Telencephalic connections of the trigeminal system in the pigeon (Columba livia): a trigeminal sensorimotor circuit

Variations in touch representation in the hummingbird and zebra finch forebrain

Somatosensation is essential for animals to perceive the external world through touch, allowing them to detect physical contact, temperature, pain, and body position. Studies on rodent vibrissae have highlighted the organization and processing in mammalian somatosensory pathways.1,2 Comparative research across vertebrates is vital for understanding evolutionary influences and ecological specialization on somatosensory systems. Birds, with their diverse morphologies, sensory abilities, and behaviors, serve as ideal models for investigating the evolution of somatosensation. Prior studies have uncovered tactile-responsive areas within the avian telencephalon, particularly in pigeons,3,4,5,6 parrots,7 and finches,8 but variations in somatosensory maps and responses across avian species are not fully understood. This study aims to explore somatotopic organization and neural coding in the telencephalon of Anna's hummingbirds (Calypte anna) and zebra finches (Taeniopygia guttata) by using in vivo extracellular electrophysiology to record activity in response to controlled tactile stimuli on various body regions. These findings reveal unique representations of body regions across distinct forebrain somatosensory nuclei, indicating significant differences in the extent of areas dedicated to certain body surfaces, which may correlate with their behavioral importance.

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.

Morphology of the "prefrontal" nidopallium caudolaterale in the long-distance night-migratory Eurasian blackcap (Sylvia atricapilla)

Migrating birds have developed remarkable navigational capabilities to successfully master biannual journeys between their breeding and wintering grounds. To reach their intended destination, they need to calculate navigational goals from a large variety of natural directional and positional cues to set a meaningful motor output command. One brain area, which has been associated with such executive functions, is the nidopallium caudolaterale (NCL), which, due to its striking similarities in terms of neurochemistry, connectivity and function, is considered analogous to the mammalian prefrontal cortex. To establish a baseline for further analyses elucidating the neuronal correlates underlying avian navigation, we performed quantitative and qualitative analyses of dopaminergic fibres in the brains of long-distance night-migratory Eurasian blackcaps (Sylvia atricapilla). We identified four regions in the caudal telencephalon, each of which was characterized by its specific dopaminergic innervation pattern. At least three of them presumably constitute subareas of the NCL in Eurasian blackcaps and could thus be involved in integrating navigational input from different sensory systems. The observed heterogeneity and parcellation of the NCL subcompartments in this migratory species could be a consequence of the special demands related to navigation.

“Prefrontal” Neuronal Foundations of Visual Asymmetries in Pigeons

This study was conducted in order to reveal the possibly lateralized processes in the avian nidopallium caudolaterale (NCL), a functional analogue to the mammalian prefrontal cortex, during a color discrimination task. Pigeons are known to be visually lateralized with a superiority of the left hemisphere/right eye for visual feature discriminations. While animals were working on a color discrimination task, we recorded single visuomotor neurons in left and right NCL. As expected, pigeons learned faster and responded more quickly when seeing the stimuli with their right eyes. Our electrophysiological recordings discovered several neuronal properties of NCL neurons that possibly contributed to this behavioral asymmetry. We found that the speed of stimulus encoding was identical between left and right NCL but action generation was different. Here, most left hemispheric NCL neurons reached their peak activities shortly before response execution. In contrast, the majority of right hemispheric neurons lagged behind and came too late to control the response. Thus, the left NCL dominated the animals’ behavior not by a higher efficacy of encoding, but by being faster in monopolizing the operant response. A further asymmetry concerned the hemisphere-specific integration of input from the contra- and ipsilateral eye. The left NCL was able to integrate and process visual input from the ipsilateral eye to a higher degree and thus achieved a more bilateral representation of two visual fields. We combine these novel findings with those from previous publications to come up with a working hypothesis that could explain how hemispheric asymmetries for visual feature discrimination in birds are realized by a sequential buildup of lateralized neuronal response properties in the avian forebrain.

In Search for the Avian Trigeminal Magnetic Sensor: Distribution of Peripheral and Central Terminals of Ophthalmic Sensory Neurons in the Night-Migratory Eurasian Blackcap (Sylvia atricapilla)

In night-migratory songbirds, neurobiological and behavioral evidence suggest the existence of a magnetic sense associated with the ophthalmic branch of the trigeminal nerve (V1), possibly providing magnetic positional information. Curiously, neither the unequivocal existence, structural nature, nor the exact location of any sensory structure has been revealed to date. Here, we used neuronal tract tracing to map both the innervation fields in the upper beak and the detailed trigeminal brainstem terminations of the medial and lateral V1 subbranches in the night-migratory Eurasian Blackcap (Sylvia atricapilla). The medial V1 subbranch takes its course along the ventral part of the upper beak to innervate subepidermal layers and the mucosa of the nasal cavity, whereas the lateral V1 subbranch runs along dorsolateral levels until the nostrils to innervate mainly the skin of the upper beak. In the trigeminal brainstem, medial V1 terminals innervate both the dorsal part and the ventral, magnetically activated part of the principal sensory trigeminal brainstem nuclei (PrV). In contrast, the lateral V1 subbranch innervates only a small part of the ventral PrV. The spinal sensory trigeminal brainstem nuclei (SpV) receive topographically ordered projections. The medial V1 subbranch mainly innervates rostral and medial parts of SpV, whereas the lateral V1 subbranch mainly innervates the lateral and caudal parts of SpV. The present findings could provide valuable information for further analysis of the trigeminal magnetic sense of birds.

A canonical interlaminar circuit in the sensory dorsal ventricular ridge of birds: The anatomical organization of the trigeminal pallium

The dorsal ventricular ridge (DVR), which is the largest component of the avian pallium, contains discrete partitions receiving tectovisual, auditory, and trigeminal ascending projections. Recent studies have shown that the auditory and the tectovisual regions can be regarded as complexes composed of three highly interconnected layers: an internal senso-recipient one, an intermediate afferent/efferent one, and a more external re-entrant one. Cells located in homotopic positions in each of these layers are reciprocally linked by an interlaminar loop of axonal processes, forming columnar-like local circuits. Whether this type of organization also extends to the trigemino-recipient DVR is, at present, not known. This question is of interest, since afferents forming this sensory pathway, exceptional among amniotes, are not thalamic but rhombencephalic in origin. We investigated this question by placing minute injections of neural tracers into selected locations of vital slices of the chicken telencephalon. We found that neurons of the trigemino-recipient nucleus basorostralis pallii (Bas) establish reciprocal, columnar and homotopical projections with cells located in the overlying ventral mesopallium (MV). "Column-forming" axons originated in B and MV terminate also in the intermediate strip, the fronto-trigeminal nidopallium (NFT), in a restricted manner. We also found that the NFT and an internal partition of B originate substantial, coarse-topographic projections to the underlying portion of the lateral striatum. We conclude that all sensory areas of the DVR are organized according to a common neuroarchitectonic motif, which bears a striking resemblance to that of the radial/laminar intrinsic circuits of the sensory cortices of mammals.

Endocranial Anatomy of the Giant Extinct Australian Mihirung Birds (Aves, Dromornithidae)

Dromornithids are an extinct group of large flightless birds from the Cenozoic of Australia. Their record extends from the Eocene to the late Pleistocene. Four genera and eight species are currently recognised, with diversity highest in the Miocene. Dromornithids were once considered ratites, but since the discovery of cranial elements, phylogenetic analyses have placed them near the base of the anseriforms or, most recently, resolved them as stem galliforms. In this study, we use morphometric methods to comprehensively describe dromornithid endocranial morphology for the first time, comparing Ilbandornis woodburnei and three species of Dromornis to one another and to four species of extant basal galloanseres. We reveal that major endocranial reconfiguration was associated with cranial foreshortening in a temporal series along the Dromornis lineage. Five key differences are evident between the brain morphology of Ilbandornis and Dromornis, relating to the medial wulst, the ventral eminence of the caudoventral telencephalon, and morphology of the metencephalon (cerebellum + pons). Additionally, dromornithid brains display distinctive dorsal (rostral position of the wulst), and ventral morphology (form of the maxillomandibular [V2+V3], glossopharyngeal [IX], and vagus [X] cranial nerves), supporting hypotheses that dromornithids are more closely related to basal galliforms than anseriforms. Functional interpretations suggest that dromornithids were specialised herbivores that likely possessed well-developed stereoscopic depth perception, were diurnal and targeted a soft browse trophic niche.

A cortex-like canonical circuit in the avian forebrain

Although the avian pallium seems to lack an organization akin to that of the cerebral cortex, birds exhibit extraordinary cognitive skills that are comparable to those of mammals. We analyzed the fiber architecture of the avian pallium with three-dimensional polarized light imaging and subsequently reconstructed local and associative pallial circuits with tracing techniques. We discovered an iteratively repeated, column-like neuronal circuitry across the layer-like nuclear boundaries of the hyperpallium and the sensory dorsal ventricular ridge. These circuits are connected to neighboring columns and, via tangential layer-like connections, to higher associative and motor areas. Our findings indicate that this avian canonical circuitry is similar to its mammalian counterpart and might constitute the structural basis of neuronal computation.

Event-related functional MRI of awake behaving pigeons at 7T

Animal-fMRI is a powerful method to understand neural mechanisms of cognition, but it remains a major challenge to scan actively participating small animals under low-stress conditions. Here, we present an event-related functional MRI platform in awake pigeons using single-shot RARE fMRI to investigate the neural fundaments for visually-guided decision making. We established a head-fixated Go/NoGo paradigm, which the animals quickly learned under low-stress conditions. The animals were motivated by water reward and behavior was assessed by logging mandibulations during the fMRI experiment with close to zero motion artifacts over hundreds of repeats. To achieve optimal results, we characterized the species-specific hemodynamic response function. As a proof-of-principle, we run a color discrimination task and discovered differential neural networks for Go-, NoGo-, and response execution-phases. Our findings open the door to visualize the neural fundaments of perceptual and cognitive functions in birds-a vertebrate class of which some clades are cognitively on par with primates.

Brain Lateralization: A Comparative Perspective

Comparative studies on brain asymmetry date back to the 19th century but then largely disappeared due to the assumption that lateralization is uniquely human. Since the reemergence of this field in the 1970s, we learned that left-right differences of brain and behavior exist throughout the animal kingdom and pay off in terms of sensory, cognitive, and motor efficiency. Ontogenetically, lateralization starts in many species with asymmetrical expression patterns of genes within the Nodal cascade that set up the scene for later complex interactions of genetic, environmental, and epigenetic factors. These take effect during different time points of ontogeny and create asymmetries of neural networks in diverse species. As a result, depending on task demands, left- or right-hemispheric loops of feedforward or feedback projections are then activated and can temporarily dominate a neural process. In addition, asymmetries of commissural transfer can shape lateralized processes in each hemisphere. It is still unclear if interhemispheric interactions depend on an inhibition/excitation dichotomy or instead adjust the contralateral temporal neural structure to delay the other hemisphere or synchronize with it during joint action. As outlined in our review, novel animal models and approaches could be established in the last decades, and they already produced a substantial increase of knowledge. Since there is practically no realm of human perception, cognition, emotion, or action that is not affected by our lateralized neural organization, insights from these comparative studies are crucial to understand the functions and pathologies of our asymmetric brain.

Sensory systems in birds: What we have learned from studying sensory specialists

"Diversity" is an apt descriptor of the research career of Jack Pettigrew as it ranged from the study of trees, to clinical conditions, to sensory neuroscience. Within sensory neuroscience, he was fascinated by the evolution of sensory systems across species. Here, we review some of his work on avian sensory specialists and research that he inspired in others. We begin with an overview of the importance of the Wulst in stereopsis and the need for further study of the Wulst in relation to binocularity across avian species. Next, we summarize recent anatomical, behavioral, and physiological studies on optic flow specializations in hummingbirds. Beyond vision, we discuss the first evidence of a tactile "fovea" in birds and how this led to detailed studies of tactile specializations in waterfowl and sensorimotor systems in parrots. We then describe preliminary studies by Pettigrew of two endemic Australian species, the plains-wanderer (Pedionomus torquatus) and letter-winged kite (Elanus scriptus), that suggest the evolution of some unique auditory and visual specializations in relation to their unique behavior and ecology. Finally, we conclude by emphasizing the importance of a comparative and integrative approach to understanding avian sensory systems and provide an example of one system that has yet to be properly examined: tactile facial bristles in birds. Through reviewing this research and offering future avenues for discovery, we hope that others also embrace the comparative approach to understanding sensory system evolution in birds and other vertebrates.

A comparative analysis of the dopaminergic innervation of the executive caudal nidopallium in pigeon, chicken, zebra finch, and carrion crow

Despite the long, separate evolutionary history of birds and mammals, both lineages developed a rich behavioral repertoire of remarkably similar executive control generated by distinctly different brains. The seat for executive functioning in birds is the nidopallium caudolaterale (NCL) and the mammalian equivalent is known as the prefrontal cortex (PFC). Both are densely innervated by dopaminergic fibers, and are an integration center of sensory input and motor output. Whereas the variation of the PFC has been well documented in different mammalian orders, we know very little about the NCL across the avian clade. In order to investigate whether this structure adheres to species-specific variations, this study aimed to describe the trajectory of the NCL in pigeon, chicken, carrion crow and zebra finch. We employed immunohistochemistry to map dopaminergic innervation, and executed a Gallyas stain to visualize the dorsal arcopallial tract that runs between the NCL and the arcopallium. Our analysis showed that whereas the trajectory of the NCL in the chicken is highly comparable to the pigeon, the two Passeriformes show a strikingly different pattern. In both carrion crow and zebra finch, we identified four different subareas of high dopaminergic innervation that span the entire caudal forebrain. Based on their sensory input, motor output, and involvement in dopamine-related cognitive control of the delineated areas here, we propose that at least three morphologically different subareas constitute the NCL in these songbirds. Thus, our study shows that comparable to the PFC in mammals, the NCL in birds varies considerably across species.

A newly identified trigeminal brain pathway in a night-migratory bird could be dedicated to transmitting magnetic map information

Night-migratory songbirds can use geomagnetic information to navigate over thousands of kilometres with great precision. A crucial part of the magnetic 'map' information used by night-migratory songbirds is conveyed via the ophthalmic branches of the trigeminal nerves to the trigeminal brainstem complex, where magnetic-driven neuronal activation has been observed. However, it is not known how this information reaches the forebrain for further processing. Here, we show that the magnetically activated region in the trigeminal brainstem of migratory Eurasian blackcaps (Sylvia atricapilla) represents a morphologically distinctive neuronal population with an exclusive and previously undescribed projection to the telencephalic frontal nidopallium. This projection is clearly different from the known trigeminal somatosensory pathway that we also confirmed both by neuronal tracing and by a thorough morphometric analysis of projecting neurons. The new pathway we identified here represents part of a brain circuit that-based on the known nidopallial connectivities in birds-could potentially transmit magnetic 'map' information to key multisensory integration centres in the brain known to be critically involved in spatial memory formation, cognition and/or controlling executive behaviour, such as navigation, in birds.

Immediate early gene fingerprints of multi-component behaviour

The ability to execute different responses in an expedient temporal order is central for efficient goal-directed actions and often referred to as multi-component behaviour. However, the underlying neural mechanisms on a cellular level remain unclear. Here we establish a link between neural activity at the cellular level within functional neuroanatomical structures to this form of goal-directed behaviour by analyzing immediate early gene (IEG) expression in an animal model, the pigeon (Columba livia). We focus on the group of zif268 IEGs and ZENK in particular. We show that when birds have to cascade separate task goals, ZENK expression is increased in the avian equivalent of the mammalian prefrontal cortex, i.e. the nidopallium caudolaterale (NCL) as well as in the homologous striatum. Moreover, in the NCL as well as in the medial striatum (MSt), the degree of ZENK expression was highly correlated with the efficiency of multi-component behaviour. The results provide the first link between cellular IEG expression and behavioural outcome in multitasking situations. Moreover, the data suggest that the function of the fronto-striatal circuitry is comparable across species indicating that there is limited flexibility in the implementation of complex cognition such as multi-component behaviour within functional neuroanatomical structures.

The chick pallium displays divergent expression patterns of chick orthologues of mammalian neocortical deep layer-specific genes

The avian pallium is organised into clusters of neurons and does not have layered structures such as those seen in the mammalian neocortex. The evolutionary relationship between sub-regions of avian pallium and layers of mammalian neocortex remains unclear. One hypothesis, based on the similarities in neural connections of the motor output neurons that project to sub-pallial targets, proposed the cell-type homology between brainstem projection neurons in neocortex layers 5 or 6 (L5/6) and those in the avian arcopallium. Recent studies have suggested that gene expression patterns are associated with neural connection patterns, which supports the cell-type homology hypothesis. However, a limited number of genes were used in these studies. Here, we showed that chick orthologues of mammalian L5/6-specific genes, nuclear receptor subfamily 4 group A member 2 and connective tissue growth factor, were strongly expressed in the arcopallium. However, other chick orthologues of L5/6-specific genes were primarily expressed in regions other than the arcopallium. Our results do not fully support the cell-type homology hypothesis. This suggests that the cell types of brainstem projection neurons are not conserved between the avian arcopallium and the mammalian neocortex L5/6. Our findings may help understand the evolution of pallium between birds and mammals.

Selective response of the nucleus taeniae of the amygdala to a naturalistic social stimulus in visually naive domestic chicks

The detection of animate beings at the onset of life is important for phylogenetically distant species, such as birds and primates. Naïve chicks preferentially approach a stimulus resembling a conspecific (a stuffed fowl) over a less naturalistic one (a scrambled version of the stuffed fowl, presenting the same low-level visual features as the fowl in an unnatural configuration). The neuronal mechanisms underlying this behavior are mostly unknown. However, it has been hypothesized that innate social predispositions may involve subpallial brain areas including the amygdala. Here we asked whether a stuffed hen would activate areas of the arcopallium/amygdala complex, in particular the nucleus taeniae of the amygdala (TnA) or septum. We measured brain activity by visualizing the immediate early gene product c-Fos. After exposure to the hen, TnA showed higher density of c-Fos expressing neurons, compared to chicks that were exposed to the scrambled stimulus. A similar trend was present in the lower portion of the arcopallium, but not in the upper portion of the arcopallium or in the septum. This demonstrates that at birth the TnA is already engaged in responses to social visual stimuli, suggesting an important role for this nucleus in the early ontogenetic development of social behavior.

Control of bill-grasping aperture with varying food size in crows

Grasping movement in primates is known to be a visually guided behavior and the aperture of hand opening is adjusted to the target size on the basis of visual information. The analogous behavior can be found in birds, called 'pecking', consisting of head-reaching and bill-grasping. Bill-grasping has been investigated mainly in pigeons and an aperture adjustment as seen in primates has been reported. This study focused on kinematics of pecking in crows, known to possess dexterous visuomotor skills, to examine whether crows adjust the grasping aperture to food diameter with a kinematic mechanism similar to that in pigeons. The pecking at a small piece of food was video recorded to analyze the grasping aperture. The results showed that the grasping aperture was proportional to food diameter. Kinematic analysis showed that the aperture adjustment was mediated by grasping velocity and grasping duration, which is consistent with the findings of previous research on pecking in pigeons. However, the relative contribution of grasping velocity was much higher than that of grasping duration. Our findings suggest the different sensorimotor mechanisms to control bill-grasping between the avian species with different foraging ecology.

Complex social ecology needs complex machineries of foraging

Uncertainty is caused not only by environmental changes, but also by social interference resulting from competition over food resources. Actually, foraging effort is socially facilitated, which, however, does not require incentive control by the dopamine system; Zajonc's "drive" theory is thus questionable. Instead, social adjustments may be pre-embedded in the limbic network responsible for decisions of appropriate effort-cost investment.

NMDA receptors in the avian amygdala and the premotor arcopallium mediate distinct aspects of appetitive extinction learning

Extinction learning is an essential mechanism that enables constant adaptation to ever-changing environmental conditions. The underlying neural circuit is mostly studied with rodent models using auditory cued fear conditioning. In order to uncover the variant and the invariant neural properties of extinction learning, we adopted pigeons as an animal model in an appetitive sign-tracking paradigm. The animals firstly learned to respond to two conditioned stimuli in two different contexts (CS-1 in context A and CS-2 in context B), before conditioned responses to the stimuli were extinguished in the opposite contexts (CS-1 in context B and CS-2 in context A). Subsequently, responding to both stimuli was tested in both contexts. Prior to extinction training, we locally injected the N-methyl-d-aspartate receptor (NMDAR) antagonist 2-Amino-5-phosphonovaleric acid (APV) in either the amygdala or the (pre)motor arcopallium to investigate their involvement in extinction learning. Our findings suggest that the encoding of extinction memory required the activation of amygdala, as visible by an impairment of extinction acquisition by concurrent inactivation of local NMDARs. In contrast, consolidation and subsequent retrieval of extinction memory recruited the (pre)motor arcopallium. Also, the inactivation of arcopallial NMDARs induced a general motoric slowing during extinction training. Thus, our results reveal a double dissociation between arcopallium and amygdala with respect to acquisition and consolidation of extinction, respectively. Our study therefore provides new insights on the two key components of the avian extinction network and their resemblance to the data obtained from mammals, possibly indicating a shared neural mechanism underlying extinction learning shaped by evolution.

Transmitter receptors reveal segregation of the arcopallium/amygdala complex in pigeons (Columba livia)

At the beginning of the 20th century it was suggested that a complex group of nuclei in the avian posterior ventral telencephalon is comparable to the mammalian amygdala. Subsequent findings, however, revealed that most of these structures share premotor characteristics, while some indeed constitute the avian amygdala. These developments resulted in 2004 in a change of nomenclature of these nuclei, which from then on were named arcopallial or amygdala nuclei and referred to as the arcopallium/amygdala complex. The structural basis for the similarities between avian and mammalian arcopallial and amygdala subregions is poorly understood. Therefore, we analyzed binding site densities for glutamatergic AMPA, NMDA and kainate, GABAergic GABA_A , muscarinic M₁ , M₂ and nicotinic acetylcholine (nACh; α₄ β₂ subtype), noradrenergic α₁ and α₂ , serotonergic 5-HT_1A and dopaminergic D_1/5 receptors using quantitative in vitro receptor autoradiography combined with a detailed analysis of the cyto- and myelo-architecture. Our approach supports a segregation of the pigeon's arcopallium/amygdala complex into the following subregions: the arcopallium anterius (AA), the arcopallium ventrale (AV), the arcopallium dorsale (AD), the arcopallium intermedium (AI), the arcopallium mediale (AM), the arcopallium posterius (AP), the nucleus posterioris amygdalopallii pars basalis (PoAb) and pars compacta (PoAc), the nucleus taeniae amgygdalae (TnA) and the area subpallialis amygdalae (SpA). Some of these subregions showed further subnuclei and each region of the arcopallium/amygdala complex are characterized by a distinct multi-receptor density expression. Here we provide a new detailed map of the pigeon's arcopallium/amygdala complex and compare the receptor architecture of the subregions to their possible mammalian counterparts.

Cortical inter-hemispheric circuits for multimodal vocal learning in songbirds

Vocal learning in songbirds and humans is strongly influenced by social interactions based on sensory inputs from several modalities. Songbird vocal learning is mediated by cortico-basal ganglia circuits that include the SHELL region of lateral magnocellular nucleus of the anterior nidopallium (LMAN), but little is known concerning neural pathways that could integrate multimodal sensory information with SHELL circuitry. In addition, cortical pathways that mediate the precise coordination between hemispheres required for song production have been little studied. In order to identify candidate mechanisms for multimodal sensory integration and bilateral coordination for vocal learning in zebra finches, we investigated the anatomical organization of two regions that receive input from SHELL: the dorsal caudolateral nidopallium (dNCL_SHELL ) and a region within the ventral arcopallium (Av). Anterograde and retrograde tracing experiments revealed a topographically organized inter-hemispheric circuit: SHELL and dNCL_SHELL , as well as adjacent nidopallial areas, send axonal projections to ipsilateral Av; Av in turn projects to contralateral SHELL, dNCL_SHELL , and regions of nidopallium adjacent to each. Av on each side also projects directly to contralateral Av. dNCL_SHELL and Av each integrate inputs from ipsilateral SHELL with inputs from sensory regions in surrounding nidopallium, suggesting that they function to integrate multimodal sensory information with song-related responses within LMAN-SHELL during vocal learning. Av projections share this integrated information from the ipsilateral hemisphere with contralateral sensory and song-learning regions. Our results suggest that the inter-hemispheric pathway through Av may function to integrate multimodal sensory feedback with vocal-learning circuitry and coordinate bilateral vocal behavior.

Connectivity and neurochemistry of the commissura anterior of the pigeon (Columba livia)

The anterior commissure (AC) and the much smaller hippocampal commissure constitute the only interhemispheric pathways at the telencephalic level in birds. Since the degeneration study from Zeier and Karten (), no detailed description of the topographic organization of the AC has been performed. This information is not only necessary for a better understanding of interhemispheric transfer in birds, but also for a comparative analysis of the evolution of commissural systems in the vertebrate classes. We therefore examined the fiber connections of the AC by using choleratoxin subunit B (CTB) and biotinylated dextran amine (BDA). Injections into subareas of the arcopallium and posterior amygdala (PoA) demonstrated contralateral projection fields within the anterior arcopallium (AA), intermediate arcopallium (AI), PoA, lateral, caudolateral and central nidopallium, dorsal and ventral mesopallium, and medial striatum (MSt). Interestingly, only arcopallial and amygdaloid projections were reciprocally organized, and all AC projections originated within a rather small area of the arcopallium and the PoA. The commissural neurons were not GABA-positive, and thus possibly not of an inhibitory nature. In sum, our neuroanatomical study demonstrates that a small group of arcopallial and amygdaloid neurons constitute a wide range of contralateral projections to sensorimotor and limbic structures. Different from mammals, in birds the neurons that project via the AC constitute mostly heterotopically organized and unidirectional connections. In addition, the great majority of pallial areas do not participate by themselves in interhemispheric exchange in birds. Instead, commissural exchange rests on a rather small arcopallial and amygdaloid cluster of neurons.

Integrating brain, behavior, and phylogeny to understand the evolution of sensory systems in birds

The comparative anatomy of sensory systems has played a major role in developing theories and principles central to evolutionary neuroscience. This includes the central tenet of many comparative studies, the principle of proper mass, which states that the size of a neural structure reflects its processing capacity. The size of structures within the sensory system is not, however, the only salient variable in sensory evolution. Further, the evolution of the brain and behavior are intimately tied to phylogenetic history, requiring studies to integrate neuroanatomy with behavior and phylogeny to gain a more holistic view of brain evolution. Birds have proven to be a useful group for these studies because of widespread interest in their phylogenetic relationships and a wealth of information on the functional organization of most of their sensory pathways. In this review, we examine the principle of proper mass in relation differences in the sensory capabilities among birds. We discuss how neuroanatomy, behavior, and phylogeny can be integrated to understand the evolution of sensory systems in birds providing evidence from visual, auditory, and somatosensory systems. We also consider the concept of a "trade-off," whereby one sensory system (or subpathway within a sensory system), may be expanded in size, at the expense of others, which are reduced in size.

Magnetic field-driven induction of ZENK in the trigeminal system of pigeons (Columba livia)

Magnetoreception remains one of the few unsolved mysteries in sensory biology. The upper beak, which is innervated by the ophthalmic branch of the trigeminal nerve (V1), has been suggested to contain magnetic sensors based on ferromagnetic structures. Recently, its existence in pigeons has been seriously challenged by studies suggesting that the previously described iron-accumulations are macrophages, not magnetosensitive nerve endings. This raised the fundamental question of whether V1 is involved in magnetoreception in pigeons at all. We exposed pigeons to either a constantly changing magnetic field (CMF), to a zero magnetic field providing no magnetic information, or to CMF conditions after V1 was cut bilaterally. Using immediate early genes as a marker of neuronal responsiveness, we report that the trigeminal brainstem nuclei of pigeons, which receive V1 input, are activated under CMF conditions and that this neuronal activation disappears if the magnetic stimuli are removed or if V1 is cut. Our data suggest that the trigeminal system in pigeons is involved in processing magnetic field information and that V1 transmits this information from currently unknown, V1-associated magnetosensors to the brain.

The anatomy of the bill tip of kiwi and associated somatosensory regions of the brain: comparisons with shorebirds

Three families of probe-foraging birds, Scolopacidae (sandpipers and snipes), Apterygidae (kiwi), and Threskiornithidae (ibises, including spoonbills) have independently evolved long, narrow bills containing clusters of vibration-sensitive mechanoreceptors (Herbst corpuscles) within pits in the bill-tip. These 'bill-tip organs' allow birds to detect buried or submerged prey via substrate-borne vibrations and/or interstitial pressure gradients. Shorebirds, kiwi and ibises are only distantly related, with the phylogenetic divide between kiwi and the other two taxa being particularly deep. We compared the bill-tip structure and associated somatosensory regions in the brains of kiwi and shorebirds to understand the degree of convergence of these systems between the two taxa. For comparison, we also included data from other taxa including waterfowl (Anatidae) and parrots (Psittaculidae and Cacatuidae), non-apterygid ratites, and other probe-foraging and non probe-foraging birds including non-scolopacid shorebirds (Charadriidae, Haematopodidae, Recurvirostridae and Sternidae). We show that the bill-tip organ structure was broadly similar between the Apterygidae and Scolopacidae, however some inter-specific variation was found in the number, shape and orientation of sensory pits between the two groups. Kiwi, scolopacid shorebirds, waterfowl and parrots all shared hypertrophy or near-hypertrophy of the principal sensory trigeminal nucleus. Hypertrophy of the nucleus basorostralis, however, occurred only in waterfowl, kiwi, three of the scolopacid species examined and a species of oystercatcher (Charadriiformes: Haematopodidae). Hypertrophy of the principal sensory trigeminal nucleus in kiwi, Scolopacidae, and other tactile specialists appears to have co-evolved alongside bill-tip specializations, whereas hypertrophy of nucleus basorostralis may be influenced to a greater extent by other sensory inputs. We suggest that similarities between kiwi and scolopacid bill-tip organs and associated somatosensory brain regions are likely a result of similar ecological selective pressures, with inter-specific variations reflecting finer-scale niche differentiation.

Large-scale network organization in the avian forebrain: a connectivity matrix and theoretical analysis

Many species of birds, including pigeons, possess demonstrable cognitive capacities, and some are capable of cognitive feats matching those of apes. Since mammalian cortex is laminar while the avian telencephalon is nucleated, it is natural to ask whether the brains of these two cognitively capable taxa, despite their apparent anatomical dissimilarities, might exhibit common principles of organization on some level. Complementing recent investigations of macro-scale brain connectivity in mammals, including humans and macaques, we here present the first large-scale "wiring diagram" for the forebrain of a bird. Using graph theory, we show that the pigeon telencephalon is organized along similar lines to that of a mammal. Both are modular, small-world networks with a connective core of hub nodes that includes prefrontal-like and hippocampal structures. These hub nodes are, topologically speaking, the most central regions of the pigeon's brain, as well as being the most richly connected, implying a crucial role in information flow. Overall, our analysis suggests that indeed, despite the absence of cortical layers and close to 300 million years of separate evolution, the connectivity of the avian brain conforms to the same organizational principles as the mammalian brain.

Identification of two forebrain structures that mediate execution of memorized sequences in the pigeon

The execution of action sequences is the basis of most behavior. However, little is known about the neural foundation of visuomotor sequence execution in birds, although pigeons are a classic model animal to study sequence learning and production. Recently, we identified two structures in the pigeon brain, the nidopallium intermedium medialis pars laterale (NIML) and the nidopallium caudolaterale (NCL), that are involved in the execution of a serial reaction time task (SRTT). In the SRTT sequence execution is always cue guided. Thus the previous study could not unambiguously clarify whether NCL and NIML contribute to a memory-based execution of sequential behavior. In addition, a possibly differential role of these two structures could not be identified. Therefore, the present study was conducted to further elucidate the role of NCL and NIML in sequence execution in a task where pigeons performed a memorized four-item sequence. Transient inactivation of each NIML and NCL severely impaired sequence execution. The results confirm and extend our previous findings. NIML and NCL seem to store sequence information in parallel. However, the results support the hypothesis that NCL, in contrast to NIML, is especially required for sequence initiation.

Afferents from vocal motor and respiratory effectors are recruited during vocal production in juvenile songbirds

Learned behaviors require coordination of diverse sensory inputs with motivational and motor systems. Although mechanisms underlying vocal learning in songbirds have focused primarily on auditory inputs, it is likely that sensory inputs from vocal effectors also provide essential feedback. We investigated the role of somatosensory and respiratory inputs from vocal effectors of juvenile zebra finches (Taeniopygia guttata) during the stage of sensorimotor integration when they are learning to imitate a previously memorized tutor song. We report that song production induced expression of the immediate early gene product Fos in trigeminal regions that receive hypoglossal afferents from the tongue and syrinx (the main vocal organ). Furthermore, unilateral lesion of hypoglossal afferents greatly diminished singing-induced Fos expression on the side ipsilateral to the lesion, but not on the intact control side. In addition, unilateral lesion of the vagus reduced Fos expression in the ipsilateral nucleus of the solitary tract in singing birds. Lesion of the hypoglossal nerve to the syrinx greatly disrupted vocal behavior, whereas lesion of the hypoglossal nerve to the tongue exerted no obvious disruption and lesions of the vagus caused some alterations to song behavior. These results provide the first functional evidence that somatosensory and respiratory feedback from peripheral effectors is activated during vocal production and conveyed to brainstem regions. Such feedback is likely to play an important role in vocal learning during sensorimotor integration in juvenile birds and in maintaining stereotyped vocal behavior in adults.

Trigeminal and telencephalic projections to jaw and other upper vocal tract premotor neurons in songbirds: sensorimotor circuitry for beak movements during singing

During singing in songbirds, the extent of beak opening, like the extent of mouth opening in human singers, is partially correlated with the fundamental frequency of the sounds emitted. Since song in songbirds is under the control of "the song system" (a collection of interconnected forebrain nuclei dedicated to the learning and production of song), it might be expected that beak movements during singing would also be controlled by this system. However, direct neural connections between the telencephalic output of the song system and beak muscle motor neurons in the brainstem are conspicuous by their absence, leaving unresolved the question of how beak movements are affected during singing. By using standard tract tracing methods, we sought to answer this question by defining beak premotor neurons and examining their afferent projections. In the caudal medulla, jaw premotor cell bodies were located adjacent to the terminal field of the output of the song system, into which many premotor neurons extended their dendrites. The premotor neurons also received a novel input from the trigeminal ganglion and an overlapping input from a lateral arcopallial component of a trigeminal sensorimotor circuit that traverses the forebrain. The ganglionic input in songbirds, which is not present in doves and pigeons that vocalize with a closed beak, may modulate the activity of beak premotor neurons in concert with the output of the song system. These inputs to jaw premotor neurons could, together, affect beak movements as a means of modulating filter properties of the upper vocal tract during singing.

Expression of the neocortical marker, RORβ, in the entopallium and field L2 of adult chicken

Two opposing hypotheses on the homology of the avian brain suggest that the dorsal ventricular ridge of birds is comparable in certain respects either to the neocortex or to the claustroamygdalar complex of mammals. To help resolve this issue, we examined in adult chicken brains the gene expression of RORβ mRNA, a selective marker for layer IV of mammalian neocortex. RORβ mRNA was expressed in neurons of the chicken's visual entopallium and auditory field L2, but not in other regions of the nidopallium, hyperpallium, mesopallium or arcopallium. Together with hodological evidence of direct thalamic projections conveying primary sensory information to the entopallium and field L2, our results support the contention that these two regions are composed of neurons comparable to those in layer IV of mammalian neocortex.

Interaction of magnetite-based receptors in the beak with the visual system underlying 'fixed direction' responses in birds

BACKGROUND

European robins, Erithacus rubecula, show two types of directional responses to the magnetic field: (1) compass orientation that is based on radical pair processes and lateralized in favor of the right eye and (2) so-called 'fixed direction' responses that originate in the magnetite-based receptors in the upper beak. Both responses are light-dependent. Lateralization of the 'fixed direction' responses would suggest an interaction between the two magnetoreception systems.

RESULTS

Robins were tested with either the right or the left eye covered or with both eyes uncovered for their orientation under different light conditions. With 502 nm turquoise light, the birds showed normal compass orientation, whereas they displayed an easterly 'fixed direction' response under a combination of 502 nm turquoise with 590 nm yellow light. Monocularly right-eyed birds with their left eye covered were oriented just as they were binocularly as controls: under turquoise in their northerly migratory direction, under turquoise-and-yellow towards east. The response of monocularly left-eyed birds differed: under turquoise light, they were disoriented, reflecting a lateralization of the magnetic compass system in favor of the right eye, whereas they continued to head eastward under turquoise-and-yellow light.

CONCLUSION

'Fixed direction' responses are not lateralized. Hence the interactions between the magnetite-receptors in the beak and the visual system do not seem to involve the magnetoreception system based on radical pair processes, but rather other, non-lateralized components of the visual system.

Connections of the auditory brainstem in a songbird, Taeniopygia guttata. III. Projections of the superior olive and lateral lemniscal nuclei

Sequential to companion articles that report the projections of the cochlear nucleus angularis (NA) and the third-order nucleus laminaris (NL) to the central nucleus of the inferior colliculus (MLd) and to the superior olive (OS) and lateral lemniscal nuclei (LLV, LLI, and LLD) (Krützfeldt et al., J Comp Neurol, this issue), we here describe the projections of the latter group of nuclei using standard tract-tracing methods. OS projects on LLV and both have further ascending projections on LLI, LLD, and MLd. LLV also provides auditory input to the song system, via nucleus uvaeformis, and to the thalamo-telencephalic auditory system, via nucleus ovoidalis (Ov), thus bypassing MLd. The two divisions of LLD (LLDa and LLDp) project across the midline via the commissure of Probst each to innervate the homologous contralateral nucleus and MLd. Both, particularly LLDp, also project on Ov. Injections in LLD and LLV resulted in anterograde labeling of caudal nucleus basorostralis (Bas) in the frontal telencephalon, but retrograde tracing so far suggests that only LLI is a real source of this projection (Wild and Farabaugh [1996] J Comp Neurol 365:306-328). OS and LLV also have descending projections on the ipsilateral NA, NM, and NL, and LLV also projects on OS. The ascending inputs to MLd and more rostral nuclei may contribute importantly to mechanisms of auditory pattern (song) recognition. Consistent with previous studies, some of the descending projections may be inhibitory.

Magnetic field changes activate the trigeminal brainstem complex in a migratory bird

The upper beak of birds, which contains putative magnetosensory ferro-magnetic structures, is innervated by the ophthalmic branch of the trigeminal nerve (V1). However, because of the absence of replicable neurobiological evidence, a general acceptance of the involvement of the trigeminal nerve in magnetoreception is lacking in birds. Using an antibody to ZENK protein to indicate neuronal activation, we here document reliable magnetic activation of neurons in and near the principal (PrV) and spinal tract (SpV) nuclei of the trigeminal brainstem complex, which represent the two brain regions known to receive primary input from the trigeminal nerve. Significantly more neurons were activated in PrV and in medial SpV when European robins (Erithacus rubecula) experienced a magnetic field changing every 30 seconds for a period of 3 h (CMF) than when robins experienced a compensated, zero magnetic field condition (ZMF). No such differences in numbers of activated neurons were found in comparison structures. Under CMF conditions, sectioning of V1 significantly reduced the number of activated neurons in and near PrV and medial SpV, but not in lateral SpV or in the optic tectum. Tract tracing of V1 showed spatial proximity and regional overlap of V1 nerve endings and ZENK-positive (activated) neurons in SpV, and partly in PrV, under CMF conditions. Together, these results suggest that magnetic field changes activate neurons in and near the trigeminal brainstem complex and that V1 is necessary for this activation. We therefore suggest that V1 transmits magnetic information to the brain in this migratory passerine bird.

A pathway for predation in the brain of the barn owl (Tyto alba): projections of the gracile nucleus to the "claw area" of the rostral wulst via the dorsal thalamus

The Wulst of birds, which is generally considered homologous with the isocortex of mammals, is an elevation on the dorsum of the telencephalon that is particularly prominent in predatory species, especially those with large, frontally placed eyes, such as owls. The Wulst, therefore, is largely visual, but a relatively small rostral portion is somatosensory in nature. In barn owls, this rostral somatosensory part of the Wulst forms a unique physical protuberance dedicated to the representation of the contralateral claw. Here we investigate whether the input to this "claw area" arises from dorsal thalamic neurons that, in turn, receive their somatosensory input from the gracile nucleus. After injections of biotinylated dextran amine into the gracile nucleus and cholera toxin B chain into the claw area, terminations from the former and retrogradely labeled neurons from the latter overlapped substantially in the thalamic nucleus dorsalis intermedius ventralis anterior. These results indicate the existence in this species of a "classical" trisynaptic somatosensory pathway from the body periphery to the telencephalic Wulst, via the dorsal thalamus, one that is likely involved in the barn owl's predatory behavior. The results are discussed in the context of somatosensory projections, primarily in this and other avian species.

Neuro-economics in chicks: foraging choices based on amount, delay and cost

Studies on the foraging choices are reviewed, with an emphasis on the neural representations of elementary factors of food (i.e., amount, delay and consumption time) in the avian brain. Domestic chicks serve as an ideal animal model in this respect, as they quickly associate cue colors with subsequently supplied food rewards, and their choices are quantitatively linked with the rewards. When a pair of such color cues was simultaneously presented, the trained chicks reliably made choices according to the profitability of food associated with each color. Two forebrain regions are involved in distinct aspects of choices; i.e., nucleus accumbens-medial striatum (Ac-MSt) and arcopallium intermedium (AI), an association area in the lateral forebrain. Localized lesions of Ac-MSt enhanced delay aversion, and the ablated chicks made impulsive choices of immediate reward more frequently than sham controls. On the other hand, lesions of AI enhanced consumption-time aversion, and the ablated chicks shifted their choices toward easily consumable reward with their impulsiveness unchanged; delay and consumption time are thus doubly dissociated. Furthermore, chicks showed distinct patterns of risk-sensitive choices depending on the factor that varied at trials. Risk aversion occurred when food amount varied, whereas consistent risk sensitivity was not found when the delay varied; amount and delay were not interchangeable. Choices are thus deviated from those predicted as optima. Instead, factors such as amount, delay and consumption time could be separately represented and processed to yield economically sub-optimal choices.

Subcortical auditory input to the primary visual cortex in anophthalmic mice

Anatomical and imaging studies show ample evidence for auditory activation of the visual cortex following early onset of blindness in both humans and animal models. Anatomical studies in animal models of early blindness clearly show intermodal pathways through which auditory information can reach the primary visual cortex. There is clear evidence for intermodal corticocortical pathways linking auditory and visual cortex and also novel connections between the inferior colliculus and the visual thalamus. A recent publication [L.K. Laemle, N.L. Strominger, D.O. Carpenter, Cross-modal innervation of primary visual cortex by auditory fibers in congenitally anophthalmic mice, Neurosci. Lett. 396 (2006) 108-112] suggested the presence of a direct reciprocal connection between the inferior colliculus and the primary visual cortex (V1) in congenitally anophthalmic ZRDCT/An mice. This implies that this mutant mouse would be the only known vertebrate having a direct tectal connection with a primary sensory cortex. The presence of this peculiar pathway was reinvestigated in the ZRDCT/An mouse with highly sensitive neuronal tracers. We found the connections normally described in the ZRDCT/An mouse between: (i) the inferior colliculus and the dorsal lateral geniculate nucleus, (ii) V1 and the superior colliculus, (iii) the lateral posterior nucleus and V1 and between (iv) the inferior colliculus and the medial geniculate nucleus. We also show unambiguously that the auditory subcortical structures do not connect the primary visual cortex in the anophthalmic mouse. In particular, we find no evidence of a direct projection from the auditory mesencephalon to the cortex in this animal model of blindness.