Respiratory units of motor production and song imitation in the zebra finch

Juvenile male zebra finches (Taeniopygia guttata) learn a stereotyped song by imitating sounds from adult male tutors. Their song is composed of a series of syllables, which are separated by silent periods. How acoustic units of song are translated into respiratory and syringeal motor gestures during the song learning process is not well understood. To learn about the respiratory contribution to the imitation process, we recorded air sac pressure in 38 male zebra finches and compared the acoustic structures and air sac pressure patterns of similar syllables qualitatively and quantitatively. Acoustic syllables correspond to expiratory pressure pulses and most often (74%) entire syllables are copied using similar air sac pressure patterns. Even notes placed within different syllables are generated with similar air sac pressure patterns when only segments of syllables are copied (9%). A few of the similar syllables (17%) are generated with a modified pressure pattern, typically involving addition or deletion of an inspiration. The high similarity of pressure patterns for like syllables indicates that generation of particular sounds is constrained to a narrow range of air sac pressure conditions. Following presentation of stroboscope flashes, song was typically interrupted at the end of an expiratory pressure pulse, confirming that expirations and, therefore, syllables are the smallest unit of motor production of song. Silent periods, which separate syllables acoustically, are generated by switching from expiration to inspiration. Switching between respiratory phases, therefore, appears to play a dominant role in organizing the stereotyped motor program for song production.     

The neuromuscular control of birdsong.

Birdsong requires complex learned motor skills involving the coordination of respiratory, vocal organ and craniomandibular muscle groups. Recent studies have added to our understanding of how these vocal subsystems function and interact during song production. The respiratory rhythm determines the temporal pattern of song. Sound is produced during expiration and each syllable is typically followed by a small inspiration, except at the highest syllable repetition rates when a pattern of pulsatile expiration is used. Both expiration and inspiration are active processes. The oscine vocal organ, the syrinx, contains two separate sound sources at the cranial end of each bronchus, each with independent motor control. Dorsal syringeal muscles regulate the timing of phonation by adducting the sound-generating labia into the air stream. Ventral syringeal muscles have an important role in determining the fundamental frequency of the sound. Different species use the two sides of their vocal organ in different ways to achieve the particular acoustic properties of their song. Reversible paralysis of the vocal organ during song learning in young birds reveals that motor practice is particularly important in late plastic song around the time of song crystallization in order for normal adult song to develop. Even in adult crystallized song, expiratory muscles use sensory feedback to make compensatory adjustments to perturbations of respiratory pressure. The stereotyped beak movements that accompany song appear to have a role in suppressing harmonics, particularly at low frequencies.     

Matters of life and death in the songbird forebrain

Male zebra finches learn a specific vocal pattern during a restricted period of development. They produce that song in stereotyped form throughout adulthood, and are unable to learn new song patterns. Development of the neural substrate for song learning and behavior is delayed relative to other brain regions, and neural song-control circuits undergo dramatic changes during the period of vocal learning due to both loss of neurons as well as incorporation of newly generated neurons. In contrast, canaries do learn new song patterns in adulthood and modify their vocal repertoires each breeding season. Adult canaries also maintain a large population of dividing cells in the ependymal zone of the telencephalon, and vast numbers of newly generated neurons migrate out to become incorporated into functional circuits and replace older neurons. We review the relationships between cellular and behavioral aspects of song learning in both zebra finches and canaries, as well as the role of gonadal hormones in regulating diverse aspects of the song-control system. © 1992 John Wiley & Sons, Inc.     

Matters of life and death in the songbird forebrain.

Male zebra finches learn a specific vocal pattern during a restricted period of development. They produce that song in stereotyped form throughout adulthood, and are unable to learn new song patterns. Development of the neural substrate for song learning and behavior is delayed relative to other brain regions, and neural song-control circuits undergo dramatic changes during the period of vocal learning due to both loss of neurons as well as incorporation of newly generated neurons. In contrast, canaries do learn new song patterns in adulthood and modify their vocal repertoires each breeding season. Adult canaries also maintain a large population of dividing cells in the ependymal zone of the telencephalon, and vast numbers of newly generated neurons migrate out to become incorporated into functional circuits and replace older neurons. We review the relationships between cellular and behavioral aspects of song learning in both zebra finches and canaries, as well as the role of gonadal hormones in regulating diverse aspects of the song-control system.     

Partial muting leads to age-dependent modification of motor patterns underlying crystallized zebra finch song

The crystallized structure of adult zebra finch (Taeniopygia guttata) song is modifiable if sensory feedback is altered during sound production. Such song plasticity has been studied by examining acoustic modifications to the motif; however, the underlying changes to the vocal motor patterns of these acoustic modifications have not been addressed. Adult birds in two age categories (young=90-120 days or middle aged 150-250 days) that sang crystallized song were used in the experiment. Vocal motor patterns were monitored by recording respiratory air sac pressure before, during, and after song plasticity was induced by partial or complete reduction of phonation (i.e., "partial muting"). Birds were recorded until changes in air sac pressure patterns underlying the song structure were observed (up to 160 days). Young adult birds were more likely to insert shorter duration (<125 ms) expiratory pulses (EPs) into the motif than middle-aged adults. These shorter duration EPs were produced with a unique pressure pattern relative to the intact song, and therefore appeared to be generated by novel motor gestures. Stuttering (atypical repetition of an EP) was observed when these novel EPs were inserted into the motif, regardless of age. The EP of the distance call, which is also a learned vocalization in zebra finches, showed a similar reduction in duration if EPs were also shortened in the song. The emergence of shorter duration EPs was not related to sound production, or nonspecific effects of the surgical procedure, which suggests an age-dependent neural process for song plasticity.     

Partial muting leads to age‐dependent modification of motor patterns underlying crystallized zebra finch song

The crystallized structure of adult zebra finch (Taeniopygia guttata) song is modifiable if sensory feedback is altered during sound production. Such song plasticity has been studied by examining acoustic modifications to the motif; however, the underlying changes to the vocal motor patterns of these acoustic modifications have not been addressed. Adult birds in two age categories (young = 90–120 days or middle aged 150–250 days) that sang crystallized song were used in the experiment. Vocal motor patterns were monitored by recording respiratory air sac pressure before, during, and after song plasticity was induced by partial or complete reduction of phonation (i.e., “partial muting”). Birds were recorded until changes in air sac pressure patterns underlying the song structure were observed (up to 160 days). Young adult birds were more likely to insert shorter duration (<125 ms) expiratory pulses (EPs) into the motif than middle‐aged adults. These shorter duration EPs were produced with a unique pressure pattern relative to the intact song, and therefore appeared to be generated by novel motor gestures. Stuttering (atypical repetition of an EP) was observed when these novel EPs were inserted into the motif, regardless of age. The EP of the distance call, which is also a learned vocalization in zebra finches, showed a similar reduction in duration if EPs were also shortened in the song. The emergence of shorter duration EPs was not related to sound production, or nonspecific effects of the surgical procedure, which suggests an age‐dependent neural process for song plasticity. © 2004 Wiley Periodicals, Inc. J Neurobiol, 2004     

Beak gape dynamics during song in the zebra finch

Bird song is a complex communication behavior that requires the coordination of several motor systems. Sound is produced in the syrinx and then modified by the upper vocal tract, but the specific nature and dynamics of this modification are not well understood. To determine the contribution of beak movements to sound modification, we studied the beak gape patterns in zebra finches (Taeniopygia guttata). Subsyringeal air sac pressure and song were recorded together with changes in beak gape, which were monitored with a magneto-sensitive transducer. Beak gape was positively correlated with fundamental frequency, peak frequency, and subsyringeal air sac pressure in all but one bird. For harmonic stacks, peak frequency increased with increasing beak gape, and the relationship between fundamental frequency and beak gape was no longer significant. Experimentally holding the beak open or closed had acoustic consequences consistent with the model in which beak movements change upper vocal tract length and, thus, the filter properties. Beak gape was positively correlated with sound amplitude in all but two birds. The relationship between beak aperture and amplitude may, however, be indirect because air sac pressure is correlated with amplitude and beak gape. The beak is opened quickly and to its widest aperture immediately prior to the onset of sound and at rapid transitions in sound, suggesting that beak movements may affect vibratory behavior of the labia.     

Seasonal activation and inactivation of song motor memories in wild canaries is not reflected in neuroanatomical changes of forebrain song areas

Seasonal, testosterone-dependent changes in sexual behaviors are common in male vertebrates. In songbirds such seasonal changes occur in a learned behavior--singing. Domesticated male canaries (Serinus canaria) appear to lose song units (syllables) after the breeding season and learn new ones until the next breeding season. Here we demonstrate in a longitudinal field study of individual, free-living nondomesticated (wild) canaries (S. canaria) a different mode of seasonal behavioral plasticity, seasonal activation, and inactivation of auditory-motor memories. The song repertoire composition of wild canaries changes seasonally: about 25% of the syllables are sung seasonally; the remainder occur year-round, despite seasonal changes in the temporal patterns of song. In the breeding season, males sing an increased number of fast frequency-modulated syllables, which are sexually attractive for females, in correlation with seasonally increased testosterone levels. About 50% of the syllables that were lost after one breeding season reappear in the following breeding season. Furthermore, some identical syllable sequences are reactivated on an annual basis. The seasonal plasticity in vocal behavior occurred despite the gross anatomical and ultrastructural stability of the forebrain song control areas HVc and RA that are involved in syllable motor control.     

Airflow and pressure during canary song: direct evidence for mini-breaths

Summary Male canaries (Serinus canaria) produce songs of long duration compared to the normal respiratory cycle. Each phrase in a song contains repetitions of a particular song syllable, with repetition rates for different syllables ranging from 3 to 35 notes/s. We measured tracheal airflow and air sac pressure in order to investigate respiratory dynamics during song.Song syllables (11–280 ms) are always accompanied by expiratory tracheal airflow. The silent intervals (15–90 ms) between successive syllables are accompanied by inspiration, except for a few phrases where airflow ceases instead of reversing. Thus, the mini-breath respiratory pattern is used most often by the five birds studied and pulsatile expiration is used only occasionally.Songs and phrases accompanied by minibreaths were of longer duration than those accompanied by pulsatile expiration, presumably because the animal's finite vital capacity is not a limiting factor when the volume of air expired for one note is replaced by inspiration prior to the next. Pulsatile expiration was used for only a few syllable types from one bird that were produced at higher repetition rates than syllables accompanied by mini-breaths. We suggest that male canaries switch to pulsatile expiration only when the syllable repetition rate is too high (greater than about 30 Hz) for them to achieve mini-breaths.Changes in syringeal configuration that may accompany song are discussed, based on the assumption that changes in the ratio of subsyringeal (air sac) pressure to tracheal flow rate reflect changes in syringeal resistance.     

Sensitive period for sensorimotor integration during vocal motor learning

Sensory experience during sensitive periods in development may direct the organization of neural substrates, thereby permanently influencing subsequent adult behavior. We report a sensitive period during the imitative motor learning phase of sensorimotor integration in birdsong development. By temporarily and reversibly blocking efference to the vocal muscles, we disrupted vocal motor practice during selected stages of song development. Motor disruption during prolonged periods early in development, which allows recovery of vocal control prior to the onset of adult song, has no effect on adult song production. However, song disruption late in development, during the emergence of adult song, results in permanent motor defects in adult song production. These results reveal a decreased ability to compensate for interference with motor function when disturbances occur during the terminal stage of vocal motor development. Temporary disruption of syringeal motor control in adults does not produce permanent changes in song production. Permanent vocal aberrations in juveniles are evident exclusively in learned song elements rather than nonlearned calls, suggesting that the sensitive period is associated with motor learning.     

Lesions targeted to the anterior forebrain disrupt vocal variability associated with testosterone‐induced sensorimotor song development in adult female canaries,Serinus canaria

Learned communication was a trait observed in a limited number of vertebrates such as humans but also songbirds (i.e., species in the suborder passeri sometimes called oscines). Robust male‐biased sex‐differences in song development and production have been observed in several songbird species. However, in some of these species treating adult females with testosterone (T) induced neuro‐behavioral changes such that females become more male‐like in brain and behavior. T‐treatment in these adult females seemed to stimulate sensorimotor song development to facilitate song masculinization. In male songbirds it was known that the lateral magnocellular nucleus of the anterior nidopallium (LMAN) played a modulatory role during song development. LMAN was androgen sensitive and may be a key target of a T‐induced recapitulation of a developmental process in adult females. This hypothesis was tested. Adult female canaries were given either a chemical lesion of LMAN or a control sham‐surgery. Prior to surgery birds were individually housed for 2‐weeks in sound‐attenuated chambers to record baseline vocal behavior. Post‐surgery birds were given 1‐week to recover before subcutaneous implantation with silastic capsules filled with crystalline‐T. Birds remained on treatment for 3‐weeks (behavioral recordings continued throughout). Birds with a lesion to LMAN had less variability in their song compared with controls. The diversity of syllable and phrase type(s) was greater in sham controls as compared with birds with LMAN lesions. Birds did not differ in song rate. These data suggested that the sustention and conclusion of T‐induced sensorimotor song development in adult female canaries required an intact LMAN. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 3–18, 2016     

Peripheral control and lateralization of birdsong

Recent studies on several species of oscine songbirds show that they achieve their varied vocal performances through coordinated activity of respiratory, syringeal, and other vocal tract muscles in ways that take maximum advantage of the acoustic flexibility made possible by the presence of two independently controlled sound sources in their bipartite syrinx (vocal organ). During song, special motor programs to respiratory muscles alter the pattern of ventilation to maintain the supply of respiratory air and oxygen to permit songs of long duration, high syllable repetition rates, or maximum spectral complexity. Each side of the syrinx receives its own motor program that, together with that sent to respiratory muscles, determines the acoustic properties of the ipsilaterally produced sound. The acoustic expression of these bilaterally distinct, phonetic motor patterns depends on the action of dorsal syringeal adductor muscles that, by opening or closing the ipsilateral side of the syrinx to airflow, determine the amount each side contributes to song. The syringeally generated sound is further modified by muscles that control the shape of the vocal tract. Different species have adopted different motor strategies that use the left and right sides of the syrinx in patterns of unilateral, bilateral, alternating, or sequential phonation to achieve the differing temporal and spectral characteristics of their songs. As a result, the degree of song lateralization probably varies between species to form a continuum from unilateral dominance to bilateral equality. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 632–652, 1997     

Acute injections of brain-derived neurotrophic factor in a vocal premotor nucleus reversibly disrupt adult birdsong stability and trigger syllable deletion

Behavioral variability serves an essential role in motor learning by enabling sensory feedback to select those motor patterns that minimize error. Birds use auditory feedback to learn how to sing, and their songs lose variability and become highly stereotyped, or crystallized, at the end of a sensitive period for sensorimotor learning. The molecular cues that regulate song variability are not well understood. In other systems, neurotrophins, and brain-derived neurotrophic factor (BDNF) in particular, can mediate various forms of neural plasticity, including sensitive period neural circuit plasticity and activity-dependent synapse formation, and may also influence learning and memory. Here, we have tested the hypothesis that neurotrophin expression in the robust nucleus of the arcopallium (RA), the telencephalic output controlling song, regulates song variability. BDNF and its receptor trkB are expressed in RA, and BDNF expression in RA appears to be highest in juveniles, when song is most variable and plastic, and synapse density highest. Thus, song variability and synaptic connectivity could be enhanced by augmented expression of BDNF in RA. In support of this idea, we found that BDNF injections into the adult RA induced the re-expression of juvenile-like phenotypes, including song variability and an increased synaptic density in RA. Furthermore, BDNF treatment also induced vocal plasticity, characterized by syllable deletions and persistent changes to the song patterns. These results suggest that endogenous BDNF could be a molecular regulator of the song variability essential to vocal plasticity and, ultimately, to song learning.     

Lesions of a telencephalic nucleus in male zebra finches: Influences on vocal behavior in juveniles and adults

Male zebra finches learn to sing during a restricted phase of juvenile development. Song learning is characterized by the progressive modification of unstable song vocalizations by juvenile birds during development, a process that leads to the production of stereotyped vocal patterns as birds reach adulthood. The medial magnocellular nucleus of the anterior neostriatum (mMAN) is a small cortical region that has been implicated in song behavior based on its neuronal projection to the High Vocal Center (HVC), a nucleus that is critical for adult vocal production and presumably also plays a role in song learning. To assess the function of mMAN in song, ibotenic acid lesions of this brain region were made in juvenile male zebra finches during the period of vocal learning (40-50 days of age) and in adult males that were producing stable song (>90 days of age). Birds lesioned as juveniles produced highly abnormal, poor quality song as adults. Although the overall song quality of birds lesioned as adults was not highly disrupted or abnormal, the postoperative song behavior of these birds was discernibly different due to slight increases in variability of vocal production, particularly at the onset of singing. These results demonstrate that mMAN plays some important role in vocal production during the sensitive period for song learning, and is also important for consistent initiation and stereotyped production of adult song behavior.     

Acute injections of brain‐derived neurotrophic factor in a vocal premotor nucleus reversibly disrupt adult birdsong stability and trigger syllable deletion

Behavioral variability serves an essential role in motor learning by enabling sensory feedback to select those motor patterns that minimize error. Birds use auditory feedback to learn how to sing, and their songs lose variability and become highly stereotyped, or crystallized, at the end of a sensitive period for sensorimotor learning. The molecular cues that regulate song variability are not well understood. In other systems, neurotrophins, and brain‐derived neurotrophic factor (BDNF) in particular, can mediate various forms of neural plasticity, including sensitive period neural circuit plasticity and activity‐dependent synapse formation, and may also influence learning and memory. Here, we have tested the hypothesis that neurotrophin expression in the robust nucleus of the arcopallium (RA), the telencephalic output controlling song, regulates song variability. BDNF and its receptor trkB are expressed in RA, and BDNF expression in RA appears to be highest in juveniles, when song is most variable and plastic, and synapse density highest. Thus, song variability and synaptic connectivity could be enhanced by augmented expression of BDNF in RA. In support of this idea, we found that BDNF injections into the adult RA induced the re‐expression of juvenile‐like phenotypes, including song variability and an increased synaptic density in RA. Furthermore, BDNF treatment also induced vocal plasticity, characterized by syllable deletions and persistent changes to the song patterns. These results suggest that endogenous BDNF could be a molecular regulator of the song variability essential to vocal plasticity and, ultimately, to song learning. © 2004 Wiley Periodicals, Inc. J Neurobiol, 2005     

Sleep, learning, and birdsong

Neuroethological research combines approaches derived from animal behavior and neurobiology to examine the neuronal mechanisms of behavior, often in the context of laboratory experiments on species chosen for particular adaptations. Typically, these species are not traditional laboratory animals yet they contribute greatly to a broad, evolutionarily diverse view of nervous system function. The surprising role of sleep in the vocal learning process of songbirds is one such example, described here. Juvenile zebra finches show sleep-dependent daytime fluctuations in their patterns of singing starting after their first exposure to tutor songs. Nighttime bursting activity in the vocal control song system also changes after the onset of tutoring, with the neuronal changes preceding the changes in objective behavior (daytime singing). After tutoring, the nighttime bursting increases and exhibits structure that depends on the particular tutor song, and the nighttime expression of these changes requires normal auditory feedback during daytime singing. These observations shed light on the information carried in neuronal activity during sleep and on the adaptive plastic mechanisms engaged during sleep. They suggest a new hypothesis of sensorimotor learning, whereby sensory memories act indirectly on sensorimotor feedback by modifying networks through plastic changes at night. Sleep may also contribute to adult song maintenance, with nighttime neuronal replay conveying information about songs produced during the day and possibly mediating daily changes in the structure of premotor bursts. Collectively, these insights contribute a comparative perspective to theories of sleep and memory, which also help to inform a developing understanding of how humans acquire and retain memories.     

Motor stereotypy and diversity in songs of mimic thrushes

The relationship between the motor and acoustic similarity of song was examined in brown thrashers (Toxostoma rufum) and grey catbirds (Dumetella carolinensis) (family Mimidae), which have very large song repertoires and sometimes mimic other species. Motor similarity was assessed by cross correlation of syringeal airflows and air sac pressures that accompany sound production. Although most syllables were sung only once in the song analyzed, some were repeated, either immediately forming a couplet, or after a period of intervening song, as a distant repetition. Both couplets and distant repetitions are produced by distinctive, stereotyped motor patterns. Their motor similarity does not decrease as the time interval between repetitions increases, suggesting that repeated syllables are stored in memory as fixed motor programs. The acoustic similarity between nonrepeated syllables, as indicated by correlation of their spectrograms, has a significant positive correlation with their motor similarity. This correlation is weak, however, suggesting that there is no simple linear relationship between motor action and acoustic output and that similar sounds may sometimes be produced by different motor mechanisms. When compared without regard to the sequence in which they are sung, syllables paired for maximum spectral similarity form a continuum with repeated syllables in terms of their acoustic and motor similarity. The prominence of couplets in the “syntax” of normal song is enhanced by the dissimilarity of successive nonrepeated syllables that make up the remainder of the song. © 1996 John Wiley & Sons, Inc.     

Testosterone-induced changes in adult canary brain are reversible

Brain nuclei that control song are larger in male canaries, which sing, than in females, which sing rarely or not at all. Treatment of adult female canaries with testosterone (T) induces song production and causes song-control nuclei to grow, approaching the volumes observed in males. For example, the higher vocal center (HVC) of adult females approximately doubles in size by 1 month following the onset of T treatment. Male HVC projects to a second telencephalic nucleus, RA (the robust nucleus of the archistriatum), which projects in turn to the vocal motor neurons. Whether HVC makes a similar connection in female canaries is not known, although HVC and RA are not functionally connected in female zebra finches, a species in which testosterone does not induce neural or behavioral changes in the adult song system. This experiment investigated whether HVC makes an efferent projection to RA in normal adult female canaries, or if T is necessary to induce the growth of this connection. In addition, we examined whether T-induced changes in adult female canary brain are reversible. Adult female canaries received systemic T implants that were removed after 4 weeks; these birds were killed 4 weeks after T removal (Testosterone-Removal, T-R). Separate groups of control birds received either (a) T implants for 4 weeks which were not removed (Testosterone-Control, T-C) or (b) empty implants (Untreated Control, øO-C). Crystals of the fluorescent tracer DiI were placed in the song-control nucleus HVC in order to anterogradely label both efferent targets of HVC, RA and Area X. Projections from HVC to RA and Area X were present in all treatment groups including untreated controls, and did not appear to differ either qualitatively or quantitatively. Thus, formation of efferent connections from HVC may be prerequisite to hormone-induced expression of song behavior in adult songbirds. The volumes of RA and Area X were measured using the distribution of anterograde label as well as their appearance in Nissl-stained tissue. RA was larger in T-treated control birds than in untreated controls. Experimental birds in which T was given and then removed (T-R) had RA volumes closer in size to untreated controls (ø-C). Because the volume of RA in T-treated controls (T-C) was larger than that of birds that did not receive T (ø-C), we conclude that the volume of RA increased in both T-C and T-R birds but regressed upon removal of T in T-R birds. Surprisingly, the volume of Area X did not increase in T-treated birds. Birds in this study were maintained on short days, suggesting that T-induced growth of Area X reported previously may have resulted from an interaction between T and another seasonal or photoperiodic factor induced by exposure to long daylengths. © 1993 John Wiley & Sons, Inc.