Avian brain
The avian brain is the central organ of the nervous system in birds. Birds possess large, complex brains, which process, integrate, and coordinate information received from the environment and make decisions on how to respond with the rest of the body. Like in all chordates, the avian brain is contained within the skull bones of the head.
The bird brain is divided into a number of sections, each with a different function. The cerebrum or telencephalon is divided into two hemispheres, and controls higher functions. The telencephalon is dominated by a large pallium, which corresponds to the mammalian cerebral cortex and is responsible for the cognitive functions of birds. The pallium is made up of several major structures: the hyperpallium, a dorsal bulge of the pallium found only in birds, as well as the nidopallium, mesopallium, and archipallium. The bird telencephalon nuclear structure, wherein neurons are distributed in three-dimensionally arranged clusters, with no large-scale separation of white matter and grey matter, though there exist layer-like and column-like connections. Structures in the pallium are associated with perception, learning, and cognition. Beneath the pallium are the two components of the subpallium, the striatum and pallidum. The subpallium connects different parts of the telencephalon and plays major roles in a number of critical behaviours. To the rear of the telencephalon are the thalamus, midbrain, and cerebellum. The hindbrain connects the rest of the brain to the spinal cord.
The size and structure of the avian brain enables prominent behaviours of birds such as flight and vocalization. Dedicated structures and pathways integrate the auditory and visual senses, strong in most species of birds, as well as the typically weaker olfactory and tactile senses. Social behaviour, widespread among birds, depends on the organisation and functions of the brain. Some birds exhibit strong abilities of cognition, enabled by the unique structure and physiology of the avian brain.
Structure and function
Telencephalon
The telencephalon, or forebrain, is generally a large structure in birds. It is made up of three primary domains: the striatal domain, the pallidal domain, and the pallial domain.Pallium
The pallial domain is divided into two main parts: the Wulst, or hyperpallium, at the dorsal surface of the pallium, and the dorso-ventricular ridge, which comprises the nidopallium, arcopallium, and archipallium. The circuitry of the avian brain is organised such that circuits for processes such as vision and cognition pass between the structures of the pallium, with connections both horizontally and vertically oriented within the pallium. In the sensory areas of the pallium, neurons display a layer-like organisation into columns, with horizontal connections; hence, the part of the pallium responsible for sensory circuitry is also called the sensory cortex in birds.Within the pallium, the nidopallium caudolaterale is thought to be a centre of goal-directed action. It is the part of the pallium that has the most innervation sensitive to dopamine, and has a structure suggesting strong dopamine control. Neural activity in the nidopallium caudolaterale is correlated with rewards, rules, categories, and information held in the memory. The nidopallium caudolaterale is also proportionally much larger in birds with stronger cognitive abilities. Showing diversity in bird brains and potentially the role of the nidopallium caudolaterale is that structure's separation into multiple structures in songbirds.
Lateralisation
The avian telencephalon, like its human counterpart, is organised into two distinct hemispheres. Functions such as song learning in songbirds, vision, auditory, and olfactory, and magnetic sensory systems are all lateralised, meaning that one hemisphere of the brain dominates a certain function. The left hemisphere of the brain dominates activities that demand the separation of important stimuli from distracting ones, while the right hemisphere is more easily distracted and has a broad attention. The right hemisphere also dominates fear and escape responses. In zebra finches, the left hemisphere also is dominant in learning tasks. Birds can sense the orientation of magnetic fields, and use them to help navigate. The right hemisphere of the brain is dominant in the magnetic sense.Subpallial telencephalon
The subpallial tenecephalon of birds, ventral to the pallium, contains neural systems with critical functions. It can be divided into the striatal domain and the pallidal domain, which are progenitor zones from which brain structures develop. The subpallial telencephalon is also grouped into several major functional systems. These are the dorsal somatomotor basal ganglia, the ventral viscerolimbic basal ganglia, subpallial extended amygdala, basal corticopetal system, and the septum. Additionally, the preoptic area is considered part of the subpallial telencephalon. Functional systems exist in both subpallial domains.Research through the 1960s demonstrated that the basal ganglia of birds occupied only the ventromedial telencephalon and not the entire forebrain, a historically held belief. The basal ganglia include the dorsal somatomotor basal ganglia, which possesses both a pallidal and a striatal component, and is made up of the medial striatum, lateral striatum, globus pallidus, and intrapeduncular nucleus. In songbirds, it also contains Area X, which is responsible for aspects of vocal function in songbirds. The DSBG has important functions in voluntary motor control. Pathways including the DSBG allow birds to effect movement when desired and to reject movement when undesired. The basal ganglia also include the ventral viscerolimbic basal ganglia. The VVBG, like the DSBG, occupies portion of both the striatum and pallidum. It contains the ventral portion of the medial striatum, the nucleus accumbens, the olfactory tubercle, and the ventral pallidum. The VVBG functions as a "reward centre" as well as a facilitator of "action selection". In these roles, the VVBG is important in supporting reward-seeking behaviour and discouraging behaviour leading to negative stimulus.
Area X, unique to songbirds, is critical to learning song, which is a reproduction-critical behaviour in songbirds. It has been described by scientists as analogous to the mammalian striatum, due to a similar composition to that part of the brain. However, it also contains brain cells from the pallidum. Area X is activated most strongly when songbirds are learning new songs, and decline thereafter.
The extended amygdala is composed of the central extended amygdala and medial extended amygdala. The central extended amygdala is connected to behaviours involving ingestion, as well as stress, anxiety, and fear. The role and structure of the medial extended amygdala are debated by scientists. The medial extended amygdala receives inputs from the olfactory bulb, as well as the rest of the olfactory system, including the olfactory organs in the front of bird heads. It is related to sexual and social behaviours. Certain structures in the median extended amygdala have been demonstrated in birds to be sexually dimorphic. In chickens, research shows that part of the median extended amygdala plays a part in male sexual behaviour.
The basal corticopetal system in birds is made up of three nuclei: the basal magnocellular nucleus, and the horizontal and vertical limbs of the nuclei of the diagonal band. The role of the basal corticopetal system is poorly known in birds, although that system is known to be correlated to memory in mammals. However, it has been shown that damage to the basal corticopetal system impairs the memory of chicks.
Based on studies of a variety of passerine birds, the avian septum is divided into four main parts: the lateral septum, medial septum, septohippocampal septum, and caudocentral septum. The septum has functions related to stress, as well as responses to stimuli such as light. The preoptic area has functions related to sexual behaviour.
Brain stem and cerebellum
Cerebellum
The cerebellum is a relatively conservative part of the brain, with circuitry tending to be similar across different types of vertebrates. Like in mammals, the avian cerebellum is a strongly folded structure. Bird cerebella are typically divided into ten different groups of folds called lobuli. In some bird groups, the cerebellum is expanded to accommodate different requirements. For instance, in birds that perform much manipulation with the beak, like crows, woodpeckers, and parrots, there is an expansion of the visual part of the cerebellum. In contrast, the part of the cerebellum associated with the tail is expanded.There is substantial variation in the foliation in the cerebella of birds, although all birds have at least some folding of the cerebellum. The folding of the cerebellum is also not strictly associated with the structure and size of the rest of the brain: penguins, seabirds, parrots, and crows, exhibit a similar degree of folding, despite very different brain characteristics. Likewise, owls, galliformes, and pigeons exhibit similar folding patterns. In general, the folding patterns of the cerebellum in birds reflect differences of behaviour, as well as variations in skull shape constraining cerebellar development and sensory and sensorimotor requirements of animals living disparate lifestyles.