Lateralization of brain function

The lateralization of brain function (or hemispheric dominance

Lateralization of brain structures is based on general trends expressed in healthy patients; however, there are numerous counterexamples to each generalization. Each human's brain develops differently, leading to unique lateralization in individuals. This is different from specialization, as lateralization refers only to the function of one structure divided between two hemispheres. Specialization is much easier to observe as a trend, since it has a stronger anthropological history.^[5]

The best example of an established lateralization is that of

Language functions such as grammar, vocabulary and literal meaning are typically lateralized to the left hemisphere, especially in right-handed individuals.^[7] 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.^[8]

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.^[9]: 69 Social interactions, demonstrating fierce emotions, and mathematical information are all provided by the right hemisphere.^[10]

Sensory processing

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.^[11] 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

Because of this functional division of the left and right sides of the body and of the space that surrounds it, the processing of information in the sensory cortices is essentially identical. That is, the processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally.^[8] Numerical estimation, comparison and online calculation depend on bilateral parietal regions^[12][13] while exact calculation and fact retrieval are associated with left parietal regions, perhaps due to their ties to linguistic processing.^[12][13]

Value systems

Rather than just being a series of places where different brain modules occur, there are running similarities in the kind of function seen in each side, for instance how right-side impairment of drawing ability making patients draw the parts of the subject matter with wholly incoherent relationships, or where the kind of left-side damage seen in language impairment not damaging the patient's ability to catch the significance of intonation in speech.

Hemisphere damage

Damage to either the right or left hemisphere, and its resulting deficits provide insight into the function of the damaged area. There is truth to the idea that some brain functions reside more on one side of the brain than the other. We know this in part from what is lost when a stroke affects a particular part of the brain. Left hemisphere damage has many effects on language production and perception. Damage or lesions to the right hemisphere can result in a lack of emotional prosody^[19] or intonation when speaking.^[20] The left hemisphere is often involved with dealing of detail oriented and perception while the right hemisphere is involved with mostly overview and an overall concept of things.[1]

Right hemisphere damage also has grave effects on understanding discourse. People with damage to the right hemisphere have a reduced ability to generate inferences, comprehend and produce main concepts, and a reduced ability to manage alternative meanings. Furthermore, people with right hemisphere damage often exhibit discourse that is abrupt and perfunctory or verbose and excessive. They can also have pragmatic deficits in situations of turn taking, topic maintenance and shared knowledge. .^[20] Although both sides of the hemisphere has different responsibilities and tasks, they both complete each other and create a bigger picture. [2] Lateral brain damage can also affect visual perceptual spatial resolution. People with left hemisphere damage may have impaired 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.

Plasticity

If a specific region of the brain, or even an entire hemisphere, is injured or destroyed, its functions can sometimes be assumed by a neighboring region in the same hemisphere or the corresponding region in the other hemisphere, depending upon the area damaged and the patient's age.^[21] When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.

Broca's aphasia

Broca's aphasia is a specific type of expressive aphasia and is so named due to the aphasia that results from damage or lesions to the Broca's area of the brain, that exists most commonly in the left inferior frontal hemisphere. Thus, the aphasia that develops from the lack of functioning of the Broca's area is an expressive and non-fluent aphasia. It is called 'non-fluent' due to the issues that arise because 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.^[22]

Wernicke's aphasia

The concept of "right-brained" or "left-brained" individuals is considered a widespread myth which oversimplifies the true nature of the brain's cerebral hemispheres (for a recent counter position, though, see below). Proof leading to the "mythbuster" of the left-/right-brained concept is increasing as more and more studies are brought to light. Harvard Health Publishing includes a study from the University of Utah in 2013, that exhibited brain scans revealing similarity on both sides of the brain, personality and environmental factors aside.[1] Although 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.^[23]

Counter position

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The debate as to being “right-brained” or “left-brained” was reopened anew in 2023.[28] This re-opening is based on neuroscientific research which has refined the underlying primary functions of the two sides of the neocortex. In particular, Elkhonon Goldberg's “novelty-routinization” theory posits that the neocortex's right and left hemispheres primarily process cognitively novel and cognitively routine tasks, respectively.^[29]

(Thus, this theory fits with [novel] aspects of visual-spatial processing predominantly occurring in the right hemisphere, as they tend to be relatively novel; similarly, [familiar] aspects of language tend to being predominantly processed in the left hemisphere, as they indeed tend to be cognitively routine to us. New/novel aspects of language, though, will fittingly be processed in the right-brain, just as familiar aspects of visual-spatial processing will be processed in the left-brain. [This explains why these functions are not exclusively tied to one hemisphere.])

As such, right-brained people favor the cognitively novel tasks/information whereas left-brained people favor the cognitively routine tasks/information. Or, in other words, right-brained people tend to prefer new and novel experiences whereas left-brained people favor familiar experiences. (Thus, this theory is the deeper understanding of many of pop psychology's “oversimplifications” of lateralization. For example, it explains why the right hemisphere has been considered creative, as creativity tends to involve new and novel information/tasks, whereas the left hemisphere has been considered analytical, as analysis tends to be the re-processing of already familiar information.)

This is not to say, though, that the tendencies of the left hemisphere cannot be performed well by a right-brained person and vice versa. For instance, right-brained people can exhibit strong analytical skills and attention to detail (generally cognitively routine, left-brained tasks), sometimes even surpassing their left-brained counterparts, but their overall tendency is more so to instead be synthesizing and to attend to the big picture (generally cognitively new, right-hemisphere-tending tasks). (Such nuances are one reason this thinking has been labeled a myth.)

Based on Goldberg's theory, the following table provides newly identified preferences of left- and right-brained people.^[28]

It is vital, though, to remember that these are only tendencies. At times we may contradict our tendency to be left- or right-brained, but often we can come to identify the underlying reason.^[28] Furthermore, although less likely, some of us may be more bilobal, but in general it seems most of demonstrate tendencies related to one side or the other—in other words, most of us prefer either new and novel experiences most of the time or we prefer familiar and known experiences most of the time.

This is not to say, though, that we do not use the whole brain. Life and personal growth require the use and development of both the right and left hemispheres, as it requires the processing of novel, unfamiliar situations as well as familiar, routine ones. Without this ability, we would struggle to survive and thrive. Interestingly, as we get older, we tend to become more set in our ways, which fits with many older adults and seniors having struggles to stay up on technology and other new and novel experiences.

Popular psychology

Some popularizations oversimplify the science about lateralization, by presenting the functional differences between hemispheres as being more absolute than is actually the case.^{[30]: 107 [31]} Interestingly, research has shown quite opposite function of brain lateralisation, i.e. left hemisphere creatively and chaotically links between concepts and right hemisphere tends to adhere to specific date and time, although generally adhering to the pattern of left-brain as linguistic interpretation and right brain as spatio-temporal.^[32][33][34]

Sex differences

In the 19th century and to a lesser extent the 20th, it was thought that each side of the brain was associated with a specific gender: the left corresponding with masculinity and the right with femininity and each half could function independently.

History

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Broca

One of the first indications of brain function lateralization resulted from the research of French physician

These seminal works on hemispheric specialization were done on patients or postmortem brains, raising questions about the potential impact of pathology on the research findings. New methods permit the in vivo comparison of the hemispheres in healthy subjects. Particularly, magnetic resonance imaging (MRI) and positron emission tomography (PET) are important because of their high spatial resolution and ability to image subcortical brain structures.

Movement and sensation

In the 1940s, neurosurgeon

Research by Michael Gazzaniga and Roger Wolcott Sperry in the 1960s on split-brain patients led to an even greater understanding of functional laterality. Split-brain patients are patients who have undergone corpus callosotomy (usually as a treatment for severe epilepsy), a severing of a large part of the corpus callosum. The corpus callosum 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. This led to many interesting behavioral phenomena that allowed Gazzaniga and Sperry to study the contributions of each hemisphere to various cognitive and perceptual processes. One of their main findings was that the right hemisphere was capable of rudimentary language processing, but often has no lexical or grammatical abilities.^[38] Eran Zaidel also studied such patients and found some evidence for the right hemisphere having at least some syntactic ability.^{[citation needed]}

Language is primarily localized in the left hemisphere. While the left hemisphere has proven to be more optimized for language, the right hemisphere has the capacity with emotions, such as sarcasm, that can express prosody in sentences when speaking. According to Sheppard and Hillis, "The right hemisphere is critical for perceiving sarcasm (Davis et al., 2016), integrating context required for understanding metaphor, inference, and humour, as well as recognizing and expressing affective or emotional prosody—changes in pitch, rhythm, rate, and loudness that convey emotions".^[39] One of the experiments carried out by Gazzaniga involved a split-brain male patient sitting in front of a computer screen while having words and images presented on either side of the screen, and the visual stimuli would go to either the right or left visual field, and thus the left or right brain, respectively. It was observed that if the patient was presented with an image to his left visual field (right brain), he would report not seeing anything. If he was able to feel around for certain objects, he could accurately pick out the correct object, despite not having the ability to verbalize what he saw.

Additional images

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  Ventricles of brain and basal ganglia. Superior view. Horizontal section. Deep dissection
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  Ventricles of brain and basal ganglia. Superior view. Horizontal section. Deep dissection

See also

^[40]

References