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IntroductionFor a long time the rat has been widely used as a subject for research in the neurosciences. This is chiefly because of its size, resistance to infection and the availability of inbred strains. In recent years, the use of rats has increased significantly, particularly in the field of neurohistochemistry. With increasing public pressure against the use of medium-sized mammals (the monkey, cat and rabbit) in experimental biology, many laboratories have opted to use only small rodents. While the mouse is ideal for studies involving development and genetic selection (because of its short generation time), the rat, having a larger brain, is more appropriate for accurate stereotaxic localization of discrete brain areas.In spite of the pre-eminent popularity of the rat for stereotaxic procedures, there is no atlas of the rat brain based on the convenient flat-skull position that is sufficiently comprehensive and stereotaxically accurate for adult (250-350 g) male rats. In addition, no current atlas has photographs of sufficient magnification to permit investigators to judge for themselves the veracity of the atlas delineations.Before beginning the construction of the present atlas, we surveyed the 181 articles reporting use of stereotaxic surgery in rats which were published during 1980 in three prominent neuroscience journals (Brain Research, Neuroscience Letters and Physiology and Behavior). We did this in order to assess current preferences in stereotaxic methods and nomenclature. In 52% of the studies surveyed, the Kônig and Klippel (1963) atlas was used; in 20%, the Pellegrino and Cushman atlas was used (Pellegrino and Cushman 1967; Pellegrino et al., 1979); and in the remainder, other atlases were used. In 79% of the studies, male rats were used, and the mean weight was 276 g. In 63% of the studies^ bregma was used as the reference point; in 25%, the interaural line; and in 12%, lambda. In 85% of the reports, English or anglicized nomenclature was used for the naming of brain structures.Because the Kônig and Klippel atlas is by far the most widely used atlas of the rat brain, we took note of the advantages of this atlas in the preparation of our own. We believe that the Kônig and Klippel atlas is popular with researchers because its reference system closely approximates the flat-skull position and brain structures are correctly identified in almost every case.

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The standard nomenclature that has been used for many telencephalic and related brainstem structures in birds is based on flawed assumptions of homology to mammals. In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is neurochemically, hodologically, and functionally comparable to the mammalian neocortex, claustrum, and pallial amygdala (all of which derive from the pallial sector of the developing telencephalon). Recognizing that this promotes misunderstanding of the functional organization of avian brains and their evolutionary relationship to mammalian brains, avian brain specialists began discussions to rectify this problem, culminating in the Avian Brain Nomenclature Forum held at Duke University in July 2002, which approved a new terminology for avian telencephalon and some allied brainstem cell groups. Details of this new terminology are presented here, as is a rationale for each name change and evidence for any homologies implied by the new names.Revisions for the brainstem focused on vocal control, catecholaminergic, cholinergic, and basal ganglia-related nuclei. For example, the Forum recognized that the hypoglossal nucleus had been incorrectly identified as the nucleus intermedius in the Karten and Hodos (1967) pigeon brain atlas, and what was identified as the hypoglossal nucleus in that atlas should instead be called the supraspinal nucleus. The locus ceruleus of this and other avian atlases was noted to consist of a caudal noradrenergic part homologous to the mammalian locus coeruleus and a rostral region corresponding to the mammalian A8 dopaminergic cell group. The midbrain dopaminergic cell group in birds known as the nucleus tegmenti pedunculopontinus pars compacta was recognized as homologous to the mammalian substantia nigra pars compacta and was renamed accordingly; a group of ␥-aminobutyric acid (GABA)ergic neurons at the lateral edge of this region was identified as homologous to the mammalian substantia nigra pars reticulata and was also renamed accordingly. A field of cholinergic neurons in the rostral avian hindbrain was named the nucleus pedunculopontinus tegmenti, whereas the anterior nucleus of the ansa lenticularis in the avian diencephalon was renamed the subthalamic nucleus, both for their evident mammalian homologues.For the basal (i.e., subpallial) telencephalon, the actual parts of the basal ganglia were given names reflecting their now evident homologues. For example, the lobus parolfactorius and paleostriatum augmentatum were acknowledged to make up the dorsal subdivision of the striatal part of the basal ganglia and were renamed as the medial and lateral striatum. The paleostriatum primitivum was recognized as homologous to the mammalian globus pallidus and renamed as such. Additionally, the rostroventral part of what was called the lobus parolfactorius was acknowledged as comparable to the mammalian nucleus accumbens, which, together with the...

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

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