Variable but not random: temporal pattern coding in a songbird brain area necessary for song modification.

Practice of a complex motor gesture involves motor exploration to attain a better match to target, but little is known about the neural code for such exploration. We examine spiking in a premotor area of the songbird brain critical for song modification and quantify correlations between spiking and time in the motor sequence. While isolated spikes code for time in song during performance of song to a female bird, extended strings of spiking and silence, particularly bursts, code for time in song during undirected (solo) singing, or "practice." Bursts code for particular times in song with more information than individual spikes, and this spike-spike synergy is significantly higher during undirected singing. The observed pattern information cannot be accounted for by a Poisson model with a matched time-varying rate, indicating that the precise timing of spikes in both bursts in undirected singing and isolated spikes in directed singing code for song with a temporal code. Temporal coding during practice supports the hypothesis that lateral magnocellular nucleus of the anterior nidopallium neurons actively guide song modification at local instances in time.NEW & NOTEWORTHY This paper shows that bursts of spikes in the songbird brain during practice carry information about the output motor pattern. The brain's code for song changes with social context, in performance versus practice. Synergistic combinations of spiking and silence code for time in the bird's song. This is one of the first uses of information theory to quantify neural information about a motor output. This activity may guide changes to the song.

Differential contributions of basal ganglia and thalamus to song initiation, tempo, and structure.

Basal ganglia-thalamocortical circuits are multistage loops critical to motor behavior, but the contributions of individual components to overall circuit function remain unclear. We addressed these issues in a songbird basal ganglia-thalamocortical circuit (the anterior forebrain pathway, AFP) specialized for singing and critical for vocal plasticity. The major known afferent to the AFP is the premotor cortical nucleus, HVC. Surprisingly, previous studies found that lesions of HVC alter song but do not eliminate the ability of the AFP to drive song production. We therefore used this AFP-driven song to investigate the role of basal ganglia and thalamus in vocal structure, tempo, and initiation. We found that lesions of the striatopallidal component (Area X) slowed song and simplified its acoustic structure. Elimination of the thalamic component (DLM) further simplified the acoustic structure of song and regularized its rhythm but also dramatically reduced song production. The acoustic structure changes imply that sequential stages of the AFP each add complexity to song, but the effects of DLM lesions on song initiation suggest that thalamus is a locus of additional inputs important to initiation. Together, our results highlight the cumulative contribution of stages of a basal ganglia-thalamocortical circuit to motor output along with distinct involvement of thalamus in song initiation or "gating."

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.

FOS is induced by singing in distinct neuronal populations in a motor network.

Mechanisms underlying the learned vocal behavior of songbirds were studied by examining expression of the protein product of the immediate early gene c-fos (Fos) in zebra finches. Auditory stimuli including the bird's own song did not induce Fos in the song system. In contrast, the motor act of singing induced Fos in two song sensorimotor nuclei, HVc and RA. This induction was independent of auditory feedback, since it occurred in deafened birds that sang. Double-labeling studies demonstrated that only one of the two sets of projection neurons in HVc expressed singing-related Fos. The motor-driven induction of Fos identifies functionally distinct cell populations in a network for singing and may point to sites of cellular plasticity necessary for song maintenance.

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.

A neural circuit specialized for vocal learning.

The anterior forebrain circuit of the songbird brain has been known for some time to play a special role in song learning. Recent work has strengthened this view and has begun to describe the specific properties of this pathway. The development of the circuit early during song learning, its auditory responsiveness, and its synaptic interaction with the vocal motor pathway all suggest that it is involved in the sensory learning and auditory-motor matching essential to normal song development. Behavioral studies point to a variety of mechanisms of action of this pathway and suggest that it is one site for steroid hormonal effects on vocal motor plasticity. Investigation of the anterior forebrain circuit promises to clarify its role in learning and to elucidate the cellular mechanisms involved.

Postlearning consolidation of birdsong: stabilizing effects of age and anterior forebrain lesions.

Birdsong is a learned, sequenced motor skill. For the zebra finch, learned song normally remains unchanging beyond early adulthood. However, stable adult song will gradually deteriorate after deafening (Nordeen and Nordeen, 1992), indicating an ongoing influence of auditory feedback on learned song. This plasticity of adult song in response to deafening gradually declines with age (Lombardino and Nottebohm, 2000), suggesting that, after song learning, there continue to be changes in the brain that progressively stabilize the song motor program. A qualitatively similar stabilization of learned song can be precipitated artificially by lesions of a basal ganglia circuit in the songbird anterior forebrain (Brainard and Doupe, 2000), raising the question of whether and how these two forms of song stabilization are related. We investigated this issue by characterizing the deterioration of song that occurs after deafening in young adult birds and the degree to which that deterioration is reduced by age or by lesions of the anterior forebrain that were directed at the lateral portion of the magnocellular nucleus of the anterior neostriatum (LMAN). In most respects, LMAN lesions stabilized song to a significantly greater extent than did aging; whereas old-deafened birds eventually exhibited significant deterioration of song, lesioned-deafened birds generally did not differ from controls. The one exception was for song tempo, which was significantly stabilized by age, but not by LMAN lesions. The results indicate that LMAN lesions do not simply mimic a normal aging process, and likewise suggest that the anterior forebrain pathway continues to play a role even in the residual song plasticity that is observed after the age-dependent stabilization of song.

Contributions of tutor and bird's own song experience to neural selectivity in the songbird anterior forebrain.

Auditory neurons of the anterior forebrain (AF) of zebra finches become selective for song during song learning. In adults, these neurons respond more to the bird's own song (BOS) than to the songs of other zebra finches (conspecifics) or BOS played in reverse. In contrast, AF neurons from young birds (30 d) respond equally well to all song stimuli. AF selectivity develops rapidly during song learning, appearing in 60-d-old birds. At this age, many neurons also respond equally well to BOS and tutor song. These similar neural responses to BOS and tutor song might reflect contributions from both song experiences to selectivity, because auditory experiences of both BOS and tutor song are essential for normal song learning. Alternatively, they may simply result from acoustic similarities between BOS and tutor song. Understanding which experience shapes selectivity could elucidate the function of song-selective AF neurons. To minimize acoustic similarity between BOS and tutor song, we induced juvenile birds to produce abnormal song by denervating the syrinx, the avian vocal organ, before song onset. We recorded single neurons extracellularly in the AF at 60 d, after birds had had substantial experience of both the abnormal BOS (tsBOS) and tutor song. Some neurons preferred the unique tsBOS over the tutor song, clearly indicating a role for BOS experience in shaping neural selectivity. In addition, a sizable proportion of neurons responded equally well to tsBOS and tutor song, despite their acoustic dissimilarity. These neurons were not simply immature, because they were selective for tsBOS and tutor song relative to conspecific and reverse song. Furthermore, their similar responses to tsBOS and tutor song could not be attributed to residual acoustic similarities between the two stimuli, as measured by several song analyses. The neural sensitivity to two very different songs suggests that single AF neurons may be shaped by both BOS and tutor song experience.

Spectral-temporal receptive fields of nonlinear auditory neurons obtained using natural sounds.

The stimulus-response function of many visual and auditory neurons has been described by a spatial-temporal receptive field (STRF), a linear model that for mathematical reasons has until recently been estimated with the reverse correlation method, using simple stimulus ensembles such as white noise. Such stimuli, however, often do not effectively activate high-level sensory neurons, which may be optimized to analyze natural sounds and images. We show that it is possible to overcome the simple-stimulus limitation and then use this approach to calculate the STRFs of avian auditory forebrain neurons from an ensemble of birdsongs. We find that in many cases the STRFs derived using natural sounds are strikingly different from the STRFs that we obtained using an ensemble of random tone pips. When we compare these two models by assessing their predictions of neural response to the actual data, we find that the STRFs obtained from natural sounds are superior. Our results show that the STRF model is an incomplete description of response properties of nonlinear auditory neurons, but that linear receptive fields are still useful models for understanding higher level sensory processing, as long as the STRFs are estimated from the responses to relevant complex stimuli.

Song- and order-selective neurons in the songbird anterior forebrain and their emergence during vocal development.

Auditory experience is critical for vocal learning in songbirds as in humans. Therefore, in a search for neural mechanisms for song learning and recognition, the auditory response properties of neurons in the anterior forebrain (AF) pathway of the songbird brain were investigated. This pathway plays an essential but poorly understood role during the period of song development when auditory feedback is most crucial. Single-unit recordings demonstrated that both the lateral magnocellular nucleus of the anterior neostriatum (LMAN) and Area X (X) contain auditory neurons in adult male 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 the BOS if it is played in reverse. In addition, X neurons are more broadly responsive than LMAN neurons, suggesting that responses to song become progressively more refined along this pathway. Both X and LMAN of young male finches early in the process of song learning (30-45 d old) also contain song-responsive auditory neurons, but these juvenile neurons lack the song and order selectivity present in adult birds. The spectral and temporal selectivity of the adult AF auditory neurons therefore arises during development in neurons that are initially broadly song-responsive. These neurons provide one of the clearest examples of experience-dependent acquisition of complex stimulus selectivity. Moreover, the auditory properties of the AF circuit suggest that one of its functions may be to mediate the auditory learning and feedback so essential to song development.

Interruption of a basal ganglia-forebrain circuit prevents plasticity of learned vocalizations.

Birdsong, like speech, is a learned vocal behaviour that relies greatly on hearing; in both songbirds and humans the removal of auditory feedback by deafening leads to a gradual deterioration of adult vocal production. Here we investigate the neural mechanisms that contribute to the processing of auditory feedback during the maintenance of song in adult zebra finches. We show that the deleterious effects on song production that normally follow deafening can be prevented by a second insult to the nervous system--the lesion of a basal ganglia-forebrain circuit. The results suggest that the removal of auditory feedback leads to the generation of an instructive signal that actively drives non-adaptive changes in song; they also suggest that this instructive signal is generated within (or conveyed through) the basal ganglia-forebrain pathway. Our findings provide evidence that cortical-basal ganglia circuits may participate in the evaluation of sensory feedback during calibration of motor performance, and demonstrate that damage to such circuits can have little effect on previously learned behaviour while conspicuously disrupting the capacity to adaptively modify that behaviour.

Singing-related neural activity in a dorsal forebrain-basal ganglia circuit of adult zebra finches.

The anterior forebrain pathway (AFP) of songbirds, a specialized dorsal forebrain-basal ganglia circuit, is crucial for song learning but has a less clear function in adults. We report here that neurons in two nuclei of the AFP, the lateral magnocellular nucleus of the anterior neostriatum (LMAN) and Area X, show marked changes in neurophysiological activity before and during singing in adult zebra finches. The presence of modulation before song output suggests that singing-related AFP activity originates, at least in part, in motor control nuclei. Some neurons in LMAN of awake birds also responded selectively to playback of the bird's own song, but neural activity during singing did not completely depend on auditory feedback in the short term, because neither the level nor the pattern of this activity was strongly affected by deafening. The singing-related activity of neurons in AFP nuclei of songbirds is consistent with a role of the AFP in adult singing or song maintenance, possibly related to the function of this circuit during initial song learning.

Song-selective auditory circuits in the vocal control system of the zebra finch.

Birdsong is a learned behavior controlled by a distinct set of brain nuclei. The song nuclei known as area X, the medial nucleus of the dorsolateral thalamus (DLM), and the lateral magnocellular nucleus of the anterior neostriatum (L-MAN) form a pathway that plays an important but unknown role in song learning. One function served by this circuit might be auditory feedback, which is critical to normal song development. We used single unit recordings to demonstrate that all three of these nuclei contain auditory neurons in adult male zebra finches (Taeniopygia guttata). These neurons are song selective: they respond more robustly to the bird's own song than to songs of conspecific individuals, and they are sensitive to the temporal structure of song. Auditory neurons so highly specialized for song within a pathway required for song learning may play a role in the auditory feedback essential in song development. Recordings in the robust nucleus of the archistriatum (RA), the nucleus to which L-MAN projects, showed that RA also contains highly song-selective neurons. RA receives a direct projection from the caudal nucleus of the ventral hyperstriatum (HVc) as well as from L-MAN. We investigated the contributions of these two inputs to auditory responses of RA neurons by selectively inactivating one or both inputs. Our results suggest that there is a song-selective pathway directly from HVc to RA in addition to the circuit via L-MAN. Thus the songbird brain contains multiple auditory pathways specialized for song, and these circuits may vary in their functional importance at different stages of learning.

Auditory feedback in learning and maintenance of vocal behaviour.

Songbirds are one of the best-studied examples of vocal learners. Learning of both human speech and birdsong depends on hearing. Once learned, adult song in many species remains unchanging, suggesting a reduced influence of sensory experience. Recent studies have revealed, however, that adult song is not always stable, extending our understanding of the mechanisms involved in song maintenance, and their similarity to those active during song learning. Here we review some of the processes that contribute to song learning and production, with an emphasis on the role of auditory feedback. We then consider some of the possible neural substrates involved in these processes, particularly basal ganglia circuitry. Although a thorough treatment of human speech is beyond the scope of this article, we point out similarities between speech and song learning, and ways in which studies of these disparate behaviours complement each other in developing an understanding of general principles that contribute to learning and maintenance of vocal behaviour.

Intrinsic and thalamic excitatory inputs onto songbird LMAN neurons differ in their pharmacological and temporal properties.

In passerine songbirds, the lateral portion of the magnocellular nucleus of the anterior neostriatum (LMAN) plays a vital role in song learning, possibly by encoding sensory information and providing sensory feedback to the vocal motor system. Consistent with this, LMAN neurons are auditory, and, as learning progresses, they evolve from a broadly tuned initial state to a state of strong preference for the bird's own song and acute sensitivity to the temporal order of this song. Moreover, normal synaptic activity in LMAN is required during sensory learning for accurate tutor song copying to occur (). To explore cellular and synaptic properties of LMAN that may contribute to this crucial stage of song acquisition, we developed an acute slice preparation of LMAN from zebra finches in the early stages of sensory learning (18-25 days posthatch). We used this preparation to examine intrinsic neuronal properties of LMAN neurons at this stage and to identify two independent excitatory inputs to these neurons and compare each input's pharmacology and short-term synaptic plasticity. LMAN neurons had immature passive membrane properties, well-developed spiking behavior, and received excitatory input from two sources: afferents from the medial portion of the dorsolateral thalamus (DLM), and recurrent axon collaterals from LMAN itself ("intrinsic" input). These two inputs differed in both their pharmacology and temporal properties. Both inputs were glutamatergic, but LMAN responses to intrinsic inputs exhibited a larger N-methyl--aspartate component than responses to DLM inputs. Both inputs elicited temporal summation in response to pairs of stimuli delivered at short intervals, but -2-amino-5-phosphonovalerate (APV) significantly reduced the temporal summation only of the responses to intrinsic inputs. Moreover, responses to DLM inputs showed consistent paired-pulse depression, whereas the responses to intrinsic inputs did not. The differences between these two inputs suggest that intrinsic circuitry plays an important role in transforming DLM input patterns into the appropriate LMAN output patterns, as has been suggested for mammalian thalamocortical networks. Moreover, in LMAN, such interactions may contribute to the profound temporal and spectral selectivity that these neurons will acquire during learning.

Temporal and spectral sensitivity of complex auditory neurons in the nucleus HVc of male zebra finches.

Complex vocalizations, such as human speech and birdsong, are characterized by their elaborate spectral and temporal structure. Because auditory neurons of the zebra finch forebrain nucleus HVc respond extremely selectively to a particular complex sound, the bird's own song (BOS), we analyzed the spectral and temporal requirements of these neurons by measuring their responses to systematically degraded versions of the BOS. These synthetic songs were based exclusively on the set of amplitude envelopes obtained from a decomposition of the original sound into frequency bands and preserved the acoustical structure present in the original song with varying degrees of spectral versus temporal resolution, which depended on the width of the frequency bands. Although both excessive temporal or spectral degradation eliminated responses, HVc neurons responded well to degraded synthetic songs with time-frequency resolutions of approximately 5 msec or 200 Hz. By comparing this neuronal time-frequency tuning with the time-frequency scales that best represented the acoustical structure in zebra finch song, we concluded that HVc neurons are more sensitive to temporal than to spectral cues. Furthermore, neuronal responses to synthetic songs were indistinguishable from those to the original BOS only when the amplitude envelopes of these songs were represented with 98% accuracy. That level of precision was equivalent to preserving the relative time-varying phase across frequency bands with resolutions finer than 2 msec. Spectral and temporal information are well known to be extracted by the peripheral auditory system, but this study demonstrates how precisely these cues must be preserved for the full response of high-level auditory neurons sensitive to learned vocalizations.

Birdsong and human speech: common themes and mechanisms.

Human speech and birdsong have numerous parallels. Both humans and songbirds learn their complex vocalizations early in life, exhibiting a strong dependence on hearing the adults they will imitate, as well as themselves as they practice, and a waning of this dependence as they mature. Innate predispositions for perceiving and learning the correct sounds exist in both groups, although more evidence of innate descriptions of species-specific signals exists in songbirds, where numerous species of vocal learners have been compared. Humans also share with songbirds an early phase of learning that is primarily perceptual, which then serves to guide later vocal production. Both humans and songbirds have evolved a complex hierarchy of specialized forebrain areas in which motor and auditory centers interact closely, and which control the lower vocal motor areas also found in nonlearners. In both these vocal learners, however, how auditory feedback of self is processed in these brain areas is surprisingly unclear. Finally, humans and songbirds have similar critical periods for vocal learning, with a much greater ability to learn early in life. In both groups, the capacity for late vocal learning may be decreased by the act of learning itself, as well as by biological factors such as the hormones of puberty. Although some features of birdsong and speech are clearly not analogous, such as the capacity of language for meaning, abstraction, and flexible associations, there are striking similarities in how sensory experience is internalized and used to shape vocal outputs, and how learning is enhanced during a critical period of development. Similar neural mechanisms may therefore be involved.

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.