# 11.8D: Hemispherical lateralization (2023)

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##### learning objectives
• To describe the impact of hemispheric lateralization on brain function.

A longitudinal fissure separates the human brain into two distinct cerebral hemispheres connected by the corpus callosum. The two sides resemble each other, and the structure of each hemisphere is usually mirrored on the other side. However, despite strong anatomical similarities, the functions of each cortical hemisphere are distinct.

The hemispheres of the cerebral cortex.: The human brain is divided into two hemispheres: left and right. Scientists continue to explore how some cognitive functions tend to be dominated by one side or the other; that is, how they are lateralized.

In popular psychology, broad generalizations are often made about a hemisphere having a broad label, such as "logical" for the left or "creative" for the right. But although there is measurable lateral dominance, most functions are present in both hemispheres. The degree of hemisphere specialization remains under investigation. If a specific region of the brain or even an entire hemisphere is damaged or destroyed, its functions can sometimes be taken over by a neighboring region, even in the opposite hemisphere, depending on the area damaged and the age of the patient. When damage interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with separate areas, despite the inefficiencies.

While many features are left out, this is just a trend. The implementation of a specific brain function varies significantly from person to person. Areas of exploration for this causal or effective difference in particular brain function include gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variation of a given brain function, between the two hemispheres of a brain or between the same hemisphere of two different brains, is still being studied. Other than undergoing a hemispherectomy (removal of one hemisphere of the brain), no one is a "left-brain only" or "right-brain only" person.

## Lateralization and laterality

Lateralization of brain function is evident in left- or right-handed phenomena, but a person's preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people are left-hemisphere dominant for language, 18.8% of left-handed people are right-hemisphere dominant for language function. Furthermore, 19.8% of left-handers have bilateral linguistic functions. Even within various language functions (eg semantics, syntax, prosody), the degree and even hemisphere of dominance can differ.

Language functions such as grammar, vocabulary, and literal meaning are usually lateralized to the left hemisphere, especially in right-handed people. While language production is lateralized to the left in up to 90% of right-handed individuals, it is more bilateral or even lateralized to the right in approximately 50% of left-handers. On the other hand, the prosodic functions of language, such as intonation and emphasis, are generally lateralized to the right hemisphere of the brain.

## Other side distinctions

Processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally, but may show right hemisphere dominance. Numerical estimation, comparison and online calculation depend on the bilateral parietal regions. Precise computation and fact retrieval are associated with the left parietal regions, perhaps because of their links to language processing. Dyscalculia is a neurological syndrome associated with damage to the left temporoparietal junction. This syndrome is associated with poor numerical manipulation, poor mental arithmetic ability, and inability to understand or apply mathematical concepts.

## Lateralization and Evolution

The specialization of the two hemispheres is general in vertebrates, including fish, frogs, reptiles, birds, and mammals, with the left hemisphere specializing in categorizing information and controlling routine behavior. The right hemisphere is responsible for responses to new events and behaviors in emergency situations, including the expression of intense emotions. Feeding is an example of routine left-hemisphere behavior, while escaping from predators is an example of right-hemisphere behavior. This suggests that the evolutionary advantage of lateralization comes from the ability to perform separate parallel tasks in each hemisphere of the brain.

## split brain phenomenon

Split-brain patients are people who have had a corpus callosum callosotomy, an amputation of a large part of the corpus callosum (usually as a treatment for severe epilepsy). The corpus callosum connects the two hemispheres of the brain and allows them to communicate. When these connections are severed, the two halves of the brain have a reduced ability to communicate with each other.

The widespread lateralization of many vertebrate animals indicates an evolutionary advantage associated with the specialization of each hemisphere. The evolutionary advantage of lateralization comes from the ability to perform separate parallel tasks in each hemisphere of the brain. In a 2011 study published in the Journal ofbrain behavior researchthe lateralization of some specific functions as opposed to the general lateralization of the brain was correlated with the efficiency of parallel tasks.

## key points

• The corpus callosum connects the hemispheres of the brain.
• Lateralization of function between the two hemispheres occurs, but after injury other regions of the cortex can often compensate.
• There is no such thing as being left-brained or right-brained.
• Functional lateralization generally varies between individuals.

## key terms

• sticky body: a broad, flat bundle of neural fibers beneath the cortex that connects the left and right cerebral hemispheres and facilitates interhemispheric communication.
• lateralization: locate a function such as speech in the right or left side of the brain.
• hemisphere: Either of the two halves of the brain.
• prosody: Properties of syllables and larger units of speech that contribute to linguistic functions such as intonation, tone, stress, and rhythm.

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