Memory in the making: localized brain activation related to song learning in young songbirds

Songbird males learn to sing their songs from an adult ‘tutor’ early in life, much like human infants learn to speak. Similar to humans, in the songbird brain there are separate neural substrates for vocal production and for auditory memory. In adult songbirds, the caudal pallium, the avian equivalent of the auditory association cortex, has been proposed to contain the neural substrate of tutor song memory, while the song system is involved in song production as well as sensorimotor learning. If this hypothesis is correct, there should be neuronal activation in the caudal pallium, and not in the song system, while the young bird is hearing the tutor song. We found increased song-induced molecular neuronal activation, measured as the expression of an immediate early gene, in the caudal pallium of juvenile zebra finch males that were in the process of learning to sing their songs. No such activation was found in the song system. Molecular neuronal activation was significantly greater in response to tutor song than to novel song or silence in the medial part of the caudomedial nidopallium (NCM). In the caudomedial mesopallium, there was significantly greater molecular neuronal activation in response to tutor song than to silence. In addition, in the NCM there was a significant positive correlation between spontaneous molecular neuronal activation and the strength of song learning during sleep. These results suggest that the caudal pallium contains the neural substrate for tutor song memory, which is activated during sleep when the young bird is in the process of learning its song. The findings provide insight into the formation of auditory memories that guide vocal production learning, a process fundamental for human speech acquisition.     

鸣禽前脑X区结构功能的研究进展

鸣禽鸣唱与人类说话一样,都是在教习和听觉反馈下形成的感知运动学习过程。鸣禽鸣唱的发育和成熟巩固依赖于发声通路和前端脑通路组成的鸣唱系统的完整。前端脑通路中的X区在鸣唱学习记忆中扮演着重要角色。本文就X区的形态组织结构、在鸣唱发育与成熟巩固中的作用、突触可塑性的研究进展进行了综述,并且将X区与哺乳动物基底神经节的学习记忆功能做了比较。     

Common features of neural activity during singing and sleep periods in a basal ganglia nucleus critical for vocal learning in a juvenile songbird

Reactivations of waking experiences during sleep have been considered fundamental neural processes for memory consolidation. In songbirds, evidence suggests the importance of sleep-related neuronal activity in song system motor pathway nuclei for both juvenile vocal learning and maintenance of adult song. Like those in singing motor nuclei, neurons in the basal ganglia nucleus Area X, part of the basal ganglia-thalamocortical circuit essential for vocal plasticity, exhibit singing-related activity. It is unclear, however, whether Area X neurons show any distinctive spiking activity during sleep similar to that during singing. Here we demonstrate that, during sleep, Area X pallidal neurons exhibit phasic spiking activity, which shares some firing properties with activity during singing. Shorter interspike intervals that almost exclusively occurred during singing in awake periods were also observed during sleep. The level of firing variability was consistently higher during singing and sleep than during awake non-singing states. Moreover, deceleration of firing rate, which is considered to be an important firing property for transmitting signals from Area X to the thalamic nucleus DLM, was observed mainly during sleep as well as during singing. These results suggest that songbird basal ganglia circuitry may be involved in the off-line processing potentially critical for vocal learning during sensorimotor learning phase.     

Neural song preference during vocal learning in the zebra finch depends on age and state

The zebra finch acquires its song by first memorizing a model song from a tutor and then matching its own vocalizations to the memory trace of the tutor song, called a template. Neural mechanisms underlying this process require a link between the neural memory trace and the premotor song circuitry, which drives singing. We now report that a premotor song nucleus responds more to the tutor song model than to every other stimulus examined, including the bird's own song (BOS). Neural tuning to the song model occurred only during waking and peaked during the template-matching period of development, when the vocal motor output is sculpted to match the tutor song. During the same developmental phase, the BOS was the most effective excitatory stimulus during sleep. The preference for BOS compared to tutor song inverted with sleep/wake state. Thus, song preference shifts with development and state.     

A Lightweight,Headphones-based System for Manipulating Auditory Feedback in Songbirds

Experimental manipulations of sensory feedback during complex behavior have provided valuable insights into the computations underlying motor control and sensorimotor plasticity¹. Consistent sensory perturbations result in compensatory changes in motor output, reflecting changes in feedforward motor control that reduce the experienced feedback error. By quantifying how different sensory feedback errors affect human behavior, prior studies have explored how visual signals are used to recalibrate arm movements^2,3 and auditory feedback is used to modify speech production^4-7. The strength of this approach rests on the ability to mimic naturalistic errors in behavior, allowing the experimenter to observe how experienced errors in production are used to recalibrate motor output.Songbirds provide an excellent animal model for investigating the neural basis of sensorimotor control and plasticity^8,9. The songbird brain provides a well-defined circuit in which the areas necessary for song learning are spatially separated from those required for song production, and neural recording and lesion studies have made significant advances in understanding how different brain areas contribute to vocal behavior^9-12. However, the lack of a naturalistic error-correction paradigm - in which a known acoustic parameter is perturbed by the experimenter and then corrected by the songbird - has made it difficult to understand the computations underlying vocal learning or how different elements of the neural circuit contribute to the correction of vocal errors¹³.The technique described here gives the experimenter precise control over auditory feedback errors in singing birds, allowing the introduction of arbitrary sensory errors that can be used to drive vocal learning. Online sound-processing equipment is used to introduce a known perturbation to the acoustics of song, and a miniaturized headphones apparatus is used to replace a songbird''s natural auditory feedback with the perturbed signal in real time. We have used this paradigm to perturb the fundamental frequency (pitch) of auditory feedback in adult songbirds, providing the first demonstration that adult birds maintain vocal performance using error correction¹⁴. The present protocol can be used to implement a wide range of sensory feedback perturbations (including but not limited to pitch shifts) to investigate the computational and neurophysiological basis of vocal learning.     

Of Mice,Birds, and Men: The Mouse Ultrasonic Song System Has Some Features Similar to Humans and Song-Learning Birds

Humans and song-learning birds communicate acoustically using learned vocalizations. The characteristic features of this social communication behavior include vocal control by forebrain motor areas, a direct cortical projection to brainstem vocal motor neurons, and dependence on auditory feedback to develop and maintain learned vocalizations. These features have so far not been found in closely related primate and avian species that do not learn vocalizations. Male mice produce courtship ultrasonic vocalizations with acoustic features similar to songs of song-learning birds. However, it is assumed that mice lack a forebrain system for vocal modification and that their ultrasonic vocalizations are innate. Here we investigated the mouse song system and discovered that it includes a motor cortex region active during singing, that projects directly to brainstem vocal motor neurons and is necessary for keeping song more stereotyped and on pitch. We also discovered that male mice depend on auditory feedback to maintain some ultrasonic song features, and that sub-strains with differences in their songs can match each other''s pitch when cross-housed under competitive social conditions. We conclude that male mice have some limited vocal modification abilities with at least some neuroanatomical features thought to be unique to humans and song-learning birds. To explain our findings, we propose a continuum hypothesis of vocal learning.     

Memory-dependent adjustment of vocal response latencies in a territorial songbird

Vocal interactions in songbirds can be used as a model system to investigate the interplay of intrinsic singing programmes (e.g. influences from vocal memories) and external variables (e.g. social factors). When characterizing vocal interactions between territorial rivals two aspects are important: (1) the timing of songs in relation to the conspecific’s singing and (2) the use of a song pattern that matches the rival’s song. Responses in both domains can be used to address a territorial rival. This study is the first to investigate the relation of the timing of vocal responses to (1) the vocal memory of a responding subject and (2) the selection of the song pattern that the subject uses as a response. To this end, we conducted interactive playback experiments with adult nightingales (Luscinia megarhynchos) that had been hand-reared and tutored in the laboratory. We analysed the subjects’ vocal response latencies towards broadcast playback stimuli that they either had in their own vocal repertoire (songs shared with playback) or that they had not heard before (unknown songs). Likewise, we compared vocal response latencies between responses that matched the stimulus song and those that did not. Our findings showed that the latency of singing in response to the playback was shorter for shared versus unknown song stimuli when subjects overlapped the playback stimuli with their own song. Moreover birds tended to overlap faster when vocally matching the stimulus song rather than when replying with a non-matching song type. We conclude that memory of song patterns influenced response latencies and discuss possible mechanisms.     

At the interface of the auditory and vocal motor systems: NIf and its role in vocal processing,production and learning

Communication between auditory and vocal motor nuclei is essential for vocal learning. In songbirds, the nucleus interfacialis of the nidopallium (NIf) is part of a sensorimotor loop, along with auditory nucleus avalanche (Av) and song system nucleus HVC, that links the auditory and song systems. Most of the auditory information comes through this sensorimotor loop, with the projection from NIf to HVC representing the largest single source of auditory information to the song system. In addition to providing the majority of HVC’s auditory input, NIf is also the primary driver of spontaneous activity and premotor-like bursting during sleep in HVC. Like HVC and RA, two nuclei critical for song learning and production, NIf exhibits behavioral-state dependent auditory responses and strong motor bursts that precede song output. NIf also exhibits extended periods of fast gamma oscillations following vocal production. Based on the converging evidence from studies of physiology and functional connectivity it would be reasonable to expect NIf to play an important role in the learning, maintenance, and production of song. Surprisingly, however, lesions of NIf in adult zebra finches have no effect on song production or maintenance. Only the plastic song produced by juvenile zebra finches during the sensorimotor phase of song learning is affected by NIf lesions. In this review, we carefully examine what is known about NIf at the anatomical, physiological, and behavioral levels. We reexamine conclusions drawn from previous studies in the light of our current understanding of the song system, and establish what can be said with certainty about NIf’s involvement in song learning, maintenance, and production. Finally, we review recent theories of song learning integrating possible roles for NIf within these frameworks and suggest possible parallels between NIf and sensorimotor areas that form part of the neural circuitry for speech processing in humans.     

The role of auditory feedback in vocal learning and maintenance

Auditory experience is critical for the acquisition and maintenance of learned vocalizations in both humans and songbirds. Despite the central role of auditory feedback in vocal learning and maintenance, where and how auditory feedback affects neural circuits important to vocal control remain poorly understood. Recent studies of singing birds have uncovered neural mechanisms by which feedback perturbations affect vocal plasticity and also have identified feedback-sensitive neurons at or near sites of auditory and vocal motor interaction. Additionally, recent studies in marmosets have underscored that even in the absence of vocal learning, vocalization remains flexible in the face of changing acoustical environments, pointing to rapid interactions between auditory and vocal motor systems. Finally, recent studies show that a juvenile songbird's initial auditory experience of a song model has long-lasting effects on sensorimotor neurons important to vocalization, shedding light on how auditory memories and feedback interact to guide vocal learning.     

An Automated Procedure for Evaluating Song Imitation

Songbirds have emerged as an excellent model system to understand the neural basis of vocal and motor learning. Like humans, songbirds learn to imitate the vocalizations of their parents or other conspecific “tutors.” Young songbirds learn by comparing their own vocalizations to the memory of their tutor song, slowly improving until over the course of several weeks they can achieve an excellent imitation of the tutor. Because of the slow progression of vocal learning, and the large amounts of singing generated, automated algorithms for quantifying vocal imitation have become increasingly important for studying the mechanisms underlying this process. However, methodologies for quantifying song imitation are complicated by the highly variable songs of either juvenile birds or those that learn poorly because of experimental manipulations. Here we present a method for the evaluation of song imitation that incorporates two innovations: First, an automated procedure for selecting pupil song segments, and, second, a new algorithm, implemented in Matlab, for computing both song acoustic and sequence similarity. We tested our procedure using zebra finch song and determined a set of acoustic features for which the algorithm optimally differentiates between similar and non-similar songs.     

Rufous‐capped Warblers Basileuterus rufifrons show seasonal,temporal and annual variation in song use

In the majority of songbird species, males have repertoires of multiple song types used for mate attraction and territory defence. The wood‐warblers (family Parulidae) are a diverse family of songbirds in which males of many migratory species use different song types or patterns of song delivery (known as ‘singing modes’) depending on context. The vocal behaviour of most tropical resident warblers remains undescribed, although these species differ ecologically and behaviourally from migratory species, and may therefore differ in their vocal behaviour. We test whether male Rufous‐capped Warblers Basileuterus rufifrons use distinct singing modes by examining song structure and context‐dependent variation in their songs. We recorded multiple song bouts from 50 male Warblers in a Costa Rican population over 3 years to describe seasonal, diel and annual variation in song structure and vocal behaviour. We found that Rufous‐capped Warbler songs are complex, with many syllable types shared both within and between males’ repertoires. Males varied their song output depending on context: they sang long songs at a high rate at dawn and during the breeding season, and shortened songs in the presence of a vocalizing female mate. Unlike many migratory species, Rufous‐capped Warblers do not appear to have different singing modes; they did not change the song variants used or the pattern of song delivery according to time of day, season or female vocal activity. Our research provides the first detailed vocal analysis of any Basileuterus warbler species, and enhances our understanding of the evolution of repertoire specialization in tropical resident songbirds.     

Age-dependent effects of song exposure: song crystallization sets a boundary between fast and delayed vocal imitation

Song crystallization is a prominent developmental phase of oscine birds in which there is a transition from a production of plastic vocal material to a performance of elaborated song patterns that are typical for adult birds. Here we show that crystallization can be related to a marked change in memory properties involved in supplementary learning of song occurring during this phase. We studied nightingales, Luscinia megarhynchos, a species renowned for its large repertoire of song types. After a period of early tutoring as fledglings, hand-reared subjects (N=8) were exposed to a set of temporally distributed training experiences as juveniles and young adults. Analyses of the birds' singing yielded clear evidence for late song learning and also a striking phase-related shift in their memory properties. New songs heard shortly before crystallization (at an age of 40-42 weeks) were imitated within a few days and their structural and syntactical ‘quality’ seemed not to be inferior to imitations developed from songs heard earlier. In contrast, none of the songs experienced soon after song crystallization (at 45-47 weeks) appeared as imitations in the repertoires of the young adults (at 48-55 weeks). However, birds had clearly memorized these songs, as they produced imitations of them in their second spring. Our findings show that auditory song acquisition in nightingales extends well into the phase of vocal production. At the same time, however, the boundary between an immediate and a delayed form of vocal imitation suggests that song crystallization reflects a marked change in memory properties. That is, song crystallization seems to constrain the ability to develop motor programs for song patterns that are heard even though these are committed to memory. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Developmentally restricted synaptic plasticity in a songbird nucleus required for song learning

We provide evidence here of long-term synaptic plasticity in a songbird forebrain area required for song learning, the lateral magnocellular nucleus of the anterior neostriatum (LMAN). Pairing postsynaptic bursts in LMAN principal neurons with stimulation of recurrent collateral synapses had two effects: spike timing- and NMDA receptor-dependent LTP of the recurrent synapses, and LTD of thalamic afferent synapses that were stimulated out of phase with the postsynaptic bursting. Both types of plasticity were restricted to the sensory critical period for song learning, consistent with a role for each in sensory learning. The properties of the observed plasticity are appropriate to establish recurrent circuitry within LMAN that reflects the spatiotemporal pattern of thalamic afferent activity evoked by tutor song. Such circuit organization could represent a tutor song memory suitable for reinforcing particular vocal sequences during sensorimotor learning.     

The song structure and seasonal patterns of vocal behavior of male and female bellbirds (Anthornis melanura)

The bellbird (Anthornis melanura) is a honeyeater endemic to New Zealand, which uses song to defend breeding territories and/or food resources year round. Both sexes sing and the song structure and singing behavior have not yet been quantified. The number of song types, spectral structure, repertoire size, and singing behavior of male and female bellbirds was investigated for a large island population. Song types differed between the sexes with males singing a number of structurally distinct song types and females producing song types that overlapped in structure. Singing behavior also differed between the sexes; males often sung long series of songs while females sung each song at relatively long and variable intervals. Singing by both sexes occurred year round but the frequency of male and female singing bouts showed contrasting seasonal patterns. The frequency of female singing bouts increased as the breeding season progressed, whereas male singing bouts decreased. In contrast to almost all studied passerines, female bellbirds exhibited significant singing behavior and sung songs of complex structure and variety that parallel male song. These results provide a quantitative foundation for further research of song in bellbirds and in particular the function of female vocal behavior.     

Former Experience Can Modify Social Selectivity during Song Learning in the Nightingale (Luscinia megarhynchos)

Changes in preference for a specific song-learning context, characterized by close proximity of a tutor, were investigated in the nightingale, Luscinia megarhynchos. Two groups of males were tutored with conspecific song for two periods, days 42 to 56, and days 57 to 71. Song types were either presented from tape alone (group A) or were played in the presence of a familiar tutor, i.e. the person who hand-raised the birds (group B). Acquisition success did not differ between the groups for period 2. However, while males of group B acquired songs during both tutoring periods, males of group A learned only during period 2. This indicates that in nightingales the preference for a specific song-learning context decreases during development. A comparison with an earlier study suggests that this shift is not age dependent but primarily due to auditory experience with the song patterns to be learned. Such a mechanism has also been described for the termination of sensitive phases for song learning. Because song acquisition in adult nightingales does not depend any more on close spatial proximity of a familiar tutor, a male in his first spring and later on can learn from a number of singing territorial neighbours, and by this means acquire the large song-type repertoire typical for the species.     

Long-time storage of song types in birds: evidence from interactive playbacks

In studies of birdsong learning, imitation-based assays of stimulus memorization do not take into account that tutored song types may have been stored, but were not retrieved from memory. Such a 'silent' reservoir of song material could be used later in the bird's life, e.g. during vocal interactions. We examined this possibility in hand-reared nightingales during their second year. The males had been exposed to songs, both as fledglings and later, during their first full song period in an interactive playback design. Our design allowed us to compare the performance of imitations from the following categories: (i) songs only experienced during the early tutoring; (ii) songs experienced both during early tutoring and interactive playbacks; and (iii) novel songs experienced only during the simulated interactions. In their second year, birds imitated song types from each category, including those from categories (i) and (ii) which they had failed to imitate before. In addition, the performance of these song types was different (category (ii) > category (i)) and more pronounced than for category (iii) songs. Our results demonstrate 'silent' song storage in nightingales and point to a graded influence of the time and the social context of experience on subsequent vocal imitation.     

Patterns of dawn singing by Buff-breasted Flycatchers

ABSTRACT.   Many passerine species exhibit a "dawn chorus"—a bout of intense singing activity before or at dawn, but our understanding of this phenomenon is poor. Tyrant flycatchers (Tyrannidae) exhibit pronounced daily bouts of dawn singing. I documented this behavior in several populations of Buff-breasted Flycatchers ( Empidonax fulvifrons ) in Arizona, tape recording >30,000 songs of 23 individuals during dawn singing. Individual males sang a dawn bout each morning, even during breeding phases when daytime song was almost completely absent. Dawn bouts began 5–10 min before local civil twilight and continued for 25–30 min. Each male possessed two song types delivered at high rates during dawn singing (four times the rate during sustained daytime singing). Song rate varied significantly over the course of dawn bouts, increasing to 55 songs / min at mid-bout, then declining to the end of the bout. Type 2 songs comprised about 30% of songs during dawn singing, and decreased significantly in proportion during the final 10 min of the bout. Songs of the two types were delivered in a nonrandom fashion. Males sang at locations near territory boundaries and pairs of neighbors engaged in counter-singing from the same locations each morning. A number of dawn singing bouts ended with attempted or successful copulations. These observations are consistent with the social dynamics hypothesis for the functional significance of dawn singing in this species.     

Deafening drives cell-type-specific changes to dendritic spines in a sensorimotor nucleus important to learned vocalizations

Hearing loss prevents vocal learning and causes learned vocalizations to deteriorate, but how vocalization-related auditory feedback acts on neural circuits that control vocalization remains poorly understood. We deafened adult zebra finches, which rely on auditory feedback to maintain their learned songs, to test the hypothesis that deafening modifies synapses on neurons in a sensorimotor nucleus important to song production. Longitudinal in vivo imaging revealed that deafening selectively decreased the size and stability of dendritic spines on neurons that provide input to a striatothalamic pathway important to audition-dependent vocal plasticity, and changes in spine size preceded and predicted subsequent vocal degradation. Moreover, electrophysiological recordings from these neurons showed that structural changes were accompanied by functional weakening of both excitatory and inhibitory synapses, increased intrinsic excitability, and changes in spontaneous action potential output. These findings shed light on where and how auditory feedback acts within sensorimotor circuits to shape learned vocalizations.     

Assessing the Importance of Social Factors in Bird Song Learning: A Test Using Computer-Simulated Tutors

We used a mixed live/synthetic tutoring design to investigate whether the social factors of eavesdropping on adult singing interactions and/or direct interaction with a tutor would influence song learning in song sparrows ( Melospiza melodia ). Males were brought into the laboratory at 4–5 d-old, hand-raised and then tutored by two pairs of adult song sparrows in June and July From January through March of the next year, subjects received tutoring from computer simulations of two of the original live tutors. The first, non-interactive, model simulated one of the earlier tutors singing 'naturalistic' bouts of song with no interaction with the subject. The second, interactive, model simulated a different early tutor that behaved similarly to the non-interactive model, but synchronized its singing with the subject, and tried to match the subject's song. Subjects learned relatively more from their interactive late tutor and his early partner, and showed no tendency to learn more from their late than their early-only tutors. These results support the eavesdropping hypothesis, and also suggest that direct interaction with the tutor is a relevant social factor.