from a short call would allow a sender to choose a specific group member to address, resulting in the possibility to form complex communication networks. However, little research has yet been carried out to discover whether soft calls contain individual identity. In this study, males and females of zebra finch pairs were tested with six vocalization types - four different soft calls, the distance call and the male song - to investigate whether they are able to distinguish individuals of the opposite sex. For both sexes, we provide the first evidence of individual vocal recognition for a zebra finch soft unlearned call. Moreover, while controlling for habituation and testing for repeatability of the findings, we quantify the effects of hitherto little studied variables such as partners’ vocal exchange previous to the experiment, spectral content of playback calls and quality of the answers. We suggest that zebra finches can recognize individuals via soft vocalizations, therefore allowing complex directed communication within vocalizing flocks.

Single Neurons in the Avian Auditory Cortex Encode Individual Identity and Propagation Distance in Naturally Degraded Communication Calls

One of the most complex tasks performed by sensory systems is "scene analysis": the interpretation of complex signals as behaviorally relevant objects. The study of this problem, universal to species and sensory modalities, is particularly challenging in audition, where sounds from various sources and localizations, degraded by propagation through the environment, sum to form a single acoustical signal. Here we investigated in a songbird model, the zebra finch, the neural substrate for ranging and identifying a single source. We relied on ecologically and behaviorally relevant stimuli, contact calls, to investigate the neural discrimination of individual vocal signature as well as sound source distance when calls have been degraded through propagation in a natural environment. Performing electrophysiological recordings in anesthetized birds, we found neurons in the auditory forebrain that discriminate individual vocal signatures despite long-range degradation, as well as neurons discriminating propagation distance, with varying degrees of multiplexing between both information types. Moreover, the neural discrimination performance of individual identity was not affected by propagation-induced degradation beyond what was induced by the decreased intensity. For the first time, neurons with distance-invariant identity discrimination properties as well as distance-discriminant neurons are revealed in the avian auditory cortex. Because these neurons were recorded in animals that had prior experience neither with the vocalizers of the stimuli nor with long-range propagation of calls, we suggest that this neural population is part of a general-purpose system for vocalizer discrimination and ranging.SIGNIFICANCE STATEMENT Understanding how the brain makes sense of the multitude of stimuli that it continually receives in natural conditions is a challenge for scientists. Here we provide a new understanding of how the auditory system extracts behaviorally relevant information, the vocalizer identity and its distance to the listener, from acoustic signals that have been degraded by long-range propagation in natural conditions. We show, for the first time, that single neurons, in the auditory cortex of zebra finches, are capable of discriminating the individual identity and sound source distance in conspecific communication calls. The discrimination of identity in propagated calls relies on a neural coding that is robust to intensity changes, signals' quality, and decreases in the signal-to-noise ratio.

Auditory Responses to Vocal Sounds in the Songbird Nucleus Taeniae of the Amygdala and the Adjacent Arcopallium

Many species of animals communicate with others through vocalizations. Over time, these species have evolved mechanisms to respond to biologically relevant vocal sounds via adaptive behaviors. Songbirds provide a good opportunity to search for the neural basis of this adaptation, because they interact with others through a variety of vocalizations in complex social relationships. The nucleus taeniae of the amygdala (TnA) is a structure located in the ventromedial arcopallium, which is akin to the mammalian medial amygdala. Studies on the anatomy and function of this nucleus have led to the speculation that the TnA is one of the possible neural substrates that represents the relevance of acoustic stimuli related to behavior. However, neural responses in this nucleus to auditory stimuli have not been studied in depth. To give a detailed description about auditory responses of the TnA in the songbird, we conducted neural recordings from the TnA and the adjacent arcopallium in adult male and female Bengalese finches under anesthesia. The birds were exposed to auditory stimuli including natural vocalizations as well as synthesized noise. We demonstrated that a substantial population of neurons in the TnA and the adjacent arcopallium responded to vocal sounds and that some neurons were selectively activated to specific stimuli. Proportions of responsive cells and stimulus-selective cells were larger in males than in females. In addition, a larger ratio of selective cells was observed in the arcopallium compared to the TnA. These findings support the idea that neuronal activity in the TnA and the neighboring area represents behavioral relevance of sounds. Further studies in electrophysiology combined with evidence from other fields, such as region-specific gene expression patterns, are required to fully understand the functions of the TnA as well as the evolution of the amygdala in songbirds and vertebrate animals.

Hierarchical emergence of sequence sensitivity in the songbird auditory forebrain

Bengalese finches (Lonchura striata var. domestica) generate more complex sequences in their songs than zebra finches. Because of this, we chose this species to explore the signal processing of sound sequence in the primary auditory forebrain area, field L, and in a secondary area, the caudomedial nidopallium (NCM). We simultaneously recorded activity from multiple single units in urethane-anesthetized birds. We successfully replicated the results of a previous study in awake zebra finches examining stimulus-specific habituation of NCM neurons to conspecific songs. Then, we used an oddball paradigm and compared the neural response to deviant sounds that were presented infrequently, with the response to standard sounds, which were presented frequently. In a single sound oddball task, two different song elements were assigned for the deviant and standard sounds. The response bias to deviant elements was larger in NCM than in field L. In a triplet sequence oddball task, two triplet sequences containing elements ABC and ACB were assigned as the deviant and standard. Only neurons in NCM that displayed broad-shaped spike waveforms had sensitivity to the difference in element order. Our results suggest the hierarchical processing of complex sound sequences in the songbird auditory forebrain.

Meaning in the avian auditory cortex: neural representation of communication calls

Understanding how the brain extracts the behavioral meaning carried by specific vocalization types that can be emitted by various vocalizers and in different conditions is a central question in auditory research. This semantic categorization is a fundamental process required for acoustic communication, and presupposes discriminative and invariance properties of the auditory system for conspecific vocalizations. Songbirds have been used extensively to study vocal learning, but the communicative function of all their vocalizations and their neural representation has yet to be examined. In this study, we first generated a library containing almost the entire zebra finch vocal repertoire, and organised communication calls along nine different categories according to their behavioral meaning. We then investigated the neural representations of these semantic categories in the primary and secondary auditory areas of six anesthetised zebra finches. To analyse how single units encode these call categories, we described neural responses in terms of their discrimination, selectivity and invariance properties. Quantitative measures for these neural properties were obtained with an optimal decoder using both spike counts and spike patterns. Information theoretic metrics show that almost half of the single units encode semantic information. Neurons achieve higher discrimination of these semantic categories by being more selective and more invariant. These results demonstrate that computations necessary for semantic categorization of meaningful vocalizations are already present in the auditory cortex, and emphasise the value of a neuro-ethological approach to understand vocal communication.

Sex differences in the representation of call stimuli in a songbird secondary auditory area

Understanding how communication sounds are encoded in the central auditory system is critical to deciphering the neural bases of acoustic communication. Songbirds use learned or unlearned vocalizations in a variety of social interactions. They have telencephalic auditory areas specialized for processing natural sounds and considered as playing a critical role in the discrimination of behaviorally relevant vocal sounds. The zebra finch, a highly social songbird species, forms lifelong pair bonds. Only male zebra finches sing. However, both sexes produce the distance call when placed in visual isolation. This call is sexually dimorphic, is learned only in males and provides support for individual recognition in both sexes. Here, we assessed whether auditory processing of distance calls differs between paired males and females by recording spiking activity in a secondary auditory area, the caudolateral mesopallium (CLM), while presenting the distance calls of a variety of individuals, including the bird itself, the mate, familiar and unfamiliar males and females. In males, the CLM is potentially involved in auditory feedback processing important for vocal learning. Based on both the analyses of spike rates and temporal aspects of discharges, our results clearly indicate that call-evoked responses of CLM neurons are sexually dimorphic, being stronger, lasting longer, and conveying more information about calls in males than in females. In addition, how auditory responses vary among call types differ between sexes. In females, response strength differs between familiar male and female calls. In males, temporal features of responses reveal a sensitivity to the bird's own call. These findings provide evidence that sexual dimorphism occurs in higher-order processing areas within the auditory system. They suggest a sexual dimorphism in the function of the CLM, contributing to transmit information about the self-generated calls in males and to storage of information about the bird's auditory experience in females.

Neural responses in songbird forebrain reflect learning rates, acquired salience, and stimulus novelty after auditory discrimination training

How do social interactions form and modulate the neural representations of specific complex signals? This question can be addressed in the songbird auditory system. Like humans, songbirds learn to vocalize by imitating tutors heard during development. These learned vocalizations are important in reproductive and social interactions and in individual recognition. As a model for the social reinforcement of particular songs, male zebra finches were trained to peck for a food reward in response to one song stimulus (GO) and to withhold responding for another (NoGO). After performance reached criterion, single and multiunit neural responses to both trained and novel stimuli were obtained from multiple electrodes inserted bilaterally into two songbird auditory processing areas [caudomedial mesopallium (CMM) and caudomedial nidopallium (NCM)] of awake, restrained birds. Neurons in these areas undergo stimulus-specific adaptation to repeated song stimuli, and responses to familiar stimuli adapt more slowly than to novel stimuli. The results show that auditory responses differed in NCM and CMM for trained (GO and NoGO) stimuli vs. novel song stimuli. When subjects were grouped by the number of training days required to reach criterion, fast learners showed larger neural responses and faster stimulus-specific adaptation to all stimuli than slow learners in both areas. Furthermore, responses in NCM of fast learners were more strongly left-lateralized than in slow learners. Thus auditory responses in these sensory areas not only encode stimulus familiarity, but also reflect behavioral reinforcement in our paradigm, and can potentially be modulated by social interactions.

The presence of an audience modulates responses to familiar call stimuli in the male zebra finch forebrain

The ability to recognize familiar individuals is crucial for establishing social relationships. The zebra finch, a highly social songbird species that forms lifelong pair bonds, uses a vocalization, the distance call, to identify its mate. However, in males, this ability depends on social conditions, requiring the presence of an audience. To evaluate whether the presence of bystanders modulates the auditory processing underlying recognition abilities, we assessed, by using a lightweight telemetry system, whether electrophysiological responses driven by familiar and unfamiliar female calls in a high-level auditory area [the caudomedial nidopallium (NCM)] were modulated by the presence of conspecific males. Males had experienced the call of their mate for several months and the call of a familiar female for several days. When they were exposed to female calls in the presence of two male conspecifics, NCM neurons showed greater responses to the playback of familiar female calls, including the mate's call, than to unfamiliar ones. In contrast, no such discrimination was observed in males when they were alone or when call-evoked responses were collected under anaesthesia. Together, these results suggest that NCM neuronal activity is profoundly influenced by social conditions, providing new evidence that the properties of NCM neurons are not simply determined by the acoustic structure of auditory stimuli. They also show that neurons in the NCM form part of a network that can be shaped by experience and that probably plays an important role in the emergence of communication sound recognition.

Mate choice in adult female Bengalese finches: females express consistent preferences for individual males and prefer female-directed song performances

In the process of mate selection by female songbirds, male suitors advertise their quality through reproductive displays in which song plays an important role. Females evaluate the quality of each signal and the associated male, and the results of that evaluation guide expression of selective courtship displays. Some studies reveal broad agreement among females in their preferences for specific signal characteristics, indicating that those features are especially salient in female mate choice. Other studies reveal that females differ in their preference for specific characteristics, indicating that in those cases female evaluation of signal quality is influenced by factors other than simply the physical properties of the signal. Thus, both the physical properties of male signals and specific traits of female signal evaluation can impact female mate choice. Here, we characterized the mate preferences of female Bengalese finches. We found that calls and copulation solicitation displays are equally reliable indicators of female preference. In response to songs from an array of males, each female expressed an individual-specific song preference, and those preferences were consistent across tests spanning many months. Across a population of females, songs of some males were more commonly preferred than others, and females preferred female-directed songs more than undirected songs, suggesting that some song features are broadly attractive. Preferences were indistinguishable for females that did or did not have social experience with the singers, indicating that female preference is strongly directed by song features rather than experiences associated with the singer. Analysis of song properties revealed several candidate parameters that may influence female evaluation. In an initial investigation of those parameters, females could be very selective for one song feature yet not selective for another. Therefore, multiple song parameters are evaluated independently. Together these findings reveal the nature of signal evaluation and mate choice in this species.

Auditory signal processing in communication: perception and performance of vocal sounds

Learning and maintaining the sounds we use in vocal communication require accurate perception of the sounds we hear performed by others and feedback-dependent imitation of those sounds to produce our own vocalizations. Understanding how the central nervous system integrates auditory and vocal-motor information to enable communication is a fundamental goal of systems neuroscience, and insights into the mechanisms of those processes will profoundly enhance clinical therapies for communication disorders. Gaining the high-resolution insight necessary to define the circuits and cellular mechanisms underlying human vocal communication is presently impractical. Songbirds are the best animal model of human speech, and this review highlights recent insights into the neural basis of auditory perception and feedback-dependent imitation in those animals. Neural correlates of song perception are present in auditory areas, and those correlates are preserved in the auditory responses of downstream neurons that are also active when the bird sings. Initial tests indicate that singing-related activity in those downstream neurons is associated with vocal-motor performance as opposed to the bird simply hearing itself sing. Therefore, action potentials related to auditory perception and action potentials related to vocal performance are co-localized in individual neurons. Conceptual models of song learning involve comparison of vocal commands and the associated auditory feedback to compute an error signal that is used to guide refinement of subsequent song performances, yet the sites of that comparison remain unknown. Convergence of sensory and motor activity onto individual neurons points to a possible mechanism through which auditory and vocal-motor signals may be linked to enable learning and maintenance of the sounds used in vocal communication. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".

The search for a neural basis of communication: Learning, memory, perception and performance of vocal signals

Brain mechanisms for communication must establish a correspondence between sensory perception and motor performance of individual signals. A class of neurons in the swamp sparrow forebrain is well suited for that task. Recordings from awake and freely behaving birds reveal that those cells express categorical auditory responses to changes in note duration, a learned feature of their songs, and the neural response boundary accurately predicts the perceptual boundary measured in field studies. Extremely precise auditory activity of those cells represents not only songs in the adult repertoire but also songs of others and tutor songs, including those imitated only very few times or perhaps not at all during development. Furthermore, recordings during singing reveal that these cells also express a temporally precise auditory-vocal correspondence, and limits on auditory responses to extremely challenging tutor songs may contribute to the emergence of a novel form of song syntax. Therefore, these forebrain neurons provide a mechanism through which sensory perception may influence motor performance to enable imitation. These cells constitute the projection from a premotor cortical-like area into the avian striatum (HVCX neurons), and data from humans implicate analogous or homologous areas in perception and performance of the sounds used in speech.

The incentive salience of courtship vocalizations: hormone-mediated 'wanting' in the auditory system

Conspecific vocalizations differ from many other sounds in that they have natural incentive salience. Our thinking about auditory responses to vocalizations may therefore benefit from models originally developed to understand reward. According to those models, the brain attributes incentive salience to rewarding stimuli via the activity of monoaminergic neuromodulators. These neuromodulators, in turn, mediate the effects of experience and internal state. Songbirds lend themselves well to this discussion because the natural incentive salience of song is clearly modulated by both factors. Their auditory responses have been well-studied, particularly the song-induced expression of plasticity-associated genes such as ZENK. Here I review evidence that ZENK responses to song are regulated by monoamine neuromodulators, and I interpret this evidence in the context of incentive salience. First, hearing conspecific song engages monoaminergic activity in the auditory system and elsewhere. Second, in females this activity may be regulated by the same hormones that regulate behavioral preferences for song. Finally, much of the evidence thought to implicate neuromodulators in song discrimination and memory suggests that they may affect incentive salience. Expanding the study of incentive salience beyond the mesolimbic reward system may reveal some new ways of thinking about its underlying neural basis. This article is part of a Special Issue entitled "Communication Sounds and the Brain: New Directions and Perspectives".