Changing views of brain evolution

Does a Vertebrate Morphotype of Pallial Subdivisions Really Exist?

BACKGROUND

Comparative neuroanatomists have long sought to determine which part of the pallium in nonmammals is homologous to the mammalian neocortex. A number of similar connectivity patterns across species have led to the idea that the basic organization of the vertebrate brain is relatively conserved; thus, efforts of the last decades have been focused on determining a vertebrate "morphotype" - a model comprising the characteristics believed to have been present in the last common ancestor of all vertebrates.

SUMMARY

The endeavor to determine the vertebrate morphotype has been riddled with controversies due to the extensive morphological diversity of the pallium among vertebrate taxa. Nonetheless, most proposed scenarios of pallial homology are variants of a common theme where the vertebrate pallium is subdivided into subdivisions homologous to the hippocampus, neocortex, piriform cortex, and amygdala, in a one-to-one manner. We review the rationales of major propositions of pallial homology and identify the source of the discrepancies behind different hypotheses. We consider that a source of discrepancies is the prevailing assumption that there is a single "morphotype of the pallial subdivisions" throughout vertebrates. Instead, pallial subdivisions present in different taxa probably evolved independently in each lineage.

KEY MESSAGES

We encounter discrepancies when we search for a single morphotype of subdivisions across vertebrates. These discrepancies can be resolved by considering that several subdivisions within the pallium were established after the divergence of the different lineages. The differences of pallial organization are especially remarkable between actinopterygians (including teleost fishes) and other vertebrates. Thus, the prevailing notion of a simple one-to-one homology between the mammalian and teleost pallia needs to be reconsidered.

Comparative regenerative mechanisms across different mammalian tissues

Stimulating regeneration of complex tissues and organs after injury to effect complete structural and functional repair, is an attractive therapeutic option that would revolutionize clinical medicine. Compared to many metazoan phyla that show extraordinary regenerative capacity, which in some instances persists throughout life, regeneration in mammalians, particularly humans, is limited or absent. Here we consider recent insights in the elucidation of molecular mechanisms of regeneration that have come from studies of tissue homeostasis and injury repair in mammalian tissues that span the spectrum from little or no self-renewal, to those showing active cell turnover throughout life. These studies highlight the diversity of factors that constrain regeneration, including immune responses, extracellular matrix composition, age, injury type, physiological adaptation, and angiogenic and neurogenic capacity. Despite these constraints, much progress has been made in elucidating key molecular mechanisms that may provide therapeutic targets for the development of future regenerative therapies, as well as previously unidentified developmental paradigms and windows-of-opportunity for improved regenerative repair.

Representational specializations of the hippocampus in phylogenetic perspective

In a major evolutionary transition that occurred more than 520 million years ago, the earliest vertebrates adapted to a life of mobile, predatory foraging guided by distance receptors concentrated on their heads. Vision and olfaction served as the principal sensory systems for guiding their search for nutrients and safe haven. Among their neural innovations, these animals had a telencephalon that included a homologue of the hippocampus. Experiments on goldfish, turtles, lizards, rodents, macaque monkeys and humans have provided insight into the initial adaptive advantages provided by the hippocampus homologue. These findings indicate that it housed specialized map-like representations of odors and sights encountered at various locations in an animal's home range, including the order and timing in which they should be encountered during a journey. Once these representations emerged in early vertebrates, they also enabled a variety of behaviors beyond navigation. In modern rodents and primates, for example, the specialized representations of the hippocampus enable the learning and performance of tasks involving serial order, timing, recency, relations, sequences of events and behavioral contexts. During primate evolution, certain aspects of these representations gained particular prominence, in part due to the advent of foveal vision in haplorhines. As anthropoid primates-the ancestors of monkeys, apes and humans-changed from small animals that foraged locally into large ones with an extensive home range, they made foraging choices at a distance based on visual scenes. Experimental evidence shows that the hippocampus of monkeys specializes in memories that reflect the representation of such scenes, rather than spatial processing in a general sense. Furthermore, and contrary to the idea that the hippocampus functions in memory to the exclusion of perception, brain imaging studies and lesion effects in humans show that its specialized representations support both the perception and memory of scenes and sequences.

Hypothesis concerning embodied calendars: A case study of number form, color spreading, and taste-color synaesthesia

We propose a hypothesis concerning the neural basis of the mental 'calendar' we all carry around in our brains, based on observations we made on a 25year old 'projector synaesthete', EA, who displays some novel and instructive features. In addition to her grapheme-color synaesthesia, she has a circular 'calendar line', laid out vividly in front of her in the horizontal plane with December 31st passing through the middle of her chest and other months arranged in clockwise sequence ending with December on her right (July was 3 feet in front of her). Her access to episodic memories felt, subjectively, as if it was facilitated by her calendar line. Intriguingly, if she turned her sideways to look to the right, the calendar remained 'stuck to the chest' - meaning that it was body centered rather than head centered. Even more surprising is how, when she rotated her head rightward, the left portion of her calendar became "fuzzy", and memories of February and March became less accessible; a striking example of embodied cognition - memories gated by information from neural networks representing the activity of neck muscles. We postulate that the human calendar is mediated by connections between the angular gyrus and hippocampal place and time cells-via the inferior longitudinal fasciculus. Other aspects of EA's synaesthesia were also explored. The colors evoked by graphemes spread outside the grapheme itself, but the spread could be blocked by real as well as illusory contours. These interactions might be mediated by cells signaling illusory contours (in V2) and color/texture sensitive cells in (V4). Tastes also evoked colors, but, intriguingly, the colors were experienced inside her mouth rather than out there in the world. We discuss the deeper implications of these findings for understanding the nature of quale, and the manner in which the self anchors itself in space and time.

Distinction in the immunoreactivities of two calcium-binding proteins and neuronal birthdates in the first and higher-order somatosensory thalamic nuclei of mice: Evolutionary implications

Comparative embryonic studies are the most effective way to discern phylogenetic changes. To gain insight into the constitution and evolution of mammalian somatosensory thalamic nuclei, we first studied how calbindin (CB) and parvalbumin (PV) immunoreactivities appear during embryonic development in the first-order relaying somatosensory nuclei, i.e., the ventral posteromedial (VPM) and posterolateral (VPL) nuclei, and their neighboring higher-order modulatory regions, including the ventromedial or ventrolateral nucleus, posterior, and the reticular nucleus. The results indicated that cell bodies that were immunoreactive for CB were found earlier (embryonic day 12 [E12]) in the dorsal thalamus than were cells positive for PV (E14), and the adult somatosensory thalamus was characterized by complementary CB and PV distributions with PV dominance in the first-order relaying nuclei and CB dominance in the higher-order regions. We then labeled proliferating cells with [(3) H]-thymidine from E11 to 19 and found that the onset of neurogenesis began later (E12) in the first-order relaying nuclei than in the higher-order regions (E11). Using double-labeling with [(3) H]-thymidine autoradiography and CB or PV immunohistochemistry, we found that CB neurons were born earlier (E11-12) than PV neurons (E12-13) in the studied areas. Thus, similar to auditory nuclei, the first and the higher-order somatosensory nuclei exhibited significant distinctions in CB/PV immunohistochemistry and birthdates during embryonic development. These data, combined with the results of a cladistic analysis of the thalamic somatosensory nuclei, are discussed from an evolutionary perspective of sensory nuclei.

Ludwig Edinger: the vertebrate series and comparative neuroanatomy

At the end of the nineteenth century, Ludwig Edinger completed the first comparative survey of the microscopic anatomy of vertebrate brains. He is regarded as the founder of the field of comparative neuroanatomy. Modern commentators have misunderstood him to have espoused an anti-Darwinian linear view of brain evolution, harkening to the metaphysics of the scala naturae. This understanding arises, in part, from an increasingly contested view of nineteenth-century morphology in Germany. Edinger did espouse a progressionist, though not strictly linear, view of forebrain evolution, but his work also provided carefully documented evidence that brain stem structures vary in complexity independently from one another and across species in a manner that is not compatible with linear progress. This led Edinger to reject progressionism for all brain structures other than the forebrain roof, based on reasoning not too dissimilar from those his successors used to dismiss it for the forebrain roof.

Postnatal brain development of the pulse type, weakly electric gymnotid fish Gymnotus omarorum

Teleosts are a numerous and diverse group of fish showing great variation in body shape, ecological niches and behaviors, and a correspondent diversity in brain morphology, usually associated with their functional specialization. Weakly electric fish are a paradigmatic example of functional specialization, as these teleosts use self-generated electric fields to sense the nearby environment and communicate with conspecifics, enabling fish to better exploit particular ecological niches. We analyzed the development of the brain of the pulse type gymnotid Gymnotus omarorum, focusing on the brain regions involved directly or indirectly in electrosensory information processing. A morphometric analysis has been made of the whole brain and of brain regions of interest, based on volumetric data obtained from 3-D reconstructions to study the growth of the whole brain and the relative growth of brain regions, from late larvae to adulthood. In the smallest studied larvae some components of the electrosensory pathway appeared to be already organized and functional, as evidenced by tract-tracing and in vivo field potential recordings of electrosensory-evoked activity. From late larval to adult stages, rombencephalic brain regions (cerebellum and electrosensory lateral line lobe) showed a positive allometric growth, mesencephalic brain regions showed a negative allometric growth, and the telencephalon showed an isometric growth. In a first step towards elucidating the role of cell proliferation in the relative growth of the analyzed brain regions, we also studied the spatial distribution of proliferation zones by means of pulse type BrdU labeling revealed by immunohistochemistry. The brain of G. omarorum late larvae showed a widespread distribution of proliferating zones, most of which were located at the ventricular-cisternal lining. Interestingly, we also found extra ventricular-cisternal proliferation zones at in the rombencephalic cerebellum and electrosensory lateral line lobe. We discuss the role of extraventricular-cisternal proliferation in the relative growth of the latter brain regions.

Evolution of bilaterian central nervous systems: a single origin?

The question of whether the ancestral bilaterian had a central nervous system (CNS) or a diffuse ectodermal nervous system has been hotly debated. Considerable evidence supports the theory that a CNS evolved just once. However, an alternative view proposes that the chordate CNS evolved from the ectodermal nerve net of a hemichordate-like ancestral deuterostome, implying independent evolution of the CNS in chordates and protostomes. To specify morphological divisions along the anterior/posterior axis, this ancestor used gene networks homologous to those patterning three organizing centers in the vertebrate brain: the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer, and subsequent evolution of the vertebrate brain involved elaboration of these ancestral signaling centers; however, all or part of these signaling centers were lost from the CNS of invertebrate chordates. The present review analyzes the evidence for and against these theories. The bulk of the evidence indicates that a CNS evolved just once - in the ancestral bilaterian. Importantly, in both protostomes and deuterostomes, the CNS represents a portion of a generally neurogenic ectoderm that is internalized and receives and integrates inputs from sensory cells in the remainder of the ectoderm. The expression patterns of genes involved in medio/lateral (dorso/ventral) patterning of the CNS are similar in protostomes and chordates; however, these genes are not similarly expressed in the ectoderm outside the CNS. Thus, their expression is a better criterion for CNS homologs than the expression of anterior/posterior patterning genes, many of which (for example, Hox genes) are similarly expressed both in the CNS and in the remainder of the ectoderm in many bilaterians. The evidence leaves hemichordates in an ambiguous position - either CNS centralization was lost to some extent at the base of the hemichordates, or even earlier, at the base of the hemichordates + echinoderms, or one of the two hemichordate nerve cords is homologous to the CNS of protostomes and chordates. In any event, the presence of part of the genetic machinery for the anterior neural ridge, the zona limitans intrathalamica and the isthmic organizer in invertebrate chordates together with similar morphology indicates that these organizers were present, at least in part, at the base of the chordates and were probably elaborated upon in the vertebrate lineage.

Avian cerebellar floccular fossa size is not a proxy for flying ability in birds

Extinct animal behavior has often been inferred from qualitative assessments of relative brain region size in fossil endocranial casts. For instance, flight capability in pterosaurs and early birds has been inferred from the relative size of the cerebellar flocculus, which in life protrudes from the lateral surface of the cerebellum. A primary role of the flocculus is to integrate sensory information about head rotation and translation to stabilize visual gaze via the vestibulo-occular reflex (VOR). Because gaze stabilization is a critical aspect of flight, some authors have suggested that the flocculus is enlarged in flying species. Whether this can be further extended to a floccular expansion in highly maneuverable flying species or floccular reduction in flightless species is unknown. Here, we used micro computed-tomography to reconstruct “virtual” endocranial casts of 60 extant bird species, to extract the same level of anatomical information offered by fossils. Volumes of the floccular fossa and entire brain cavity were measured and these values correlated with four indices of flying behavior. Although a weak positive relationship was found between floccular fossa size and brachial index, no significant relationship was found between floccular fossa size and any other flight mode classification. These findings could be the result of the bony endocranium inaccurately reflecting the size of the neural flocculus, but might also reflect the importance of the flocculus for all modes of locomotion in birds. We therefore conclude that the relative size of the flocculus of endocranial casts is an unreliable predictor of locomotor behavior in extinct birds, and probably also pterosaurs and non-avian dinosaurs.

A potential role for differential contractility in early brain development and evolution

Differences in brain structure between species have long fascinated evolutionary biologists. Understanding how these differences arise requires knowing how they are generated in the embryo. Growing evidence in the field of evolutionary developmental biology (evo-devo) suggests that morphological differences between species result largely from changes in the spatiotemporal regulation of gene expression during development. Corresponding changes in functional cellular behaviors (morphogenetic mechanisms) are only beginning to be explored, however. Here we show that spatiotemporal patterns of tissue contractility are sufficient to explain differences in morphology of the early embryonic brain between disparate species. We found that enhancing cytoskeletal contraction in the embryonic chick brain with calyculin A alters the distribution of contractile proteins on the apical side of the neuroepithelium and changes relatively round cross-sections of the tubular brain into shapes resembling triangles, diamonds, and narrow slits. These perturbed shapes, as well as overall brain morphology, are remarkably similar to those of corresponding sections normally found in species such as zebrafish and Xenopus laevis (frog). Tissue staining revealed relatively strong concentration of F-actin at vertices of hyper-contracted cross-sections, and a finite element model shows that local contraction in these regions can convert circular sections into the observed shapes. Another model suggests that these variations in contractility depend on the initial geometry of the brain tube, as localized contraction may be needed to open the initially closed lumen in normal zebrafish and Xenopus brains, whereas this contractile machinery is not necessary in chick brains, which are already open when first created. We conclude that interspecies differences in cytoskeletal contraction may play a larger role in generating differences in morphology, and at much earlier developmental stages, in the brain than previously appreciated. This study is a step toward uncovering the underlying morphomechanical mechanisms that regulate how neural phenotypic differences arise between species.

Sites of origin and developmental dynamics of the neurons in the core and shell regions of torus semicircularis in the Chinese softshell turtle (Pelodiscus sinensis)

To know the embryogenesis of the core and shell regions of the midbrain auditory nucleus, a single dose of [(3)H]-thymidine was injected into the turtle embryos at peak stages of neurogenesis in the shell and core of the torus semicircularis. Following sequential survival times, labeled neurons and the dynamics of cell proliferation were examined. The expression of vimentin (VM), reelin, calbindin, parvalbumin, and substance P were also studied. The results showed that: 1) progenitor cells for the core and shell regions were generated in different sites of the ventricular zone; 2) the length of the cell cycle or S-phase for the shell region were both longer than those for the core region (4.7 and 3.2 hours longer, respectively), suggesting that mitotic activity in the core region is higher than it is in the shell region; 3) the elongated cell bodies of the labeled core and shell cells had close apposition to VM fibers, suggesting that the migration of these cells is guided by VM fibers; 4) the germinal sites of the core and shell constructed by projecting the orientation of radial VM fibers back to the ventricular zone was consistent with those obtained by short and sequential survival [(3)H]-thymidine radiography; and 5) the beginning of positive staining for parvalbumin in the core region was interposed between those for calbindin and substance P in the shell regions. This study contributes to the understanding of how auditory nuclei are organized and how their components developed and evolved.

Evo-devo and brain scaling: candidate developmental mechanisms for variation and constancy in vertebrate brain evolution

Biologists have long been interested in both the regularities and the deviations in the relationship between brain, development, ecology, and behavior between taxa. We first examine some basic information about the observed ranges of fundamental changes in developmental parameters (i.e. neurogenesis timing, cell cycle rates, and gene expression patterns) between taxa. Next, we review what is known about the relative importance of different kinds of developmental mechanisms in producing brain change, focusing on mechanisms of segmentation, local and general features of neurogenesis, and cell cycle kinetics. We suggest that a limited set of developmental alterations of the vertebrate nervous system typically occur and that each kind of developmental change may entail unique anatomical, functional, and behavioral consequences for the organism. Thus, neuroecologists who posit a direct mapping of brain size to behavior should consider that not any change in brain anatomy is possible.

Considering the evolution of regeneration in the central nervous system

For many years the mammalian CNS has been seen as an organ that is unable to regenerate. However, it was also long known that lower vertebrate species are capable of impressive regeneration of CNS structures. How did this situation arise through evolution? Increasing cellular and molecular understanding of regeneration in different animal species coupled with studies of adult neurogenesis in mammals is providing a basis for addressing this question. Here we compare CNS regeneration among vertebrates and speculate on how this ability may have emerged or been restricted.

Evolutionary significance of delayed neurogenesis in the core versus shell auditory areas of Mus musculus

Early comparative embryogenesis can reflect the organization and evolutionary origins of brain areas. Neurogenesis in the auditory areas of sauropsids displays a clear core-to-shell distinction, but it remains unclear in mammals. To address this issue, [3H]-thymidine was injected into pregnant mice on consecutive embryonic (E) days (E10-E19) to date neuronal birthdays. Immunohistochemistry for substance P, calbindin, and parvalbumin was conducted to distinguish the core and shell auditory regions. The results showed that: 1) cell generation began at E13 in the external or dorsal nucleus of the inferior colliculus (IC), but it did not start in the caudomedial portion of the central nucleus of IC, and significantly fewer cells were produced in the medial and rostromedial portions of the central nucleus of IC; 2) cells were generated at E11 in the dorsal and medial divisions of the medial geniculate complex (MGd and MGm, respectively), whereas cell generation was absent in the medial and rostromedial portions of the ventral medial geniculate complex (MGv), and fewer cells were produced in the caudomedial portion of MGv; 3) in the telencephalic auditory cortex, cells were produced at E11 or E12 in layer I and the subplate, which receive projections from the MGd and MGm. However, cell generation occurred at E13-E18 in layers II-VI, including the area receiving projections from the MGv. The core-to-shell distinction of neurogenesis is thus present in the mesencephalic to telencephalic auditory areas in the mouse. This distinction of neurogenesis is discussed from an evolutionary perspective.

Cortical evolution and human behaviour

All mammals have complex behaviours but these are generally stereotyped in nature and lack the flexibility of human behaviour. Can the flexibility of human behaviour be understood as an evolutionary extension of previous behaviours or is it a departure? Theories pertaining to this question have a long history including, now refuted, theories on neoteny. This paper, using an evolutionary developmental biology approach, outlines some existing theories and suggests some novel ideas. Previous trends during brain evolution are determined by outlining the phylogeny and ontogeny of the six layered mammalian isocortex with particular reference to the primate lineage. These evolutionary trends are extrapolated to hominids to postulate the effect of increasingly large brains. The palaeoanthropological literature is cited to debate the nature and time course of behavioural change during hominid evolution. In particular, when was truly flexible behaviour first evident, and did it occur gradually or suddenly? The proposed isocortical and behavioural changes during hominid evolution are then equated to determine if modern human behaviour can be seen as part of a continuum. It is concluded that a continuation of previous trends in isocortical evolution maybe inadequate to explain human behavioural flexibility. Several possible departures from previous trends that would be compatible with increased behavioural flexibility are suggested. These mainly relate to evolutionary changes in the later stages of isocortical development and in particular during the activity-dependant phase when cortico-cortical connections are refined.

What makes us human: revisiting an age-old question in the genomic era

In 1970, Karl Pribram took on the immense challenge of asking the question, what makes us human? Nearly four decades later, the most significant finding has been the undeniable realization of how incredibly subtle and fine-scaled the unique biological features of our species must be. The recent explosion in the availability of large-scale sequence data, however, and the consequent emergence of comparative genomics, are rapidly transforming the study of human evolution. The field of comparative genomics is allowing us to reach unparalleled resolution, reframing our questions in reference to DNA sequence--the very unit that evolution operates on. But like any reductionist approach, it comes at a price. Comparative genomics may provide the necessary resolution for identifying rare DNA sequence differences in a vast sea of conservation, but ultimately we will have to face the challenge of figuring out how DNA sequence divergence translates into phenotypic divergence. Our goal here is to provide a brief outline of the major findings made in the study of human brain evolution since the Pribram lecture, focusing specifically on the field of comparative genomics. We then discuss the broader implications of these findings and the future challenges that are in store.

Organization and evolution of the avian forebrain

Early 20th-century comparative anatomists regarded the avian telencephalon as largely consisting of a hypertrophied basal ganglia, with thalamotelencephalic circuitry thus being taken to be akin to thalamostriatal circuitry in mammals. Although this view has been disproved for more than 40 years, only with the recent replacement of the old telencephalic terminology that perpetuated this view by a new terminology reflecting more accurate understanding of avian brain organization has the modern view of avian forebrain organization begun to become more widely appreciated. The modern view, reviewed in the present article, recognizes that the avian basal ganglia occupies no more of the telencephalon than is typically the case in mammals, and that it plays a role in motor control and motor learning as in mammals. Moreover, the vast majority of the telencephalon in birds is pallial in nature and, as true of cerebral cortex in mammals, provides the substrate for the substantial perceptual and cognitive abilities evident among birds. While the evolutionary relationship of the pallium of the avian telencephalon and its thalamic input to mammalian cerebral cortex and its thalamic input remains a topic of intense interest, the evidence currently favors the view that they had a common origin from forerunners in the stem amniotes ancestral to birds and mammals.

A new avian brain nomenclature: Why, how and what

Many of the assumptions of homology on which the standard nomenclature for the cell groups and fiber tracts of the avian brain have been based are in error, and consequently that terminology promotes misunderstanding of the functional organization of avian brain and its evolutionary relationship to mammalian brain. Recognizing this problem, a number of avian brain researchers began an effort to revise the terminology, which culminated in the Avian Brain Nomenclature Forum, held at Duke University from 18 to 20 July 2002. In the new terminology approved at this Forum, the flawed conception that the telencephalon of birds consists nearly entirely of a hypertrophied basal ganglia has been purged, and the actual parts of the basal ganglia and its brainstem afferent cell groups given names reflecting their now-evident mammalian homologues. The pallial telencephalic regions that were erroneously named to reflect presumed homology to mammalian basal ganglia were renamed as parts of pallium, using prefixes in most cases that retained established abbreviations (for continuity with the replaced nomenclature). The new nomenclature should lead to better communication among neuroscientists, especially between avian brain specialists and those not specialized in avian neurobiology. More information is available at the Avian Brain Nomenclature Exchange website ().

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

What, if anything, is the medial temporal lobe, and how can the amygdala be part of it if there is no such thing?

Should the medial temporal lobe (MTL) of primates--which includes allocortical structures such as the hippocampus, neocortical structures such as the parahippocampal cortex, and nuclear structures such as the basolateral amygdala--be considered a single "thing"? According to the prevailing view, here termed the reification theory, the answer is yes. According to this theory, the MTL functions as an amalgamated entity that provides the neuronal mechanisms for declarative memory; the greater the damage to the MTL or any of its components, the greater the deleterious effects on declarative memory. A countervailing view, here called the balkanization theory, holds that the various components of the MTL process and store different kinds of information. According to this theory, damage to each part of the MTL causes a unique set of behavioral deficits-some involving memory, others involving perception, and yet others involving response selection. The empirical neuropsychological evidence favors the balkanization theory, as do some new concepts in theoretical neuroanatomy.

Songbirds and the revised avian brain nomenclature

It has become increasingly clear that the standard nomenclature for many telencephalic and related brainstem structures of the avian brain is based on flawed once-held assumptions of homology to mammalian brain structures, greatly hindering functional comparisons between avian and mammalian brains. This has become especially problematic for those researchers studying the neurobiology of birdsong, the largest single group within the avian neuroscience community. To deal with the many communication problems this has caused among researchers specializing in different vertebrate classes, the Avian Brain Nomenclature Forum, held at Duke University from July 18-20, 2002, set out to develop a new terminology for the avian telencephalon and some allied brainstem cell groups. In one major step, the erroneous conception that the avian telencephalon consists mainly of a hypertrophied basal ganglia has been purged from the telencephalic terminology, and the actual parts of the basal ganglia and its brainstem afferent cell groups have been given new names to reflect their now-evident homologies. The telencephalic regions that were incorrectly named to reflect presumed homology to mammalian basal ganglia have been renamed as parts of the pallium. The prefixes used for the new names for the pallial subdivisions have retained most established abbreviations, in an effort to maintain continuity with the pre-existing nomenclature. Here we present a brief synopsis of the inaccuracies in the old nomenclature, a summary of the nomenclature changes, and details of changes for specific songbird vocal and auditory nuclei. We believe this new terminology will promote more accurate understanding of the broader neurobiological implications of song control mechanisms and facilitate the productive exchange of information between researchers studying avian and mammalian systems.