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A canonical chain, similar to a bark, in the forebrain of birds



Basic principles of the brain of birds and mammals

Mammals can be very smart. They also have a brain with a cortex. Therefore, advanced mammalian cognitive skills are often thought to be closely related to the evolution of the cerebral cortex. However, birds can also be very intelligent and several species of birds show amazing cognitive abilities. Although birds lack a cerebral cortex, they have palium and this is considered analogous, if not homologous, to the cerebral cortex. An exceptional feature of the mammalian bark is its layered architecture. In a detailed anatomical examination of the bird Pali, Stacho and others. describe a similarly layered architecture. Despite the nuclear organization of the pali bird, it has a cytoarchitectonic organization that resembles the bark of mammals.

Science, this issue page eabc5534

Structured abstract

INTRODUCTION

For more than a century, the forebrain of birds has been a mystery to neurologists. Birds exhibit exceptional cognitive abilities comparable to those of mammals, but their forebrain organization is radically different. While mammalian knowledge emerges from the canonical chains of the six-layered neocortex, the avian forebrain appears to indicate a simple nuclear organization. Only one of these nuclei, Woolst, is generally accepted to be homologous to the neocortex. Most of the remaining pallium consists of a multinucleated structure called the dorsal ventricular spine (DVR), which has no direct analogue in mammals. Nevertheless, a long-standing theory, along with the latest scientific evidence, supports the idea that some parts of the sensory DVR can display connectivity patterns, physiological signatures, and cell-specific markers that resemble the neocortex. However, it remains unknown whether the entire Woolst and the sensory DVR contain a canonical circuit that structurally resembles the cortical organization of mammals.

RATIONALE

The neocortex of mammals includes a columnar and laminar organization with orthogonally organized fibers that move in radial and tangential directions. These fibers represent repetitive canonical chains as computing units that process information along the radial domain and connect it tangentially. In this study, we first analyzed the architecture of palliative fibers with three-dimensional polarized light imaging (3D-PLI) in pigeons and subsequently reconstructed Woolst’s local sensor circuits and the sensor DVR in pigeons and weeds using in vivo or in vitro applications of neuronal markers. We focused on two remotely related bird species to prove the hypothesis that the canonical chain, comparable to the neocortex, is a true characteristic of the bird’s sensory forebrain.

RESULTS

Analysis of the 3D-PLI fibers showed that both the Wulst and the sensor DVR showed an orthogonal organization of radially and tangentially organized fibers throughout their range. In contrast, the pointless components of a DVR show a complex mosaic arrangement with fiber spots of different orientations. The tracing of the fibers reveals an iterative motif of the chain, which is present in different modalities (somatosensory, visual and auditory), brain regions (sensory DVR and Wulst) and species (pigeon and owl). Although both species showed comparable columnar and laminate-like organization of the chains, small species differences were observed, especially for Woolst, who was more differentiated in owls, which fits well with stereopsis processing combined with Wulst’s high visual acuity than this. kind. The primary sensory zones of the DVR were closely interconnected with the intercalated nidopallial layers and the upper mesopalium. In addition, nidopal and some hyperpalial lamina-like regions have given rise to tangential long-distance projections connecting sensory, associative, and motor structures.

CONCLUSION

Our study reveals a hitherto unknown neuroarchitecture of the avian sensory forebrain, which is composed of iteratively organized canonical chains in tangentially organized laminate-like and orthogonally arranged columnar formations. Our findings suggest that it is likely that an ancient chip that already existed in the last common amniotic trunk was evolutionarily conserved and partially modified in birds and mammals. The avian version of this binding plan could generate computational properties reminiscent of the neocortex and thus provide a neurobiological explanation for the comparable and exceptional perceptions and cognitive feats that occur in both taxa.

Architecture of the fibers of the forebrains of mammals and birds.

Schematic drawings of a rat brain (left) and a pigeon brain (right) depict their overall palliative organization. The mammalian dorsal pallium houses the six-layered neocortex with granular inlet layer IV (purple) and supra- and infragranular layer II / III and V / VI, respectively (blue). Avian Pali includes Wulst and DVR, which at first glance show a nuclear organization. Their primary sensory input zones are shown in purple, comparable to layer IV. According to this study, both mammals and birds show an orthogonal fiber architecture composed of radially (dark blue) and tangential (white) oriented fibers. Tangential fibers connect distant pallial areas. While this model dominates the entire mammalian neocortex, in birds only the sensory DVR and Wulst (light green) show such an architecture, and the associative and motor areas (dark green), as in the caudal DVR, lack this bark-like fiber architecture. NC, caudal nidopalium.

3D RAT BRAIN (LEFT): MASCABLE BRAIN ATLAS, RESEARCH RESOURCES IDENTIFIER (RRID) SCR_006934

Summary

Although avian palias do not appear to have an organization similar to that of the cerebral cortex, birds exhibit extraordinary cognitive skills comparable to those of mammals. We analyzed the fibrous architecture of avian pallium with three-dimensional representation of polarized light and subsequently reconstructed local and associative palliative chains with tracking techniques. We found an iteratively repeating columnar neural circuit across the layered nuclear boundaries of hyperpalia and the sensory dorsal ventricular spine. These chains are connected to adjacent columns and by tangential layer connections with higher associative and motor areas. Our findings show that this avian canonical scheme is similar to its mammalian pair and may be the structural basis of neuronal calculations.


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