Basic principles of the brain of birds and mammals
Mammals can be very intelligent. They also have a brain with a cortex. Thus, it is often suggested that the advanced cognitive skills of mammals are 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 do not have a cerebral cortex, they have a pile, and this is considered an analogue, if not homologous, of the cerebral cortex. An outstanding feature of mammalian bark is multilayered architecture. In a detailed anatomical study of avian birds, Stacho etc. describe a similar layered architecture. Despite the nuclear organization of the avian bird, it has a cytoarchitectonic organization resembling the bark of mammals.
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For more than a century, the bird’s forebrain has been a mystery to neurologists. Birds show exceptional cognitive abilities, comparable to those of mammals, but their organization of the forebrain is radically different. While cognition of mammals comes from the canonical chains of the six-layered neocortex, the avian forebrain seems to demonstrate a simple nuclear organization. Only one of these nuclei, Woolst, was a universally recognized homologous neocortex. Most of the remaining pallium consists of a multinucleated structure called the dorsal ventricular spine (DVR), which has no direct analogue in mammals. However, an old theory, along with recent scientific evidence, supports the idea that some parts of the DVR can display communication patterns, physiological signatures, and cell-specific markers that resemble a neocortex. However, it remains unknown whether the entire Woolst and the DVR contain a canonical scheme that structurally resembles the cortical organization of mammals.
The neocortex of mammals consists of a columnar-laminar organization with orthogonally organized fibers that run in the radial and tangential directions. These fibers form repetitive canonical schemes as computational units that process information along the radial region and associate it tangentially. In this study, we first analyzed the palliative fiber architecture using three-dimensional polarized light imaging (3D-PLI) in pigeons, and then reconstructed local Woolst sensory circuits and a DVR in pigeons and barn owls using in vivo or in vitro neural indicators. We focused on two remotely related bird species to prove the hypothesis that the canonical chain, comparable to the neocortex, is a true feature of the sensitive forebrain of birds.
Analysis of 3D-PLI fibers showed that both the Woolst and the DVR reflect the orthogonal organization of radially and tangentially organized fibers throughout. In contrast, the nonsense components of the DVR exhibited a complex mosaic composition with areas of fibers with different orientations. Fiber tracking revealed an iterative chain motif that was present in different modalities (somatosensory, visual, and auditory), brain regions (sensory DVR, and Woolst) and species (pigeon and barn owl). Although both species showed comparable columnar and lamellar circulatory organization, small species differences were noticeable, especially for Woolst, who was more differentiated in barn owl, which goes well with stereopsis treatment combined with Wulst’s high visual acuity. The primary sensory zones of the DVR were closely interconnected with the intercalated nidopalial layers and the upper mesopallium. In addition, nidopalial and some plate-like hyperpalial plates have given rise to tangential projections connecting sensory, associative, and motor structures.
Our study reveals the hitherto unknown neuroarchitecture of the sensitive forebrain of birds, which consists of iteratively organized canonical chains in tangentially organized plate-like and orthogonally arranged columnar structures. Our findings suggest that it is likely that the ancient chip, which already existed in the last common stem amniotic fluid, may have been evolutionarily preserved and partially modified in birds and mammals. The avian version of this communication plan could create computational properties resembling a neocortex, and thus provide a neurobiological explanation of comparable and outstanding perceptual and cognitive feats occurring in both taxa.
Although the bird’s eye appears to lack an organization similar to that of the cerebral cortex, birds exhibit extraordinary cognitive skills comparable to mammals. We analyzed the fiber architecture of avian pallium using a three-dimensional polarized image of light and subsequently reconstructed local and associative palliative contours using tracing methods. We detected a repetitive repetitive columnar neural circuit across the spherical nuclear boundaries of the hyperpalium and the sensory spinal ventricle. These schemes are connected with adjacent columns, and through tangential spherical connections – with higher associative and motor zones. Our findings indicate that this canonical scheme of birds is similar to that of mammals and may form the structural basis of neuronal calculations.