Brain evolution by brain pathway duplication

Bridging the fields of cognition and birdsong with corvids

Corvids, readily adaptable across social and ecological contexts, successfully inhabit almost the entire world. They are seen as highly intelligent birds, and current research examines their cognitive abilities. Despite being songbirds with a complete 'song system', corvids have historically received less attention in studies of song production, learning, and perception compared to non-corvid songbirds. However, recent neurobiological studies have demonstrated that songbird vocal production and its neuronal representations are regularly influenced by environmental and cognitive factors. This opinion article discusses the literature on 'corvid song' before introducing other flexible vocal behaviors of corvids in both the wild and controlled laboratory studies. We suggest corvids with their flexible vocal control as promising model species to study the links between brain networks for cognition and vocalization. Studying corvid vocal flexibility and associated cognitive processes in both ecological and lab settings offers complementary insights, crucial for bridging the fields of cognition and birdsong.

Automatic imitation of intransitive actions in macaws

Automatic imitation is the involuntary tendency of humans to copy others' actions even when counterproductive. We examined the automatic imitation of intransitive actions in blue-throated macaws (Ara glaucogularis), employing a stimulus-response-compatibility task. After training seven macaws to perform two different actions with legs and wings upon specific hand commands, the subjects were divided into a compatible and incompatible group. We rewarded the subjects for performing the same action as the conspecific model in the compatible group and the opposite action in the incompatible group. Involuntarily imitating the demonstrated actions, the incompatible group made more errors than the compatible group and took longer to eventually respond correctly. The study provides evidence for the automatic imitation of intransitive actions in non-human animals- parrots, suggesting that arbitrary action imitation facilitated by a mirror-neuron system in parrot brain may be adaptive in the ever-changing complex social environment of parrots and possibly drive cultural evolution.

Beat-based dancing to music has evolutionary foundations in advanced vocal learning

Dancing to music is ancient and widespread in human cultures. While dance shows great cultural diversity, it often involves nonvocal rhythmic movements synchronized to musical beats in a predictive and tempo-flexible manner. To date, the only nonhuman animals known to spontaneously move to music in this way are parrots. This paper proposes that human-parrot similarities in movement to music and in the neurobiology of advanced vocal learning hold clues to the evolutionary foundations of human dance. The proposal draws on recent research on the neurobiology of parrot vocal learning by Jarvis and colleagues and on a recent cortical model for speech motor control by Hickock and colleagues. These two lines of work are synthesized to suggest that gene regulation changes associated with the evolution of a dorsal laryngeal pitch control pathway in ancestral humans fortuitously strengthened auditory-parietal cortical connections that support beat-based rhythmic processing. More generally, the proposal aims to explain how and why the evolution of strong forebrain auditory-motor integration in the service of learned vocal control led to a capacity and proclivity to synchronize nonvocal movements to the beat. The proposal specifies cortical brain pathways implicated in the origins of human beat-based dancing and leads to testable predictions and suggestions for future research.

A modular circuit coordinates the diversification of courtship strategies

Mate recognition systems evolve rapidly to reinforce the reproductive boundaries between species, but the underlying neural mechanisms remain enigmatic. Here we leveraged the rapid coevolution of female pheromone production and male pheromone perception in Drosophila1,2 to gain insight into how the architecture of mate recognition circuits facilitates their diversification. While in some Drosophila species females produce unique pheromones that act to arouse their conspecific males, the pheromones of most species are sexually monomorphic such that females possess no distinguishing chemosensory signatures that males can use for mate recognition3. We show that Drosophila yakuba males evolved the ability to use a sexually monomorphic pheromone, 7-tricosene, as an excitatory cue to promote courtship. By comparing key nodes in the pheromone circuits across multiple Drosophila species, we reveal that this sensory innovation arises from coordinated peripheral and central circuit adaptations: a distinct subpopulation of sensory neurons has acquired sensitivity to 7-tricosene and, in turn, selectively signals to a distinct subset of P1 neurons in the central brain to trigger courtship. Such a modular circuit organization, in which different sensory inputs can independently couple to parallel courtship control nodes, may facilitate the evolution of mate recognition systems by allowing novel sensory modalities to become linked to male arousal. Together, our findings suggest how peripheral and central circuit adaptations can be flexibly coordinated to underlie the rapid evolution of mate recognition strategies across species.

Selective expansion of motor cortical projections in the evolution of vocal novelty

Deciphering how cortical architecture evolves to drive behavioral innovations is a long-standing challenge in neuroscience and evolutionary biology. Here, we leverage a striking behavioral novelty in the Alston's singing mouse (Scotinomys teguina), compared to the laboratory mouse (Mus musculus), to quantitatively test models of motor cortical evolution. We used bulk tracing, serial two-photon tomography, and high-throughput DNA sequencing of over 76,000 barcoded neurons to discover a specific and substantial expansion (200%) of orofacial motor cortical (OMC) projections to the auditory cortical region (AudR) and the midbrain periaqueductal gray (PAG), both implicated in vocal behaviors. Moreover, analysis of individual OMC neurons' projection motifs revealed preferential expansion of exclusive projections to AudR. Our results imply that selective expansion of ancestral motor cortical projections can underlie behavioral divergence over short evolutionary timescales, suggesting potential mechanisms for the evolution of enhanced cortical control over vocalizations-a crucial preadaptation for human language.

Evolution of Telencephalon Anterior-Posterior Patterning through Core Endogenous Network Bifurcation

How did the complex structure of the telencephalon evolve? Existing explanations are based on phenomena and lack a first-principles account. The Darwinian dynamics and endogenous network theory-established decades ago-provides a mathematical and theoretical framework and a general constitutive structure for theory-experiment coupling for answering this question from a first-principles perspective. By revisiting a gene network that explains the anterior-posterior patterning of the vertebrate telencephalon, we found that upon increasing the cooperative effect within this network, fixed points gradually evolve, accompanied by the occurrence of two bifurcations. The dynamic behavior of this network is informed by the knowledge obtained from experiments on telencephalic evolution. Our work provides a quantitative explanation for how telencephalon anterior-posterior patterning evolved from the pre-vertebrate chordate to the vertebrate and provides a series of verifiable predictions from a first-principles perspective.

Neuronal Circuit Evolution: From Development to Structure and Adaptive Significance

Neuronal circuits represent the functional units of the brain. Understanding how the circuits are generated to perform computations will help us understand how the brain functions. Nevertheless, neuronal circuits are not engineered, but have formed through millions of years of animal evolution. We posit that it is necessary to study neuronal circuit evolution to comprehensively understand circuit function. Here, we review our current knowledge regarding the mechanisms that underlie circuit evolution. First, we describe the possible genetic and developmental mechanisms that have contributed to circuit evolution. Then, we discuss the structural changes of circuits during evolution and how these changes affected circuit function. Finally, we try to put circuit evolution in an ecological context and assess the adaptive significance of specific examples. We argue that, thanks to the advent of new tools and technologies, evolutionary neurobiology now allows us to address questions regarding the evolution of circuitry and behavior that were unimaginable until very recently.

The partial upward migration of the laryngeal motor cortex: A window to the human brain evolution

The pioneer cortical electrical stimulation studies of the last century did not explicitly mark the location of the human laryngeal motor cortex (LMC), but only the "vocalization area" in the lower half of the lateral motor cortex. In the final years of 2010́s, neuroimaging studies did demonstrate two human cortical laryngeal representations, located at the opposing ends of the orofacial motor zone, therefore termed dorsal (LMCd) and ventral laryngeal motor cortex (LMCv). Since then, there has been a continuing debate regarding the origin, function and evolutionary significance of these areas. The "local duplication model" posits that the LMCd evolved by a duplication of an adjacent region of the motor cortex. The "duplication and migration model" assumes that the dorsal LMCd arose by a duplication of motor regions related to vocalization, such as the ancestry LMC, followed by a migration into the orofacial region of the motor cortex. This paper reviews the basic arguments of these viewpoints and suggests a new explanation, declaring that the LMCd in man is rather induced through the division of the unitary LMC in nonhuman primates, upward shift and relocation of its motor part due to the disproportional growth of the head, face, mouth, lips, and tongue motor areas in the ventral part of the human motor homunculus. This explanation may be called "expansion-division and relocation model".

The evolution of social play in songbirds, parrots and cockatoos - emotional or highly complex cognitive behaviour or both?

Social play has been described in many animals. However, much of this social behaviour among birds, particularly in adults, is still relatively unexplored in terms of the environmental, psychological, and social dynamics of play. This paper provides an overview of what we know about adult social play in birds and addresses areas in which subtleties and distinctions, such as in play initiation and social organisation and its relationship to expressions of play, are considered in detail. The paper considers emotional, social, innovative, and cognitive aspects of play, then the environmental conditions and affiliative bonds, suggesting a surprisingly complex framework of criteria awaiting further research. Adult social play has so far been studied in only a small number of avian species, exclusively in those with a particularly large brain relative to body size without necessarily addressing brain functions and lateralization. When lateralization of brain function is considered, it can further illuminate a possibly significant relevance of play behaviour to the evolution of cognition, to management of emotions, and the development of sociality.

The emergence of enhanced intelligence in a brain-inspired cognitive architecture

The Causal Cognitive Architecture is a brain-inspired cognitive architecture developed from the hypothesis that the navigation circuits in the ancestors of mammals duplicated to eventually form the neocortex. Thus, millions of neocortical minicolumns are functionally modeled in the architecture as millions of "navigation maps." An investigation of a cognitive architecture based on these navigation maps has previously shown that modest changes in the architecture allow the ready emergence of human cognitive abilities such as grounded, full causal decision-making, full analogical reasoning, and near-full compositional language abilities. In this study, additional biologically plausible modest changes to the architecture are considered and show the emergence of super-human planning abilities. The architecture should be considered as a viable alternative pathway toward the development of more advanced artificial intelligence, as well as to give insight into the emergence of natural human intelligence.

Unsolved morphogenesis problems and the hidden order

In this work, the morphogenesis mechanisms are considered from the complexity perspective. It is shown that both morphogenesis and the functioning of organs should be unstable in the case of short-range interaction potentials. The repeatability of forms during evolution is a strong argument for its directionality. The formation of organs during evolution can occur only in the presence of a priori information about the structure of such an organ. The focus of the discussion is not merely on constraining potential possibilities but on the concept of directed evolution itself. A morphogenesis model was constructed based on nontrivial quantum effects. These interaction effects between biologically important molecules ensure the accurate synthesis of cells, tissues, and organs.

Evolution of neural circuitry and cognition

Neural circuits govern the interface between the external environment, internal cues and outwardly directed behaviours. To process multiple environmental stimuli and integrate these with internal state requires considerable neural computation. Expansion in neural network size, most readily represented by whole brain size, has historically been linked to behavioural complexity, or the predominance of cognitive behaviours. Yet, it is largely unclear which aspects of circuit variation impact variation in performance. A key question in the field of evolutionary neurobiology is therefore how neural circuits evolve to allow improved behavioural performance or innovation. We discuss this question by first exploring how volumetric changes in brain areas reflect actual neural circuit change. We explore three major axes of neural circuit evolution-replication, restructuring and reconditioning of cells and circuits-and discuss how these could relate to broader phenotypes and behavioural variation. This discussion touches on the relevant uses and limitations of volumetrics, while advocating a more circuit-based view of cognition. We then use this framework to showcase an example from the insect brain, the multi-sensory integration and internal processing that is shared between the mushroom bodies and central complex. We end by identifying future trends in this research area, which promise to advance the field of evolutionary neurobiology.

Parallel executive pallio-motor loops in the pigeon brain

A core component of the avian pallial cognitive network is the multimodal nidopallium caudolaterale (NCL) that is considered to be analogous to the mammalian prefrontal cortex (PFC). The NCL plays a key role in a multitude of executive tasks such as working memory, decision-making during navigation, and extinction learning in complex learning environments. Like the PFC, the NCL is positioned at the transition from ascending sensory to descending motor systems. For the latter, it sends descending premotor projections to the intermediate arcopallium (AI) and the medial striatum (MSt). To gain detailed insight into the organization of these projections, we conducted several retrograde and anterograde tracing experiments. First, we tested whether NCL neurons projecting to AI (NCLarco neurons) and MSt (NCLMSt neurons) are constituted by a single neuronal population with bifurcating neurons, or whether they form two distinct populations. Here, we found two distinct projection patterns to both target areas that were associated with different morphologies. Second, we revealed a weak topographic projection toward the medial and lateral striatum and a strong topographic projection toward AI with clearly distinguishable sensory termination fields. Third, we investigated the relationship between the descending NCL pathways to the arcopallium with those from the hyperpallium apicale, which harbors a second major descending pathway of the avian pallium. We embed our findings within a system of parallel pallio-motor loops that carry information from separate sensory modalities to different subpallial systems. Our results also provide insights into the evolution of the avian motor system from which, possibly, the song system has emerged.

Anterior forebrain pathway in parrots is necessary for producing learned vocalizations with individual signatures

Parrots have enormous vocal imitation capacities and produce individually unique vocal signatures. Like songbirds, parrots have a nucleated neural song system with distinct anterior (AFP) and posterior forebrain pathways (PFP). To test if song systems of parrots and songbirds, which diverged over 50 million years ago, have a similar functional organization, we first established a neuroscience-compatible call-and-response behavioral paradigm to elicit learned contact calls in budgerigars (Melopsittacus undulatus). Using variational autoencoder-based machine learning methods, we show that contact calls within affiliated groups converge but that individuals maintain unique acoustic features, or vocal signatures, even after call convergence. Next, we transiently inactivated the outputs of AFP to test if learned vocalizations can be produced by the PFP alone. As in songbirds, AFP inactivation had an immediate effect on vocalizations, consistent with a premotor role. But in contrast to songbirds, where the isolated PFP is sufficient to produce stereotyped and acoustically normal vocalizations, isolation of the budgerigar PFP caused a degradation of call acoustic structure, stereotypy, and individual uniqueness. Thus, the contribution of AFP and the capacity of isolated PFP to produce learned vocalizations have diverged substantially between songbirds and parrots, likely driven by their distinct behavioral ecology and neural connectivity.

A modular circuit architecture coordinates the diversification of courtship strategies in Drosophila

Identifying a mate is a central imperative for males of most species but poses the challenge of distinguishing a suitable partner from an array of potential male competitors or females of related species. Mate recognition systems are thus subject to strong selective pressures, driving the rapid coevolution of female sensory cues and male sensory preferences. Here we leverage the rapid evolution of female pheromones across the Drosophila genus to gain insight into how males coordinately adapt their detection and interpretation of these chemical cues to hone their mating strategies. While in some Drosophila species females produce unique pheromones that act to attract and arouse their conspecific males, the pheromones of most species are sexually monomorphic such that females possess no distinguishing chemosensory signatures that males can use for mate recognition. By comparing several close and distantly-related Drosophila species, we reveal that D. yakuba males have evolved the distinct ability to use a sexually-monomorphic pheromone, 7-tricosene (7-T), as an excitatory cue to promote courtship, a sensory innovation that enables D. yakuba males to court in the dark thereby expanding their reproductive opportunities. To gain insight into the neural adaptations that enable 7-T to act as an excitatory cue, we compared the functional properties of two key nodes within the pheromone circuits of D. yakuba and a subset of its closest relatives. We show that the instructive role of 7-T in D. yakuba arises from concurrent peripheral and central circuit changes: a distinct subpopulation of sensory neurons has acquired sensitivity to 7-T which in turn selectively signals to a distinct subset of P1 neurons in the central brain that trigger courtship behaviors. Such a modular circuit organization, in which different sensory inputs can independently couple to multiple parallel courtship control nodes, may facilitate the evolution of mate recognition systems by allowing males to take advantage of novel sensory modalities to become aroused. Together, our findings suggest how peripheral and central circuit adaptations can be flexibly linked to underlie the rapid evolution of mate recognition and courtship strategies across species.

Songbird species that display more-complex vocal learning are better problem-solvers and have larger brains

Complex vocal learning, a critical component of human spoken language, has been assumed to be associated with more-advanced cognitive abilities. Tests of this hypothesis between individuals within a species have been inconclusive and have not been done across species. In this work, we measured an array of cognitive skills-namely, problem-solving, associative and reversal learning, and self-control-across 214 individuals of 23 bird species, including 19 wild-caught songbird species, two domesticated songbird species, and two wild-caught vocal nonlearning species. We found that the greater the vocal learning abilities of a species, the better their problem-solving skills and the relatively larger their brains. These conclusions held when controlling for noncognitive variables and phylogeny. Our results support a hypothesis of shared genetic and cognitive mechanisms between vocal learning, problem-solving, and bigger brains in songbirds.

Rapid expansion and visual specialisation of learning and memory centres in the brains of Heliconiini butterflies

Changes in the abundance and diversity of neural cell types, and their connectivity, shape brain composition and provide the substrate for behavioral evolution. Although investment in sensory brain regions is understood to be largely driven by the relative ecological importance of particular sensory modalities, how selective pressures impact the elaboration of integrative brain centers has been more difficult to pinpoint. Here, we provide evidence of extensive, mosaic expansion of an integration brain center among closely related species, which is not explained by changes in sites of primary sensory input. By building new datasets of neural traits among a tribe of diverse Neotropical butterflies, the Heliconiini, we detected several major evolutionary expansions of the mushroom bodies, central brain structures pivotal for insect learning and memory. The genus Heliconius, which exhibits a unique dietary innovation, pollen-feeding, and derived foraging behaviors reliant on spatial memory, shows the most extreme enlargement. This expansion is primarily associated with increased visual processing areas and coincides with increased precision of visual processing, and enhanced long term memory. These results demonstrate that selection for behavioral innovation and enhanced cognitive ability occurred through expansion and localized specialization in integrative brain centers.

Evolutionary dynamics of mushroom body Kenyon cell types in hymenopteran brains from multifunctional type to functionally specialized types

Evolutionary dynamics of diversification of brain neuronal cell types that have underlain behavioral evolution remain largely unknown. Here, we compared transcriptomes and functions of Kenyon cell (KC) types that compose the mushroom bodies between the honey bee and sawfly, a primitive hymenopteran insect whose KCs likely have the ancestral properties. Transcriptome analyses show that the sawfly KC type shares some of the gene expression profile with each honey bee KC type, although unique gene expression profiles have also been acquired in each honey bee KC type. In addition, functional analysis of two sawfly genes suggested that the functions in learning and memory of the ancestral KC type were heterogeneously inherited among the KC types in the honey bee. Our findings strongly suggest that the functional evolution of KCs in Hymenoptera involved two previously hypothesized processes for evolution of cell function: functional segregation and divergence.

Neural correlates of cognitively controlled vocalizations in a corvid songbird

The neuronal basis of the songbird's song system is well understood. However, little is known about the neuronal correlates of the executive control of songbird vocalizations. Here, we record single-unit activity from the pallial endbrain region "nidopallium caudolaterale" (NCL) of crows that vocalize to the presentation of a visual go-cue but refrain from vocalizing during trials without a go-cue. We find that the preparatory activity of single vocalization-correlated neurons, but also of the entire population of NCL neurons, before vocal onset predicts whether or not the crows will produce an instructed vocalization. Fluctuations in baseline neuronal activity prior to the go-cue influence the premotor activity of such vocalization-correlated neurons and seemingly bias the crows' decision to vocalize. Neuronal response modulation significantly differs between volitional and task-unrelated vocalizations. This suggests that the NCL can take control over the vocal motor network during the production of volitional vocalizations in a corvid songbird.

A non-canonical retina-ipRGCs-SCN-PVT visual pathway for mediating contagious itch behavior

Contagious itch behavior informs conspecifics of adverse environment and is crucial for the survival of social animals. Gastrin-releasing peptide (GRP) and its receptor (GRPR) in the suprachiasmatic nucleus (SCN) of the hypothalamus mediates contagious itch behavior in mice. Here, we show that intrinsically photosensitive retina ganglion cells (ipRGCs) convey visual itch information, independently of melanopsin, from the retina to GRP neurons via PACAP-PAC1R signaling. Moreover, GRPR neurons relay itch information to the paraventricular nucleus of the thalamus (PVT). Surprisingly, neither the visual cortex nor superior colliculus is involved in contagious itch. In vivo calcium imaging and extracellular recordings reveal contagious itch-specific neural dynamics of GRPR neurons. Thus, we propose that the retina-ipRGC-SCN-PVT pathway constitutes a previously unknown visual pathway that probably evolved for motion vision that encodes salient environmental cues and enables animals to imitate behaviors of conspecifics as an anticipatory mechanism to cope with adverse conditions.

It has been shown that GRP-GRPR neuropeptide signaling in the SCN is important for contagious itch behavior in mice. Gao et al. find that SCN-projecting ipRGCs are sufficient to relay itch information from the retina to the SCN by releasing neuropeptide PACAP to activate the GRP-GRPR pathway.

Forebrain nuclei linked to woodpecker territorial drum displays mirror those that enable vocal learning in songbirds

Vocal learning is thought to have evolved in 3 orders of birds (songbirds, parrots, and hummingbirds), with each showing similar brain regions that have comparable gene expression specializations relative to the surrounding forebrain motor circuitry. Here, we searched for signatures of these same gene expression specializations in previously uncharacterized brains of 7 assumed vocal non-learning bird lineages across the early branches of the avian family tree. Our findings using a conserved marker for the song system found little evidence of specializations in these taxa, except for woodpeckers. Instead, woodpeckers possessed forebrain regions that were anatomically similar to the pallial song nuclei of vocal learning birds. Field studies of free-living downy woodpeckers revealed that these brain nuclei showed increased expression of immediate early genes (IEGs) when males produce their iconic drum displays, the elaborate bill-hammering behavior that individuals use to compete for territories, much like birdsong. However, these specialized areas did not show increased IEG expression with vocalization or flight. We further confirmed that other woodpecker species contain these brain nuclei, suggesting that these brain regions are a common feature of the woodpecker brain. We therefore hypothesize that ancient forebrain nuclei for refined motor control may have given rise to not only the song control systems of vocal learning birds, but also the drumming system of woodpeckers.

Vocal learning is thought to have evolved in three orders of birds (songbirds, parrots, and hummingbirds). This study shows that woodpeckers have evolved a set of brain nuclei to mediate their drum displays, and these regions closely mirror those that underlie song learning in songbirds.

Vocal babbling in a wild parrot shows life history and endocrine affinities with human infants

Prelinguistic babbling is a critical phase in infant language development and is best understood in temperate songbirds where it occurs primarily in males at reproductive maturity and is modulated by sex steroids. Parrots of both sexes are icons of tropical vocal plasticity, but vocal babbling is unreported in this group and whether the endocrine system is involved is unknown. Here we show that vocal babbling is widespread in a wild parrot population in Venezuela, ensues in both sexes during the nestling stage, occurs amidst a captive audience of mixed-aged siblings, and is modulated by corticosteroids. Spectrographic analysis and machine learning found phoneme diversity and combinatorial capacity increased precipitously for the first week, thereafter, crystalizing into a smaller repertoire, consistent with the selective attrition model of language development. Corticosterone-treated nestlings differed from unmanipulated birds and sham controls in several acoustic properties and crystallized a larger repertoire post-treatment. Our findings indicate babbling occurs during an early life-history stage in which corticosteroids help catalyse the transition from a universal learning programme to one finely tuned for the prevailing ecological environment, a potentially convergent scenario in human prelinguistic development.

Organizational Conservation and Flexibility in the Evolution of Birdsong and Avian Motor Control

Birds and mammals have independently evolved complex behavioral and cognitive capabilities yet have markedly different brain structures. An open question is to what extent, despite these differences in anatomy, birds and mammals have evolved similar neural solutions to complex motor control and at what level of organization these similarities might lie. Courtship song in songbirds, a learned motor skill that is similar to the fine motor skills of many mammals including human speech, provides a powerful system in which to study the links connecting the development and evolution of cells, circuits, and behavior. Until recently, obtaining cellular-resolution views of the specialized neural circuitry that subserves birdsong was impossible due to a lack of molecular tools for songbirds. However, the ongoing revolution in cellular profiling and genomics offers unprecedented opportunities for molecular analysis in organisms that lack a traditional genetic infrastructure but have tractable, well-defined behaviors. Here, I describe recent efforts to understand the evolutionary relationships between birdsong control circuitry and mammalian neocortical circuitry using new approaches to measure gene expression in single cells. These results, combined with foundational work relating avian and mammalian brains at a range of biological levels, present an emerging view that amniote pallium evolution is a story of diverse neural circuit architectures employing conserved neuronal elements within a conserved topological framework. This view suggests that one locus of pallial neural circuit evolution lies at the intersection between the gene regulatory programs that regulate regional patterning and those that specify functional identity. Modifications to this intersection may underlie the evolution of pallial motor control in birds in general and to the evolutionary and developmental relationships of these circuits to the avian pallial amygdala.

Vocal changes in a zebra finch model of Parkinson's disease characterized by alpha-synuclein overexpression in the song-dedicated anterior forebrain pathway

Deterioration in the quality of a person's voice and speech is an early marker of Parkinson's disease (PD). In humans, the neural circuit that supports vocal motor control consists of a cortico-basal ganglia-thalamo-cortico loop. The basal ganglia regions, striatum and globus pallidus, in this loop play a role in modulating the acoustic features of vocal behavior such as loudness, pitch, and articulatory rate. In PD, this area is implicated in pathogenesis. In animal models of PD, the accumulation of toxic aggregates containing the neuronal protein alpha-synuclein (αsyn) in the midbrain and striatum result in limb and vocal motor impairments. It has been challenging to study vocal impairments given the lack of well-defined cortico-basal ganglia circuitry for vocalization in rodent models. Furthermore, whether deterioration of voice quality early in PD is a direct result of αsyn-induced neuropathology is not yet known. Here, we take advantage of the well-characterized vocal circuits of the adult male zebra finch songbird to experimentally target a song-dedicated pathway, the anterior forebrain pathway, using an adeno-associated virus expressing the human wild-type αsyn gene, SNCA. We found that overexpression of αsyn in this pathway coincides with higher levels of insoluble, monomeric αsyn compared to control finches. Impairments in song production were also detected along with shorter and poorer quality syllables, which are the most basic unit of song. These vocal changes are similar to the vocal abnormalities observed in individuals with PD.

Chronic recording of cortical activity underlying vocalization in awake minipigs

BACKGROUND

Investigating brain dynamics underlying vocal production in animals is a powerful way to inform on the neural bases of human speech. In particular, brain networks underlying vocal production in non-human primates show striking similarities with the human speech production network. However, despite increasing findings also in birds and more recently in rodents, the extent to which the primate vocal cortical network model generalizes to other non-primate mammals remains unclear. Especially, no domestic species has yet been proposed to investigate vocal brain activity using electrophysiological approaches.

NEW METHOD

In the present study, we introduce a novel experimental paradigm to identify the cortical dynamics underlying vocal production in behaving minipigs. A key problem to chronically implant cortical probes in pigs is the presence and growth of frontal sinuses extending caudally to the parietal bone and preventing safe access to neural structures with conventional craniotomy in adult animals.

RESULTS

Here we first show that implantations of soft ECoG grids can be done safely using conventional craniotomy in minipigs younger than 5 months, a period when sinuses are not yet well developed. Using wireless recordings in behaving animals, we further show activation of the motor and premotor cortex around the onset of vocal production of grunts, the most common vocalization of pigs.

CONCLUSION

These results suggest that minipigs, which are very loquacious and social animals, can be a good experimental large animal model to study the cortical bases of vocal production.

A dual larynx motor networks hypothesis

Humans are vocal modulators par excellence. This ability is supported in part by the dual representation of the laryngeal muscles in the motor cortex. Movement, however, is not the product of motor cortex alone but of a broader motor network. This network consists of brain regions that contain somatotopic maps that parallel the organization in motor cortex. We therefore present a novel hypothesis that the dual laryngeal representation is repeated throughout the broader motor network. In support of the hypothesis, we review existing literature that demonstrates the existence of network-wide somatotopy and present initial evidence for the hypothesis' plausibility. Understanding how this uniquely human phenotype in motor cortex interacts with broader brain networks is an important step toward understanding how humans evolved the ability to speak. We further suggest that this system may provide a means to study how individual components of the nervous system evolved within the context of neuronal networks. This article is part of the theme issue 'Voice modulation: from origin and mechanism to social impact (Part I)'.

Extensive GJD2 Expression in the Song Motor Pathway Reveals the Extent of Electrical Synapses in the Songbird Brain

Birdsong is a precisely timed animal behavior. The connectivity of song premotor neural networks has been proposed to underlie the temporal patterns of neuronal activity that control vocal muscle movements during singing. Although the connectivity of premotor nuclei via chemical synapses has been characterized, electrical synapses and their molecular identity remain unexplored. We show with in situ hybridizations that GJD2 mRNA, coding for the major channel-forming electrical synapse protein in mammals, connexin 36, is expressed in the two nuclei that control song production, HVC and RA from canaries and zebra finches. In canaries' HVC, GJD2 mRNA is extensively expressed in GABAergic and only a fraction of glutamatergic cells. By contrast, in RA, GJD2 mRNA expression is widespread in glutamatergic and GABAergic neurons. Remarkably, GJD2 expression is similar in song nuclei and their respective embedding brain regions, revealing the widespread expression of GJD2 in the avian brain. Inspection of a single-cell sequencing database from zebra and Bengalese finches generalizes the distributions of electrical synapses across cell types and song nuclei that we found in HVC and RA from canaries, reveals a differential GJD2 mRNA expression in HVC glutamatergic subtypes and its transient increase along the neurogenic lineage. We propose that songbirds are a suitable model to investigate the contribution of electrical synapses to motor skill learning and production.

Positive selection in noncoding genomic regions of vocal learning birds is associated with genes implicated in vocal learning and speech functions in humans

Vocal learning, the ability to imitate sounds from conspecifics and the environment, is a key component of human spoken language and learned song in three independently evolved avian groups—oscine songbirds, parrots, and hummingbirds. Humans and each of these three bird clades exhibit specialized behavioral, neuroanatomical, and brain gene expression convergence related to vocal learning, speech, and song. To understand the evolutionary basis of vocal learning gene specializations and convergence, we searched for and identified accelerated genomic regions (ARs), a marker of positive selection, specific to vocal learning birds. We found avian vocal learner-specific ARs, and they were enriched in noncoding regions near genes with known speech functions or brain gene expression specializations in humans and vocal learning birds, including FOXP2, NEUROD6, ZEB2, and MEF2C, and near genes with major neurodevelopmental functions, including NR2F1, NRP2, and BCL11B. We also found enrichment near the SFARI class S genes associated with syndromic vocal communication forms of autism spectrum disorders. These findings reveal strong candidate noncoding regions near genes for the evolutionary adaptations that distinguish vocal learning species from their close vocal nonlearning relatives and provide further evidence of molecular convergence between birdsong and human spoken language.

Vocal learning as a preadaptation for the evolution of human beat perception and synchronization

The human capacity to synchronize movements to an auditory beat is central to musical behaviour and to debates over the evolution of human musicality. Have humans evolved any neural specializations for music processing, or does music rely entirely on brain circuits that evolved for other reasons? The vocal learning and rhythmic synchronization hypothesis proposes that our ability to move in time with an auditory beat in a precise, predictive and tempo-flexible manner originated in the neural circuitry for complex vocal learning. In the 15 years, since the hypothesis was proposed a variety of studies have supported it. However, one study has provided a significant challenge to the hypothesis. Furthermore, it is increasingly clear that vocal learning is not a binary trait animals have or lack, but varies more continuously across species. In the light of these developments and of recent progress in the neurobiology of beat processing and of vocal learning, the current paper revises the vocal learning hypothesis. It argues that an advanced form of vocal learning acts as a preadaptation for sporadic beat perception and synchronization (BPS), providing intrinsic rewards for predicting the temporal structure of complex acoustic sequences. It further proposes that in humans, mechanisms of gene-culture coevolution transformed this preadaptation into a genuine neural adaptation for sustained BPS. The larger significance of this proposal is that it outlines a hypothesis of cognitive gene-culture coevolution which makes testable predictions for neuroscience, cross-species studies and genetics. This article is part of the theme issue 'Synchrony and rhythm interaction: from the brain to behavioural ecology'.

From Cell Types to an Integrated Understanding of Brain Evolution: The Case of the Cerebral Cortex

With the discovery of the incredible diversity of neurons, Cajal and coworkers laid the foundation of modern neuroscience. Neuron types are not only structural units of nervous systems but also evolutionary units, because their identities are encoded in the genome. With the advent of high-throughput cellular transcriptomics, neuronal identities can be characterized and compared systematically across species. The comparison of neurons in mammals, reptiles, and birds indicates that the mammalian cerebral cortex is a mosaic of deeply conserved and recently evolved neuron types. Using the cerebral cortex as a case study, this review illustrates how comparing neuron types across species is key to reconciling observations on neural development, neuroanatomy, circuit wiring, and physiology for an integrated understanding of brain evolution.

Cockatiels sing human music in synchrony with a playback of the melody

It is known among aviculturists that cockatiels imitate human music with their whistle-like vocal sounds. The present study examined whether cockatiels are also able to sing “in unison”, or, line up their vocalizations with a musical melody so that they occur at the same time. Three hand-raised cockatiels were exposed to a musical melody of human whistling produced by an experimenter. All the birds learned to sing the melody. Then, two out of these three birds spontaneously joined in singing during an ongoing melody, so that the singing by the bird and the whistling by the human were nearly perfectly synchronous. Further experiments revealed that the birds actively adjusted their vocal timing to playback of a recording of the same melody. This means cockatiels have a remarkable ability for flexible vocal control similar to what is seen in human singing. The proximate/ultimate factors for this behavior and implications for musicality in humans are discussed.

Architectures of neuronal circuits

[Figure: see text].

Defining the multidimensional phenotype: New opportunities to integrate the behavioral ecology and behavioral neuroscience of vocal learning

Vocal learning has evolved independently in several lineages. This complex cognitive trait is commonly treated as binary: species either possess or lack it. This view has been a useful starting place to examine the origins of vocal learning, but is also incomplete and potentially misleading, as specific components of the vocal learning program - such as the timing, extent and nature of what is learned - vary widely among species. In our review we revive an idea first proposed by Beecher and Brenowitz (2005) by describing six dimensions of vocal learning: (1) which vocalizations are learned, (2) how much is learned, (3) when it is learned, (4) who it is learned from, (5) what is the extent of the internal template, and (6) how is the template integrated with social learning and innovation. We then highlight key examples of functional and mechanistic work on each dimension, largely from avian taxa, and discuss how a multi-dimensional framework can accelerate our understanding of why vocal learning has evolved, and how brains became capable of this important behaviour.

Colony expansions underlie the evolution of army ant mass raiding

The mass raids of army ants are an iconic collective phenomenon, in which many thousands of ants spontaneously leave their nest to hunt for food, mostly other arthropods. While the structure and ecology of these raids have been relatively well studied, how army ants evolved such complex cooperative behavior is not understood. Here, we show that army ant mass raiding has evolved from a different form of cooperative hunting called group raiding, in which a scout directs a small group of ants to a specific target through chemical communication. We describe the structure of group raids in the clonal raider ant, a close relative of army ants in the subfamily Dorylinae. We find evidence that the coarse structure of group raids and mass raids is highly conserved and that all doryline ants likely follow similar behavioral rules for raiding. We also find that the evolution of army ant mass raiding occurred concurrently with expansions in colony size. By experimentally increasing colony size in the clonal raider ant, we show that mass raiding gradually emerges from group raiding without altering individual behavioral rules. This suggests that increasing colony size can explain the evolution of army ant mass raids and supports the idea that complex social behaviors may evolve via mechanisms that need not alter the behavioral interaction rules that immediately underlie the collective behavior of interest.

A computational model of the shrimp-goby escape and communication system

Fish escape from approaching threats via a stereotyped escape behavior. This behavior, and the underlying neural circuit organized around the Mauthner cell command neurons, have both been extensively investigated experimentally, mainly in two laboratory model organisms, the goldfish and the zebrafish. However, fish biodiversity is enormous, a number of variants of the basal escape behavior exist. In marine gobies (a family of small benthic fishes) which share burrows with alpheid shrimp, the escape behavior has likely been partially modified into a tactile communication system which allow the fish to communicate the approach of a predatory fish to the shrimp. In this communication system, the goby responds to intermediate-strength threats with a brief tail-flick which the shrimp senses with its antennae.We investigated the shrimp goby escape and communication system with computational models. We asked how the circuitry of the basal escape behavior could be modified to produce behavior akin to the shrimp-goby communication system. In a simple model, we found that mutual inhibitions between Mauthner cells can be tuned to produce an oscillatory response to intermediate strength inputs, albeit only in a narrow parameter range.Using a more detailed model, we found that two modifications of the fish locomotor system transform it into a model reproducing the shrimp goby behavior. These modifications are: 1. modifying the central pattern generator which drives swimming such that it is quiescent when receiving no inputs; 2. introducing a direct sensory input to this central pattern generator, bypassing the Mauthner cells.

Brain Size Associated with Foot Preferences in Australian Parrots

Since foot preference of cockatoos and parrots to hold and manipulate food and other objects has been associated with better ability to perform certain tasks, we predicted that either strength or direction of foot preference would correlate with brain size. Our study of 25 psittacine species of Australia found that species with larger absolute brain mass have stronger foot preferences and that percent left-footedness is correlated positively with brain mass. In a sub-sample of 11 species, we found an association between foot preference and size of the nidopallial region of the telencephalon, an area equivalent to the mammalian cortex and including regions with executive function and other higher-level functions. Our analysis showed that percent left-foot use correlates positively and significantly with size of the nidopallium relative to the whole brain, but not with the relative size of the optic tecta. Psittacine species with stronger left-foot preferences have larger brains, with the nidopallium making up a greater proportion of those brains. Our results are the first to show an association between brain size and asymmetrical limb use by parrots and cockatoos. Our results support the hypothesis that limb preference enhances brain capacity and higher (nidopallial) functioning.

Memory-specific correlated neuronal activity in higher-order auditory regions of a parrot

Male budgerigars (Melopsittacus undulatus) are open-ended learners that can learn to produce new vocalisations as adults. We investigated neuronal activation in male budgerigars using the expression of the protein products of the immediate early genes zenk and c-fos in response to exposure to conspecific contact calls (CCs: that of the mate or an unfamiliar female) in three subregions (CMM, dNCM and vNCM) of the caudomedial pallium, a higher order auditory region. Significant positive correlations of Zenk expression were found between these subregions after exposure to mate CCs. In contrast, exposure to CCs of unfamiliar females produced no such correlations. These results suggest the presence of a CC-specific association among the subregions involved in auditory memory. The caudomedial pallium of the male budgerigar may have functional subdivisions that cooperate in the neuronal representation of auditory memory.

Cerebellar nuclei evolved by repeatedly duplicating a conserved cell-type set

How have complex brains evolved from simple circuits? Here we investigated brain region evolution at cell-type resolution in the cerebellar nuclei, the output structures of the cerebellum. Using single-nucleus RNA sequencing in mice, chickens, and humans, as well as STARmap spatial transcriptomic analysis and whole-central nervous system projection tracing, we identified a conserved cell-type set containing two region-specific excitatory neuron classes and three region-invariant inhibitory neuron classes. This set constitutes an archetypal cerebellar nucleus that was repeatedly duplicated to form new regions. The excitatory cell class that preferentially funnels information to lateral frontal cortices in mice becomes predominant in the massively expanded human lateral nucleus. Our data suggest a model of brain region evolution by duplication and divergence of entire cell-type sets.

Of Great Apes and Magpies: Initiations into Animal Behaviour

Simple Summary

Animal encounters have been favourite subjects for a long time and it would scarcely be novel to report such stories for their own sake, even though the ones told here are dramatic enough to stand on their own. The questions addressed in this paper are twofold. The first question is: What influence may particular and dramatic animal encounters have on the human observer and how dependent is such a response on previously held attitudes? This paper provides three cases studies of extraordinary moments that changed the lives of the human participants and turned them into advocates of the species they had encountered. The next question asked is how we can be respectful of animals without anthropomorphising them and study them in ways that help us understand their abilities and their needs rather than impose questions that mean much to the human researcher but could be irrelevant to the species? The examples given here compare and contrast species that are especially close to us (great apes) with studies of those that are distant from us in their evolution (birds) and show how different attitudes change the questions that can be asked by scientists, demonstrably leading to new and even stunning results.

Abstract

This paper presents three case studies of exceptional human encounters with animals. These particular examples were selected because they enabled analysis of the underlying reasons that led the human participants to respond in new ways to their animal counterparts. The question asked here is whether sudden insights into the needs and abilities of an animal arises purely from an anthropocentric position as empathy because of genetic closeness (e.g., chimpanzees) or is something else and whether new insights can be applied to other phylogenetic orders not close to us, e.g., birds, and change research questions and implicit prejudices and stereotypes. Particularly in avian species, phylogenetically distant from humans, the prejudices (anthroprocentric position) and the belief in human uniqueness (human exceptionalism) might be greater than in the reactions to primates. Interestingly, in studies of great apes, contradictory opinions and controversies about cognitive abilities, especially when compared with humans, tend to be pronounced. Species appropriateness in test designs are desirable present and future goals but here it is suggested how different experiences can also lead to different questions that explode the myth of human uniqueness and then arrive at entirely different and new results in cognitive and affective abilities of the species under investigation.

Fruitless mutant male mosquitoes gain attraction to human odor

The Aedesaegypti mosquito shows extreme sexual dimorphism in feeding. Only females are attracted to and obtain a blood-meal from humans, which they use to stimulate egg production. The fruitless gene is sex-specifically spliced and encodes a BTB zinc-finger transcription factor proposed to be a master regulator of male courtship and mating behavior across insects. We generated fruitless mutant mosquitoes and showed that males failed to mate, confirming the ancestral function of this gene in male sexual behavior. Remarkably, fruitless males also gain strong attraction to a live human host, a behavior that wild-type males never display, suggesting that male mosquitoes possess the central or peripheral neural circuits required to host-seek and that removing fruitless reveals this latent behavior in males. Our results highlight an unexpected repurposing of a master regulator of male-specific sexual behavior to control one module of female-specific blood-feeding behavior in a deadly vector of infectious diseases.

Mapping Human Laryngeal Motor Cortex during Vocalization

The representations of the articulators involved in human speech production are organized somatotopically in primary motor cortex. The neural representation of the larynx, however, remains debated. Both a dorsal and a ventral larynx representation have been previously described. It is unknown, however, whether both representations are located in primary motor cortex. Here, we mapped the motor representations of the human larynx using functional magnetic resonance imaging and characterized the cortical microstructure underlying the activated regions. We isolated brain activity related to laryngeal activity during vocalization while controlling for breathing. We also mapped the articulators (the lips and tongue) and the hand area. We found two separate activations during vocalization-a dorsal and a ventral larynx representation. Structural and quantitative neuroimaging revealed that myelin content and cortical thickness underlying the dorsal, but not the ventral larynx representation, are similar to those of other primary motor representations. This finding confirms that the dorsal larynx representation is located in primary motor cortex and that the ventral one is not. We further speculate that the location of the ventral larynx representation is in premotor cortex, as seen in other primates. It remains unclear, however, whether and how these two representations differentially contribute to laryngeal motor control.

Fate of Duplicated Neural Structures

Statistical physics determines the abundance of different arrangements of matter depending on cost-benefit balances. Its formalism and phenomenology percolate throughout biological processes and set limits to effective computation. Under specific conditions, self-replicating and computationally complex patterns become favored, yielding life, cognition, and Darwinian evolution. Neurons and neural circuits sit at a crossroads between statistical physics, computation, and (through their role in cognition) natural selection. Can we establish a statistical physics of neural circuits? Such theory would tell what kinds of brains to expect under set energetic, evolutionary, and computational conditions. With this big picture in mind, we focus on the fate of duplicated neural circuits. We look at examples from central nervous systems, with stress on computational thresholds that might prompt this redundancy. We also study a naive cost-benefit balance for duplicated circuits implementing complex phenotypes. From this, we derive phase diagrams and (phase-like) transitions between single and duplicated circuits, which constrain evolutionary paths to complex cognition. Back to the big picture, similar phase diagrams and transitions might constrain I/O and internal connectivity patterns of neural circuits at large. The formalism of statistical physics seems to be a natural framework for this worthy line of research.

Spontaneity and diversity of movement to music are not uniquely human

Spontaneous movement to music occurs in every human culture and is a foundation of dance [1]. This response to music is absent in most species (including monkeys), yet it occurs in parrots, perhaps because they (like humans, and unlike monkeys) are vocal learners whose brains contain strong auditory-motor connections, conferring sophisticated audiomotor processing abilities [2,3]. Previous research has shown that parrots can bob their heads or lift their feet in synchrony with a musical beat [2,3], but humans move to music using a wide variety of movements and body parts. Is this also true of parrots? If so, it would constrain theories of how movement to music is controlled by parrot brains. Specifically, as head bobbing is part of parrot courtship displays [4] and foot lifting is part of locomotion, these may be innate movements controlled by central pattern generators which become entrained by auditory rhythms, without the involvement of complex motor planning. This would be unlike humans, where movement to music engages cortical networks including frontal and parietal areas [5]. Rich diversity in parrot movement to music would suggest a strong contribution of forebrain regions to this behavior, perhaps including motor learning regions abutting the complex vocal-learning 'shell' regions that are unique to parrots among vocal learning birds [6]. Here we report that a sulphur-crested cockatoo (Cacatua galerita eleonora) responds to music with remarkably diverse spontaneous movements employing a variety of body parts, and suggest why parrots share this response with humans.

Distributed physiology and the molecular basis of social life in eusocial insects

The traditional focus of physiological and functional genomic research is on molecular processes that play out within a single multicellular organism. In the colonial (eusocial) insects such as ants, bees, and termites, molecular and behavioral responses of interacting nestmates are tightly linked, and key physiological processes are regulated at the scale of the colony. Such colony-level physiological processes regulate nestmate physiology in a distributed fashion, through various social communication mechanisms. As a result of physiological decentralization over evolutionary time, organismal mechanisms, for example related to pheromone detection, hormone signaling, and neural signaling pathways, are deployed in novel contexts to influence nestmate and colony traits. Here we explore how functional genomic, physiological, and behavioral studies can benefit from considering the traits of eusocial insects in this light. We highlight functional genomic work exploring how nestmate-level and colony-level traits arise and are influenced by interactions among physiologically-specialized nestmates of various developmental stages. We also consider similarities and differences between nestmate-level (organismal) and colony-level (superorganismal) physiological processes, and make specific hypotheses regarding the physiology of eusocial taxa. Integrating theoretical models of distributed systems with empirical functional genomics approaches will be useful in addressing fundamental questions related to the evolution of eusociality and collective behavior in natural systems.