Vocal development in a Waddington landscape

Prenatal development of neonatal vocalizations

Human and non-human primates produce rhythmical sounds as soon as they are born. These early vocalizations are important for soliciting the attention of caregivers. How they develop, remains a mystery. The orofacial movements necessary for producing these vocalizations have distinct spatiotemporal signatures. Therefore, their development could potentially be tracked over the course of prenatal life. We densely and longitudinally sampled fetal head and orofacial movements in marmoset monkeys using ultrasound imaging. We show that orofacial movements necessary for producing rhythmical vocalizations differentiate from a larger movement pattern that includes the entire head. We also show that signature features of marmoset infant contact calls emerge prenatally as a distinct pattern of orofacial movements. Our results establish that aspects of the sensorimotor development necessary for vocalizing occur prenatally, even before the production of sound.

A mechanism for punctuating equilibria during mammalian vocal development

Evolution and development are typically characterized as the outcomes of gradual changes, but sometimes (states of equilibrium can be punctuated by sudden change. Here, we studied the early vocal development of three different mammals: common marmoset monkeys, Egyptian fruit bats, and humans. Consistent with the notion of punctuated equilibria, we found that all three species undergo at least one sudden transition in the acoustics of their developing vocalizations. To understand the mechanism, we modeled different developmental landscapes. We found that the transition was best described as a shift in the balance of two vocalization landscapes. We show that the natural dynamics of these two landscapes are consistent with the dynamics of energy expenditure and information transmission. By using them as constraints for each species, we predicted the differences in transition timing from immature to mature vocalizations. Using marmoset monkeys, we were able to manipulate both infant energy expenditure (vocalizing in an environment with lighter air) and information transmission (closed-loop contingent parental vocal playback). These experiments support the importance of energy and information in leading to punctuated equilibrium states of vocal development.

Species can sometimes evolve suddenly; their appearance is preceded and followed by long periods of stability. This process is known as “punctuated equilibrium”. Our data show that for three mammalian species—marmoset monkeys, fruit bats, and humans—early vocal development trajectories can also be characterized as different equilibrium states punctuated by sharp transitions; transitions indicate the advent of a new vocal behavior. To better understand the putative mechanism behind such transitions, we show that a balance model, in which variables trade-off in their importance over time, captured this change by accurately simulating the shape of the developmental trajectory and predicting the timing of the transition between immature and mature vocal states for all three species. Two variables—energy and information—were hypothesized to trade-off during development. We tested and found support for this hypothesis in analyses of two marmoset monkey experiments, one which manipulated energy metabolic costs and another which manipulated information transmission.

Arousal elevation drives the development of oscillatory vocal output

Adult behaviors, such as vocal production, often exhibit temporal regularity. In contrast, their immature forms are more irregular. We ask whether the coupling of motor behaviors with arousal changes gives rise to temporal regularity: Do they drive the transition from variable to regular motor output over the course of development? We used marmoset monkey vocal production to explore this putative influence of arousal on the nonlinear changes in their developing vocal output patterns. Based on a detailed analysis of vocal and arousal dynamics in marmosets, we put forth a general model incorporating arousal and auditory feedback loops for spontaneous vocal production. Using this model, we show that a stable oscillation can emerge as the baseline arousal increases, predicting the transition from stochastic to periodic oscillations observed during marmoset vocal development. We further provide a solution for how this model can explain vocal development as the joint consequence of energetic growth and social feedback. Together, we put forth a plausible mechanism for the development of arousal-mediated adaptive behavior.NEW & NOTEWORTHY The development of motor behaviors, and the influence of energetic and social factors on it, has long been of interest, yet we lack an integrated picture of how these different systems may interact. Through the lens of vocal development in infant marmosets, this study offers a solution for social behavior development by linking motor production with arousal states. Increases in arousal can drive the system out of stochastic states toward oscillatory dynamics ready for communication.

The maturational gradient of infant vocalizations: Developmental stages and functional modules

Stage models have been influential in characterizing infant vocalizations in the first year of life. These models are basically descriptive and do not explain why certain types of vocal behaviors occur within a particular stage or why successive patterns of vocalization occur. This review paper summarizes and elaborates a theory of Developmental Functional Modules (DFMs) and discusses how maturational gradients in the DFMs explain age typical vocalizations as well as the transitions between successive stages or other static forms. Maturational gradients are based on biological processes that effect the reconfiguration and remodeling of the respiratory, laryngeal, and craniofacial systems during infancy. From a dynamic systems perspective, DFMs are part of a complex system with multiple degrees of freedom that can achieve stable performance with relatively few control variables by relying on principles such as synergies, self-organization, nonlinear performance, and movement variability.

A Hierarchy of Autonomous Systems for Vocal Production

Vocal production is hierarchical in the time domain. These hierarchies build upon biomechanical and neural dynamics across various timescales. We review studies in marmoset monkeys, songbirds, and other vertebrates. To organize these data in an accessible and across-species framework, we interpret the different timescales of vocal production as belonging to different levels of an autonomous systems hierarchy. The first level accounts for vocal acoustics produced on short timescales; subsequent levels account for longer timescales of vocal output. The hierarchy of autonomous systems that we put forth accounts for vocal patterning, sequence generation, dyadic interactions, and context dependence by sequentially incorporating central pattern generators, intrinsic drives, and sensory signals from the environment. We then show the framework's utility by providing an integrative explanation of infant vocal production learning in which social feedback modulates infant vocal acoustics through the tuning of a drive signal.

Vocal state change through laryngeal development

Across vertebrates, progressive changes in vocal behavior during postnatal development are typically attributed solely to developing neural circuits. How the changing body influences vocal development remains unknown. Here we show that state changes in the contact vocalizations of infant marmoset monkeys, which transition from noisy, low frequency cries to tonal, higher pitched vocalizations in adults, are caused partially by laryngeal development. Combining analyses of natural vocalizations, motorized excised larynx experiments, tensile material tests and high-speed imaging, we show that vocal state transition occurs via a sound source switch from vocal folds to apical vocal membranes, producing louder vocalizations with higher efficiency. We show with an empirically based model of descending motor control how neural circuits could interact with changing laryngeal dynamics, leading to adaptive vocal development. Our results emphasize the importance of embodied approaches to vocal development, where exploiting biomechanical consequences of changing material properties can simplify motor control, reducing the computational load on the developing brain.

Vocal and locomotor coordination develops in association with the autonomic nervous system

In adult animals, movement and vocalizations are coordinated, sometimes facilitating, and at other times inhibiting, each other. What is missing is how these different domains of motor control become coordinated over the course of development. We investigated how postural-locomotor behaviors may influence vocal development, and the role played by physiological arousal during their interactions. Using infant marmoset monkeys, we densely sampled vocal, postural and locomotor behaviors and estimated arousal fluctuations from electrocardiographic measures of heart rate. We found that vocalizations matured sooner than postural and locomotor skills, and that vocal-locomotor coordination improved with age and during elevated arousal levels. These results suggest that postural-locomotor maturity is not required for vocal development to occur, and that infants gradually improve coordination between vocalizations and body movement through a process that may be facilitated by arousal level changes.

Transition state characteristics during cell differentiation

Models describing the process of stem-cell differentiation are plentiful, and may offer insights into the underlying mechanisms and experimentally observed behaviour. Waddington's epigenetic landscape has been providing a conceptual framework for differentiation processes since its inception. It also allows, however, for detailed mathematical and quantitative analyses, as the landscape can, at least in principle, be related to mathematical models of dynamical systems. Here we focus on a set of dynamical systems features that are intimately linked to cell differentiation, by considering exemplar dynamical models that capture important aspects of stem cell differentiation dynamics. These models allow us to map the paths that cells take through gene expression space as they move from one fate to another, e.g. from a stem-cell to a more specialized cell type. Our analysis highlights the role of the transition state (TS) that separates distinct cell fates, and how the nature of the TS changes as the underlying landscape changes-change that can be induced by e.g. cellular signaling. We demonstrate that models for stem cell differentiation may be interpreted in terms of either a static or transitory landscape. For the static case the TS represents a particular transcriptional profile that all cells approach during differentiation. Alternatively, the TS may refer to the commonly observed period of heterogeneity as cells undergo stochastic transitions.

Limiting parental interaction during vocal development affects acoustic call structure in marmoset monkeys

Human vocal development is dependent on learning by imitation through social feedback between infants and caregivers. Recent studies have revealed that vocal development is also influenced by parental feedback in marmoset monkeys, suggesting vocal learning mechanisms in nonhuman primates. Marmoset infants that experience more contingent vocal feedback than their littermates develop vocalizations more rapidly, and infant marmosets with limited parental interaction exhibit immature vocal behavior beyond infancy. However, it is yet unclear whether direct parental interaction is an obligate requirement for proper vocal development because all monkeys in the aforementioned studies were able to produce the adult call repertoire after infancy. Using quantitative measures to compare distinct call parameters and vocal sequence structure, we show that social interaction has a direct impact not only on the maturation of the vocal behavior but also on acoustic call structures during vocal development. Monkeys with limited parental interaction during development show systematic differences in call entropy, a measure for maturity, compared with their normally raised siblings. In addition, different call types were occasionally uttered in motif-like sequences similar to those exhibited by vocal learners, such as birds and humans, in early vocal development. These results indicate that a lack of parental interaction leads to long-term disturbances in the acoustic structure of marmoset vocalizations, suggesting an imperative role for social interaction in proper primate vocal development.

Internal states and extrinsic factors both determine monkey vocal production

A key question for understanding speech evolution is whether or not the vocalizations of our closest living relatives-nonhuman primates-represent the precursors to speech. Some believe that primate vocalizations are not volitional but are instead inextricably linked to internal states like arousal and thus bear little resemblance to human speech. Others disagree and believe that since many primates can use their vocalizations strategically, this demonstrates a degree of voluntary vocal control. In the current study, we present a behavioral paradigm that reliably elicits different types of affiliative vocalizations from marmoset monkeys while measuring their heart rate fluctuations using noninvasive electromyography. By modulating both the physical distance between marmosets and the sensory information available to them, we find that arousal levels are linked, but not inextricably, to vocal production. Different arousal levels are, generally, associated with changes in vocal acoustics and the drive to produce different call types. However, in contexts where marmosets are interacting, the production of these different call types is also affected by extrinsic factors such as the timing of a conspecific's vocalization. These findings suggest that variability in vocal output as a function of context might reflect trade-offs between the drive to perpetuate vocal contact and conserving energy.

Vocal development through morphological computation

The vocal behavior of infants changes dramatically during early life. Whether or not such a change results from the growth of the body during development-as opposed to solely neural changes-has rarely been investigated. In this study of vocal development in marmoset monkeys, we tested the putative causal relationship between bodily growth and vocal development. During the first two months of life, the spontaneous vocalizations of marmosets undergo (1) a gradual disappearance of context-inappropriate call types and (2) an elongation in the duration of context-appropriate contact calls. We hypothesized that both changes are the natural consequences of lung growth and do not require any changes at the neural level. To test this idea, we first present a central pattern generator model of marmoset vocal production to demonstrate that lung growth can affect the temporal and oscillatory dynamics of neural circuits via sensory feedback from the lungs. Lung growth qualitatively shifted vocal behavior in the direction observed in real marmoset monkey vocal development. We then empirically tested this hypothesis by placing the marmoset infants in a helium-oxygen (heliox) environment in which air is much lighter. This simulated a reversal in development by decreasing the effort required to respire, thus increasing the respiration rate (as though the lungs were smaller). The heliox manipulation increased the proportions of inappropriate call types and decreased the duration of contact calls, consistent with a brief reversal of vocal development. These results suggest that bodily growth alone can play a major role in shaping the development of vocal behavior.

Constraints and flexibility during vocal development: Insights from marmoset monkeys

Human vocal development is typically conceived as a sequence of two processes-an early maturation phase where vocal sounds change as a function of body growth ("constraints") followed by a period during which social experience can influence vocal sound production ("flexibility"). However, studies of other behaviors (e.g., locomotion) reveal that growth and experience are interactive throughout development. As it turns out, vocal development is not exceptional; it is also the on-going result of the interplay between an infant's growing biological system of production (the body and the nervous system) and experience with caregivers. Here, we review work on developing marmoset monkeys - a species that exhibits strikingly similar vocal developmental processes to those of prelinguistic human infants - that demonstrates how constraints and flexibility are parallel and interactive processes.