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.One hundred years ago, Edinger, the father of comparative neuroanatomy, formulated a unified theory of brain evolution that formed the basis of a nomenclature that has been used to define the cerebral subdivisions of all vertebrates 1 . This resulted in terms and associated concepts such as palaeostriatum, archistriatum, neostriatum and neocortex that are still in common use. According to this theory, the avian cerebrum is almost entirely composed of basal ganglia, the basal ganglia is involved in only instinctive behaviour, and the malleable behaviour that is thought to typify mammals exclusively requires the so-called neocortex. However, towards the end of the twentieth century, there accumulated a wealth of evidence that these viewpoints were incorrect. The avian cerebrum has a large pallial territory that performs functions similar to those of the mammalian cortex. Although the avian pallium is nuclear, and the mammalian cortex is laminar in organization, the avian pallium supports cognitive abilities similar to, and for some species more advanced than, those of many mammals. To eliminate these misconceptions, an international forum of neuroscientists (BOX 1) has, for the first time in 100 years, developed new terminology that more accurately reflects our current understanding of the avian cerebrum and its homologies with mammals. This change in terminology is part of a new understanding of vertebrate brain evolution.In this article, we summarize the traditional view of telencephalic evolution before reviewing more recent findings and insights. We then present the new nomenclature that has been Correspondence to Erich Jarvis at the
It is well known that the density of neurons varies within the adult brain. In neocortex, this includes variations in neuronal density between different lamina as well as between different regions. Yet the concomitant variation of the microvessels is largely uncharted. Here we present automated histological, imaging, and analysis tools to simultaneously map the locations of all neuronal and non-neuronal nuclei and the centerlines and diameters of all blood vessels within thick slabs of neocortex from mice. Based on total inventory measurements of different cortical regions (~ 107 cells vectorized across brains), these methods revealed: (1) In three dimensions, the mean distance of the center of neuronal somata to the closest microvessel was 14 μm. (2) Volume samples within lamina of a given region show that the density of microvessels does not match the strong laminar variation in neuronal density. This holds for both agranular and granular cortex. (3) Volume samples in successive radii from the midline to the ventral-lateral edge, where each volume summed the number of cells and microvessels from the pia to the white matter, show a significant correlation between neuronal and microvessel densities. These data show that while neuronal and vascular densities do not track each other on the 100 μm scale of cortical lamina, they do track each other on the 1 – 10 mm scale of the cortical mantle. The absence of a disproportionate density of blood vessels in granular lamina is argued to be consistent with the initial locus of functional brain imaging signals.
Retinal fibers in both the pigeon and owl terminate in a multinucleate complex of the dorsal thalamus, including the nuclei lateralis anterior, dorsolateralis-anterior, dorsolateralis anterior, pars lateralis et pars magnocellularis, and collectively designated the nucleus opticus principalis thalami (OPT) I Efferent projections of OPT were traced with the Fink-Heimer method into the ipsilateral lateral forebrain bundle, and via the dorsal supraoptic decussation, into the contralateral lateral forebrain bundle. OPT projections terminate within an elevation. or "Wulst" on the dorsum of the telencephalon. The Wulst is a multilaminate structure containing a deep lying layer of large cells the hyperstriatum dorsale (HD), a dispersed cell layer -the hyperstriatum intercalatus suprema (HISm), a granule cell layer or nucleus intercalatus hyperstriatum accessorium (IHA), an overlying hyperstriatum accessorium (HA) consisting of a broad layer of medium sized neurons, and an overlying fibro-molecular layer. Each of these laminae are particularly well developed in the owl, where the granule cell layer is divisible into inner and outer bands (IHAex and IHAint). The projections of OPT terminate in the HD, HISm and IHA. A homotopic projection was also found in the contralateral Wulst. The pattern of termination was similar in both the pigeon and the owl, though the pattern of distribution was more apparent in the owl with its massive OPT and Wulst. Medial, nonvisual, thalamic cell groups in the pigeon (nuclei dorsolateralis pars medialis and dorsomedialis anterior) also project bilaterally upon the U'ulst, but terminate in a more medial, nonvisual and cytologically different, portion of HD. The projections of the medial thalamic nuclei did not overlap with those of OPT and appear to be a separate functional system of still undetermined nature.Efferent axons of the "visual Wulst" of the pigeon and owl project upon the ipsilateral lateral hyperstriatum ventrale, neostriatum and upon the peri-ectostriatal belt (Karten and Hodos, '70). Extratelencephalic projections via the septomesencephalic tract (TSM) terminate in OPT, the internal lamina of the ventral lateral geniculate nucleus (LGv), pretectal nuclei and optic tectum. A small contingent of fibers of the TSM cross to the opposite side in the dorsal supraoptic decussation, to terminate in LGv, and, in the owl, in the contralateral ventromedial tectum. Dense terminal degeneration has also been observed in the deeper layers of the ipsilateral optic tectum of pigeon whereas in the owl the projection of the Wulst also extends to the more superficial layers of the tectum, and appears to be topographically arranged.The numerous similarities between the system described above and the genicdostriate visual pathways of mammals seems apparent. These findings clearly indicate that the geniculo-striate type of system may attain elaborate degrees of development in nonmammalian as well as mammalian brains.
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