Neural pathways for the control of birdsong production

Vocal learning-associated convergent evolution in mammalian proteins and regulatory elements

Vocal production learning ("vocal learning") is a convergently evolved trait in vertebrates. To identify brain genomic elements associated with mammalian vocal learning, we integrated genomic, anatomical, and neurophysiological data from the Egyptian fruit bat (Rousettus aegyptiacus) with analyses of the genomes of 215 placental mammals. First, we identified a set of proteins evolving more slowly in vocal learners. Then, we discovered a vocal motor cortical region in the Egyptian fruit bat, an emergent vocal learner, and leveraged that knowledge to identify active cis-regulatory elements in the motor cortex of vocal learners. Machine learning methods applied to motor cortex open chromatin revealed 50 enhancers robustly associated with vocal learning whose activity tended to be lower in vocal learners. Our research implicates convergent losses of motor cortex regulatory elements in mammalian vocal learning evolution.

Steroid-dependent plasticity in the song control system: Perineuronal nets and HVC neurogenesis

The vocal control nucleus HVC in songbirds has emerged as a widespread model system to study adult brain plasticity in response to changes in the hormonal and social environment. I review here studies completed in my laboratory during the last decade that concern two aspects of this plasticity: changes in aggregations of extracellular matrix components surrounding the soma of inhibitory parvalbumin-positive neurons called perineuronal nets (PNN) and the production/incorporation of new neurons. Both features are modulated by the season, age, sex and endocrine status of the birds in correlation with changes in song structure and stability. Causal studies have also investigated the role of PNN and of new neurons in the control of song. Dissolving PNN with chondroitinase sulfate, a specific enzyme applied directly on HVC or depletion of new neurons by focalized X-ray irradiation both affected song structure but the amplitude of changes was limited and deserves further investigations.

Convergent and divergent neural circuit architectures that support acoustic communication

Vocal communication is used across extant vertebrates, is evolutionarily ancient, and been maintained, in many lineages. Here I review the neural circuit architectures that support intraspecific acoustic signaling in representative anuran, mammalian and avian species as well as two invertebrates, fruit flies and Hawaiian crickets. I focus on hindbrain motor control motifs and their ties to respiratory circuits, expression of receptors for gonadal steroids in motor, sensory, and limbic neurons as well as divergent modalities that evoke vocal responses. Hindbrain and limbic participants in acoustic communication are highly conserved, while forebrain participants have diverged between anurans and mammals, as well as songbirds and rodents. I discuss the roles of natural and sexual selection in driving speciation, as well as exaptation of circuit elements with ancestral roles in respiration, for producing sounds and driving rhythmic vocal features. Recent technical advances in whole brain fMRI across species will enable real time imaging of acoustic signaling partners, tying auditory perception to vocal production.

A brain atlas of the carrion crow (Corvus corone)

Corvidae, passerine songbirds such as jays, crows, and ravens known as corvids, have become model systems for the study of avian cognition. The superior cognitive capabilities of corvids mainly emerge from a disproportionally large telencephalon found in these species. However, a systematic mapping of the neuroanatomy of the corvid brain, and the telencephalon in particular, is lacking so far. Here, we present a brain atlas of the carrion crow, Corvus corone, with special emphasis on the telencephalic pallium. We applied four staining techniques to brain slices (Nissl, myelin, combination of Nissl and myelin, and tyrosine hydroxylase targeting catecholaminergic neurons). This allowed us to identify brain nuclei throughout the brain and delineate the known pallial subdivisions termed hyperpallium, entopallium, mesopallium, nidopallium, arcopallium, and hippocampal complex. The extent of these subdivisions and brain nuclei are described according to stereotaxic coordinates. In addition, 3D depictions of pallial regions were reconstructed from these slices. While the overall organization of the carrion crow's brain matches other songbird brains, the relative proportions and expansions of associative pallial areas differ considerably in agreement with enhanced cognitive skills found in corvids. The presented global organization of the crow brain in stereotaxic coordinates will help to guide future neurobiological studies in corvids.

Durations of preparatory motor activity in the avian basal ganglia for songs and calls in a species of songbirds

Male songbirds are highly motivated to sing undirected song (US) as juveniles during song learning, and as adults. Given that singing US is a self-driven, elaborated behavior, we would expect to see preparatory activity in the striatal area prior to vocalization, and this preparatory activity could have different characteristics compared to activity driven by calls. In general, songs are longer, complex and influenced by learning while calls are shorter, simpler, and less influenced by experience. The present study recorded neural activity in Area X, a nucleus of the basal ganglia, in male Java sparrows (Lonchura oryzivora) in a sound-proof box and analyzed differences in activity change before US and trill-calls. Trill-calls were often emitted in social arousal, but occasionally emitted when alone. We saw a gradual increase in firing rate for about 2.3seconds prior to the onset of US, and a shorter increase of about 1.3seconds in firing rate prior to the onset of trill-calls. The results reveal that initiating US may be influenced by a prolonged and specific activity increase in the extent that is not seen with trill-calls. Results suggest that direct or indirect projections to Area X, which may reflect motivational state, could be the cause of this activity change.

A Microclip Peripheral Nerve Interface (μcPNI) for Bioelectronic Interfacing with Small Nerves

Peripheral nerves carry sensory (afferent) and motor (efferent) signals between the central nervous system and other parts of the body. The peripheral nervous system (PNS) is therefore rich in targets for therapeutic neuromodulation, bioelectronic medicine, and neuroprosthetics. Peripheral nerve interfaces (PNIs) generally suffer from a tradeoff between selectivity and invasiveness. This work describes the fabrication, evaluation, and chronic implantation in zebra finches of a novel PNI that breaks this tradeoff by interfacing with small nerves. This PNI integrates a soft, stretchable microelectrode array with a 2-photon 3D printed microclip (μcPNI). The advantages of this μcPNI compared to other designs are: a) increased spatial resolution due to bi-layer wiring of the electrode leads, b) reduced mismatch in biomechanical properties with the nerve, c) reduced disturbance to the host tissue due to the small size, d) elimination of sutures or adhesives, e) high circumferential contact with small nerves, f) functionality under considerable strain, and g) graded neuromodulation in a low-threshold stimulation regime. Results demonstrate that the μcPNIs are electromechanically robust, and are capable of reliably recording and stimulating neural activity in vivo in small nerves. The μcPNI may also inform the development of new optical, thermal, ultrasonic, or chemical PNIs as well.

The expression of delta opioid receptor mRNA in adult male zebra finches (Taenopygia guttata)

The endogenous opioid system is evolutionarily conserved across reptiles, birds and mammals and is known to modulate varied brain functions such as learning, memory, cognition and reward. To date, most of the behavioral and anatomical studies in songbirds have mainly focused on μ-opioid receptors (ORs). Expression patterns of δ-ORs in zebra finches, a well-studied species of songbird have not yet been reported, possibly due to the high sequence similarity amongst different opioid receptors. In the present study, a specific riboprobe against the δ-OR mRNA was used to perform fluorescence in situ hybridization (FISH) on sections from the male zebra finch brain. We found that δ-OR mRNA was expressed in different parts of the pallium, basal ganglia, cerebellum and the hippocampus. Amongst the song control and auditory nuclei, HVC (abbreviation used as a formal name) and NIf (nucleus interfacialis nidopallii) strongly express δ-OR mRNA and stand out from the surrounding nidopallium. Whereas the expression of δ-OR mRNA is moderate in LMAN (lateral magnocellular nucleus of the anterior nidopallium), it is low in the MSt (medial striatum), Area X, DLM (dorsolateral nucleus of the medial thalamus), RA (robust nucleus of the arcopallium) of the song control circuit and Field L, Ov (nucleus ovoidalis) and MLd (nucleus mesencephalicus lateralis, pars dorsalis) of the auditory pathway. Our results suggest that δ-ORs may be involved in modulating singing, song learning as well as spatial learning in zebra finches.

Neurally driven synthesis of learned, complex vocalizations

Brain machine interfaces (BMIs) hold promise to restore impaired motor function and serve as powerful tools to study learned motor skill. While limb-based motor prosthetic systems have leveraged nonhuman primates as an important animal model,^1-4 speech prostheses lack a similar animal model and are more limited in terms of neural interface technology, brain coverage, and behavioral study design.^5-7 Songbirds are an attractive model for learned complex vocal behavior. Birdsong shares a number of unique similarities with human speech,^8-10 and its study has yielded general insight into multiple mechanisms and circuits behind learning, execution, and maintenance of vocal motor skill.^11-18 In addition, the biomechanics of song production bear similarity to those of humans and some nonhuman primates.^19-23 Here, we demonstrate a vocal synthesizer for birdsong, realized by mapping neural population activity recorded from electrode arrays implanted in the premotor nucleus HVC onto low-dimensional compressed representations of song, using simple computational methods that are implementable in real time. Using a generative biomechanical model of the vocal organ (syrinx) as the low-dimensional target for these mappings allows for the synthesis of vocalizations that match the bird's own song. These results provide proof of concept that high-dimensional, complex natural behaviors can be directly synthesized from ongoing neural activity. This may inspire similar approaches to prosthetics in other species by exploiting knowledge of the peripheral systems and the temporal structure of their output.

Brain Size Associated with Foot Preferences in Australian Parrots

Since foot preference of cockatoos and parrots to hold and manipulate food and other objects has been associated with better ability to perform certain tasks, we predicted that either strength or direction of foot preference would correlate with brain size. Our study of 25 psittacine species of Australia found that species with larger absolute brain mass have stronger foot preferences and that percent left-footedness is correlated positively with brain mass. In a sub-sample of 11 species, we found an association between foot preference and size of the nidopallial region of the telencephalon, an area equivalent to the mammalian cortex and including regions with executive function and other higher-level functions. Our analysis showed that percent left-foot use correlates positively and significantly with size of the nidopallium relative to the whole brain, but not with the relative size of the optic tecta. Psittacine species with stronger left-foot preferences have larger brains, with the nidopallium making up a greater proportion of those brains. Our results are the first to show an association between brain size and asymmetrical limb use by parrots and cockatoos. Our results support the hypothesis that limb preference enhances brain capacity and higher (nidopallial) functioning.

Blocking Opioid Receptors in a Songbird Cortical Region Modulates the Acoustic Features and Levels of Female-Directed Singing

The organization of the anterior forebrain pathway (AFP) of songbirds important for context-dependent singing is similar to that of cortical basal ganglia loops (CBG) in mammals, which underlie motor behaviors including vocalization. Since different components of the AFP express high levels of μ-opioid receptors (μ-ORs) as do CBG loops, songbirds act as model systems to study the role of opioid modulation on vocalization and the motivation to sing. The AFP in songbirds includes the cortical/pallial region LMAN (lateral magnocellular nucleus of the anterior nidopallium) which projects to Area X, a nucleus of the avian basal ganglia. In the present study, microdialysis was used to infuse different doses of the opioid antagonist naloxone in LMAN of adult male zebra finches. Whereas all doses of naloxone led to significant decreases in the number of FD (female-directed) songs, only 100 and 200 ng/ml of naloxone affected their acoustic properties. The decrease in FD song was not accompanied by changes in levels of attention toward females or those of neurotransmitters (dopamine, glutamate, and GABA) in LMAN. An earlier study had shown that similar manipulations in Area X did not lead to alterations in the number of FD songs but had significantly greater effects on their acoustic properties. Taken together, our results suggest that there are reciprocal effects of OR modulation on cortical and basal ganglia components of the AFP in songbirds.

Increasing Muscle Speed Drives Changes in the Neuromuscular Transform of Motor Commands during Postnatal Development in Songbirds

Progressive changes in vocal behavior over the course of vocal imitation leaning are often attributed exclusively to developing neural circuits, but the effects of postnatal body changes remain unknown. In songbirds, the syrinx transforms song system motor commands into sound and exhibits changes during song learning. Here we test the hypothesis that the transformation from motor commands to force trajectories by syringeal muscles functionally changes over vocal development in zebra finches. Our data collected in both sexes show that, only in males, muscle speed significantly increases and that supralinear summation occurs and increases with muscle contraction speed. Furthermore, we show that previously reported submillisecond spike timing in the avian cortex can be resolved by superfast syringeal muscles and that the sensitivity to spike timing increases with speed. Because motor neuron and muscle properties are tightly linked, we make predictions on the boundaries of the yet unknown motor code that correspond well with cortical activity. Together, we show that syringeal muscles undergo essential transformations during song learning that drastically change how neural commands are translated into force profiles and thereby acoustic features. We propose that the song system motor code must compensate for these changes to achieve its acoustic targets. Our data thus support the hypothesis that the neuromuscular transformation changes over vocal development and emphasizes the need for an embodied view of song motor learning.SIGNIFICANCE STATEMENT Fine motor skill learning typically occurs in a postnatal period when the brain is learning to control a body that is changing dramatically due to growth and development. How the developing body influences motor code formation and vice versa remains largely unknown. Here we show that vocal muscles in songbirds undergo critical transformations during song learning that drastically change how neural commands are translated into force profiles and thereby acoustic features. We propose that the motor code must compensate for these changes to achieve its acoustic targets. Our data thus support the hypothesis that the neuromuscular transformation changes over vocal development and emphasizes the need for an embodied view of song motor learning.

Sex differences and similarities in the neural circuit regulating song and other reproductive behaviors in songbirds

In the 1970s, Nottebohm and Arnold reported marked male-biased sex differences in the volume of three song control nuclei in songbirds. Subsequently a series of studies on several songbird species suggested that there is a positive correlation between the degree to which there is a sex difference in the volume of these song control nuclei and in song behavior. This correlation has been questioned in recent years. Furthermore, it has become clear that the song circuit is fully integrated into a more comprehensive neural circuit that regulates multiple courtship and reproductive behaviors including song. Sex differences in songbirds should be evaluated in the context of the full complement of behaviors produced by both sexes in relation to reproduction and based on the entire circuit in order to understand the functional significance of variation between males and females in brain and behavior. Variation in brain and behavior exhibited among living songbird species provides an excellent opportunity to understand the functional significance of sex differences related to social behaviors.

Inspiring song: The role of respiratory circuitry in the evolution of vertebrate vocal behavior

Vocalization is a common means of communication across vertebrates, but the evolutionary origins of the neural circuits controlling these behaviors are not clear. Peripheral mechanisms of sound production vary widely: fish produce sounds with a swimbladder or pectoral fins; amphibians, reptiles, and mammalians vocalize using a larynx; birds vocalize with a syrinx. Despite the diversity of vocal effectors across taxa, there are many similarities in the neural circuits underlying the control of these organs. Do similarities in vocal circuit structure and function indicate that vocal behaviors first arose in a single common ancestor, or have similar neural circuits arisen independently multiple times during evolution? In this review, we describe the hindbrain circuits that are involved in vocal production across vertebrates. Given that vocalization depends on respiration in most tetrapods, it is not surprising that vocal and respiratory hindbrain circuits across distantly related species are anatomically intermingled and functionally linked. Such vocal-respiratory circuit integration supports the hypothesis that vocal evolution involved the expansion and functional diversification of breathing circuits. Recent phylogenetic analyses, however, suggest vocal behaviors arose independently in all major tetrapod clades, indicating that similarities in vocal control circuits are the result of repeated co-options of respiratory circuits in each lineage. It is currently unknown whether vocal circuits across taxa are made up of homologous neurons, or whether vocal neurons in each lineage arose from developmentally and evolutionarily distinct progenitors. Integrative comparative studies of vocal neurons across brain regions and taxa will be required to distinguish between these two scenarios.

In vivo assessment of the neural substrate linked with vocal imitation accuracy

Human speech and bird song are acoustically complex communication signals that are learned by imitation during a sensitive period early in life. Although the brain areas indispensable for speech and song learning are known, the neural circuits important for enhanced or reduced vocal performance remain unclear. By combining in vivo structural Magnetic Resonance Imaging with song analyses in juvenile male zebra finches during song learning and beyond, we reveal that song imitation accuracy correlates with the structural architecture of four distinct brain areas, none of which pertain to the song control system. Furthermore, the structural properties of a secondary auditory area in the left hemisphere, are capable to predict future song copying accuracy, already at the earliest stages of learning, before initiating vocal practicing. These findings appoint novel brain regions important for song learning outcome and inform that ultimate performance in part depends on factors experienced before vocal practicing.

The neurobiology of innate, volitional and learned vocalizations in mammals and birds

Vocalization is an ancient vertebrate trait essential to many forms of communication, ranging from courtship calls to free verse. Vocalizations may be entirely innate and evoked by sexual cues or emotional state, as with many types of calls made in primates, rodents and birds; volitional, as with innate calls that, following extensive training, can be evoked by arbitrary sensory cues in non-human primates and corvid songbirds; or learned, acoustically flexible and complex, as with human speech and the courtship songs of oscine songbirds. This review compares and contrasts the neural mechanisms underlying innate, volitional and learned vocalizations, with an emphasis on functional studies in primates, rodents and songbirds. This comparison reveals both highly conserved and convergent mechanisms of vocal production in these different groups, despite their often vast phylogenetic separation. This similarity of central mechanisms for different forms of vocal production presents experimentalists with useful avenues for gaining detailed mechanistic insight into how vocalizations are employed for social and sexual signalling, and how they can be modified through experience to yield new vocal repertoires customized to the individual's social group. This article is part of the theme issue 'What can animal communication teach us about human language?'

The expression of DARPP-32 in adult male zebra finches (Taenopygia guttata)

Although the catecholaminergic circuitry in the zebra finch brain has been well studied, there is little information regarding the postsynaptic targets of dopamine. To answer this question, we looked at overall patterns of immunoreactivity for DARPP-32 (a dopamine and cAMP-regulated phosphoprotein, present mostly in dopaminoceptive neurons) in adult male zebra finches. Our results demonstrated that as in mammals and other avian species, DARPP-32 expression was highest in both medial and lateral striatum. Interestingly, a specific pattern of immunoreactivity was observed in the song control system, with 'core' song control regions, that is, LMANcore (lateral magnocellular nucleus of the anterior nidopallium), RA (nucleus robustus arcopallialis) and HVC being less immunoreactive for DARPP-32 than 'shell' areas such as LMANshell, RAcup, AId (intermediate arcopallium) and HVCshelf. Our results suggest that whereas dopamine may modulate the shell pathways at various levels of the AFP, dopaminergic modulation of the core pathway occurs mainly through Area X, a basal ganglia nucleus. Further, secondary sensory cortices including the perientopallial belt, Fields L1 and L3 had higher DARPP-32-immunoreactivity than primary sensory cortical areas such as the pallial basolateral nucleus, entopallium proper and Field L2, corresponding to somatosensory, visual and auditory systems, respectively. We also found DARPP-32-rich axon terminals surrounding dopaminergic neurons in the ventral tegmental area-substantia nigra complex which in turn project to the striatum, suggesting that there may be a reciprocal modulation between these regions. Overall, DARPP-32 expression appears to be higher in areas involved in integrating sensory information, which further supports the role of this protein as a molecular integrator of different signal processing pathways.

Language Origins Viewed in Spontaneous and Interactive Vocal Rates of Human and Bonobo Infants

From the first months of life, human infants produce "protophones," speech-like, non-cry sounds, presumed absent, or only minimally present in other apes. But there have been no direct quantitative comparisons to support this presumption. In addition, by 2 months, human infants show sustained face-to-face interaction using protophones, a pattern thought also absent or very limited in other apes, but again, without quantitative comparison. Such comparison should provide evidence relevant to determining foundations of language, since substantially flexible vocalization, the inclination to explore vocalization, and the ability to interact socially by means of vocalization are foundations for language. Here we quantitatively compare data on vocalization rates in three captive bonobo (Pan paniscus) mother-infant pairs with various sources of data from our laboratories on human infant vocalization. Both humans and bonobos produced distress sounds (cries/screams) and laughter. The bonobo infants also produced sounds that were neither screams nor laughs and that showed acoustic similarities to the human protophones. These protophone-like sounds confirm that bonobo infants share with humans the capacity to produce vocalizations that appear foundational for language. Still, there were dramatic differences between the species in both quantity and function of the protophone and protophone-like sounds. The bonobo protophone-like sounds were far less frequent than the human protophones, and the human protophones were far less likely to be interpreted as complaints and more likely as vocal play. Moreover, we found extensive vocal interaction between human infants and mothers, but no vocal interaction in the bonobo mother-infant pairs-while bonobo mothers were physically responsive to their infants, we observed no case of a bonobo mother vocalization directed to her infant. Our cross-species comparison focuses on low- and moderate-arousal circumstances because we reason the roots of language entail vocalization not triggered by excitement, for example, during fighting or intense play. Language appears to be founded in flexible vocalization, used to regulate comfortable social interaction, to share variable affective states at various levels of arousal, and to explore vocalization itself.

Cholecystokinin induces crowing in chickens

Animals that communicate using sound are found throughout the animal kingdom. Interestingly, in contrast to human vocal learning, most animals can produce species-specific patterns of vocalization without learning them from their parents. This phenomenon is called innate vocalization. The underlying molecular basis of both vocal learning in humans and innate vocalization in animals remains unknown. The crowing of a rooster is also innately controlled, and the upstream center is thought to be localized in the nucleus intercollicularis (ICo) of the midbrain. Here, we show that the cholecystokinin B receptor (CCKBR) is a regulatory gene involved in inducing crowing in roosters. Crowing is known to be a testosterone (T)-dependent behavior, and it follows that roosters crow but not hens. Similarly, T-administration induces chicks to crow. By using RNA-sequencing to compare gene expression in the ICo between the two comparison groups that either crow or do not crow, we found that CCKBR expression was upregulated in T-containing groups. The expression of CCKBR and its ligand, cholecystokinin (CCK), a neurotransmitter, was observed in the ICo. We also showed that crowing was induced by intracerebroventricular administration of an agonist specific for CCKBR. Our findings therefore suggest that the CCK system induces innate vocalization in roosters.

Simulation of the song motor pathway in birds: A single neuron initiates a chain of events that produces birdsong with realistic spectra properties

Birdsong is a complex learned behavior regulated by Neuromuscular coordination of different muscle sets necessary for producing relevant sounds. We developed a heterogeneous and stochastically connected neural network representing the pathway from the high vocal center (HVC) to the robust nucleus of the arcopallium (RA) neurons that drive the muscles to generate sounds. We show that a single active neuron is sufficient to initiate a chain of spiking events that results to excite the entire network system. The network could synthesize realistic bird sounds spectra, with spontaneous generation of intermittent sound bursts typical of birdsong (song syllables). This study confirms experiments on animals and on humans, where results have shown that single neurons are responsible for the activation of complex behavior or are associated with high-level perception events.

Vocal practice regulates singing activity-dependent genes underlying age-independent vocal learning in songbirds

The development of highly complex vocal skill, like human language and bird songs, is underlain by learning. Vocal learning, even when occurring in adulthood, is thought to largely depend on a sensitive/critical period during postnatal development, and learned vocal patterns emerge gradually as the long-term consequence of vocal practice during this critical period. In this scenario, it is presumed that the effect of vocal practice is thus mainly limited by the intrinsic timing of age-dependent maturation factors that close the critical period and reduce neural plasticity. However, an alternative, as-yet untested hypothesis is that vocal practice itself, independently of age, regulates vocal learning plasticity. Here, we explicitly discriminate between the influences of age and vocal practice using a songbird model system. We prevented zebra finches from singing during the critical period of sensorimotor learning by reversible postural manipulation. This enabled to us to separate lifelong vocal experience from the effects of age. The singing-prevented birds produced juvenile-like immature song and retained sufficient ability to acquire a tutored song even at adulthood when allowed to sing freely. Genome-wide gene expression network analysis revealed that this adult vocal plasticity was accompanied by an intense induction of singing activity-dependent genes, similar to that observed in juvenile birds, rather than of age-dependent genes. The transcriptional changes of activity-dependent genes occurred in the vocal motor robust nucleus of the arcopallium (RA) projection neurons that play a critical role in the production of song phonology. These gene expression changes were accompanied by neuroanatomical changes: dendritic spine pruning in RA projection neurons. These results show that self-motivated practice itself changes the expression dynamics of activity-dependent genes associated with vocal learning plasticity and that this process is not tightly linked to age-dependent maturational factors.

How is plasticity associated with vocal learning regulated during a critical period? Although there are abundant studies on the critical period in sensory systems, which are passively regulated by the external environment, few studies have manipulated the sensorimotor experience through the entire critical period. Thus, it is a commonly held belief that age or intrinsic maturation is a crucial factor for the closure of the critical period of vocal learning. Contrary to this idea, our study using songbirds provides a new insight that self-motivated vocal practice, not age, regulates vocal learning plasticity during the critical period. To examine the effects of vocal practice on vocal learning, we prevented juvenile zebra finches from singing during the critical period by postural manipulation, which separated the contribution of lifelong vocal experience from that of age. When these birds were allowed to freely sing as adults, they generated highly plastic songs and maintained the ability to mimic tutored songs, as normal juveniles did. Genome-wide transcriptome analysis revealed that both juveniles and singing-prevented adults, but not normally reared adults, expressed a similar set of singing-dependent genes in a song nucleus in the brain that regulates syllable acoustics. However, age-dependent genes were still similarly expressed in both singing-prevented and normally reared adult birds. These results exhibit that vocal learning plasticity is actively controlled by self-motivated vocal practice.

Volumetric development of the zebra finch brain throughout the first 200 days of post-hatch life traced by in vivo MRI

The first months of life are characterized by massive neuroplastic processes that parallel the acquisition of skills and abilities vital for proper functioning in later life. Likewise, juvenile songbirds learn the song sung by their tutor during the first months after hatching. To date, most studies targeting brain development in songbirds exclusively focus on the song control and auditory pathways. To gain a comprehensive insight into structural developmental plasticity of the entire zebra finch brain throughout the different subphases of song learning, we designed a longitudinal study in a group of male (16) and female (19) zebra finches. We collected T2-weighted 3-dimensional anatomical scans at six developmental milestones throughout the process of song learning, i.e. 20, 30, 40, 65, 90 and 120 days post hatching (dph), and one additional time point well after song crystallization, i.e. 200 dph. We observed that the total brain volume initially increases, peaks around 30-40 dph and decreases towards the end of the study. Further, we performed brain-wide voxel-based volumetric analyses to create spatio-temporal maps indicating when specific brain areas increase or decrease in volume, relative to the subphases of song learning. These maps informed (1) that most areas implicated in song control change early, i.e. between 20 and 65 dph, and are embedded in large clusters that cover major subdivisions of the zebra finch brain, (2) that volume changes between consecutive subphases of vocal learning appear highly similar in males and females, and (3) that only more rostrally situated brain regions change in volume towards later ages. Lastly, besides detecting sex differences in local tissue volume that align with previous studies, we uncovered two additional brain loci that are larger in male compared to female zebra finches. These volume differences co-localize with areas related to the song control and auditory pathways and can therefore be associated to the behavioral difference as only male zebra finches sing. In sum, our data point to clear heterochronous patterns of brain development similar to brain development in mammalian species and this work can serve as a reference for future neurodevelopmental imaging studies in zebra finches.

Neuroplasticity in the cerebello-thalamo-basal ganglia pathway: A longitudinal in vivo MRI study in male songbirds

Similar to human speech, bird song is controlled by several pathways including a cortico-basal ganglia-thalamo-cortical (C-BG-T-C) loop. Neurotoxic disengagement of the basal ganglia component, i.e. Area X, induces long-term changes in song performance, while most of the lesioned area regenerates within the first months. Importantly however, the timing and spatial extent of structural neuroplastic events potentially affecting other constituents of the C-BG-T-C loop is not clear. We designed a longitudinal MRI study where changes in brain structure were evaluated relative to the time after neurotoxic lesioning or to vocal performance. By acquiring both Diffusion Tensor Imaging and 3-dimensional anatomical scans, we were able to track alterations in respectively intrinsic tissue properties and local volume. Voxel-based statistical analyses revealed structural remodeling remote to the lesion, i.e. in the thalamus and, surprisingly, the cerebellum, both peaking within the first two months after lesioning Area X. Voxel-wise correlations between song performance and MRI parameters uncovered intriguing brain-behavior relationships in several brain areas pertaining to the C-BG-T-C loop supervising vocal motor control. Our results clearly point to structural neuroplasticity in the cerebellum induced by basal ganglia (striatal) damage and might point to the existence of a human-like cerebello-thalamic-basal ganglia pathway capable of modifying vocal motor output.

Remodeling of Dendritic Spines in the Avian Vocal Motor Cortex Following Deafening Depends on the Basal Ganglia Circuit

Deafening elicits a deterioration of learned vocalization, in both humans and songbirds. In songbirds, learned vocal plasticity has been shown to depend on the basal ganglia-cortical circuit, but the underlying cellular basis remains to be clarified. Using confocal imaging and electron microscopy, we examined the effect of deafening on dendritic spines in avian vocal motor cortex, the robust nucleus of the arcopallium (RA), and investigated the role of the basal ganglia circuit in motor cortex plasticity. We found rapid structural changes to RA dendritic spines in response to hearing loss, accompanied by learned song degradation. In particular, the morphological characters of RA spine synaptic contacts between 2 major pathways were altered differently. However, experimental disruption of the basal ganglia circuit, through lesions in song-specialized basal ganglia nucleus Area X, largely prevented both the observed changes to RA dendritic spines and the song deterioration after hearing loss. Our results provide cellular evidence to highlight a key role of the basal ganglia circuit in the motor cortical plasticity that underlies learned vocal plasticity.

Androgen and estrogen sensitivity of bird song: a comparative view on gene regulatory levels

Singing of songbirds is sensitive to testosterone and its androgenic and estrogenic metabolites in a species-specific way. The hormonal effects on song pattern are likely mediated by androgen receptors (AR) and estrogen receptor alpha (ERα), ligand activated transcription factors that are expressed in neurons of various areas of the songbirds' vocal control circuit. The distribution of AR in this circuit is rather similar between species while that of ERα is species variant and concerns a key vocal control area, the HVC (proper name). We discuss the regulation of the expression of the cognate AR and ERα and putative splice variants. In particular, we suggest that transcription factor binding sites in the promoter of these receptors differ between bird species. Further, we suggest that AR- and ERα-dependent gene regulation in vocal areas differs between species due to species-specific DNA binding sites of putative target genes that are required for the transcriptional activity of the receptors. We suggest that species differences in the distribution of AR and ERα in vocal areas and in the genomic sensitivity to these receptors contribute to species-specific hormonal regulation of the song.

Vocal learning promotes patterned inhibitory connectivity

Skill learning is instantiated by changes to functional connectivity within premotor circuits, but whether the specificity of learning depends on structured changes to inhibitory circuitry remains unclear. We used slice electrophysiology to measure connectivity changes associated with song learning in the avian analog of primary motor cortex (robust nucleus of the arcopallium, RA) in Bengalese Finches. Before song learning, fast-spiking interneurons (FSIs) densely innervated glutamatergic projection neurons (PNs) with apparently random connectivity. After learning, there was a profound reduction in the overall strength and number of inhibitory connections, but this was accompanied by a more than two-fold enrichment in reciprocal FSI–PN connections. Moreover, in singing birds, we found that pharmacological manipulations of RA's inhibitory circuitry drove large shifts in learned vocal features, such as pitch and amplitude, without grossly disrupting the song. Our results indicate that skill learning establishes nonrandom inhibitory connectivity, and implicates this patterning in encoding specific features of learned movements.

Complex motor behaviors such as birdsong are learned through practice and are thought to depend on specific excitatory connectivity in premotor circuits. Here the authors show that song learning in Bengalese Finches is associated with enrichment of inhibitory network connectivity that can affect specific song features.

'Out of tune': consequences of inbreeding on bird song

The expression of bird song is expected to signal male quality to females. 'Quality' is determined by genetic and environmental factors, but, surprisingly, there is very limited evidence if and how genetic aspects of male quality are reflected in song. Here, we manipulated the genetic make-up of canaries (Serinus canaria) via inbreeding, and studied its effects upon song output, complexity, phonetics and, for the first time, song learning. To this end, we created weight-matched inbred and outbred pairs of male fledglings, which were subsequently exposed to the same tutor male during song learning. Inbreeding strongly affected syllable phonetics, but there were little or no effects on other song features. Nonetheless, females discriminated among inbred and outbred males, as they produced heavier clutches when mated with an outbred male. Our study highlights the importance of song phonetics, which has hitherto often been overlooked.

A distributed neural network model for the distinct roles of medial and lateral HVC in zebra finch song production

Male zebra finches produce a song consisting of a canonical sequence of syllables, learned from a tutor and repeated throughout its adult life. Much of the neural circuitry responsible for this behavior is located in the cortical premotor region HVC (acronym is name). In a recent study from our laboratory, we found that partial bilateral ablation of the medial portion of HVC has effects on the song that are qualitatively different from those of bilateral ablation of the lateral portion. In this report we describe a neural network organization that can explain these data, and in so doing suggests key roles for other brain nuclei in the production of song. We also suggest that syllables and the gaps between them are each coded separately by neural chains within HVC, and that the timing mechanisms for syllables and gaps are distinct. The design principles underlying this model assign distinct roles for medial and lateral HVC circuitry that explain the data on medial and lateral ablations. In addition, despite the fact that the neural coding of song sequence is distributed among several brain nuclei in our model, it accounts for data showing that cooling of HVC stretches syllables uniformly and to a greater extent than gaps. Finally, the model made unanticipated predictions about details of the effects of medial and lateral HVC ablations that were then confirmed by reanalysis of these previously acquired behavioral data.NEW & NOTEWORTHY Zebra finch song consists of a string of syllables repeated in a nearly invariant sequence. We propose a neural network organization that can explain recent data indicating that the medial and lateral portions of the premotor cortical nucleus HVC have different roles in zebra finch song production. Our model explains these data, as well as data on the effects on song of cooling HVC, and makes predictions that we test in the singing bird.

Temperature Manipulation in Songbird Brain Implicates the Premotor Nucleus HVC in Birdsong Syntax

Variable motor sequences of animals are often structured and can be described by probabilistic transition rules between action elements. Examples include the songs of many songbird species such as the Bengalese finch, which consist of stereotypical syllables sequenced according to probabilistic rules (song syntax). The neural mechanisms behind such rules are poorly understood. Here, we investigate where the song syntax is encoded in the brain of the Bengalese finch by rapidly and reversibly manipulating the temperature in the song production pathway. Cooling the premotor nucleus HVC (proper name) slows down the song tempo, consistent with the idea that HVC controls moment-to-moment timings of acoustic features in the syllables. More importantly, cooling HVC alters the transition probabilities between syllables. Cooling HVC reduces the number of repetitions of long-repeated syllables and increases the randomness of syllable sequences. In contrast, cooling the downstream motor area RA (robust nucleus of the acropallium), which is critical for singing, does not affect the song syntax. Unilateral cooling of HVC shows that control of syllables is mostly lateralized to the left HVC, whereas transition probabilities between the syllables can be affected by cooling HVC in either hemisphere to varying degrees. These results show that HVC is a key site for encoding song syntax in the Bengalese finch. HVC is thus involved both in encoding timings within syllables and in sequencing probabilistic transitions between syllables. Our finding suggests that probabilistic selections and fine-grained timings of action elements can be integrated within the same neural circuits.SIGNIFICANCE STATEMENT Many animal behaviors such as birdsong consist of variable sequences of discrete actions. Where and how the probabilistic rules of such sequences are encoded in the brain is poorly understood. We locally and reversibly cooled brain areas in songbirds during singing. Mild cooling of area HVC in the Bengalese finch brain-a premotor area homologous to the mammalian premotor cortex-alters the statistics of the syllable sequences, suggesting that HVC is critical for birdsong sequences. HVC is also known for controlling moment-to-moment timings within syllables. Our results show that timing and probabilistic sequencing of actions can share the same neural circuits in local brain areas.

An integrated model for motor control of song in Serinus canaria

Birdsong is a learned motor behavior controlled by an interconnected structure of neural nuclei. This pathway is bilaterally organized, with anatomically indistinguishable structures in each brain hemisphere. In this work, we present a computational model whose variables are the average activities of different neural nuclei of the song system of oscine birds. Two of the variables are linked to the air sac pressure and the tension of the labia during canary song production. We show that these time dependent gestures are capable of driving a model of the vocal organ to synthesize realistic canary like songs.

Exploring sex differences in the adult zebra finch brain: In vivo diffusion tensor imaging and ex vivo super-resolution track density imaging

Zebra finches are an excellent model to study the process of vocal learning, a complex socially-learned tool of communication that forms the basis of spoken human language. So far, structural investigation of the zebra finch brain has been performed ex vivo using invasive methods such as histology. These methods are highly specific, however, they strongly interfere with performing whole-brain analyses and exclude longitudinal studies aimed at establishing causal correlations between neuroplastic events and specific behavioral performances. Therefore, the aim of the current study was to implement an in vivo Diffusion Tensor Imaging (DTI) protocol sensitive enough to detect structural sex differences in the adult zebra finch brain. Voxel-wise comparison of male and female DTI parameter maps shows clear differences in several components of the song control system (i.e. Area X surroundings, the high vocal center (HVC) and the lateral magnocellular nucleus of the anterior nidopallium (LMAN)), which corroborate previous findings and are in line with the clear behavioral difference as only males sing. Furthermore, to obtain additional insights into the 3-dimensional organization of the zebra finch brain and clarify findings obtained by the in vivo study, ex vivo DTI data of the male and female brain were acquired as well, using a recently established super-resolution reconstruction (SRR) imaging strategy. Interestingly, the SRR-DTI approach led to a marked reduction in acquisition time without interfering with the (spatial and angular) resolution and SNR which enabled to acquire a data set characterized by a 78μm isotropic resolution including 90 diffusion gradient directions within 44h of scanning time. Based on the reconstructed SRR-DTI maps, whole brain probabilistic Track Density Imaging (TDI) was performed for the purpose of super resolved track density imaging, further pushing the resolution up to 40μm isotropic. The DTI and TDI maps realized atlas-quality anatomical maps that enable a clear delineation of most components of the song control and auditory systems. In conclusion, this study paves the way for longitudinal in vivo and high-resolution ex vivo experiments aimed at disentangling neuroplastic events that characterize the critical period for vocal learning in zebra finch ontogeny.

An Adapting Auditory-motor Feedback Loop Can Contribute to Generating Vocal Repetition

Consecutive repetition of actions is common in behavioral sequences. Although integration of sensory feedback with internal motor programs is important for sequence generation, if and how feedback contributes to repetitive actions is poorly understood. Here we study how auditory feedback contributes to generating repetitive syllable sequences in songbirds. We propose that auditory signals provide positive feedback to ongoing motor commands, but this influence decays as feedback weakens from response adaptation during syllable repetitions. Computational models show that this mechanism explains repeat distributions observed in Bengalese finch song. We experimentally confirmed two predictions of this mechanism in Bengalese finches: removal of auditory feedback by deafening reduces syllable repetitions; and neural responses to auditory playback of repeated syllable sequences gradually adapt in sensory-motor nucleus HVC. Together, our results implicate a positive auditory-feedback loop with adaptation in generating repetitive vocalizations, and suggest sensory adaptation is important for feedback control of motor sequences.

Repetitions are common in animal vocalizations. Songs of many songbirds contain syllables that repeat a variable number of times, with non-Markovian distributions of repeat counts. The neural mechanism underlying such syllable repetitions is unknown. In this work, we show that auditory feedback plays an important role in sustaining syllable repetitions in the Bengalese finch. Deafening reduces syllable repetitions and skews the repeat number distribution towards short repeats. These effects are explained with our computational model, which suggests that syllable repeats are initially sustained by auditory feedback to the neural networks that drive the syllable production. The feedback strength weakens as the syllable repeats, increasing the likelihood that the syllable repetition stops. Neural recordings confirm such adaptation of auditory feedback to the auditory-motor circuit in the Bengalese finch. Our results suggests that sensory feedback can directly impact repetitions in motor sequences, and may provide insights into neural mechanisms of speech disorders such as stuttering.

The basal ganglia within a cognitive system in birds and mammals

The primate basal ganglia are fundamental to Ackermann et al.'s proposal. However, primates and rodents are models for human cognitive functions involving basal ganglia circuits, and links between striatal function and vocal communication come from songbirds. We suggest that the proposal is better integrated in cognitive and/or motor theories on spoken language origins and with more analogous nonhuman animal models.

Testing the evolutionary conservation of vocal motoneurons in vertebrates

Medullary motoneurons drive vocalization in many vertebrate lineages including fish, amphibians, birds, and mammals. The developmental history of vocal motoneuron populations in each of these lineages remains largely unknown. The highly conserved transcription factor Paired-like Homeobox 2b (Phox2b) is presumed to be expressed in all vertebrate hindbrain branchial motoneurons, including laryngeal motoneurons essential for vocalization in humans. We used immunohistochemistry and in situ hybridization to examine Phox2b protein and mRNA expression in caudal hindbrain and rostral spinal cord motoneuron populations in seven species across five chordate classes. Phox2b was present in motoneurons dedicated to sound production in mice and frogs (bullfrog, African clawed frog), but not those in bird (zebra finch) or bony fish (midshipman, channel catfish). Overall, the pattern of caudal medullary motoneuron Phox2b expression was conserved across vertebrates and similar to expression in sea lamprey. These observations suggest that motoneurons dedicated to sound production in vertebrates are not derived from a single developmentally or evolutionarily conserved progenitor pool.

Convergent differential regulation of SLIT-ROBO axon guidance genes in the brains of vocal learners

Only a few distantly related mammals and birds have the trait of complex vocal learning, which is the ability to imitate novel sounds. This ability is critical for speech acquisition and production in humans, and is attributed to specialized forebrain vocal control circuits that have several unique connections relative to adjacent brain circuits. As a result, it has been hypothesized that there could exist convergent changes in genes involved in neural connectivity of vocal learning circuits. In support of this hypothesis, expanding on our related study (Pfenning et al. [2014] Science 346: 1256846), here we show that the forebrain part of this circuit that makes a relatively rare direct connection to brainstem vocal motor neurons in independent lineages of vocal learning birds (songbird, parrot, and hummingbird) has specialized regulation of axon guidance genes from the SLIT-ROBO molecular pathway. The SLIT1 ligand was differentially downregulated in the motor song output nucleus that makes the direct projection, whereas its receptor ROBO1 was developmentally upregulated during critical periods for vocal learning. Vocal nonlearning bird species and male mice, which have much more limited vocal plasticity and associated circuits, did not show comparable specialized regulation of SLIT-ROBO genes in their nonvocal motor cortical regions. These findings are consistent with SLIT and ROBO gene dysfunctions associated with autism, dyslexia, and speech sound language disorders and suggest that convergent evolution of vocal learning was associated with convergent changes in the SLIT-ROBO axon guidance pathway.

Speech, vocal production learning, and the comparative method

The faith that "comparative analysis of the behaviour of modern primates, in conjunction with an accurate phylogenetic tree of relatedness, has the power to chart the early history of human cognitive evolution" (Byrne 2000 p. 543) runs afoul of the fact that no other primate besides humans is capable of vocal production learning. This basic enabling adaptation for articulate speech bears crucially on the reconstruction of language origins.

A neural circuit mechanism for regulating vocal variability during song learning in zebra finches

Motor skill learning is characterized by improved performance and reduced motor variability. The neural mechanisms that couple skill level and variability, however, are not known. The zebra finch, a songbird, presents a unique opportunity to address this question because production of learned song and induction of vocal variability are instantiated in distinct circuits that converge on a motor cortex analogue controlling vocal output. To probe the interplay between learning and variability, we made intracellular recordings from neurons in this area, characterizing how their inputs from the functionally distinct pathways change throughout song development. We found that inputs that drive stereotyped song-patterns are strengthened and pruned, while inputs that induce variability remain unchanged. A simple network model showed that strengthening and pruning of action-specific connections reduces the sensitivity of motor control circuits to variable input and neural ‘noise’. This identifies a simple and general mechanism for learning-related regulation of motor variability.

DOI:http://dx.doi.org/10.7554/eLife.03697.001

‘Practice makes perfect’ captures the essence of how we learn new skills. When learning to play a musical instrument, for example, it often takes hours of practice before we can play a single piece of music properly for the first time. And as we get better, the variability in our performance—which is an advantage during the early stages of learning—becomes less. Likewise, songbirds need lots of practice in order to master the intricate songs they need to sing to attract mates.

Studies in songbirds show that the neural circuits in the brain that are responsible for producing song and for generating vocal variability both converge on a motor control region called the robust nucleus of the arcopallium (or RA for short). However, the details of how learning a song leads to reduced variability in vocal performance are poorly understood.

Now Garst-Orozco et al. have investigated the relationship between learning and variability by studying brain slices of zebra finches. Their experiments reveal that the inputs received by RA neurons from a higher-order brain region that controls song change with practice, with some inputs becoming stronger and others being eliminated as the birds' singing ability improves. However, inputs received by RA neurons from the circuit that generates vocal variability do not change despite the song becoming increasingly precise.

Using a computer simulation, Garst-Orozco et al. show that the sensitivity of RA neurons to variable or ‘noisy’ input is reduced when inputs from the brain region that controls song are adaptively strengthened and eliminated. This ensures that when the notes and syllables that make up the bird's song have finally been learned, they will be uttered with high fidelity and precision. Intriguingly, motor skill learning in mammals have been associated with neural connectivity changes very similar to those described by Garst-Orozco et al., suggesting that insights from songbirds may lead to a better understanding of how ‘practice makes perfect’ also works in humans.

DOI:http://dx.doi.org/10.7554/eLife.03697.002

Regulatory mechanisms of testosterone-stimulated song in the sensorimotor nucleus HVC of female songbirds

Background

In male birds, influence of the sex steroid hormone testosterone and its estrogenic metabolites on seasonal song behavior has been demonstrated for many species. In contrast, female song was only recently recognized to be widespread among songbird species, and to date, sex hormone effects on singing and brain regions controlling song development and production (song control nuclei) have been studied in females almost exclusively using domesticated canaries (Serinus canaria). However, domesticated female canaries hardly sing at all in normal circumstances and exhibit only very weak, if any, song seasonally under the natural photoperiod. By contrast, adult female European robins (Erithacus rubecula) routinely sing during the winter season, a time when they defend feeding territories and show elevated circulating testosterone levels. We therefore used wild female European robins captured in the fall to examine the effects of testosterone administration on song as well as on the anatomy and the transcriptome of the song control nucleus HVC (sic). The results obtained from female robins were compared to outcomes of a similar experiment done in female domesticated canaries.

Results

Testosterone treatment induced abundant song in female robins. Examination of HVC transcriptomes and histological analyses of song control nuclei showed testosterone-induced differentiation processes related to neuron growth and spacing, angiogenesis and neuron projection morphogenesis. Similar effects were found in female canaries treated with testosterone. In contrast, the expression of genes related to synaptic transmission was not enhanced in the HVC of testosterone treated female robins but was strongly up-regulated in female canaries. A comparison of the testosterone-stimulated transcriptomes indicated that brain-derived neurotrophic factor (BDNF) likely functions as a common mediator of the testosterone effects in HVC.

Conclusions

Testosterone-induced singing of female robins correlated with cellular differentiation processes in the HVC that were partially similar to those seen in the HVC of testosterone-treated female canaries. Other modes of testosterone action, notably related to synaptic transmission, appeared to be regulated in a more species-specific manner in the female HVC. Divergent effects of testosterone on the HVC of different species might be related to differences between species in regulatory mechanisms of the singing behavior.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-014-0128-0) contains supplementary material, which is available to authorized users.

The respiratory-vocal system of songbirds: anatomy, physiology, and neural control

This wide-ranging review presents an overview of the respiratory-vocal system in songbirds, which are the only other vertebrate group known to display a degree of respiratory control during song rivalling that of humans during speech; this despite the fact that the peripheral components of both the respiratory and vocal systems differ substantially in the two groups. We first provide a brief description of these peripheral components in songbirds (lungs, air sacs and respiratory muscles, vocal organ (syrinx), upper vocal tract) and then proceed to a review of the organization of central respiratory-related neurons in the spinal cord and brainstem, the latter having an organization fundamentally similar to that of the ventral respiratory group of mammals. The second half of the review describes the nature of the motor commands generated in a specialized "cortical" song control circuit and how these might engage brainstem respiratory networks to shape the temporal structure of song. We also discuss a bilaterally projecting "respiratory-thalamic" pathway that links the respiratory system to "cortical" song control nuclei. This necessary pathway for song originates in the brainstem's primary inspiratory center and is hypothesized to play a vital role in synchronizing song motor commands both within and across hemispheres.

Maximized song learning of juvenile male zebra finches following BDNF expression in the HVC

During song learning, vocal patterns are matched to an auditory memory acquired from a tutor, a process involving sensorimotor feedback. Song sensorimotor learning and song production of birds is controlled by a set of interconnected brain nuclei, the song control system. In male zebra finches, the beginning of the sensorimotor phase of song learning parallels an increase of the brain-derived neurotrophic factor (BDNF) in just one part of the song control system, the forebrain nucleus HVC. We report here that transient BDNF-mRNA upregulation in the HVC results in a maximized copying of song syllables. Each treated bird shows motor learning to an extent similar to that of the selected best learners among untreated zebra finches. Because this result was not found following BDNF overexpression in the target areas of HVC within the song system, HVC-anchored mechanisms are limiting sensorimotor vocal learning.

The songbird syrinx morphome: a three-dimensional, high-resolution, interactive morphological map of the zebra finch vocal organ

BACKGROUND

Like human infants, songbirds learn their species-specific vocalizations through imitation learning. The birdsong system has emerged as a widely used experimental animal model for understanding the underlying neural mechanisms responsible for vocal production learning. However, how neural impulses are translated into the precise motor behavior of the complex vocal organ (syrinx) to create song is poorly understood. First and foremost, we lack a detailed understanding of syringeal morphology.

RESULTS

To fill this gap we combined non-invasive (high-field magnetic resonance imaging and micro-computed tomography) and invasive techniques (histology and micro-dissection) to construct the annotated high-resolution three-dimensional dataset, or morphome, of the zebra finch (Taeniopygia guttata) syrinx. We identified and annotated syringeal cartilage, bone and musculature in situ in unprecedented detail. We provide interactive three-dimensional models that greatly improve the communication of complex morphological data and our understanding of syringeal function in general.

CONCLUSIONS

Our results show that the syringeal skeleton is optimized for low weight driven by physiological constraints on song production. The present refinement of muscle organization and identity elucidates how apposed muscles actuate different syringeal elements. Our dataset allows for more precise predictions about muscle co-activation and synergies and has important implications for muscle activity and stimulation experiments. We also demonstrate how the syrinx can be stabilized during song to reduce mechanical noise and, as such, enhance repetitive execution of stereotypic motor patterns. In addition, we identify a cartilaginous structure suited to play a crucial role in the uncoupling of sound frequency and amplitude control, which permits a novel explanation of the evolutionary success of songbirds.

Mouse vocal communication system: are ultrasounds learned or innate?

Mouse ultrasonic vocalizations (USVs) are often used as behavioral readouts of internal states, to measure effects of social and pharmacological manipulations, and for behavioral phenotyping of mouse models for neuropsychiatric and neurodegenerative disorders. However, little is known about the neurobiological mechanisms of rodent USV production. Here we discuss the available data to assess whether male mouse song behavior and the supporting brain circuits resemble those of known vocal non-learning or vocal learning species. Recent neurobiology studies have demonstrated that the mouse USV brain system includes motor cortex and striatal regions, and that the vocal motor cortex sends a direct sparse projection to the brainstem vocal motor nucleus ambiguous, a projection previously thought be unique to humans among mammals. Recent behavioral studies have reported opposing conclusions on mouse vocal plasticity, including vocal ontogeny changes in USVs over early development that might not be explained by innate maturation processes, evidence for and against a role for auditory feedback in developing and maintaining normal mouse USVs, and evidence for and against limited vocal imitation of song pitch. To reconcile these findings, we suggest that the trait of vocal learning may not be dichotomous but encompass a broad spectrum of behavioral and neural traits we call the continuum hypothesis, and that mice possess some of the traits associated with a capacity for limited vocal learning.