Lateralization of brain function

The brain is divided into two hemispheres–left and right. 

Some cognitive functions tend to be dominated by one side or the other; meaning they are lateralized.

The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. 

The median longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. 

The macrostructure of the two brain hemispheres appears to be almost identical, but different composition of neuronal networks allows for specialized function that is different in each hemisphere.

Each human’s brain develops differently, leading to unique lateralization in individuals. 

Lateralization refers only to the function of one structure divided between two hemispheres. 

Broca’s and Wernicke’s areas, both are often found exclusively on the left hemisphere.

Semantics, intonation, accentuation, and prosody are largely found to have a neuronal basis in both hemispheres.

Each hemisphere in the brain tends to represent one side of the body. 

The cerebellum it is is the same body side, but in the forebrain this is predominantly the contralateral side.

Language functions such as grammar, vocabulary and literal meaning are typically lateralized to the left hemisphere, especially in right-handed individuals.

While language production is left-lateralized in up to 90% of right-handers, it is more bilateral, or even right-lateralized, in approximately 50% of left-handers.

Broca’s area and Wernicke’s area, associated with the production of speech and comprehension of speech, respectively, are located in the left cerebral hemisphere for about 95% of right-handers but about 70% of left-handers.

Individuals who speak multiple languages demonstrate separate speech areas for each language.

The processing of basic sensory information is lateralized by being divided into left and right sides of the body or the space around the body.

In vision, about half the neurons of the optic nerve from each eye cross to project to the opposite hemisphere, and about half do not cross to project to the hemisphere on the same side.

This means that the left side of the visual field is processed largely by the visual cortex of the right hemisphere and vice versa for the right side of the visual field.

In hearing, about 90% of the neurons of the auditory nerve from one ear cross to project to the auditory cortex of the opposite hemisphere.

In the sense of touch, most of the neurons from the skin cross to project to the somatosensory cortex of the opposite hemisphere.

Processing of information in the sensory cortices is essentially identical: processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally.

The two cerebral hemispheres have different value systems: the left hemisphere tends to reduce complex matters such as ethics to rules and measures, and the right hemisphere is disposed to the holistic and metaphorical.

Depression is linked with a hyperactive right hemisphere, with evidence of selective involvement in processing negative emotions, pessimistic thoughts and unconstructive thinking, vigilance, arousal and self-reflection.

A relatively hypoactive left hemisphere, is involved in processing pleasurable experiences and “relatively more involved in decision-making processes.

The left hemisphere lesions result in an omissive response bias or error pattern whereas right hemisphere lesions result in a commissive response bias or error pattern.

The delusional misidentification syndromes, reduplicative paramnesia and Capgras delusion are also often the result of right hemisphere lesions.

Left hemisphere damage has many effects on language production and perception.

Lesions to the right hemisphere can result in a lack of intonation when speaking, adverse effects on understanding discourse, reduced ability to generate inferences, comprehend and produce main concepts, reduced ability to manage alternative meanings, often exhibit discourse that is abrupt and perfunctory or verbose and excessive, and can have deficits in situations of turn taking, topic maintenance and shared knowledge.

Lateral brain damage can also affect spatial visual perception, impair perception of high resolution, or detailed aspects of an image. 

People with right hemisphere damage may have impaired perception of low resolution, or big picture, aspects of an image.

If a specific region of the brain, or even an entire hemisphere, is injured its functions can sometimes be taken over by a neighboring region in the same hemisphere or the corresponding region in the other hemisphere.

This possibility of coverage depends upon the area damaged and the patient’s age, with the develop ment of altered pathways to communicate information with detached areas.

Broca’s aphasia is a specific type of expressive aphasia that results from damage or lesions to the Broca’s area of the brain most commonly in the left inferior frontal hemisphere. 

Broca’s area aphasia is an expressive and non-fluent aphasia, as Broca’s area is critical for language pronunciation and production. 

The area controls some motor aspects of speech production and articulation of thoughts to words and as such lesions to the area result in specific non-fluent aphasia.

Wernicke’s aphasia is the result of damage to the area of the brain that is commonly in the left hemisphere above the Sylvian fissure., which 

causes primarily a deficit in language comprehension. 

The language produced by a person with Wernicke’s aphasia has semantic errors and may sound nonsensical.

Wernicke’s aphasia is characterized by phonemic paraphasias, neologism or jargon, and they are unconcerned by the mistakes that they are making.

Certain functions show a degree of lateralization in the brain – with language predominantly processed in the left hemisphere, and spatial and nonverbal reasoning in the right, these functions are not exclusively tied to one hemisphere.

There is an oversimplification about lateralization, by presenting the functional differences between hemispheres as being more absolute than is actually the case.

Damage to the left posterior, superior temporal gyrus (Wernicke’s area) causes language comprehension deficits rather than speech production deficits, a syndrome known as receptive aphasia.

Stimulation of one hemisphere’s motor cortex produces muscle contraction on the opposite side of the body.

Split-brain patients are patients who have undergone corpus callosotomy a severing of a large part of the corpus callosum which  connects the two hemispheres of the brain and allows them to communicate. 

When these connections are cut, the two halves of the brain have a reduced capacity to communicate with each other. 

The right hemisphere is capable of rudimentary language processing, but often has no lexical or grammatical abilities.

Language is primarily localized in the left hemisphere. 

The right hemisphere has the capacity with emotions, such as sarcasm, that can express poetic rhythm in sentences when speaking. 

The right hemisphere is critical for perceiving sarcasm, integrating context,  understanding metaphor, inference, and humour, as well as recognizing and expressing affective or emotional changes in pitch, rhythm, rate, and loudness that convey emotions.

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