neuroanatomy Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...

, the arcuate fasciculus (AF; ) is a bundle of

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

s that generally connects

Broca's area Broca's area, or the Broca area (, also , ), is a region in the frontal lobe of the dominant Cerebral hemisphere, hemisphere, usually the left, of the Human brain, brain with functions linked to speech production. Language processing in the brai ...

and

Wernicke's area Wernicke's area (; ), also called Wernicke's speech area, is one of the two parts of the cerebral cortex that are linked to speech, the other being Broca's area. It is involved in the comprehension of written and spoken language, in contrast to ...

in the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

. It is an association fiber tract connecting caudal

temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in pr ...

and inferior

frontal lobe The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a Sulcus (neur ...

Structure

The arcuate fasciculus is a

white matter White matter refers to areas of the central nervous system that are mainly made up of myelinated axons, also called Nerve tract, tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distr ...

tract that runs parallel to the superior longitudinal fasciculus. Due to their proximity, they are sometimes referred to interchangeably. They can be distinguished by the location and function of their endpoints in the

frontal cortex The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a groove betw ...

. The arcuate fasciculus terminates in Broca's area (specifically BA 44) which is linked to processing complex syntax. However, the superior longitudinal fasciculus ends in the premotor cortex which is implicated in acoustic-motor mapping.

Connection

Historically, the arcuate fasciculus has been understood to connect two important areas for language use:

in the

inferior frontal gyrus The inferior frontal gyrus (IFG; also gyrus frontalis inferior) is the lowest positioned gyrus of the frontal gyri, of the frontal lobe, and is part of the prefrontal cortex. Its superior border is the inferior frontal sulcus (which divides it ...

and

in the posterior

superior temporal gyrus The superior temporal gyrus (STG) is one of three (sometimes two) gyri in the temporal lobe of the human brain, which is located laterally to the head, situated somewhat above the external ear. The superior temporal gyrus is bounded by: * the l ...

. It is mostly considered to be an oversimplification, but this model is still utilized because a satisfactory replacement has not been developed. The topographical relationships between independent measures of white matter and gray matter integrity suggest that rich developmental or environmental interactions influence brain structure and function. The presence and strength of such associations may elucidate pathophysiological processes influencing systems such as language and

motor planning In psychology and neuroscience, motor planning is a set of processes related to the preparation of a movement that occurs during the reaction time (the time between the presentation of a stimulus to a person and that person's initiation of a motor ...

. As the technique of

diffusion MRI Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast (vision), contrast ...

has improved, this has become a testable hypothesis. Research indicates more diffuse termination of the fibers of the arcuate than previously thought. While the main caudal source of the fiber tract appears to be posterior superior temporal cortex, the rostral terminations are mostly in

premotor cortex The premotor cortex is an area of the motor cortex lying within the frontal lobe of the brain just anterior to the primary motor cortex. It occupies part of Brodmann's area 6. It has been studied mainly in primates, including monkeys and human ...

, part of

Brodmann area 44 Brodmann area 44, or BA44, is part of the frontal cortex in the human brain. Situated just anterior to premotor cortex ( BA6) and on the lateral surface, inferior to BA9. This area is also known as pars opercularis (of the inferior frontal gyr ...

Developmental differences

Myelination is a process by which axons are covered with a protective substance called

myelin Myelin Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...

that drastically increases the signaling efficiency of the neuron. The arcuate fasciculus is heavily myelinated in healthy adult brains. The density of this myelination has been found to predict the accuracy and speed to which one can comprehend sentences and alterations in structure can impact receptive language abilities. However, the arcuate fasciculus of newborns is unmyelinated. The myelination process occurs gradually during childhood; myelin density has been shown to increase between the age of 3 and 10. A study comparing a group of 6-year-olds to a group of 3-year-olds found that the 6-year-olds had stronger functional connectivity of the arcuate fasciculus. The arcuate fasciculus is similarly undeveloped in non-human primates such as chimpanzees and macaques. This supports the theory that the arcuate fasciculus is a critical component in language.

Dorsal stream

The

two-streams hypothesis The two-streams hypothesis is a model of the neural processing of vision as well as hearing. The hypothesis, given its initial characterisation in a paper by David Milner and Melvyn A. Goodale in 1992, argues that humans possess two distinct vis ...

of language proposes that there are two streams by which the brain processes language information: the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

and ventral streams. The basis of this model is generally accepted, however the details of it are highly contentious. The dorsal pathway consists of multiple fiber tracts, one of which is the arcuate fasciculus. The dorsal pathway as a whole is implicated in sensory-to-motor mapping and processing complex syntax.

Role in language

Syntax

Syntax In linguistics, syntax ( ) is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure (constituenc ...

refers to a set of rules by which we order words within a language. Some researchers argue that syntax is what distinguishes language as a uniquely human capacity. Though the exact function of the arcuate fasciculus is still debated, the predominant theory is that it is involved with processing complex sequences of syntax. Studies indicate that as the arcuate fasciculus matures and undergoes myelination, there is a corresponding increase in the ability to process syntax. Furthermore, lesions in the arcuate fasciculus often result in difficulties with syntax. Researchers have found that when subjects are confronted with difficult syntactic structures, there is high synchronicity between the left frontal and parietal regions due to their connection by the arcuate fasciculus. This research further supports the arcuate fasciculus as the key component of human language.

Lateralization

The arcuate fasciculus is a bilateral structure; this means that it is present in both the right and left hemispheres of the brain. These fiber tracts are asymmetrical; the left arcuate fasciculus is stronger than the right. While the left arcuate fasciculus is thought to be the one involved with syntax processing, the right arcuate fasciculus has been implicated in prosody processing. Studies further suggest that the right arcuate fasciculus is involved with the ability to read emotion from human facial expression.

Clinical significance

Conduction aphasia

Historically the arcuate fasciculus has been linked to

conduction aphasia Conduction aphasia, also called associative aphasia, is an uncommon form of aphasia caused by damage to the parietal lobe of the brain. An acquired language disorder, it is characterized by intact auditory comprehension, coherent (yet paraphasi ...

, which is usually the result of damage to the

inferior parietal lobule The inferior parietal lobule (subparietal district) lies below the horizontal portion of the intraparietal sulcus, and behind the lower part of the postcentral sulcus. Also known as Geschwind's territory after Norman Geschwind, an American neu ...

that extends into the subcortical white matter and compromises the arcuate fasciculus. This type of

aphasia Aphasia, also known as dysphasia, is an impairment in a person's ability to comprehend or formulate language because of dysfunction in specific brain regions. The major causes are stroke and head trauma; prevalence is hard to determine, but aph ...

is characterized by difficulty with repetition and prevalent phonemic paraphasias. Patients otherwise exhibit a relatively normal control of language. The symptoms of

suggest that the connection between the posterior temporal cortex and frontal cortex plays a vital role in short-term memory of words and speech sounds that are new or have just been heard. The arcuate fasciculus is the main connection between these two regions. Studies that challenge the claim that the arcuate fasciculus is responsible for repetition cite that in some cases lesions to the arcuate fasciculus nor total

agenesis In medicine, agenesis () refers to the failure of an organ to develop during embryonic growth and development due to the absence of primordial tissue. Many forms of agenesis are referred to by individual names, depending on the organ affected: ...

produce conduction aphasia.

Progressive aphasia

Progressive aphasia is a type of

that slowly worsens over time. It can affect both the production and comprehension of language. Progressive aphasic patients that have

lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by injury or diseases. The term ''Lesion'' is derived from the Latin meaning "injury". Lesions may occur in both plants and animals. Types There is no de ...

s in their arcuate fasciculus were especially deficient in their syntax processing abilities. Worsened syntax processing correlated with the degree of degradation in the arcuate fasciculus.

Tone deafness

In nine out of ten people with tone deafness, the superior arcuate fasciculus in the right hemisphere could not be detected, suggesting a disconnection between the posterior superior temporal gyrus and the posterior inferior frontal gyrus. Researchers suggested the posterior superior temporal gyrus was the origin of the disorder.

Stuttering

In stutterers, the arcuate fasciculus appears to have bilateral deficits that reduce it by one-third or more relative to non-stutterers. However, there is ongoing debate concerning the contribution of each hemisphere. Diffusion-based evidence of differences between stutterers and controls is not isolated to the arcuate fasciculus.

Specific language impairment

Specific language impairment is a disorder that prevents children from developing language normally. These children particularly have difficulty with the syntactic and hierarchal structures of language. Damage to the arcuate fasciculus is implicated as a possible cause of specific language impairment, however further data is required to validate this claim.

Dyslexia

Dyslexia Dyslexia (), previously known as word blindness, is a learning disability that affects either reading or writing. Different people are affected to different degrees. Problems may include difficulties in spelling words, reading quickly, wri ...

is a disorder that is primarily characterized by reading deficits. Research has shown that decreases in the integrity of the arcuate fasciculus coincide with worsened reading ability in dyslexic subjects.

References

External links

* https://web.archive.org/web/20070528184714/http://www.lib.mcg.edu/edu/eshuphysio/program/section8/8ch15/s8c15_14.htm * http://thebrain.mcgill.ca/flash/d/d_10/d_10_cr/d_10_cr_lan/d_10_cr_lan.html

{{DEFAULTSORT:Arcuate Fasciculus Cerebral white matter Central nervous system pathways