Avian brains and a new understanding of vertebrate brain evolution

Abstract

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.

Figure 1. Avian and mammalian brain relationships

a | Side view of a songbird (zebra finch) and human brain to represent avian and mammalian species. In this view, the songbird cerebrum covers the thalamus; the human cerebrum covers the thalamus and midbrain. Inset (left) next to the human brain is the zebra finch brain to the same scale. Human brain image reproduced, with permission, courtesy of John W. Sundsten, Digital Anatomist Project. b | Classic view of avian and mammalian brain relationships. Although past authors had different opinions about which brain regions are pallium versus subpallium, we have coloured individual brain regions according to the meaning of the names given to those brain regions. Ac, accumbens; B, nucleus basalis; Cd, caudate nucleus; CDL, dorsal lateral corticoid area; E, ectostriatum; GP, globus pallidus (i, internal segment; e, external segment); HA, hyperstriatum accessorium; HV, hyperstriatum ventrale; IHA, interstitial hyperstriatum accessorium; L2, field L2; LPO, lobus parolfactorius; OB, olfactory bulb; Pt, putamen; TuO, olfactory tubercle. c | Modern consensus view of avian and mammalian brain relationships according to the conclusions of the Avian Brain Nomenclature Forum. Solid white lines are lamina (cell-sparse zones separating brain subdivisions). Large white areas in the human cerebrum are axon pathways called white matter. Dashed grey lines divide regions that differ by cell density or cell size; dashed white lines separate primary sensory neuron populations from adjacent regions. Abbreviations where different from b: E, entopallium; B, basorostralis; HA, hyperpallium apicale; Hp, hippocampus; IHA, interstitial hyperpallium apicale; MV, mesopallium ventrale.

Figure 2. Simplified modern view of vertebrate evolution

The diagram begins with the fish group that contains the most recent ancestors of land vertebrates. This differs from the classic view in that instead of giving rise to reptiles, ancestral amphibians are thought to have given rise to stem amniotes. Stem amniotes then split into at least two groups: the sauropsids, which gave rise to all modern reptiles as we know them today; and the therapsids, which, through a series of now-extinct intermediate forms, evolved into mammals. Many sauropsids (reptiles) are currently living. Solid horizontal lines indicate temporal fossil evidence. Dashed lines indicate proposed ancestral links based on other types of data. MYA, million years ago Based on REFS ,.

Figure 3. Auditory and vocal pathways of the songbird brain within the context of the new consensus view of avian brain organization

Only the most prominent and/or most studied connections are indicated. a | The auditory pathway. Most of the hindbrain connectivity is extrapolated from non-songbird species. For clarity, reciprocal connections in the pallial auditory areas are not indicated. b | The vocal pathways. Black arrows show connections of the posterior vocal pathway (or vocal motor pathway), white arrows indicate the anterior vocal pathway (or pallial–basal ganglia–thalamic–pallial loop) and dashed lines show connections between the two pathways. Av, avalanche; B, basorostralis; CLM, caudal lateral mesopallium; CMM, caudal medial mesopallium; CN, cochlear nucleus; CSt, caudal striatum; DLM, dorsal lateral nucleus of the medial thalamus; DM, dorsal medial nucleus; E, entopallium; HVC (a letter-based name); L1, L2, L3, fields L1, L2 and L3; LAreaX, lateral AreaX of the striatum; LLD, lateral lemniscus, dorsal nucleus; LLI, lateral lemniscus, intermediate nucleus; LLV, lateral lemniscus, ventral nucleus; LMAN, lateral magnocellular nucleus of the anterior nidopallium; LMO, lateral oval nucleus of the mesopallium; MLd, dorsal lateral nucleus of the mesencephalon; NCM, caudal medial nidopallium; NIf, interfacial nucleus of the nidopallium; nXIIts, nucleus XII, tracheosyringeal part; OB, olfactory bulb; Ov, ovoidalis; PAm, para-ambiguus; RA, robust nucleus of the arcopallium; RAm, retroambiguus; SO, superior olive; Uva, nucleus uvaeformis.