Bird's Own Song Research Articles

Synaptic Interactions Underlying Song-Selectivity in the Avian Nucleus HVC Revealed by Dual Intracellular Recordings

Stimulus-dependent synaptic interactions underlying selective sensory representations in neural circuits specialized for sensory processing and sensorimotor integration remain poorly understood. The songbird telencephalic nucleus HVC is a sensorimotor area essential to learned vocal control with one projection neuron (PN) type (HVC(RA)) innervating a song premotor pathway, another PN (HVC(X)) innervating a basal ganglia pathway essential to vocal plasticity, and interneurons (HVC(Int)). Playback of the bird's own song (BOS), but not other songs, evokes action potential bursts from both PNs, but HVC(RA) and HVC(X) display distinct BOS-evoked subthreshold responses. To characterize synaptic interactions underlying HVC's BOS-selective responses and assess stimulus-evoked changes in functional interactions between HVC neurons, we made simultaneous in vivo intracellular recordings from various HVC neuron pairs in urethan-anesthetized zebra finches. Spike-triggered averaging revealed that all HVC neuron types receive common excitation and that the onset of this excitation occurs during a narrower time window in projection neurons during BOS playback. To distinguish local from extrinsic contributions to HVC subthreshold response patterns, we inactivated the HVC local circuit with GABA or occluded inhibition in single HVC(X) cells. After either treatment, BOS-evoked responses in HVC(X) neurons became purely depolarizing and subthreshold responses of HVC(X) and HVC(RA) cells became remarkably similar to one another while retaining BOS selectivity. Therefore both PN types receive a common extrinsic source of BOS-selective excitation, and local inhibition specifically alters processing of auditory information in HVC(X) cells. In HVC, excitatory and inhibitory synaptic interactions are recruited in a stimulus-dependent fashion, affecting auditory representations of the BOS locally and in other song nuclei important to song learning and production.

Selectivity of Canary HVC Neurons for the Bird's Own Song: Modulation by Photoperiodic Conditions

To what extent seasonal factors modify the neuronal functional properties within the nuclei of the avian song system remains an open question. In adult songbirds, neurons of the song premotor nucleus HVC (used as a proper name) exhibit selective responses for the bird's own song (BOS). Here we examine whether, outside the breeding season, when songs are less stereotyped, HVC neurons of male canaries still respond selectively to the BOS produced during this period. In an initial experiment, single-unit recordings (n = 114) revealed that the neuronal selectivity for the current BOS was attenuated in males exposed to a short-day photoperiod (typical of the nonbreeding season) compared with that found in males exposed to a long-day photoperiod. In long-day conditions, 35% of the cells responded to the BOS, whereas only 12% did in short-day conditions; there were four times more selective cells (d' > 1) in long-day than in short-day conditions. To determine whether these effects were the consequence of differences in acoustic features between breeding and nonbreeding songs, neurons (n = 72) recorded in short-day conditions were tested with both a short-day BOS and a long-day BOS. A low percentage of neurons exhibited responses to short-day or to long-day BOS (11% for each song). Responses of putative interneurons (spike duration < 0.4 ms) and of putative relay cells were similarly attenuated by the short-day conditions. These results strongly suggest that, in canary, rather than being a fixed property, the selectivity for the BOS moves along a continuum and peaks when the day length mimics the breeding conditions.

Neural auditory selectivity develops in parallel with song

The zebra finch learns his song by memorizing a tutor's vocalization and then using auditory feedback to match his current vocalization to this memory, or template. The neural song system of adult and young birds responds to auditory stimuli, and exhibits selective tuning to the bird's own song (BOS). We have directly examined the development of neural tuning in the song motor system. We measured song system responses to vocalizations produced at various ages during sleep. We now report that the auditory response of the song motor system and motor output are linked early in song development. During sleep, playback of the current BOS induced a response in the song nucleus HVC during the song practice period, even when the song consisted of little more than repeated begging calls. Halfway through the sensorimotor period when the song was not yet in its final form, the response to BOS already exceeded that to all other auditory stimuli tested. Moreover, responses to previous, plastic versions of BOS decayed over time. This indicates that selective tuning to BOS mirrors the vocalization that the bird is currently producing.

Experimental test of the birdsong error-correction model

Adult zebra finches require auditory feedback to maintain their songs. It has been proposed that the lateral magnocellular nucleus of the anterior nidopallium (LMAN) mediates song plasticity based on auditory feedback. In this model, neurons in LMAN, tuned to the spectral and temporal properties of the bird's own song (BOS), are thought to compute the difference between the auditory feedback from the bird's vocalizations and an internal song template. This error-correction signal is then used to initiate changes in the motor system that make future vocalizations a better match to the song template. This model was tested by recording from single LMAN neurons while manipulating the auditory feedback heard by singing birds. In contrast to the model predictions, LMAN spike patterns are insensitive to manipulations of auditory feedback. These results suggest that BOS tuning in LMAN is not used for error detection and constrain the nature of any error signal from LMAN to the motor system. Finally, LMAN neurons produce spikes locked precisely to the bird's song, independent of the auditory feedback heard by the bird. This finding suggests that a large portion of the input to this nucleus is from the motor control signals that generate the song rather than from auditory feedback.

Sequential Learning From Multiple Tutors and Serial Retuning of Auditory Neurons in a Brain Area Important to Birdsong Learning

Songbirds hear many vocal models during a juvenile sensitive period, transiently imitating some while retaining imitations of others in their repertoires. Despite subsequent conflicting experiences, early experience can exert lasting effects on neural structure and function, raising the possibility that transiently expressed vocalizations or their relevant models are stored in the adult songbird's brain. One site where learned song representations could be stored is the lateral magnocellular nucleus of the anterior nidopallium (LMAN), which in the adult songbird contains neurons responsive to playback of the bird's own song (BOS) and the tutor song (TS). To test whether LMAN neurons develop and retain responses to transiently learned songs, we exposed zebra finch hatchlings [posthatch day 0 (PHD0)] to a first TS (TS1) for about 30 days, isolated them for about 30 days, then exposed them to a second TS (TS2) for 30 days starting at PHD 60. Behavioral analysis showed that PHD 60 juveniles had started to copy TS1, although this copying was transient, because the adult BOS resembled TS2 and not TS1. We found that LMAN auditory responses paralleled these behavioral changes: LMAN neurons at PHD 60 responded strongly and selectively to both the juvenile BOS and TS1, whereas LMAN neurons in adults responded to the adult BOS and TS2, but not to the transiently learned song or its model. Therefore LMAN auditory responses can be lost or overwritten as the juvenile copies a new song, suggesting that the adult LMAN does not store information about transiently learned songs or their models.

Cellular, circuit, and synaptic mechanisms in song learning.

Songbirds, much like humans, learn their vocal behavior, and must be able to hear both themselves and others to do so. Studies of the brain areas involved in singing and song learning could reveal the underlying neural mechanisms. Here we describe experiments that explore the properties of the songbird anterior forebrain pathway (AFP), a basal ganglia-forebrain circuit known to be critical for song learning and for adult modification of vocal output. First, neural recordings in anesthetized, juvenile birds show that auditory AFP neurons become selectively responsive to the song stimuli that are compared during sensorimotor learning. Individual AFP neurons develop tuning to the bird's own song (BOS), and in many cases to the tutor song as well, even when these stimuli are manipulated to be very different from each other. Such dual selectivity could be useful in the BOS-tutor song comparison critical to song learning. Second, simultaneous neural recordings from the AFP and its target nucleus in the song motor pathway in anesthetized adult birds reveal correlated activity that is preserved through multiple steps of the circuits for song, including the AFP. This suggests that the AFP contains highly functionally interconnected neurons, an architecture that can preserve information about the timing of firing of groups of neurons. Finally, in vitro studies show that recurrent synapses between neurons in the AFP outflow nucleus, which are expected to contribute importantly to AFP correlation, can undergo activity-dependent and timing-sensitive strengthening. This synaptic enhancement appears to be restricted to birds in the sensory critical and early sensorimotor phases of learning. Together, these studies show that the AFP contains cells that reflect learning of both BOS and tutor song, as well as developmentally regulated synaptic and circuit mechanisms well-suited to create temporally organized assemblies of such cells. Such experience-dependent sensorimotor assemblies are likely to be critical to the AFP's role in song learning. Moreover, studies of such mechanisms in this basal ganglia circuit specialized for song may shed light more generally on how basal ganglia circuits function in guiding motor learning using sensory feedback signals.

Auditory responses in multiple sensorimotor song system nuclei are co-modulated by behavioral state.

Auditory responsiveness in nucleus HVC, a high-order sensorimotor area of the avian song system, is modulated by changes in behavioral state. Modulation is not observed in the primary thalamo-recipient auditory area Field L, the indirect source of auditory input to HVC. In this study, we show that auditory responsiveness in nucleus interfacialis (NIf), the immediate auditory afferent to HVC, is modulated by behavioral state. While auditory responsiveness is generally greater in NIf during wakefulness and in HVC during sedation, simultaneous recordings reveal a co-variation of auditory response magnitude. This co-variation is observed both in awake birds, where responses are spontaneously variable, and in sedated birds during manipulations of arousal levels. Auditory responses in NIf and HVC, which are selective for the bird's own song (BOS) during sedation, become predominantly unselective during wakefulness. This loss of selectivity is accompanied by a decrease in the similarity of NIf and HVC response patterns. To explore the role of NIf in shaping HVC auditory responses, we pharmacologically manipulated NIf while recording in HVC. Injection of the GABA(A) agonist muscimol into NIf eliminated most spontaneous activity and all auditory responses in the ipsilateral HVC, while injections of the GABA(A) antagonist bicuculline increased HVC auditory responsiveness and selectivity. These findings indicate that HVC is not the initial site of behavioral state-dependent modulation in the song system. Together with the suppression of HVC auditory responses by muscimol in NIf, these results suggest that NIf plays an important role in the flow of auditory information to HVC.

Inhibitory and Excitatory Mechanisms Underlying Auditory Responses to Learned Vocalizations in the Songbird Nucleus HVC

Speech and birdsong require auditory feedback for their development and maintenance, necessitating precise auditory encoding of vocal sounds. In songbirds, the telencephalic song premotor nucleus HVC contains neurons that respond highly selectively to the bird's own song (BOS), a property distinguishing HVC from its auditory afferents. We examined the contribution of inhibitory and excitatory synaptic inputs to BOS-evoked firing in those HVC neurons innervating a pathway essential for audition-dependent vocal plasticity. Using in vivo intracellular techniques, we found that G protein-coupled, potassium-mediated inhibition, tuned to the BOS, interacts with BOS-tuned excitation through several mechanisms to shape neuronal firing patterns. Furthermore, in the absence of this inhibition, the response bias to the BOS increases, reminiscent of cancellation mechanisms in other sensorimotor systems.

Song selectivity and sensorimotor signals in vocal learning and production.

Bird song, like human speech, is a learned vocal behavior that requires auditory feedback. Both as juveniles, while they learn to sing, and as adults, songbirds use auditory feedback to compare their own vocalizations with an internal model of a target song. Here we describe experiments that explore a role for the songbird anterior forebrain pathway (AFP), a basal ganglia-forebrain circuit, in evaluating song feedback and modifying vocal output. First, neural recordings in anesthetized, juvenile birds show that single AFP neurons are specialized to process the song stimuli that are compared during sensorimotor learning. AFP neurons are tuned to both the bird's own song and the tutor song, even when these stimuli are manipulated to be very different from each other. Second, behavioral experiments in adult birds demonstrate that lesions to the AFP block the deterioration of song that normally follows deafening. This observation suggests that deafening results in an instructive signal, indicating a mismatch between feedback and the internal song model, and that the AFP is involved in generating or transmitting this instructive signal. Finally, neural recordings from behaving birds reveal robust singing-related activity in the AFP. This activity is likely to originate from premotor areas and could be modulated by auditory feedback of the bird's own voice. One possibility is that this activity represents an efference copy, predicting the sensory consequences of motor commands. Overall, these studies illustrate that sensory and motor processes are highly interrelated in this circuit devoted to vocal learning, as is true for brain areas involved in speech.

Different Subthreshold Mechanisms Underlie Song Selectivity in Identified HVc Neurons of the Zebra Finch

Songbirds learn and maintain their songs via auditory experience. Neurons in many telencephalic nuclei important to song production and development are song selective, firing more to forward auditory playback of the bird's own song (BOS) than to reverse BOS or conspecific songs. Elucidating circuits that generate these responses can localize where auditory experience influences vocalization, bridging cellular and systems analyses of song learning. Song-selective responses in many song nuclei, including the vocal premotor nucleus robustus archistriatalis (RA) and the basal ganglia homolog area X, are thought to originate in nucleus HVc (used as a proper name), which contains interneurons and relay cells that innervate either RA or area X. Previous studies indicated that only X-projecting neurons have auditory responses, leaving open the source of RA's auditory input and the degree to which song selectivity may be refined in HVc. Here, in vivo intracellular recordings from morphologically and electrophysiologically identified HVc neurons revealed that both relay cell types fire song-selectively. However, their firing arises via markedly different subthreshold processes, and only X-projecting neurons appear to be sites for auditory refinement. RA-projecting neurons exhibited purely depolarizing subthreshold responses that were highly song selective and that were excitatory. In contrast, subthreshold responses of X-projecting neurons included less-selective depolarizing and highly selective hyperpolarizing components. Within individual birds, these BOS-evoked hyperpolarizations closely matched interneuronal firing, suggesting that HVc interneurons make restricted inputs onto X-projecting neurons. Because of the two relay cell types' subthreshold differences, factors affecting their resting membrane potentials could enable them to transmit distinct song representations to their targets.

Compromised Neural Selectivity for Song in Birds with Impaired Sensorimotor Learning

Anterior forebrain (AF) neurons become selective for song as songbirds learn to produce a copy of a memorized tutor song. We report that development of selectivity is compromised when birds are prevented from matching their output to the tutor song. Finches with denervated vocal organs developed stable song, but it usually did not resemble the tutor song. In those birds, numerous neurons in Area X responded selectively to both tutor and bird's own song (BOS), indicating the importance of both in shaping AF responses. The degree of selectivity for BOS was less, however, than that of normal adults. In contrast, neurons in denervated birds that successfully mimicked tutor song exhibited normal adult selectivity for BOS. Thus, during sensorimotor learning, selectivity for complex stimuli may be influenced by how well motor output matches internal sensory models.

Song‐ and order‐selective neurons develop in the songbird anterior forebrain during vocal learning

The anterior forebrain (AF) pathway of songbirds has an essential but poorly understood function during song learning, a process requiring auditory experience. Consistent with a role in processing auditory information, two nuclei of the AF, the lateral magnocellular nucleus of the anterior neostriatum (lMAN) and Area X (X), contain some of the most complex auditory neurons known. In adult zebra finches, these neurons are strongly selective for both spectral and temporal properties of song: They respond more robustly to the bird's own song (BOS) than to songs of conspecific individuals, and they respond less well to BOS if it is played in reverse. lMAN and X neurons of young finches early in the process of song learning (30–45 days of age) are also song responsive, but lack the song and order selectivity present in adult birds. By an intermediate stage of learning (60 days), when birds have experience of both tutor song and their own developing (plastic) song, AF neurons have significant song and order selectivity for both tutor song and BOS (in this case, plastic song). The degree of BOS selectivity is still less than that found in adults, however. In addition, neurons at 60 days are heterogenous in their preference for BOS versus tutor song: Most prefer BOS, some prefer tutor song, and others respond equally to both songs. The selectivity of adult AF auditory neurons therefore arises rapidly during development from neurons that are initially unselective. These neurons are one of the clearest examples of experience-dependent acquisition of complex stimulus selectivity. Moreover, the neural selectivity for both BOS and tutor song at 60 days raises the possibility that experience of both songs during learning contributes to the properties of individual AF neurons. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 694–709, 1997

Song- and order-selective neurons develop in the songbird anterior forebrain during vocal learning

The anterior forebrain (AF) pathway of songbirds has an essential but poorly understood function during song learning, a process requiring auditory experience. Consistent with a role in processing auditory information, two nuclei of the AF, the lateral magnocellular nucleus of the anterior neostriatum (IMAN) and Area X (X), contain some of the most complex auditory neurons known. In adult zebra finches, these neurons are strongly selective for both spectral and temporal properties of song: They respond more robustly to the bird's own song (BOS) than to songs of conspecific individuals, and they respond less well to BOS if it is played in reverse. IMAN and X neurons of young finches early in the process of song learning (30-45 days of age) are also song responsive, but lack the song and order selectivity present in adult birds. By an intermediate stage of learning (60 days), when birds have experience of both tutor song and their own developing (plastic) song, AF neurons have significant song and order selectivity for both tutor song and BOS (in this case, plastic song). The degree of BOS selectivity is still less than that found in adults, however. In addition, neurons at 60 days are heterogenous in their preference for BOS versus tutor song: Most prefer BOS, some prefer tutor song, and others respond equally to both songs. The selectivity of adult AF auditory neurons therefore arises rapidly during development in neurons that are initially unselective. These neurons are one of the clearest examples of experience-dependent acquisition of complex stimulus selectivity. Moreover, the neural selectivity for both BOS and tutor song at 60 days raises the possibility that experience of both songs during learning contributes to the properties of individual AF neurons.

Functional organization of forebrain pathways for song production and perception.

This article reviews the organization of the forebrain nuclei of the avian song system. Particular emphasis is placed on recent physiologic recordings from awake behaving adult birds while they sing, call, and listen to broadcasts of acoustic stimuli. The neurons in the descending motor pathway (HVc and RA) are organized in a hierarchical arrangement of temporal units of song production, with HVc neurons representing syllables and RA neurons representing notes. The nuclei Uva and NIf, which are afferent to HVc, may help organize syllables into larger units of vocalization. HVc and RA are also active during production of all calls. The patterns of activity associated with calls differ between learned calls and those that are innately specified, and give insight into the interactions between the forebrain and midbrain during calling, as well as into the evolutionary origins of the song system. Neurons in Area X, the first part of the anterior forebrain pathway leading from HVc to RA, are also active during singing. Many HVc neurons are also auditory, exhibiting selectivity for learned acoustic parameters of the individual bird's own song (BOS). Similar auditory responses are also observed in RA and Area X in anesthetized birds. In contrast to HVc, however, auditory responses in RA are very weak or absent in awake birds under our experimental paradigm, but are uncovered when birds are anesthetized. Thus, the roles of both pathways beyond HVc in adult birds is under review. In particular, theories hypothesizing a role for the descending motor pathway (RA and below) in adult song perception do not appear to obtain. The data also suggest that the anterior forebrain pathway has a greater motor role than previously considered. We suggest that a major role of the anterior forebrain pathway is to resolve the timing mismatch between motor program readout and sensory feedback, thereby facilitating motor programming during birdsong learning. Pathways afferent to HVc may participate more in sensory acquisition and sensorimotor learning during song development than is commonly assumed.

Anterior Forebrain Neurons Develop Selectivity by an Intermediate Stage of Birdsong Learning

Auditory neurons of the anterior forebrain (AF) in adult zebra finches are highly selective for the bird's own song (BOS): they respond more to BOS than to songs of other zebra finches (conspecifics) and to BOS played in reverse. In contrast, juvenile AF neurons are not selective at 30 d of age, responding equally well to all song stimuli. Both BOS and tutor song experience are required by juveniles for normal song learning and may produce the selective properties of adult neurons. Because such selectivity could subserve song learning, it is important to determine when it arises. Birds were therefore studied at an intermediate stage of learning, after substantial experience of both tutor song and their own developing (plastic) song. Extracellular single neuron recordings in 60-d-old zebra finches revealed that AF neurons had significant song and order selectivity for both tutor song and BOS (the bird's plastic song). The degree of BOS selectivity was less than that found in adults, as indicated in part by 60 d neurons that were sensitive to the local order within syllables but not yet to the global order of syllables within a song. When responses to BOS and tutor song were compared, most neurons preferred BOS, some preferred tutor song, and others responded equally to both stimuli. The latter type of neuron was not simply immature, because many of these neurons responded significantly more to BOS and tutor song than to conspecific and reverse songs. The selectivity of AF neurons at 60 d is markedly different from the unselective properties of neurons at 30 d and may function in vocal learning at this stage. Moreover, the selectivity for both BOS and tutor song raises the possibility that both aspects of the birds' sensory experience during learning are reflected in properties of AF neurons.

Song‐selective auditory input to a forebrain vocal control nucleus in the zebra finch

Neurons in nuclei on the motor pathway for vocalizations in songbirds are known to respond to sound stimuli. The auditory responses in one such nucleus, robustus archistriatalis (RA), were characterized by making multi-unit recordings in awake and anesthetized adult male zebra finches and in birds that had received lesions of the input to RA from the lateral part of the magnocellular nucleus of the anterior neostriatum (LMAN) or the Higher Vocal Center (HVC). In awake birds, RA neurons have a high level of spontaneous activity and vigorous auditory responses to song stimuli. Significantly greater responses are seen to the bird's own song (BOS) than to BOS played in reverse (REV) or to the songs of conspecifics (CON). Under ketamine-xylazine anesthesia, spontaneous activity is reduced, response latency increases and responses to BOS, REV and CON are indistinguishable. Responses obtained under urethane anesthesia are similar to those seen in awake birds. Thus, the pattern and selectivity of auditory responses in RA depend on the animal's state. Auditory responses in RA are qualitatively unchanged following lesion of the input to RA from LMAN, indicating that this pathway is not required for the sensory processing that underlies the preference for BOS on the vocal production pathway. Our results show that an input other than that from LMAN must be primarily responsible for auditory responses in RA. The direct projection from HVC is the most likely pathway by which song selective auditory information arrives in RA, since lesioning HVC abolished auditory responses in RA.