Cat intelligence

From Wikipedia, the free encyclopedia

Photo of a male tabby

 

Cat intelligence is the capacity of the domesticated cat to solve problems and adapt to its environment. Researchers have also shown feline intelligence to include the ability to acquire new behavior that applies previously learned knowledge to new situations, communicating needs and desires within a social group, and responding to training cues.

The brain of a cat

Brain size

The brain of the domesticated cat is about five centimetres (2.0 in) long, and weighs 25–30 g (0.88–1.06 oz). If a typical cat is taken to be 60 cm (24 in) long with a weight of 3.3 kg (7.3 lb), then the brain would be at 0.91% of its total body mass, compared to 2.33% of total body mass in the average human. Within the encephalization quotient proposed by Jerison in 1973, values above 1 are classified big brained, while values lower than 1 are small brained. The domestic cat is attributed a value of between 1–1.71 relative to human value that is 7.44–7.8. The largest brains in the Felidae family are those of the tigers in Java and Bali, of which the largest relative brain size within the panthera is the tigris. It is debated whether there exists a causal relationship between brain size and intelligence in vertebrates. Correlations have been shown between these factors in a number^[] of experiments. However, correlation does not imply causation. Most experiments involving the relevance of brain size to intelligence hinge on the assumption that complex behavior requires a complex (and therefore intelligent) brain; however, this connection has not been consistently demonstrated.

The surface area of a cat's cerebral cortex is approximately 83 cm² (13 in²) whereas the human brain has a surface area of about 2,500 cm² (390 in²). Furthermore, a theoretical cat weighing 2.5 kg (5.5 lb) has a cerebellum weighing 5.3 g (0.19 oz), 0.17% of the total weight.

Brain structures

A cat's brain

 

According to researchers at Tufts University School of Veterinary Medicine, the physical structure of the brains of humans and cats is very similar. The human brain and the cat brain both have cerebral cortices with similar lobes.

The number of cortical neurons contained in the brain of the cat is reported to be 763 million. Area 17 of the visual cortex was found to contain about 51,400 neurons per mm³. Area 17 is the primary visual cortex.

Both human and feline brains are gyrencephalic, i.e. they have a surface folding.

Analyses of cat brains have shown they are divided into many areas with specialized tasks that are extremely interconnected and share sensory information in a kind of hub-and-spoke network, with a large number of specialized hubs and many alternative paths between them. This exchange of sensory information allows the brain to construct a complex perception of the real world and to react to and manipulate its environment.

The thalamus of the cat includes a hypothalamus, an epithalamus, a lateral geniculate nucleus, and additional secondary nuclear structures.

Secondary brain structures

The domestic cat brain also contains the hippocampus, amygdala, frontal lobes (which comprise 3 to 3.5% of the total brain in cats compared to about 25% in humans), corpus callosum, anterior commissure, pineal gland, caudate nucleus, septal nuclei and midbrain.

Neuroplasticity

Grouse et al. 1979 ascertained the neuroplasticity of kittens' brains, with respect to control of visual stimulus correlated with changes in RNA structures. In a later study, it was found that cats possess visual-recognition memory, and have flexibility of cerebral encoding from visual information.

Brain and diet

A cognitive support diet for felines is a food that is formulated to improve mental processes like attention, short and long-term memory, learning, and problem solving. Claims for cognitive support appear on a number of kitten formulations to help with brain development, as well as diets aimed at seniors to help prevent cognitive disorders. These diets typically focus on supplying Omega-3 fatty acids, omega-6 fatty acids, taurine, vitamins, and other supporting supplements that have positive effects on cognition. 

The omega-3 fatty acids are a key nutrient in cognition for felines. They are essential for felines as they cannot be synthesized naturally and must be obtained from the diet. Omega-3 fatty acids that support brain development and function are alpha-linolenic acid, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Fish oils, fish and other marine sources provide a very rich source of DHA and EPA. Alpha-linolenic acid can be acquired from oils and seeds.

Omega-6 fatty acids are also needed in feline cognition diets. The important omega-6 fatty acid that plays a role in brain support and cognition is arachidonic acid. Arachidonic acid or AA is found in animal sources such as meat and eggs. AA is required in cat diets, as felines convert insignificant amounts of it from linoleic acid due to the limited delta-6 desaturase. Like DHA, arachidonic acid is often found in the brain tissues of cats and seems to have a supporting role in brain function. In a 2000 study completed by Contreras et al., it was found that DHA and AA made up 20% of the fatty acids in the mammalian brain. Arachidonic acid makes up high amounts in the membrane of most cells and has many pro-inflammatory actions.

Taurine is an amino acid, which is essential in cat diets due to their low capacity to synthesize it. Because taurine has the ability to cross the blood–brain barrier in the brain, it has been found to have a role in many neurological functions, especially in the visual development. Without taurine, felines can have an abnormal morphology in the cerebellum and visual cortex. When cats were fed a diet deficient in taurine, this leads to a decrease in the concentration of taurine in the retina of the eye. This results in deterioration of the photoreceptors, followed by complete blindness.

Choline is a water-soluble nutrient that prevents and improves epilepsy and cognitive disorders. Supplementation is part of therapy for cats with seizures and Feline cognitive dysfunction, despite this treatment being mostly based on anecdotal evidence and research done on dogs. It is the precursor to nerve chemicals like dopamine and acetylcholine, making it important for proper functioning of the nervous system.

Intelligence

A sleeping cat. Much like humans, cats experience complex dreams while sleeping, involving long sequences of events that can be retained and recalled.

 

Intelligence through behavioural observation is defined as a composite of skills and abilities. The WAIS test is a measure of intelligence in adult homo sapiens. The test scores on four criteria: verbal comprehension, perceptual organization, working memory and processing speed. In a comparative evaluation from WAIS criteria, cats are generally fair in intelligence.

In controlled experiments, cats showed that they had fully developed concepts of object permanence, meaning that sensorimotor intelligence is completely developed in cats. For human infants, tests involving multiple invisible displacements of an object are used to assess the beginning of mental representation in the sixth and last stage of sensorimotor intelligence. The cats' searches on these tasks were consistent with representation of an unsensed object and fully developed sensorimotor intelligence. The working memory for object permanence of the domesticated cat is surmised from experiment as being of 16 hours.

In 2009, an experiment was conducted where cats could pull on a string to retrieve a treat under a plastic screen. When presented with one string, cats had no trouble getting the treats, but when presented with multiple strings, some of which were not connected to treats, the cats were unable to consistently choose the correct strings, leading to the conclusion that cats do not understand cause and effect in the same way that humans do.

Cats have complex dreams while sleeping, retaining and recalling long sequences of events while they are asleep, as many other animals do. A dreaming cat will usually have rapid, uncontrolled facial, whisker, paw, and abdominal movements.

Memory

Taken as a whole, cats have excellent memories. In experimental conditions, the memory of a cat was demonstrated as having an information-retention or recall, of a duration totalling as much as 10 years. However, relationships with humans, individual differences in intelligence, and age may all affect memory. Cats easily adapt to their current environment because they can adapt their memories of past environments throughout their lives.

In kittens

The period during which the cat is a kitten is the time when the cat learns and memorizes survival skills, which are acquired through observation of their mothers and playing with other cats. Playing, in fact, constitutes more than fun for a kitten, for it is essential for ranking social order, building hunting skills, and generally exercising for the adult roles. 

The first two to seven weeks are a particularly critical time for kittens, for it is during this period that they bond with other cats. It has been suspected that without any human contact during this time, the cat would forever mistrust humans.

In older cats

Just as in humans, advancing age may affect memory in cats. Some cats may experience a weakening of both learning ability and memory that affects them adversely in ways similar to those occurring in poorly aging humans. A slowing of function is normal, and this includes memory. Aging may affect memory by changing the way their brain stores information and by making it harder to recall stored information. Cats lose brain cells as they age, just as humans do. The older the cat, the more these changes can affect its memory. There have been no studies done on the memories of aging cats, but there is some speculation that, just like people, short-term memory is more affected by aging. In one test of where to find food, cats' short-term memory lasted about 16 hours.

Diseases

Disease, such as feline cognitive dysfunction (FCD) – a condition similar to Alzheimer's disease in humans – could also affect cat memory. Symptoms of FCD include disorientation, reduced social interaction, sleep disturbances, and loss of house training. FCD causes degenerative changes in the brain that are the source of the functional impairment.

Learning capacities

Edward Thorndike conducted some key experiments on cat's learning capacity. In one of Thorndike's experiments, cats were placed in various boxes approximately 20 in × 15 in × 12 in (51 cm × 38 cm × 30 cm) with a door opened by pulling a weight attached to it. The cats were observed to free themselves from the boxes by "trial and error with accidental success." Though cats did perform worse on occasion, Thorndike generally found that as cats continued the trials, the time taken to escape the boxes decreased in most cases. Thorndike considered the cat to follow the law of effect, which states that responses followed by satisfaction (i.e. a reward) become more likely responses to the same stimulus in the future. Thorndike was generally skeptical of the presence of intelligence in cats, criticising sources of the contemporary writing of the sentience of animals as "partiality in deductions from facts and more especially in the choice of facts for investigation."

An experiment was done to identify possible observational learning in kittens. Kittens that were able to observe their mothers performing an experimentally organised act were able to perform the same act sooner than kittens that had observed a non-related adult cat, and sooner than the ones who, being placed in trial and error conditions, observed no other cat performing the act.

Domestication effects

Cat intelligence study is mostly from consideration of the domesticated cat. The process of domestication has allowed for closer observation of cat behaviour and in the increased incidence of interspecies communication, and the inherent plasticity of the cat's brain has become apparent as the number of studies in this have increased scientific insight. Changes in the genetic structure of a number of cats have been identified as a consequence of both domestication practises and the activity of breeding, so that the species has undergone genetic evolutionary change due to human selection (although this human selection has been coupled with an initial naturally occurring selective set of cats possessing characteristics desirable for the sharing of human habitation and living in Neolithic urban environments).

Cats' intelligence may have increased during their semi-domestication: urban living may have provided an enriched and stimulating environment requiring novel adaptive behaviours. This scavenging behaviour would only have produced slow changes in evolutionary terms, but such changes would have been comparable to the changes to the brain of early primitive hominids who co-existed with primitive cats (like, for example, Machairodontinae, Megantereon and Homotherium) and adapted to savannah conditions.

Cat's urban living is, however, unlikely to indefinitely improve the animal's intelligence: consider the fossil-based family tree of placental mammals above; the feline line diverged many years previously from the primate line; the cat both feral and domesticated is likely to be maintained in an evolutionary stasis by its niche position in the food web.

Exploitation possibilities

Cats are known to be trained to perform as circus animals. An example of this is The Yuri Kuklachev Cat Theatre based in Moscow, the owner of which has been training cats for many years to perform a range of circus-style tricks.

In artificial intelligence

In November 2009, scientists claimed to simulate a cat's brain using a supercomputer containing 24,576 processors. This experiment did not simulate the function of the individual neurons in the brain, nor their synaptic patterns. It was intended to demonstrate that the problem of simulating a biological brain could be scaled to very large supercomputer platforms. However, the approach has been criticised as flawed.

There are a number of reasons for why the cat brain is a goal of computer simulations. Cats are familiar and easily kept animals, so the physiology of cats has been particularly well studied. The physical structures of human brains and cat brains are very similar. Cats, like humans, have binocular vision that gives them depth perception. Moreover, trying to build artificial mammal brains advances the research of both neuroscience and artificial intelligence.

Basal ganglia

From Wikipedia, the free encyclopedia

Basal ganglia
Basal ganglia labeled at top right.
Basal ganglia on underneath view of brain
Details
Part of	Cerebrum
Identifiers
Latin	nuclei basales
MeSH	D001479
NeuroNames	224, 2677
NeuroLex ID	birnlex_826
TA	A14.1.09.501
FMA	84013

The basal ganglia (or basal nuclei) are a group of subcortical nuclei, of varied origin, in the brains of vertebrates, including humans, which are situated at the base of the forebrain and top of the midbrain. There are some differences in the basal ganglia of primates. Basal ganglia are strongly interconnected with the cerebral cortex, thalamus, and brainstem, as well as several other brain areas. The basal ganglia are associated with a variety of functions, including control of voluntary motor movements, procedural learning, habit learning, eye movements, cognition, and emotion.

The main components of the basal ganglia – as defined functionally – are the striatum; both dorsal striatum (caudate nucleus and putamen) and ventral striatum (nucleus accumbens and olfactory tubercle), globus pallidus, ventral pallidum, substantia nigra, and subthalamic nucleus. Each of these components has a complex internal anatomical and neurochemical organization. The largest component, the striatum (dorsal and ventral), receives input from many brain areas beyond the basal ganglia, but only sends output to other components of the basal ganglia. The pallidum receives input from the striatum, and sends inhibitory output to a number of motor-related areas. The substantia nigra is the source of the striatal input of the neurotransmitter dopamine, which plays an important role in basal ganglia function. The subthalamic nucleus receives input mainly from the striatum and cerebral cortex, and projects to the globus pallidus.

Popular theories implicate the basal ganglia primarily in action selection – in helping to decide which of several possible behaviors to execute at any given time. In more specific terms, the basal ganglia's primary function is likely to control and regulate activities of the motor and premotor cortical areas so that voluntary movements can be performed smoothly. Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The "behavior switching" that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.

The basal ganglia are of major importance for normal brain function and behaviour. Their dysfunction results in a wide range of neurological conditions including disorders of behaviour control and movement. Those of behaviour include Tourette syndrome, obsessive–compulsive disorder, and addiction. Movement disorders include, most notably Parkinson's disease, which involves degeneration of the dopamine-producing cells in the substantia nigra, Huntington's disease, which primarily involves damage to the striatum, dystonia, and more rarely hemiballismus. The basal ganglia have a limbic sector whose components are assigned distinct names: the nucleus accumbens, ventral pallidum, and ventral tegmental area (VTA). There is considerable evidence that this limbic part plays a central role in reward learning as well as cognition and frontal lobe functioning, via the mesolimbic pathway from the VTA to the nucleus accumbens that uses the neurotransmitter dopamine, and the mesocortical pathway. A number of highly addictive drugs, including cocaine, amphetamine, and nicotine, are thought to work by increasing the efficacy of this dopamine signal. There is also evidence implicating overactivity of the VTA dopaminergic projection in schizophrenia.

Structure

In terms of development, the human central nervous system is often classified based on the original three primitive vesicles from which it develops: These primary vesicles form in the normal development of the neural tube of the embryo and initially include the prosencephalon, mesencephalon, and rhombencephalon, in rostral to caudal (from head to tail) orientation. Later in development of the nervous system each section itself turns into smaller components. During development, the cells that migrate tangentially to form the basal ganglia are directed by the lateral and medial ganglionic eminences. The following table demonstrates this developmental classification and traces it to the anatomic structures found in the basal ganglia. The structures relevant to the basal ganglia are shown in bold. 

Primary division of the neural tube	Secondary subdivision	Final segments in a human adult
Prosencephalon	Telencephalon Diencephalon	On each side of the brain: the cerebral cortices, caudate, putamen Globus pallidus, ventral pallidum, thalamus, subthalamus, epithalamus, hypothalamus, subthalamic nucleus
Mesencephalon	Mesencephalon	Mesencephalon (midbrain): substantia nigra pars compacta (SNc), substantia nigra pars reticulata (SNr)
Rhombencephalon	Metencephalon Myelencephalon	Pons and cerebellum Medulla

Coronal slices of human brain showing the basal ganglia. White matter is shown in dark gray, gray matter is shown in light gray.
Anterior: striatum, globus pallidus (GPe and GPi)
Posterior: subthalamic nucleus (STN), substantia nigra (SN)

The basal ganglia form a fundamental component of the cerebrum. In contrast to the cortical layer that lines the surface of the forebrain, the basal ganglia are a collection of distinct masses of gray matter lying deep in the brain not far from the junction of the thalamus. They lie to the side of and surround the thalamus. Like most parts of the brain, the basal ganglia consist of left and right sides that are virtual mirror images of each other.

In terms of anatomy, the basal ganglia are divided into four distinct structures, depending on how superior or rostral they are (in other words depending on how close to the top of the head they are): Two of them, the striatum and the pallidum, are relatively large; the other two, the substantia nigra and the subthalamic nucleus, are smaller. In the illustration to the right, two coronal sections of the human brain show the location of the basal ganglia components. Of note, and not seen in this section, the subthalamic nucleus and substantia nigra lie farther back (posteriorly) in the brain than the striatum and pallidum.

Striatum

Basal ganglia

 

The striatum is a subcortical structure generally divided into the dorsal striatum and ventral striatum, although a medial lateral classification has been suggested to be more relevant behaviorally^[10] and is being more widely used.

The striatum is composed mostly of medium spiny neurons. These GABAergic neurons project to the external (lateral) globus pallidus and internal (medial) globus pallidus as well as the substantia nigra pars reticulata. The projections into the globus pallidus and substantia nigra are primarily dopaminergic, although enkephalin, dynorphin and substance P are expressed. The striatum also contains interneurons that are classified into nitrergic neurons (due to use of nitric oxide as a neurotransmitter), tonically active cholinergic interneurons, parvalbumin-expressing neurons and calretinin-expressing neurons. The dorsal striatum receives significant glutamatergic inputs from the cortex, as well as dopaminergic inputs from the substantia nigra pars compacta. The dorsal striatum is generally considered to be involved in sensorimotor activities. The ventral striatum receives glutamatergic inputs from the limbic areas as well as dopaminergic inputs from the VTA, via the mesolimbic pathway. The ventral striatum is believed to play a role in reward and other limbic functions. The dorsal striatum is divided into the caudate and putamen by the internal capsule while the ventral striatum is composed of the nucleus accumbens and olfactory tubercle. The caudate has three primary regions of connectivity, with the head of the caudate demonstrating connectivity to the prefrontal cortex, cingulate cortex and amygdala. The body and tail show differentiation between the dorsolateral rim and ventral caudate, projecting to the sensorimotor and limbic regions of the striatum respectively. Striatopallidal fibres connect the striatum to the pallidus.

Pallidum

The pallidum consists of a large structure called the globus pallidus ("pale globe") together with a smaller ventral extension called the ventral pallidum. The globus pallidus appears as a single neural mass, but can be divided into two functionally distinct parts, called the internal (or medial) and external (lateral) segments, abbreviated GPi and GPe. Both segments contain primarily GABAergic neurons, which therefore have inhibitory effects on their targets. The two segments participate in distinct neural circuits. The GPe receives input mainly from the striatum, and projects to the subthalamic nucleus. The GPi receives signals from the striatum via the "direct" and "indirect" pathways. Pallidal neurons operate using a disinhibition principle. These neurons fire at steady high rates in the absence of input, and signals from the striatum cause them to pause or reduce their rate of firing. Because pallidal neurons themselves have inhibitory effects on their targets, the net effect of striatal input to the pallidum is a reduction of the tonic inhibition exerted by pallidal cells on their targets (disinhibition) with an increased rate of firing in the targets.

Substantia nigra

Location of the substantia nigra within the basal ganglia

 

The substantia nigra is a midbrain gray matter portion of the basal ganglia that has two parts – the pars compacta (SNc) and the pars reticulata (SNr). SNr often works in unison with GPi, and the SNr-GPi complex inhibits the thalamus. Substantia nigra pars compacta (SNc) however, produces the neurotransmitter dopamine, which is very significant in maintaining balance in the striatal pathway. The circuit portion below explains the role and circuit connections of each of the components of the basal ganglia.

Subthalamic nucleus

The subthalamic nucleus is a diencephalic gray matter portion of the basal ganglia, and the only portion of the ganglia that produces an excitatory neurotransmitter, glutamate. The role of the subthalamic nucleus is to stimulate the SNr-GPi complex and it is part of the indirect pathway. The subthalamic nucleus receives inhibitory input from the external part of the globus pallidus and sends excitatory input to the GPi.

Circuit connections

Connectivity diagram showing excitatory glutamatergic pathways as red, inhibitory GABAergic pathways as blue, and modulatory dopaminergic pathways as magenta. (Abbreviations: GPe: globus pallidus external; GPi: globus pallidus internal; STN: subthalamic nucleus; SNc: substantia nigra pars compacta; SNr: substantia nigra pars reticulata)

 

Connectivity of the basal ganglia as revealed by diffusion spectrum imaging based on thirty subjects from the Human Connectome Project. Direct, indirect and hyperdirect pathways are visualized in different colors (see legend). Subcortical structures are rendered based on the Harvard-Oxford subcortical thalamus as well as the Basal Ganglia atlas (other structures). Rendering was generated using TrackVis software.

 

The left side of Fig.1 shows a region of the prefrontal cortex receiving multiple inputs from other regions, as cortico-cortical activity. The input from B is the strongest of these. The right side of Fig. 1 shows the input signals also being fed to the basal ganglia circuitry. The output from here, back to the same region, is shown to modify the strength of the input from B, by adding strength to the input from C thereby modifying the strongest signal from B to C. (Thalamic involvement is implicit but not shown).

 

Multiple models of basal ganglia circuits and function have been proposed, however there have been questions raised about the strict divisions of the direct and indirect pathways, their possible overlap and regulation. The circuitry model has evolved since the first proposed model in the 1990s by DeLong in the parallel processing model, in which the cortex and substantia nigra pars compacta project into the dorsal striatum giving rise to an inhibitory indirect and excitatory direct pathway.

• The inhibitory indirect pathway involved the inhibition of the globus pallidus externus, allowing for the disinhibition of the globus pallidus internus (through STN) allowing it to inhibit the thalamus.
• The direct or excitatory pathway involved the disinhibition of the thalamus through the inhibition of the GPi/SNr. However the speed of the direct pathway would not be concordant with the indirect pathway in this model leading to problems with it. To get over this, a hyperdirect pathway where the cortex sends glutamatergic projections through the subthalamic nucleus exciting the inhibitory GPe under the center surround model, as well as a shorter indirect pathway have been proposed.

Generally, the basal ganglia circuitry is divided into five pathways: one limbic, two associative (prefrontal), one oculomotor, and one motor pathway. (The motor and oculomotor pathways are sometimes grouped into one motor pathway.) The five general pathways are organized as follows:

• The motor loop involving projections from the supplementary motor area, arcuate premotor area, motor cortex and somatosensory cortex into the putamen, which projects into the ventrolateral GPi and caudolateral SNr which projects into the cortex through the ventralis lateralis pars medialis and ventralis lateralis pars orialis.
• The oculomotor loop involved projections from the frontal eye fields, the dorsolateral prefrontal cortex (DLPFC), and the posterior parietal cortex into the caudate, into the caudal dorsomedial GPi and ventrolateral SNr, finally looping back into the cortex through the lateral ventralis anterior pars magnocellularis(VAmc).
• The first cognitive/associative pathway proposes a pathway from the DLPFC, into the dorsolateral caudate, followed by a projection into the lateral dorsomedial GPi, and rostral SNr before projecting into the lateral VAmc and medial pars magnocellularis.
• The second cognitive/associative pathway proposed is a circuit projecting from the lateral orbitofrontal cortex, the temporal gyrus, and anterior cingulate cortex into the ventromedial caudate, followed by a projection into the lateromedial GPi, and rostrolateral SNr before looping into the cortex via the medial VAmc and medial magnocellularis.
• The limbic circuit involving the projections from the ACC, hippocampus, entorhinal cortex, and insula into the ventral striatum, then into the rostrodorsal GPi, ventral palladium and rostrodorsal SNr, followed by a loop back into the cortex through the posteromedial part of the medial dorsal nucleus. However, more subdivisions of loops have been proposed, up to 20,000.

The direct pathway, originating in the dorsal striatum inhibits the GPi and SNr, resulting in a net disinhibition or excitation of the thalamus. This pathway consist of medium spiny neurons (MSNs) that express dopamine receptor D1, muscarinic acetylcholine receptor M4, and adenosine receptor A1. The direct pathway has been proposed to facilitate motor actions, timing of motor actions, gating of working memory, and motor responses to specific stimuli.

The (long) indirect pathway originates in the dorsal striatum and inhibits the GPe, resulting in disinhibition of the GPi which is then free to inhibit the thalamus. This pathway consists of MSNs that express dopamine receptor D2, muscarinic acetylcholine receptor M1, and adenosine receptor A2a. This pathway has been proposed to result in global motor inhibition(inhibition of all motor activity), and termination of responses. Another shorter indirect pathway has been proposed, which involves cortical excitation of the subthalamic nucleus resulting in direct excitation of the GPe, and inhibition of the thalamus. This pathway is proposed to result in inhibition of specific motor programs based on associative learning.

A combination of these indirect pathways resulting in a hyperdirect pathway that results in inhibition of basal ganglia inputs besides one specific focus has been proposed as part of the center surround theory. This hyperdirect pathway is proposed to inhibit premature responses, or globally inhibit the basal ganglia to allow for more specific top down control by the cortex.

The interactions of these pathways are currently under debate. Some say that all pathways directly antagonize each other in a "push pull" fashion, while others support the center surround theory, in which one focused input into the cortex is protected by inhibition of competing inputs by the rest of the indirect pathways.

Diagram shows two coronal slices that have been superimposed to include the involved basal ganglia structures. Green arrows (+) refer to excitatory glutamatergic pathways, red arrows (–) refer to inhibitory GABAergic pathways and turquoise arrows refer to dopaminergic pathways that are excitatory on the direct pathway and inhibitory on the indirect pathway.

Neurotransmitters

The basal ganglia contains many afferent glutamatergic inputs, with predominantly GABAergic efferent fibers, modulatory cholinergic pathways, significant dopamine in the pathways originating in the ventral tegmental area and substantia nigra, as well as various neuropeptides. Neuropeptides found in the basal ganglia include substance P, neurokinin A, cholecystokinin, neurotensin, neurokinin B, neuropeptide Y, somatostatin, dynorphin, enkephaline. Other neuromodulators found in the basal ganglia include nitric oxide, carbon monoxide, and phenylethylamine.

Functional connectivity

The functional connectivity, measured by regional co-activation during functional neuroimaging studies, is broadly consistent with the parallel processing models of basal ganglia function. The putamen was generally coactivated with motor areas such as the supplementary motor area, caudal anterior cingulate cortex and primary motor cortex, while the caudate and rostral putamen were more frequently coactivated with the rostral ACC and DLPFC. The ventral striatum was significantly associated with the amygdala and hippocampus, which although was not included in the first formulations of basal ganglia models, has been an addition to more recent models.

Function

Eye movements

One intensively studied function of the basal ganglia is its role in controlling eye movements. Eye movement is influenced by an extensive network of brain regions that converges on a midbrain area called the superior colliculus (SC). The SC is a layered structure whose layers form two-dimensional retinotopic maps of visual space. A "bump" of neural activity in the deep layers of the SC drives an eye movement directed toward the corresponding point in space.

The SC receives a strong inhibitory projection from the basal ganglia, originating in the substantia nigra pars reticulata (SNr). Neurons in the SNr usually fire continuously at high rates, but at the onset of an eye movement they "pause", thereby releasing the SC from inhibition. Eye movements of all types are associated with "pausing" in the SNr; however, individual SNr neurons may be more strongly associated with some types of movements than others. Neurons in some parts of the caudate nucleus also show activity related to eye movements. Since the great majority of caudate cells fire at very low rates, this activity almost always shows up as an increase in firing rate. Thus, eye movements begin with activation in the caudate nucleus, which inhibits the SNr via the direct GABAergic projections, which in turn disinhibits the SC.

Role in motivation

Extracellular dopamine in the basal ganglia has been linked to motivational states in rodents, with high levels being linked to satiated "euphoria", medium levels with seeking, and low with aversion. The limbic basal ganglia circuits are influenced heavily by extracellular dopamine. Increased dopamine results in inhibition of the Ventral pallidum, entopeduncular nucleus, and substantia nigra pars reticulata, resulting in disinhibition of the thalamus. This model of direct D1, and indirect D2 pathways explain why selective agonists of each receptor are not rewarding, as activity at both pathways is required for disinhibition. The disinhibition of the thalamus leads to activation of the prefrontal cortex and ventral striatum, selective for increased D1 activity leading to reward. There is also evidence from non-human primate and human electrophysiology studies that other basal ganglia structures including the globus pallidus internus and subthalamic nucleus are involved in reward processing.

Decision making

Two models have been proposed for the basal ganglia, one being that actions are generated by a "critic" in the ventral striatum and estimates value, and the actions are carried out by an "actor" in the dorsal striatum. Another model proposes the basal ganglia acts as a selection mechanism, where actions are generated in the cortex and are selected based on context by the basal ganglia. The CBGTC loop is also involved in reward discounting, with firing increasing with an unexpected or greater than expected reward. One review supported the idea that the cortex was involved in learning actions regardless of their outcome, while the basal ganglia was involved in selecting appropriate actions based on associative reward based trial and error learning.

Working memory

The basal ganglia has been proposed to gate what enters and what doesn't enter working memory. One hypothesis proposes that the direct pathway (Go, or excitatory) allows information into the PFC, where it stays independent of the pathway, however another theory proposes that in order for information to stay in the PFC the direct pathway needs to continue reverberating. The short indirect pathway has been proposed to, in a direct push pull antagonism with the direct pathway, close the gate to the PFC. Together these mechanisms regulate working memory focus.

Clinical significance

Basal ganglia disease is a group of movement disorders that result from either excessive output from the basal ganglia to the thalamus – hypokinetic disorders, or from insufficient output – hyperkinetic disorders. Hypokinetic disorders arise from an excessive output from the basal ganglia, which inhibits the output from the thalamus to the cortex, and thus limits voluntary movement. Hyperkinetic disorders result from a low output from the basal ganglia to the thalamus which gives not enough inhibition to the thalamic projections to the cortex and thus gives uncontrolled/involuntary movements. Dysfunction of the basal ganglia circuitry can also lead to other disorders.

The following is a list of disorders that have been linked to the basal ganglia:

• Addiction
• Athetosis
• Athymhormic syndrome (PAP syndrome)
• Attention-deficit hyperactivity disorder (ADHD)
• Blepharospasm
• Bruxism
• Cerebral palsy: basal ganglia damage during second and third trimester of pregnancy
• Chorea
• Dystonia
• Fahr's disease
• Foreign accent syndrome (FAS)
• Huntington's disease
• Kernicterus
• Lesch–Nyhan syndrome
• Major Depressive Disorder 
• Obsessive-compulsive disorder
• Other anxiety disorders 
• PANDAS
• Parkinson's disease
• Spasmodic dysphonia
• Stuttering
• Sydenham's chorea
• Tardive dyskinesia, caused by chronic antipsychotic treatment
• Tourette's disorder
• Wilson's disease

History

The acceptance that the basal ganglia system constitutes one major cerebral system took time to arise. The first anatomical identification of distinct subcortical structures was published by Thomas Willis in 1664. For many years, the term corpus striatum was used to describe a large group of subcortical elements, some of which were later discovered to be functionally unrelated. For many years, the putamen and the caudate nucleus were not associated with each other. Instead, the putamen was associated with the pallidum in what was called the nucleus lenticularis or nucleus lentiformis.

A thorough reconsideration by Cécile and Oskar Vogt (1941) simplified the description of the basal ganglia by proposing the term striatum to describe the group of structures consisting of the caudate nucleus, the putamen, and the mass linking them ventrally, the nucleus accumbens. The striatum was named on the basis of the striated (striped) appearance created by radiating dense bundles of striato-pallido-nigral axons, described by anatomist Samuel Alexander Kinnier Wilson (1912) as "pencil-like".

The anatomical link of the striatum with its primary targets, the pallidum and the substantia nigra, was discovered later. The name globus pallidus was attributed by Déjerine to Burdach (1822). For this, the Vogts proposed the simpler "pallidum". The term "locus niger" was introduced by Félix Vicq-d'Azyr as tache noire in (1786), though that structure has since become known as the substantia nigra, due to contributions by Von Sömmering in 1788. The structural similarity between the substantia nigra and globus pallidus was noted by Mirto in 1896. Together, the two are known as the pallidonigral ensemble, which represents the core of the basal ganglia. Altogether, the main structures of the basal ganglia are linked to each other by the striato-pallido-nigral bundle, which passes through the pallidum, crosses the internal capsule as the "comb bundle of Edinger", and finally reaches the substantia nigra. 

Additional structures that later became associated with the basal ganglia are the "body of Luys" (1865) (nucleus of Luys on the figure) or subthalamic nucleus, whose lesion was known to produce movement disorders. More recently, other areas such as the centromedian nucleus and the pedunculopontine complex have been thought to be regulators of the basal ganglia. 

Near the beginning of the 20th century, the basal ganglia system was first associated with motor functions, as lesions of these areas would often result in disordered movement in humans (chorea, athetosis, Parkinson's disease).

Terminology

The nomenclature of the basal ganglia system and its components has always been problematic. Early anatomists, seeing the macroscopic anatomical structure but knowing nothing of the cellular architecture or neurochemistry, grouped together components that are now believed to have distinct functions (such as the internal and external segments of the globus pallidus), and gave distinct names to components that are now thought to be functionally parts of a single structure (such as the caudate nucleus and putamen).

The term "basal" comes from the fact that most of its elements are located in the basal part of the forebrain. The term ganglia is a misnomer: In modern usage, neural clusters are called "ganglia" only in the peripheral nervous system; in the central nervous system they are called "nuclei". For this reason, the basal ganglia are also occasionally known as the "basal nuclei". Terminologia anatomica (1998), the international authority for anatomical naming, retained "nuclei basales", but this is not commonly used. 

The International Basal Ganglia Society (IBAGS) informally considers the basal ganglia to be made up of the striatum, the pallidum (with two nuclei), the substantia nigra (with its two distinct parts), and the subthalamic nucleus, whereas Terminologia anatomica excludes the last two. Some neurologists have included the centromedian nucleus of the thalamus as part of the basal ganglia, and some have also included the pedunculopontine nucleus.

Other animals

The basal ganglia form one of the basic components of the forebrain, and can be recognized in all species of vertebrates. Even in the lamprey (generally considered one of the most primitive of vertebrates), striatal, pallidal, and nigral elements can be identified on the basis of anatomy and histochemistry.

The names given to the various nuclei of the basal ganglia are different in different species. In cats and rodents the internal globus pallidus is known as the entopeduncular nucleus. In birds the striatum is called the paleostriatum augmentatum and the external globus pallidus is called the paleostriatum primitivum. 

A clear emergent issue in comparative anatomy of the basal ganglia is the development of this system through phylogeny as a convergent cortically re-entrant loop in conjunction with the development and expansion of the cortical mantle. There is controversy, however, regarding the extent to which convergent selective processing occurs versus segregated parallel processing within re-entrant closed loops of the basal ganglia. Regardless, the transformation of the basal ganglia into a cortically re-entrant system in mammalian evolution occurs through a re-direction of pallidal (or "paleostriatum primitivum") output from midbrain targets such as the superior colliculus, as occurs in sauropsid brain, to specific regions of the ventral thalamus and from there back to specified regions of the cerebral cortex that form a subset of those cortical regions projecting into the striatum. The abrupt rostral re-direction of the pathway from the internal segment of the globus pallidus into the ventral thalamus—via the path of the ansa lenticularis—could be viewed as a footprint of this evolutionary transformation of basal ganglia outflow and targeted influence.