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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.
The apparent pallial relationships between these mammalian and avian brain regions were also supported by molecular embryology studies28,68,69. During development, both the avian hyperstriatum and neostriatum and the mammalian pallium express the pallium-specific transcription factors EMX1, PAX6 and TBR1. The developmental data led to uncertainties about how much of the archistriatum is pallial28,30. However, comparisons of the expression of the brain-derived neurotrophic factor (BDNF ) and the glutamate receptor mGluR2 in adult birds and mammals indicated that the entire avian archistriatum, as defined in brain atlases17,70, expresses these pallium-specific mRNAs34,36. Further studies of the comparative expression patterns of other glutamate receptors in adult birds and mammals36 support these conclusions. Together, these studies indicate that the avian hyperstriatum, neostriatum, and archistriatum might be homologous to mammalian pallial regions.
An example of how avian pallial and sub-pallial areas can interact to produce complex behaviour in the context of the new view of avian brain organization can be seen in the brain pathways that control learned vocal communication (FIG. 3). Most of the telencephalic auditory processing areas are in the pallium, adjacent to a smaller auditory area in the striatum (FIG. 3a). Likewise, most of the telencephalic vocal control nuclei are in the pallium, with one vocal nucleus in the striatum (FIG. 3b). The vocal nuclei that are involved in the production of learned vocalizations, including human speech in parrots111, make up a pathway that directly innervates brainstem motor neurons (FIG. 3b, black arrows). This vocal motor pathway is similar to mammalian motor corticobulbar pathways106. The vocal nuclei that are involved in the imitation of vocalizations form a pallial–basal ganglia–thalamic–pallial loop (FIG. 3b, white arrows). This vocal learning pathway is similar to mammalian cortical–basal ganglia–thalamic–cortical loops27,106,114, which are involved in motor learning, sensorimotor integration and addictive behaviours. Other avian sensory and motor systems that are used for cognitive behaviours share a common circuit organization with the auditory and vocal pathways63,64.