Exploring the Avian Brain
Exploring the Avian Brain
Birds are often bestowed with an undeserving reputation of having a diminished mental capacity. Contrary to popular belief, birds such as the corvids (including crows, ravens and jays) and parrots have a cognitive ability that is comparable to that of mammals. To name a few, the crows of New Caledonia make tools out of leaves to retrieve grubs and are also able to teach other crows to manipulate tools to obtain food. Magpies show self-recognition in a mirror, which was the first evidence of such behavior in a non mammal. Scrub jays are able to plan for the future and show episodic memory which allows them to remember events that took place at a specific time or place. This allows them to gather and store food in preparation for times of scarcity. Herons have been sighted catching insects and placing them on the water surface as bait for fish and Gulls drop rocks on shells to break them open. Parrots can learn words and communicate with humans and African grey parrots are able to use words in numerical and relational concepts. Owls are able to learn sound localization for hunting and songbirds have the ability of vocal learning. Pigeons are capable of memorizing 725 words and are able to differentiate between human made and natural objects, and also have the ability to make a distinction between cubist and impressionistic styles of painting.
The goal of this paper is to explore avian neuroanatomy and account for the similarities and differences between avian and mammalian brains that allow birds the ability to carry out complex cognitive functions. The explorations of avian neuroanatomy also lead to the development of a new system of nomenclature that would make it easier to understand the functioning of the avian brain. This is why we have an old or classical system of nomenclature and a modern system of nomenclature.
The Avian Brain:
Birds and reptiles have a common ancestor, resulting in a similarity in brain structure. The difference between bird and reptile brains is that birds possess larger cerebral hemispheres and cerebella.
The avian brain consists of the medulla oblongata which is the lower half of the brainstem responsible for controlling functions such as breathing, heart rate and blood pressure; the cerebellum which is responsible for motor control and cognitive functions such as attention and language; the optic lobe which is part of the midbrain and relatively large in birds; and the cerebrum which consists of the two cerebral hemispheres and the olfactory lobes. The cerebrum is responsible for communication, sensory processing, learning and memory whereas the olfactory lobes are required for smell.
Figure1: The Dorsal and Ventral View of the Avian Brain
Relationship of Avian and Mammalian brains:
Source: (From: Jarvis et al. 2005).
Figure2: Classical view of Avian (left) and mammalian brain (right)
Similarities between avian and mammalian brains:
Studies have shown that there are several structures in avian brains that are homologous to structures in the mammalian brain. Figure 2, illustrates the similarities in avian and mammalian brains in the old or classical system of nomenclature. For example, the avian neostriatum and hyperstiatum receive auditory, visual and somatosensory input from the thalamus in the same way as the mammalian brain. These regions of the avian brain carry out the same kinds of sensory information processing as the neocotex in mammalian brains. Also the avian hyperstriatum accessorium and the archistriatum play a very important role in motor control and sensorimotor learning, similar to the mammalian neocortex.
Source: (From: Jarvis et al. 2005).
Figure 3: Modern View of Avian (left) and mammalian brain (right)
The telencephalon is located in the anterior portion of the brain and consists of the cerebral cortex, basal ganglia, hippocampus, amygdale and the olfactory bulb. It is responsible for movement, sensory processing, language and communication, olfaction and learning and memory.
The avian telencephalon is organized into three distinct divisions that are developmentally homologous in fish, amphibians, reptiles, birds and mammals. These are the pallial, striatal and pallidial domains.In the striatal and pallidial domains, the avian pallidum, like the mammalian pallidum has a sparse distribution of cells resulting in a pale appearance. The dorsal region of the avian pallidum is found to be homologous with the globius pallidum in mammals and the ventral region of the avian pallidum is homologous with the mammalian ventral pallidum. The avian pallial domain consists of four divisions called hyperpallium, mesopallium, nidopallium and the arcopallium. The main function of the pallidum is to link sensory inputs and motor outputs and serve as an interface between sensory and perceptual processing and mechanisms that regulate behavior. The function of the pallium is the same as the mammalian cortex. The archistriatum is found to be homologous to the pallial and subpallial regions of the mammalian amygdala and is renamed as the amygdaloid complex. Some of the other regions that are found to be homologous among vertebrates were the hippocampus, olfactory bulb and olfactory cortex.
Studies have shown that the organization of the basal ganglia among birds and other vertebrates including mammals has remained the same.
Differences in avian brains:
The avian hyperpallium has a very unique organization comprising of semi-layered subdivisions that scientists think may have evolved more recently than the six-layered mammalian neocortex. Also the Dorsal ventricular Ridge (DVR) which consists of the mesopallium, nidopallium and the arcopallium has nuclear grey matter formation that is found only in birds and reptiles. The six layered neocortex found only in mammals is thought to be inherited from a common therapsid ancestor.
These discoveries about the avian brain has lead to the realization that birds have a relatively well developed avian pallidum that allows information processing in a manner that is similar to the mammalian sensory and motor cortices. Also the cognitive abilities of some birds exceed that of some mammals. We know that the cognitive functions of mammals are carried out by the six layered neocortex, but in birds it is carried out by the pallidum. This indicates that the six layered cortical architecture of the mammalian neocortex in not the only architecturally possible solution for generating cognitive behaviors. Thus it may not be surprising that birds are capable of performing very complex cognitive functions.
2) Emery and Clayton Current Biology, Volume 15, No23, R946