Fibre architecture and song activation rates of syringeal muscles are not lateralized in the European starling.

The songbird vocal organ, the syrinx, is composed of two sound generators, which are independently controlled by sets of two extrinsic and four intrinsic muscles. These muscles rank among the fastest vertebrate muscles, but the molecular and morphological foundations of this rapid physiological performance are unknown. Here we show that the four intrinsic muscles in the syrinx of male European starlings (Sturnus vulgaris) are composed of fast oxidative and superfast fibres. Dorsal and ventral tracheobronchialis muscles contain slightly more superfast fibres relative to the number of fast oxidative fibres than dorsal and ventral syringealis muscles. This morphological difference is not reflected in the highest, burst-like activation rate of the two muscle groups during song as assessed with electromyographic recordings. No difference in fibre type ratio was found between the corresponding muscles of the left and right sound generators. Airflow and electromyographic measurements during song indicate that maximal activation rate and speed of airflow regulation do not differ between the two sound sources. Whereas the potential for high-speed muscular control exists on both sides, the two sound generators are used differentially for modulation of acoustic parameters. These results show that large numbers of superfast fibre types are present in intrinsic syringeal muscles of a songbird, providing further confirmation of rapid contraction kinetics. However, syringeal muscles are composed of two fibre types which raises questions about the neuromuscular control of this heterogeneous muscle architecture.

Low-dimensional dynamical model for the diversity of pressure patterns used in canary song.

During song production, oscine birds produce large air sac pressure pulses. During those pulses, energy is transferred to labia located at the juncture between the bronchii and the trachea, inducing the high frequency labial oscillations which are responsible for airflow modulations, i.e., the uttered sound. In order to generate diverse syllables, canaries (Serinus canaria) use a set of air sac pressure patterns with characteristic shapes. In this work we show that these different shapes can be approximated by the subharmonic solutions of a forced normal form. This simple model is built from identifying dynamical elements which allow to reproduce the shape of the pressure pattern corresponding to one syllable type. Remarkably, integrating that simple model for other parameters allows to recover the other pressure patterns used during song. Interpreting the diversity of these physiological gestures as subharmonic solutions of a simple nonlinear system allows us to account simultaneously for their morphological features as well as for the syllabic timing and suggests a strategy for the generation of complex motor patterns.

Dynamical origin of spectrally rich vocalizations in birdsong.

Birdsong is a model system for learned vocal behavior with remarkable parallels to human vocal development and sound production mechanisms. Upper vocal tract filtering plays an important role in human speech, and its importance has recently also been recognized in birdsong. However, the mechanisms of how the avian sound source might contribute to spectral richness are largely unknown. Here we show in the most widely studied songbird, the zebra finch (Taeniopygia guttata), that the broad range of upper harmonic content in different low-frequency song elements is the fingerprint of the dynamics displayed by its vocal apparatus, which can be captured by a two-dimensional dynamical model. As in human speech and singing, the varying harmonic content of birdsong is not only the result of vocal tract filtering but of a varying degree of tonality emerging from the sound source. The spectral content carries a strong signature of the intrinsic dynamics of the sound source.

Lateralization as a symmetry breaking process in bird song.

The singing by songbirds is a most convincing example in the animal kingdom of functional lateralization of the brain, a feature usually associated with human language. Lateralization is expressed as one or both of the bird's sound sources being active during the vocalization. Normal songs require high coordination between the vocal organ and respiratory activity, which is bilaterally symmetric. Moreover, the physical and neural substrate used to produce the song lack obvious asymmetries. In this work we show that complex spatiotemporal patterns of motor activity controlling airflow through the sound sources can be explained in terms of spontaneous symmetry breaking bifurcations. This analysis also provides a framework from which to study the effects of imperfections in the system's symmetries. A physical model of the avian vocal organ is used to generate synthetic sounds, which allows us to predict acoustical signatures of the song and compare the predictions of the model with experimental data.

Synthesizing bird song.

In this work we present an electronic syrinx: an analogical integrator of the equations describing a model for sound production by oscine birds. The model depends on time varying parameters with clear biological interpretation: the air sac pressure and the tension of ventral syringeal muscles. We test the hypothesis that these physiological parameters can be reconstructed from the song. In order to do so, we built two transducers. The input for these transducers is an acoustic signal. The first transducer generates an electric signal that we use to reconstruct the bronchial pressure. The second transducer allows us to reconstruct the syringeal tension (in both cases, for the time intervals where phonation takes place). By driving the electronic syrinx with the output of the transducers we generate synthetic song. Important qualitative features of the acoustic input signal are reproduced by the synthetic song. These devices are especially useful to carry out altered feedback experiences, and applications as biomimetic resources are discussed.

Experimental support for a model of birdsong production.

In this work we present an experimental validation of a recently proposed model for the production of birdsongs. We have previously observed that driving the model with simple functions of time, which represent tensions in vocal muscles, produces a wide variety of sounds resembling birdsongs. In this work we drive the model with functions whose time dependence comes from recordings of muscle activities and air sac pressure. We simultaneously recorded the birds' songs and compared them with the synthetic songs. The model produces recognizable songs. Beyond finding a qualitative agreement, we also test some predictions of the model concerning the relative levels of activity in the gating muscles at the beginning and end of a syllable.

New perspectives on mechanisms of sound generation in songbirds.

The physical mechanisms of sound generation in the vocal organ, the syrinx, of songbirds have been investigated mostly with indirect methods. Recent direct endoscopic observation identified vibrations of the labia as the principal sound source. This model suggests sound generation in a pulse-tone mechanism similar to human phonation with the labia forming a pneumatic valve. The classical avian model proposed that vibrations of the thin medial tympaniform membranes are the primary sound generating mechanism. As a direct test of these two hypotheses we ablated the medial tympaniform membranes in two species (cardinal and zebra finch) and found that both were still able to phonate and sing without functional membranes. Small changes in song structure (harmonic emphasis, frequency control) occurred after medial tympaniform membrane ablation and suggest that the medial tympaniform membranes play a role in adjusting tension on the labia. Such a role is consistent with the fact that the medial tympaniform membranes are directly attached to the medial labia. There is no experimental support for a third hypothesis, proposing an aerodynamic model for generation of tonal sounds. Indirect tests (song in heliox atmosphere) as well as direct (labial vibration during tonal sound) measurements of syringeal vibrations support a vibration-based sound-generating mechanism even for tonal sounds.

Novel motor gestures for phonation during inspiration enhance the acoustic complexity of birdsong.

Sound generation based on a pulmonary mechanism typically occurs during the expiratory phase of respiration. Phonation during inspiration has been postulated for the calls of some amphibians and for exceptional sounds in some human languages. No direct evidence exists for phonation during inspiration in birds, but such a mechanism has been proposed to explain very long uninterrupted songs. Here, we report the first physiological evidence for inspiratory sound production in the song of the zebra finch (Taeniopygia guttata). Motor gestures of the vocal and respiratory muscles leading to the production of inspiratory phonation differ from those of silent inspirations during song as well as from those leading to phonation during expiration. Inspiratory syllables have a high fundamental frequency, which makes them acoustically distinct from all other zebra finch song syllables. Furthermore, young zebra finches copy these inspiratory syllables from their tutor song, producing them during inspiration. This suggests that physical limitations confine the production of these sounds to the inspiratory phase in zebra finches. These findings directly demonstrate how novel respiratory-vocal coordination can enhance the acoustic structure of birdsong, and thus provide insight into the evolution of song complexity.

The metabolic cost of birdsong production.

The metabolic cost of birdsong production has not been studied in detail but is of importance in our understanding of how selective pressures shape song behavior. We measured rates of oxygen consumption during song in three songbird species, zebra finches (Taeniopygia guttata), Waterslager canaries (Serinus canaria) and European starlings (Sturnus vulgaris). These species sing songs with different acoustic and temporal characteristics: short stereotyped song (zebra finch), long song with high temporal complexity (canary) and long song with high acoustic, but low temporal, complexity (starling). In all three species, song slightly increased the rate of oxygen consumption over pre-song levels (1.02-1.36-fold). In zebra finches, the metabolic cost per song motif averaged 1.2 microl g(-1). This cost per motif did not change over the range of song duration measured for the four individuals. Surprisingly, the metabolic cost of song production in the species with the temporally most complex song, the canary, was no greater than in the other two species. In starlings, a 16 dB increase in sound intensity was accompanied by a 1.16-fold increase in the rate of oxygen consumption. These data indicate that the metabolic cost of song production in the songbird species studied is no higher than that for other types of vocal behavior in various bird groups. Our analysis shows that the metabolic cost of singing is also similar to that of calling in frogs and of human speech production. However, difficulties with measurements on freely behaving birds in a small respirometry chamber limit the depth of analysis that is possible.

Bilaterally symmetrical respiratory activity during lateralized birdsong.

We investigated whether activity of expiratory muscles reflects lateralized activity of the vocal organ during production of birdsong. Respiration and syringeal motor activity were assessed in brown thrashers by monitoring bilateral airflow and subsyringeal air sac pressure, together with the electromyographic activity of expiratory abdominal muscles and vocal output. Activity of expiratory muscles was always present on both sides, regardless of whether song was produced bilaterally or on only one side of the syrinx. The average amplitude of expiratory EMG of one side does not change significantly, even if that side is silent during phonation. The temporal pattern of the electromyogram (EMG) was similar on both sides. Bilateral bursts of EMG activity on both sides accompanied changes in the rate of syringeal airflow, even when these flow fluctuations were generated only by one side of the syrinx. Motor commands to the respiratory muscles therefore appear to be bilaterally distributed, in contrast to the lateralized motor control of the syrinx.

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.

Inspiratory muscle activity during bird song.

The apparently continuous flow of bird song is in reality punctuated by brief periods of silence during which there are short inspirations called minibreaths. To determine whether these minibreaths are accompanied, and thus perhaps caused, by activity in inspiratory muscles, electromyographic (EMG) activity was recorded in M. scalenus in zebra finches and in M. scalenus and Mm. levatores costarum in cowbirds, together with EMGs from the abdominal expiratory muscles, air sac pressure and tracheal airflow. EMG activity in Mm. scalenus and levatores costarum consistently preceded the onset of negative air sac pressure by approximately 11 ms during both quiet respiration and singing in both species. The electrical activity of these two muscles was very similar. Compared with during quiet respiration, the amplitude of inspiratory muscle EMG during singing was increased between five- and 12-fold and its duration was decreased from >200 ms to on average 41 ms during minibreaths, again for both species, but inspiratory muscle activity did not overlap with that of the expiratory muscles. Thus, there was no indication that the inspiratory muscles acted either to shorten the duration of expiration or to reduce the expiratory effort as might occur if both expiratory and inspiratory muscles were simultaneously active. Inspiratory and expiratory muscle activities were highly stereotyped during song to the extent that together, they defined the temporal pattern of the songs and song types of individual birds.

In situ biomechanics of the syrinx and sound generation in pigeons.

The in situ biomechanics of the vocal organ, the syrinx, was studied in anesthetized pigeons using fiberoptic instruments. The role of syringeal muscles was determined by electrical stimulation, and phonation was induced by injecting gas into the subsyringeal air sacs. This study presents the first direct observations of the biomechanical processes that occur in an intact syrinx. Contraction of one of the syringeal muscles, the m. tracheolateralis (TL), withdraws the lateral tympaniform membranes (LTM) from the syringeal lumen, causing opening of the syringeal airways. Shortening of a second muscle, the sternotrachealis (ST), draws the syringeal cartilages closer to each other, causing the LTM to fold into the syringeal lumen. Maximal ST contraction does not lead to complete closure of the syrinx. As air-sac pressure is increased by the injection of gas, the LTM are drawn into the syringeal lumen and balloon in a rostral direction until they touch, thus forming a fold-like valve. Air-induced phonation is always associated with vibrations of the membrane folds, suggesting that pulsatile release of air into the trachea by vibratory motion of the LTM generates sound. During air-induced phonation, strong stimulation of the TL terminates sound generation by abducting the LTM, whereas weak stimulation changes the geometry of the membrane folds, which is accompanied by changes in the acoustic structure of the sound. Stimulation of the ST has little effect on air-induced sounds. The LTM appear to be the main sound generators, since disabling the medial tympaniform membranes (MTM) with tissue adhesive does not prevent phonation or change the frequency and amplitude structure of display coos in spontaneously vocalizing pigeons. Moreover, the activity of the syringeal muscles appears to have a mainly modulatory function, suggesting that the basic sound-generating mechanism is similar in both air-induced and natural phonation.

A new mechanism of sound generation in songbirds.

Our current understanding of the sound-generating mechanism in the songbird vocal organ, the syrinx, is based on indirect evidence and theoretical treatments. The classical avian model of sound production postulates that the medial tympaniform membranes (MTM) are the principal sound generators. We tested the role of the MTM in sound generation and studied the songbird syrinx more directly by filming it endoscopically. After we surgically incapacitated the MTM as a vibratory source, zebra finches and cardinals were not only able to vocalize, but sang nearly normal song. This result shows clearly that the MTM are not the principal sound source. The endoscopic images of the intact songbird syrinx during spontaneous and brain stimulation-induced vocalizations illustrate the dynamics of syringeal reconfiguration before phonation and suggest a different model for sound production. Phonation is initiated by rostrad movement and stretching of the syrinx. At the same time, the syrinx is closed through movement of two soft tissue masses, the medial and lateral labia, into the bronchial lumen. Sound production always is accompanied by vibratory motions of both labia, indicating that these vibrations may be the sound source. However, because of the low temporal resolution of the imaging system, the frequency and phase of labial vibrations could not be assessed in relation to that of the generated sound. Nevertheless, in contrast to the previous model, these observations show that both labia contribute to aperture control and strongly suggest that they play an important role as principal sound generators.

Role of syringeal muscles in controlling the phonology of bird song.

1. The contribution of syringeal muscles to controlling the phonology of song was studied by recording bilateral airflow, subsyringeal air sac pressure, electromyograms (EMGs) of six syringeal muscles, and vocal output in spontaneously singing brown thrashers (Toxostoma rufum). 2. EMG activity in musculus syringealis ventralis (vS), the largest syringeal muscle, increases exponentially with the fundamental frequency of the ipsilaterally generated sound and closely parallels frequency modulation. 3. The EMG activity of other syringeal muscles is also positively correlated with sound frequency, but the amplitude of their EMGs changes only a small amount compared with variation in the amplitude of their EMGs correlated with changing syringeal resistance. The elevated activity in all syringeal muscles during high-frequency sounds may reflect an increased need for structural stability during the strong contractions of the largest syringeal muscle (vS). 4. Several syringeal mechanisms are used to generate amplitude modulation (AM). The most common of these involves modulating the rate of syringeal airflow, through activity by adductor (m. syringealis dorsalis and m. tracheobronchialis dorsalis) and abductor (m. tracheobronchialis ventralis) muscles, which change syringeal resistance, switch sound production from one side of the syrinx to the other, or produce rapid oscillatory flow changes. Variation in the phase relationship between AM and EMG bursts during oscillatory airflow suggests complex biomechanical interaction between antagonistic muscles. 5. AM can also arise from acoustic interactions of two independently generated sounds (beat notes) including cross talk signals between the two syringeal halves. In this latter mechanism, sound generated on one side radiates slightly out of phase with the source from the contralateral side, resulting in lateralized AM generation.

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.

Role of syringeal muscles in gating airflow and sound production in singing brown thrashers.

1. The role of syringeal muscles in song production, particularly in regulating airflow through the syrinx, was studied in singing brown thrashers (Toxostoma rufum). In nine individuals, muscle activity was recorded electromyographically together with bilateral syringeal airflow, subsyringeal air sac pressure, and vocal output. 2. Dorsal muscles, m. syringealis dorsalis (dS) and m. tracheolateral dorsalis (dTB), are consistently activated during ipsilateral closing of the syrinx or increasing syringeal resistance, suggesting that their main role is adduction. This interpretation is supported by the motor patterns accompanying syllables with rapid oscillations in the rate of airflow. Bursts of electrical activity (2-10 ms) in dorsal muscles are precisely synchronized with decreasing airflow. 3. Electrical activity in m. tracheobronchialis ventralis (vTB) and m. tracheolateralis (TL) is associated with active abduction. An important contribution of vTB is to open the syringeal lumen for short inspirations in between syllables. In syllables with oscillatory flow modulations, vTB bursts show variable alignment with the phase of increasing flow. From this and activity during other syllables, it appears that, during phonation, vTB activity fine tunes the syringeal configuration, which is set by action of the dorsal muscles into a partially constricted state. 4. Activity in the ventral portion of TL, an extrinsic muscle, is strikingly similar to that of vTB, an intrinsic muscle, suggesting that the two muscles have a similar functional role. This supports the notion that intrinsic syringeal muscles of songbirds evolved from extrinsic muscles of nonpasserines. 5. M. syringealis ventralis (vS) does not appear to contribute directly to gating of airflow. Its activity is not consistently correlated with active changes in syringeal resistance. 6. Activity in m. sternotrachealis (ST) is most prominent during rapid changes in the rate of airflow or when switching between expiratory and inspiratory flow, suggesting a role in stabilizing the syringeal framework.

Motor dynamics of song production by mimic thrushes.

In brown thrashers (Toxostoma rufum) and grey catbirds (Dumetella carolinensis) neither side of the syrinx has a consistently dominant role in song production. During song, the two sides operate independently, but in close cooperation with each other and with the respiratory muscles which are capable of adjusting expiratory effort to maintain a constant rate of syringeal airflow despite sudden changes in syringeal resistance. Phonation is frequently switched from one side of the syrinx to the other, both between syllables and within a syllable. When both sides of the syrinx produce sound simultaneously, their respective contributions are seldom harmonically related. The resulting "two-voice" syllables sometimes contain difference tones with prominent sinusoidal amplitude modulation (AM). Rarely, both sides simultaneously produce the same sound. In general, however, the frequency range of sound contributed by the right syrinx is higher than that of the left syrinx. The right syrinx is also primarily responsible for producing a rapid cyclical amplitude modulation which is a characteristic feature of some syllables. This kind of AM is generated by either repetitive brief bursts of sound from the right side that modulate the amplitude of a continuous sound arising on the left side or cyclically opening the right syrinx, allowing unmodulated expiratory air to bypass the phonating left side.